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Main
Date: 19 Aug 2006 08:31:04
From: Jim
Subject: Will an O III help ?
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Hello everyone,
I have only been into astronomy for about 6 mos now.
I recently aquired an 8" orion skyquest dob at the local pawn shop. I Dont
think it has been used much if at all.
It appears to be in collimation according to the manual and all Ive read on
the internet.
My question is about fiilters, When I look at things like the andromeda
galaxy (and others I dont know the names of yet) there dosnt seem to be any
definition. They all look kinda like patches of fog in the sky and I have
found several on my own so far. Will an oxygen III filter help or is that
mainly for photography ? What other filters should I look into buying ?
By the way I traded an Etx-60 AT and 150.00 cash for the 8 inch, I think I
did ok.
Jim
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Date: 19 Aug 2006 09:10:55
From: RMOLLISE
Subject: Re: Will an O III help ?
|
Stephen Paul wrote:
>
> The OIII is primarily for Planetary Nebulae.
> The UHC is primarily for reflection and emission nebulae, but also helps
> with PNs.
>
Hi:
Quite true. However...there are other nebulae that respond incredibly
well to the OIII, too. Mainly supernova remnants like the Veil and the
Crab. But also, for example, things like the Crescent Nebula (NGC 6888,
associated with a Wolf-Rayet star), and even some "normal" emission
nebulae.
Peace,
Rod Mollise
Author of:
Choosing and Using a Schmidt Cassegrain Telescope
and
The Urban Astronomer's Guide
<http://skywatch.brainiac.com/astroland >
The Annual SCT User Imaging Contest is Underway!
<http://www.rothritter.com/contest/2006/ >
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Date: 19 Aug 2006 07:56:47
From: Don't Be Evil
Subject: Re: Will an O III help ?
|
Jim wrote:
> Hello everyone,
> I have only been into astronomy for about 6 mos now.
> I recently aquired an 8" orion skyquest dob at the local pawn shop. I Dont
> think it has been used much if at all.
> It appears to be in collimation according to the manual and all Ive read on
> the internet.
> My question is about fiilters, When I look at things like the andromeda
> galaxy (and others I dont know the names of yet) there dosnt seem to be any
> definition. They all look kinda like patches of fog in the sky and I have
> found several on my own so far. Will an oxygen III filter help or is that
> mainly for photography ? What other filters should I look into buying ?
> By the way I traded an Etx-60 AT and 150.00 cash for the 8 inch, I think I
> did ok.
>
> Jim
Galaxies emit many wavelengths of light. An O-III filter will dim them
to the point of making them invisible. They're really only for certain
nebulae (and they're wonderful for that). The only thing that helps
galaxies much is dark skies.
I'd say you got a great deal. Maybe add a 32mm eyepiece.
Greg
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Date: 19 Aug 2006 10:55:54
From: Stephen Paul
Subject: Re: Will an O III help ?
|
Jim wrote:
> Hello everyone,
> I have only been into astronomy for about 6 mos now.
> I recently aquired an 8" orion skyquest dob at the local pawn shop. I Dont
> think it has been used much if at all.
> It appears to be in collimation according to the manual and all Ive read on
> the internet.
> My question is about fiilters, When I look at things like the andromeda
> galaxy (and others I dont know the names of yet) there dosnt seem to be any
> definition. They all look kinda like patches of fog in the sky and I have
> found several on my own so far. Will an oxygen III filter help or is that
> mainly for photography ? What other filters should I look into buying ?
> By the way I traded an Etx-60 AT and 150.00 cash for the 8 inch, I think I
> did ok.
>
> Jim
>
Jim,
There's not much in the way of galaxy filters. There is one being
prototyped by DGM Optics, not sure if it's available in production.
The OIII is primarily for Planetary Nebulae.
The UHC is primarily for reflection and emission nebulae, but also helps
with PNs.
Each of these filters darkens the background sky by blocking wavelengths
of artificial light, while attempting to preserve (by passing) the
primary wavelengths of light emitted by the target object.
If you can not afford to have both an OIII and a UHC, the UHC is
generally the recommended choice as it is useful on all types of
nebulae. That's not to say equally, however. The OIII is better for PNs,
the UHC for emission/reflection.
HTH,
Stephen Paul
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Date: 19 Aug 2006 14:41:34
From: CeeBee
Subject: Re: Will an O III help ?
|
"Jim" <ki7ll@centurytel.net > wrote in sci.astro.amateur:
> My question is about fiilters, When I look at things like the andromeda
> galaxy (and others I dont know the names of yet) there dosnt seem to be
> any definition. They all look kinda like patches of fog in the sky and I
> have found several on my own so far. Will an oxygen III filter help or
> is that mainly for photography ? What other filters should I look into
> buying ?
Visual all galaxies are "patches of fog". For increasing the contrast you
could use a UHC-filter to eliminate (if applicable) neon light from city
lanterns and backgroup light from cities etc. Don't expect to resolve
individual stars in a galaxies - not with a filer, nor with an amateur
scope.
OIII is especially helpfull for planetary nebula. The effect is really
stunning if you use it for the first time. The planetary nebula's pop up
from the background of stars if you compare it with a view without OIII.
I don't have a good introduction to filters at hand somewhere on the net
right now, but no doubt they're out there and known here. Rememeber the
price of a filter, however; they don't come cheap.
--
CeeBee
*** The Cookie Has Spoken ***
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Date: 19 Aug 2006 20:19:57
From: Joe S.
Subject: Re: Will an O III help ?
|
"Jim" <ki7ll@centurytel.net > wrote in message
news:nc6dnWD7F782vXrZnZ2dnUVZ_sidnZ2d@centurytel.net...
> Hello everyone,
> I have only been into astronomy for about 6 mos now.
> I recently aquired an 8" orion skyquest dob at the local pawn shop. I Dont
> think it has been used much if at all.
> It appears to be in collimation according to the manual and all Ive read
> on the internet.
> My question is about fiilters, When I look at things like the andromeda
> galaxy (and others I dont know the names of yet) there dosnt seem to be
> any definition. They all look kinda like patches of fog in the sky and I
> have found several on my own so far. Will an oxygen III filter help or is
> that mainly for photography ? What other filters should I look into buying
> ?
> By the way I traded an Etx-60 AT and 150.00 cash for the 8 inch, I think I
> did ok.
>
> Jim
>
You did okay. In fact, you did quite well. The XT-8 is a fine scope.
There is a reason that such objects as M31 are referred to as "faint
fuzzies."
Keep looking -- atmospheric conditions will change from day to day -- one
day you may not be able to detect such objects as M31 then one day you'll
see distinct areas in it -- same with other objects.
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Date: 19 Aug 2006 14:02:27
From: RMOLLISE
Subject: Re: Will an O III help ?
|
G.T. wrote:
> I was going to ask a similar question to Jim's after viewing Andromeda
> last weekend in the bright moonlight. With less moon and city light
> pollution will I get better contrast viewing galaxies and globular
> clusters?
>
Hi:
An LPR filter will NOT help with anything composed of stars:
galaxies
open clusters
globular clusters
They also won't help with reflection nebulae like the Merope nebula or
M78 unless said nebula has an emission component (which many do).
Peace,
Rod Mollise
Author of:
Choosing and Using a Schmidt Cassegrain Telescope
and
The Urban Astronomer's Guide
<http://skywatch.brainiac.com/astroland >
The Annual SCT User Imaging Contest is Underway!
<http://www.rothritter.com/contest/2006/ >
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Date: 19 Aug 2006 19:24:36
From: G.T.
Subject: Re: Will an O III help ?
|
RMOLLISE wrote:
> G.T. wrote:
>
>>I was going to ask a similar question to Jim's after viewing Andromeda
>>last weekend in the bright moonlight. With less moon and city light
>>pollution will I get better contrast viewing galaxies and globular
>>clusters?
>>
>
> Hi:
>
> An LPR filter will NOT help with anything composed of stars:
>
> galaxies
> open clusters
> globular clusters
Yes, I understood that from the previous posts. I was just wondering
about my next trip to dark skies on a moonless night. Will galaxies and
clusters stand out better in prime seeing conditions. Even though we've
had our telescope for a year I'm still a beginner because it's rare that
we've taken it away from the city. We went up to 7000ft in the Sierras
twice this summer but they were both mtn bike trips so I didn't have
room for the telescope.
Greg
--
"All my time I spent in heaven
Revelries of dance and wine
Waking to the sound of laughter
Up I'd rise and kiss the sky" - The Mekons
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Date: 20 Aug 2006 01:47:58
From: AstroApp
Subject: Re: Will an O III help ?
|
On 19 Aug 2006 14:02:27 -0700, "RMOLLISE" <rmollise@hotmail.com >
wrote:
>Hi:
>An LPR filter will NOT help with anything composed of stars:
>
>galaxies
>open clusters
>globular clusters
>
>They also won't help with reflection nebulae like the Merope nebula or
>M78 unless said nebula has an emission component (which many do).
>
>Peace,
>Rod Mollise
Really, Rod?
With either my 10" f/4.7 Dob, or my C-11, a light pollution reduction
filter (in my case, an Orion SkyGlow model, though I also have owned a
Lumicon Deep-Sky) will *definitely* help a number of faint galaxies.
'Barnard's Galaxy', NGC-6822, or the Wolf-Lundmark-Melotte Galaxy (WLM
Galaxy), are helped significantly. I can trace out their diameters
about twice as far with the filter, compared to observing without the
filter.
I have also used the filter on a number of extremely faint globular
clusters; in my opinion it did help although starlight is -- overall
-- somewhat dimmed. But contrast against the darker sky background is
somewhat improved!
I think also that the dark lanes in M-31, which the OP mentioned as
being just a patch of fog, are more easily seen in some conditions
with an LPR filter. Recently I used an LPR with M-51 and got a bit
more definition in a small refractor of the barely-seen spiral arms.
During all these observations the naked eye stellar magnitude limit
was around 6th or even fainter, so the sky could have been fairly
described as being quite dark.
The Merope nebula is hard to see *clearly* in the glow around the
bright star, and I think I've only traced it out unmistakably once.
But, an LPR filter often helps render the general eflection nebulosity
around the Pleaides members with amazing effectiveness. Sometimes,
with the filter, I can even see the fine lines that show up in very
sharply resolved photographs.
And I disagree about M78, with in various apertures from 3" to 17"
often seems better with an LPR filter. M20, in particular, is helped
and one can often see the difference between the reflection and
emission components by altering views with, and without, the LPR
filter, or by comparing views with an LPR and a hydrogen-line filter.
On a typical observing session hunting faint 13th-15th mag. galaxies
with my 11" GOTO scope, I have the LPR filter at the ready. Sometimes
I can see details that are not visible without it. Sometimes the
edge-on galaxies like NGC 4565 have a lot more definition. M104's
dust lane is almost always more clearly seen with an LPR filter.
In fact, Jack Marling originally designed the Deep-Sky filter for
galaxies. He states:
"My analysis of sky pollution, the spectral characteristics deep-sky
objects, the spectral response of the human eye to faint light, and
the optical characteristics of the telescopes and filters themselves,
I arrived at design criteria for optimum nebula filters and also
indicated that a 'Galaxy Filter' design would offer a significant
improvement in contrast for spiral galaxies and open clusters
(Population I stars), and would work almost as well for globular
clusters (Population II stars).
As an active amateur astronomer, I arranged to have sky light
pollution-reduction filters of proper design manufactured under the
tradename Lumicon, since the desired characteristics were lacking in
affordable filters available to amateur astronomers."
See the complete paper at this URL:
http://home.earthlink.net/~astro-app/horsehead/Marling.htm
Of special interest is the table at the end of the paper, indicating
which filters are appropriate for specific objects -- and I agree
entirely with his stipulations, having tested them over and over
during the past 15 years.
I wonder if the differing opinions about filters are often due to the
exit pupils being employed for various tests? Perhaps persons finding
filters unsatisfactory are using them with too large, or too small,
exit pupils. (Not meaning Rod, of course, but beginners who haven't
researched the correct use of filters.)
The O-III filter is often useful at very small exit pupils for
discerning bright but narrow diameter "stellar-like" planetaries; but
those same small exit pupils will be worthless for large extended
nebulae: almost any filter will cause a severe extinction. If the
exit pupil is already quite small, the Deep-Sky or UHC filter won't
work very well. Marling's criteria for proper exit pupil size per
filter are pretty well-tested and correspond with the opinions of many
other observers who have tested the same filters on the same objects.
For some informative photos of sky background brightness done to test
the effectiveness of filters, see this article:
http://home.earthlink.net/~astro-app/horsehead/NebFilters.htm
In that article, "visual" filters were used photographically on a
tripod-mounted fixed camera for time exposures. The dark adapted
human eye does not respond the same way that color film does, but you
can sure see the bandpass/bandstop characteristics of the filters in
the 15-minute exposures we did for this article.
To the original poster, I'd summarize all of this in my opinion as
being the following general claims:
1. A "wideband light pollution reduction filter" will help increase
contrast on nebulae (emission and reflection), and to a much lesser
extent on galaxies or globular clusters (depending on the
magnification used and the sky darkness.)
2. A narrowband "nebular filter" will greatly increase contrast on
objects that radiate a specific wavelength of light that the filter
passes unattenuated (such as hydrogen or oxygen lines), but will
greatly darken star images and galaxies.
3. All such filters work over a fairly narrow critical range of exit
pupil. Calculate the exit pupil for each of your eyepieces, and
consult the recommendations of Marling (shown in the calculations of
the free "Eyepiece" computer program, mentioned on my website; I
assume these data are still given in the instruction sheets for
Lumicon filters although other makers -- such as Orion -- don't seem
to discuss the issue.)
AstroApp
http://home.earthlink.net/~astro-app/essays/full-moon-essays.htm
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Date: 22 Aug 2006 20:35:35
From: AstroApp
Subject: Re: Will an O III help ?
|
On Sun, 20 Aug 2006 01:47:58 GMT, AstroApp <AstroApp@blocked.net >
wrote:
>On 19 Aug 2006 14:02:27 -0700, "RMOLLISE" <rmollise@hotmail.com>
>wrote:
>
>>Hi:
>>An LPR filter will NOT help with anything composed of stars:
>>
>>galaxies
>>open clusters
>>globular clusters
>>
>>They also won't help with reflection nebulae like the Merope nebula or
>>M78 unless said nebula has an emission component (which many do).
>>
>>Peace,
>>Rod Mollise
>
>I think also that the dark lanes in M-31, which the OP mentioned as
>being just a patch of fog, are more easily seen in some conditions
>with an LPR filter.
I confirmed this at about 3:50am local west coast time this morning,
using my C-11 (not the ideal scope for the galaxy as the field is too
narrow). At any rate, the dark lane was almost invisible, just being
a gentle diminishment of the central brightness that faded into the
background light, with the gx almost at the meridian.
I added the Orion Sky-Glow LPR filter, and using my 21mm and 32mm
eyepieces (113x and 87x respectively) the dark lane was *much* better
seen and more distinct: not quite approaching the discrete narrow line
seen in a good sharp photo, but very clearly not merely a gentle
brightness rolloff from the nuclear region. It was darker now than
the backgroun light glow in the region around the gx.
This is not the best season to test M78 but it gave me a sort of
worst-case situation, the objet being only perhaps 12 degrees above
the western horizon, with some milky light pollution from Morgan Hill,
CA. that washed out the sky. With my GOTO scope I zeroed in on M78
but saw only the two ~9.9 M stars that are the bright ones in the
heart of the richest area of nebulosity in the region. Around them to
the edge of the EP field was consistent gray milky background glow at
87x. I added the Sky-Glow LPR filter and *BINGO* there was the
recognizable shape of M78! The background light, which consisted
largely of streetlights reflected in haze, was hugely diminished and
the nebula itself stood out.
Now, it is important NOT to take any one person's opinions about
filter performance as being definitive. I did a series of double
blind tests of various types and brands of filters while working for
Orion in the late 1980s, using four extremely well skilled observers
and matched pairs of eyepieces. Usually for any given pair of filters
of a certain type (such as two identical models with slightly
different transmissivity peaks, or two competing brands of the same
type) I got FOUR distinct opinions, differing by a considerable
amount. Sometimes there would be a general consensus but not much
more than that. Everyone saw slightly different results and had
different preferences. Some of us liked the filters with relatively
gentle passband curves; others found them "ineffective" and preferred
the models with STEEP curves. Sometimes there was no preference in
comparing, say, Brand A with Brand B. Each filter gave a different
visual appearance to an object, and certain viewers liked the variety.
In fact, after that test I ultimately ended up with some overlaps in
my own complement of filters, since the Lumicon and Orion models of
the same type *did not look exactly similar* on any one object.
This was a controlled test with a specific series of optical devices
that were of known performance, not just a casual check as might be
done in a star party where people gather round a scope and stick in
different filters to see what happens.
In an even more highly controlled test, such as one that used a filter
wheel or bar allowing for RAPID change with one eyepiece, you can make
distinctions of just a few percent of transmission efficiency by
alternating filters in a second or two. Even then, opinions tend to
vary since every person has both neurophysical and aesthetic issues
that come into play: "the personal equation".
This is why I said earlier that I would make "claims" for the general
characteristics of filters. They are no more than assertions based on
what information that *I* have at hand. In my case they are backed by
tests, Marling's research, and the ideas of close viewing associates
who, by definition, are observing in the *same place* under identical
sky and weather conditions.
Now, if you compare opinions of people all over the world, in diverse
conditions, you will only come to a very coarse, fragmented consensus
-- if there is any consensus at all.
So: people who want to know about filters SHOULD TRY THEM OUT, not
just take advice from articles or usenet posts.
AstroApp
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Date: 20 Aug 2006 01:05:13
From: David Knisely
Subject: Re: Will an O III help ?
|
Rod Mollise wrote:
> An LPR filter will NOT help with anything composed of stars:
>
> galaxies
> open clusters
> globular clusters
>
> They also won't help with reflection nebulae like the Merope nebula or
> M78 unless said nebula has an emission component (which many do).
Well, I'm afraid that I have to disagree somewhat here. I have found
that some broad-band "light pollution" filters *do* help with some of
the larger and more diffuse galaxies under mild light pollution or even
under some dark sky conditions where airglow is a problem. I have found
this particularly true for NGC 253, as the dark detail in that galaxy
seems to come out a bit better with the Lumicon Deep-sky filter than
without it. From my driveway, I notice a small gain in contrast with
M101, M33 and M31, although in some cases, increasing the magnification
can help almost as much. As for reflection nebulae, again, there is a
gain in contrast over non-filtered use, as the filter will kill the
airglow lines as well as some of the main emission lines of light
pollution. I have used the Lumicon Deep-sky on a number of reflection
nebulae such as the Merope Nebula, and I have even used the UHC on that
object, although the UHC tended to dim it a bit more than the Deep-sky
filter did. It isn't exactly a huge gain, but it is there. Whether
this degree of improvement is worth the cost is a judgement call, but to
just dismiss LPR filters for use on continuum sources is probably going
somewhat too far. Clear skies to you.
--
David W. Knisely KA0CZC@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/
**********************************************
* Attend the 13th Annual NEBRASKA STAR PARTY *
* July 23-28, 2006, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************
|
| | | |
Date: 20 Aug 2006 08:23:10
From: Greg Crinklaw
Subject: Re: Will an O III help ?
|
David Knisely wrote:
> Rod Mollise wrote:
>
>> An LPR filter will NOT help with anything composed of stars:
>>
>> galaxies
>> open clusters
>> globular clusters
>>
>> They also won't help with reflection nebulae like the Merope nebula or
>> M78 unless said nebula has an emission component (which many do).
>
> Well, I'm afraid that I have to disagree somewhat here.
I think several people have lost the original context of this thread.
The original poster is a beginner who is disappointed that deep sky
objects look like fuzzy blobs. To him I say forget about the filters.
The answer important to *you* is that deep sky objects will still look
like fuzzy blobs at the eyepiece of a telescope. Filters are useful,
but only after one has learned to appreciate what one is looking at.
If you are observing from a light polluted location, try going to a dark
site. Also, try looking at globular clusters and some of the brighter
planetary nebulae.
For a list of interesting objects to observe, and what you can expect to
see, see the "In the Eyepiece" section of my web site each month:
<http://www.skyhound.com/sh/skyhound.html >
Remember, most of all, that the beauty of what you see in a telescope
(and there is great beauty) is in your mind not your eye. Don't forget
to stop and think about what you are looking at and how far away it is.
Clear skies,
Greg
--
Greg Crinklaw
Astronomical Software Developer
Cloudcroft, New Mexico, USA (33N, 106W, 2700m)
SkyTools: http://www.skyhound.com/cs.html
Observing: http://www.skyhound.com/sh/skyhound.html
Comets: http://www.skyhound.com/sh/comets.html
To reply take out your eye
|
| |
Date: 19 Aug 2006 14:00:52
From: RMOLLISE
Subject: Re: Will an O III help ?
|
Willie R. Meghar wrote:
>
> One of the major decisions after deciding to get one of these filters
> is the size: 2 inch or 1.25 inch? Both filters perform best at
> relatively low magnifications. They'll cut down on the number of
> stars seen and add color to those still visible; but they'll also
> darken the background sky significantly more than they'll darken most
> nebulae -- thus significantly improving contrast (and visibility) for
> the nebulae. These filters perform well regardless of the presence or
> absence of light pollution.
>
>
Hi:
This is not as much of a problem as it used to be. All the 1.25 -
2-inch adapters I've seen lately have been threaded for filters,
allowing you to use 2-inch filters with 1.25-inch eyepieces. So there's
no real reason to choose 1.25-inch filters if you're set up for 2-inch
eyepieces...other than cost, that is. ;-)
Peace,
Rod Mollise
Author of:
Choosing and Using a Schmidt Cassegrain Telescope
and
The Urban Astronomer's Guide
<http://skywatch.brainiac.com/astroland >
The Annual SCT User Imaging Contest is Underway!
<http://www.rothritter.com/contest/2006/ >
|
| |
Date: 19 Aug 2006 12:37:45
From: Willie R. Meghar
Subject: Re: Will an O III help ?
|
"Jim" wrote:
>My question is about fiilters, When I look at things like the andromeda
>galaxy (and others I dont know the names of yet) there dosnt seem to be any
>definition. They all look kinda like patches of fog in the sky and I have
>found several on my own so far. Will an oxygen III filter help or is that
>mainly for photography ? What other filters should I look into buying ?
> By the way I traded an Etx-60 AT and 150.00 cash for the 8 inch, I think I
>did ok.
It looks like you did well with the trade!
An Oxygen III filter will not help you with galaxies; but it will help
with many nebulae. The usual alternative to an OIII is a UHC
(Ultra-High Contrast) filter. The UHC is similar to the OIII in that
it will not help with galaxies; but will help with many nebulae.
Personally, and not everyone is agrees, I prefer the OIII over the
UHC.
One of the major decisions after deciding to get one of these filters
is the size: 2 inch or 1.25 inch? Both filters perform best at
relatively low magnifications. They'll cut down on the number of
stars seen and add color to those still visible; but they'll also
darken the background sky significantly more than they'll darken most
nebulae -- thus significantly improving contrast (and visibility) for
the nebulae. These filters perform well regardless of the presence or
absence of light pollution.
Experience, the use of averted-vision, careful attention to the dark
adaptation of your eyes, and a dark sky will all help with seeing
galaxies, etc. (I probably should have capitalized this paragraph.
It's THAT important!)
I rarely use these filters (though I have one of each type), not
because they don't work; but because I have a personal preference for
the 'natural' view; and I'm lucky enough to have a dark enough sky to
allow me to see 'enough' without the use of a filter. (I probably
should use these filters more often than I do!)
Willie R. Meghar
|
| | |
Date: 19 Aug 2006 12:11:26
From: G.T.
Subject: Re: Will an O III help ?
|
Willie R. Meghar wrote:
> "Jim" wrote:
>
>
>>My question is about fiilters, When I look at things like the andromeda
>>galaxy (and others I dont know the names of yet) there dosnt seem to be any
>>definition. They all look kinda like patches of fog in the sky and I have
>>found several on my own so far. Will an oxygen III filter help or is that
>>mainly for photography ? What other filters should I look into buying ?
>>By the way I traded an Etx-60 AT and 150.00 cash for the 8 inch, I think I
>>did ok.
>
>
> It looks like you did well with the trade!
>
> An Oxygen III filter will not help you with galaxies; but it will help
> with many nebulae. The usual alternative to an OIII is a UHC
> (Ultra-High Contrast) filter. The UHC is similar to the OIII in that
> it will not help with galaxies; but will help with many nebulae.
> Personally, and not everyone is agrees, I prefer the OIII over the
> UHC.
>
> One of the major decisions after deciding to get one of these filters
> is the size: 2 inch or 1.25 inch? Both filters perform best at
> relatively low magnifications. They'll cut down on the number of
> stars seen and add color to those still visible; but they'll also
> darken the background sky significantly more than they'll darken most
> nebulae -- thus significantly improving contrast (and visibility) for
> the nebulae. These filters perform well regardless of the presence or
> absence of light pollution.
>
> Experience, the use of averted-vision, careful attention to the dark
> adaptation of your eyes, and a dark sky will all help with seeing
> galaxies, etc. (I probably should have capitalized this paragraph.
> It's THAT important!)
>
I was going to ask a similar question to Jim's after viewing Andromeda
last weekend in the bright moonlight. With less moon and city light
pollution will I get better contrast viewing galaxies and globular
clusters?
Thanks,
Greg
--
"All my time I spent in heaven
Revelries of dance and wine
Waking to the sound of laughter
Up I'd rise and kiss the sky" - The Mekons
|
| | | |
Date: 19 Aug 2006 20:40:37
From: Willie R. Meghar
Subject: Re: Will an O III help ?
|
"G.T." wrote:
>I was going to ask a similar question to Jim's after viewing Andromeda
>last weekend in the bright moonlight. With less moon and city light
>pollution will I get better contrast viewing galaxies and globular
>clusters?
Views will improve some with less light; but the really dramatic
change occurs when you reach a sky that's dark enough to see stars
that are around magnitude 6.5, give or take a little, with the naked
eye.
In addition to light pollution, atmospheric particulates (dust, smoke,
fog, etc.) can compromise a sky. Such particulates tend to
dramatically magnify the nasty effects of light pollution. Best is a
clean, dark sky.
Willie R. Meghar
|
| |
Date: 20 Aug 2006 06:06:10
From: RMOLLISE
Subject: Re: Will an O III help ?
|
AstroApp wrote:
> Really, Rod?
>
> With either my 10" f/4.7 Dob, or my C-11, a light pollution reduction
> filter (in my case, an Orion SkyGlow model, though I also have owned a
> Lumicon Deep-Sky) will *definitely* help a number of faint galaxies.
>
> 'Barnard's Galaxy', NGC-6822, or the Wolf-Lundmark-Melotte Galaxy (WLM
> Galaxy), are helped significantly. I can trace out their diameters
> about twice as far with the filter, compared to observing without the
> filter.
>
> I have also used the filter on a number of extremely faint globular
> clusters; in my opinion it did help although starlight is -- overall
> -- somewhat dimmed. But contrast against the darker sky background is
> somewhat improved!
>
Hi:
Well, a lot of folks have told me over the years that a very mild
filter like a Deepsky or Skyglow "helps" with galaxies and globulars in
that it darkens the bright sky background without dimmin' the object
itself too much. I've never noticed that. Each and every time I've
tried this trick, the object _still_ looks worse with the filter and
better without. But that's just me...you may certainly find the view
better "with." But there's no doubt that the filter is dimming the DSO.
The light of the stars lies in the same range of wavelengths these
filters must block in order to be effective. Not much way around that.
This all applies to reflection nebulae, too. I do agree that M78 is
helped, to some extent by a filter. Why? As I noted, some reflection
nebulae do have a strong emission component. Ever seen a really deep
shot of M78? You'll seen plenty of red hues.
OTOH, as I said in my last book, DON'T BE AFRAID TO EXPERIMENT. Not
long after I bought my first OIII, I was observing M101. I knew that
the filter would basically destroy that distant giant. But I was
possessed of a strange urge to try it anyway. And out popped many HII
regions!
;-)
Peace,
Rod Mollise
Author of:
Choosing and Using a Schmidt Cassegrain Telescope
and
The Urban Astronomer's Guide
<http://skywatch.brainiac.com/astroland >
The Annual SCT User Imaging Contest is Underway!
<http://www.rothritter.com/contest/2006/ >
|
| | |
Date: 22 Aug 2006 18:03:40
From: David Knisely
Subject: Re: Will an O III help ?
|
Rod Mollise wrote:
> The light of the stars lies in the same range of wavelengths these
> filters must block in order to be effective. Not much way around that.
>
> This all applies to reflection nebulae, too. I do agree that M78 is
> helped, to some extent by a filter. Why? As I noted, some reflection
> nebulae do have a strong emission component. Ever seen a really deep
> shot of M78? You'll seen plenty of red hues.
Well, in the case of the brighter O, B, and A stars which are the
primary illuminators of reflection nebulosity, they tend to have
somewhat more light output in the blue and blue-green parts of the
spectrum than in the far yellow and red wavelengths that the broadband
filters tend to block (usually the broadband's "notch" is between 5300
and 6400 Angstroms, with a second notch in the deep blue below 4500
Angstroms). In addition, the dust from most reflection nebulae tends to
scatter the more bluish parts of the spectrum somewhat better than in
the yellow and red. In most color images of reflection nebulae, these
objects tend to appear somewhat pale bluish or bluish-white, so again,
the area of the spectrum which the broadband filters are eliminating is
not necessarily the main component of the light from these objects.
Light gets blocked to be sure, but so do the faint airglow lines, the
light from unresolved starlight from cooler stars, and the emission
lines from terrestrial light pollution. Under moderate to heavy light
pollution, these broadband filters do tend to get a little "swamped"
resulting in a noticable reduction in their benefit. However, under
mild light pollution or under a dark sky (and especially at the right
magnification), the broadband filters can indeed be somewhat helpful.
In fact, on galaxies, the spiral arm "tracers" are again mostly the hot
O, B, and A stars. Thus, the spiral arms in some of the larger and more
diffuse galaxies may show just a bit more contrast when using a
broadband filter than without one, as the light from between the arms
tends to be from the fainter cooler stars in the galaxy. I have
observed enough galaxies to see which ones tend to be helped by a
broad-band filter and which aren't helped much by them. To me, the
improvements are fairly mild, but sometimes, the effects are just enough
to justify the use of a broadband filter. The effects of the broadband
filters are indeed somewhat mild in magnitude compared with those seen
when true emission nebulae are viewed in narrow-band and line filters,
but the improvements still do exist under the right conditions and with
the right objects. Clear skies to you.
--
David W. Knisely KA0CZC@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/
**********************************************
* Attend the 13th Annual NEBRASKA STAR PARTY *
* July 23-28, 2006, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************
|
| |
Date: 20 Aug 2006 00:55:21
From: David Knisely
Subject: Re: Will an O III help ?
|
USEFUL FILTERS FOR VIEWING DEEP-SKY OBJECTS
by David Knisely
Prairie Astronomy Club
There are a number of different filters available on the market
today for improving the views of various Deep-Sky objects, with most
coming in one of three classes: 1. Broad-Band "light pollution" filters,
2. Narrow-Band "Nebula" filters, and 3: Line filters.
BROAD-BAND "LIGHT-POLLUTION" FILTERS
The broad-band "Light-pollution Reduction (LPR) filters are
designed to improve the visibility of a variety of Deep-Sky objects by
blocking out the common Mercury vapor, Sodium, and some other emission
lines from man-made or natural sources which contribute to light
pollution, while letting through a broad range of other more useful
wavelengths. Since the eye is mainly a "contrast detector", this
selective screening out of some of the background skyglow increases the
contrast and helps Deep-sky objects stand out more noticably. While
these broad-band filters do not eliminate the effects of light pollution
or make the objects brighter, in many cases, these filters can improve
the visibility of some deep-sky objects to at least some degree. The
greatest improvement in the overall view is often found with emission
nebulae, but broadband filters can also give a slight contrast boost to
some reflection nebulae and a few of the larger more diffuse galaxies.
In addition, larger versions of these broadband filters which fit over
camera lenses can be somewhat useful for photography of wide star fields
when some skyglow is present.
Some available broad-band filters are the Lumicon Deep-Sky, the Meade
Series 4000 Broadband, the Celestron LPR, the Thousand Oaks Type 1, and
the Orion SkyGlow. The broadband filters can offer a mild to moderate
boost in contrast and visibility of the fainter outer detail in emisson
nebulae over non-filtered views for objects like the Orion Nebula (M42),
the Lagoon Nebula (M8), the Merope Nebula, the Trifid (M20), and a
number of others. However, the improvement is not as noticable on star
clusters or galaxies. I have found that using the filter on larger and
more diffuse galaxies like M33, M81, M101, NGC 253 and NGC 2403 in my 10
inch when weak skyglow is present will help boost the visibility of the
detail, but the effect is fairly mild. On star clusters, there is even
less of an effect, since some of their emission falls in the portions of
the spectrum blocked by these filters. In that case it may be better to
use slightly higher power on some of the smaller objects to dilute the
light pollution effect a bit. Since some light is blocked by the
filters, there can be times when a few objects may even look fainter
from a dark sky site when using a broad-band filter than without
one. Severe levels of light pollution may also be too much for the
broad-band filters to handle effectively, so you still want to find as
dark an observing site as you can and use averted vision. The broadband
filter has an additional bonus, as it does work fairly well as a blue
filter for observing Jupiter and for bringing out the white clouds and
polar caps of Mars. In summary, the broad band "light pollution" filter
can be useful in compensating for some light pollution, but may not be
the most impressive filter intended for deep-sky use.
NARROW-BAND "NEBULA" FILTERS
Narrow-band "Nebula" filters, as the name implies, are mainly
designed for viewing many emission nebulae. These filters allow only
the bright pair of emission lines of Oxygen III, the Hydrogen Beta
emission line, and wavelengths between H-beta and the OIII lines to get
through. Narrow-band filters darken the background skyglow
significantly without hurting the nebula, and are often of considerable
help when observing in mild to moderate light pollution. The filter's
improvement of the view of emission nebulae is usually superior to that
of the broadband filters, as many faint nebular objects become much
easier to see (without the filter, some may not be visible at all!).
Even the more prominent nebulae which are visible without filters gain
considerable detail and contrast with the narrow band units. However,
you still need to use proper dark adaptation, averted vision and low
to moderate powers (3.7x to 9.9x per inch of aperture) to get the most
out of these filters.
Some available narrow-band filters are the Lumicon UHC, Astronomik
UHC, DGM Optics NPB, Meade Series 4000 Narrowband, Thousand Oaks Type 2,
and Orion Ultrablock. The UHC, NPB, and Meade Narrowband also have a
deep-red passband for the Hydrogen Alpha line. These filters will, for
example, often show the Rosette Nebula TO THE UNAIDED EYE when you look
through them from a dark sky site. Even under a really dark sky, the
contrast and detail improvements are impressive, and most observers
continue to use their narrow-band filters at such dark-sky sites. One
neat trick for finding tiny planetary nebulae is to "blink" the objects
by holding a narrow-band filter between the eyepiece and the eye. The
stars in the field will dim somewhat, but the planetary nebula will
remain undimmed, thus standing out from the background stars.
Many of these narrowband filters will perform quite well, and the
overall difference between them can be very slight. However, these
"nebula" filters usually slightly reduce the brightness of most star
clusters, reflection nebulae, and galaxies, although in moderate light
pollution, a narrowband filter may still be of some use on these objects
with larger apertures. Photographic use of these narrow band filters is
also not recommended.
LINE FILTERS
Line Filters are very narrow passband specialty units which are
designed to let in only one or two spectral lines from emission nebulae,
such as the close pair of Oxygen III lines or the Hydrogen-Beta line.
In the line filter category, the Oxygen III (OIII) filter is the real
standout. Its very narrow bandwidth allows only the pair of emission
lines of Oxygen to get to the eye, and for many planetary and some
diffuse emission nebulae, the boost in contrast has to be seen to be
believed! The Veil and Helix Nebulae look like photographs in a 10"
with the OIII filter, and some of the "green box" emission nebulae in
SKY ATLAS 2000.0 jump out at you. You may even see some nebulae which
are not shown on some atlases. This filter is often the best one for
many planetary nebulae, with the "blinking" technique becoming vastly
more effective, as the stars nearly vanish, leaving the planetary
standing out like a sore thumb. However, since the bandwidth of the
OIII filter is so narrow, it may hurt some nebulae with significant
H-beta emission somewhat, like the nebulae around Gamma Cygni or the
Horsehead. Differences between this filter and narrow-band filters like
the Lumicon UHC are mainly in nebula visibility and contrast. Many
nebulae show a slightly larger area of nebulosity in the UHC filter with
slightly higher brightness, but in the OIII filter, they will often have
more contrast and dark detail. However, the OIII filter really dims the
view of star clusters and galaxies even more than the narrow band
filters do, although observers with large telescopes may find the OIII
useful for bringing out a few emission nebulae in other galaxies, like
the HII regions in M33. The early Lumicon and Meade models also have a
substantial red passband, and on bright emission nebulae like M42 and
M8, weak red color in parts of the nebulae have been reported visually
using moderate to large apertures. Thousand Oaks, Astronomik, and Orion
all now also have OIII filters. The Tele Vue "Bandmate" OIII is,
however, somewhat broader than the standard OIII filters, so it is more
of a narrowband one and may not provide the same level of contrast on
some objects that the "true" Oxygen III filters do.
Another somewhat less-used line filter is the H-Beta. As the name
indicates, the filter only lets through the H-Beta emission line of
Hydrogen, and is best known for its effect on the Horsehead Nebula, the
California Nebula, the Coccoon Nebula, and a few others. On an 8" to
10" scope, the Horsehead Nebula goes from near invisibility to
visibility, and the California Nebula becomes fairly easy, gaining a
great deal of contrast and filamentary detail. An improvement over
non-filter use for additional objects like M42/43, the North America
Nebula, and a few others can also be noted, but in many cases, these
other objects can appear somewhat better overall in the UHC or OIII
filters. The H-beta can also be used to observe some of the structural
details of some brighter nebulae by comparing the H-beta view with that
in other filters. However, the H-beta does not usually work well on
most planetary nebulae, as it nearly wipes out some of them and greatly
dims most of the rest. The total number of emission nebulae which
the H-beta will significantly improve is somewhat limited. Many of
these "H-beta" objects tend to be fairly faint to begin with (like the
Horsehead) and require larger apertures for decent views even with the
filter. Unless you REALLY like looking at these faint H-beta targets,
you may be able to do without the H-Beta filter. Thousand Oaks also
makes their "Type-4" version of the H-beta filter, as does Astronomik.
For recommendations, if you can afford only one filter, get a
narrowband filter like the Lumicon UHC, Meade 4000 Series Narrowband, or
Orion Ultrablock (whichever is least expensive at the time). If you can
afford to get two filters, the OIII makes a good companion filter to a
narrowband one but remember to use them with an eye that is properly
dark adapted and employ averted vision. Filters won't make the objects
brighter, but in many cases, they can make many of them a lot easier to
see. Have fun!
--
David W. Knisely KA0CZC@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/
**********************************************
* Attend the 13th Annual NEBRASKA STAR PARTY *
* July 23-28, 2006, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************
|
| | |
Date: 20 Aug 2006 14:54:05
From: John Banister
Subject: Re: Will an O III help ?
|
Pardon my ignorance, but what are "green box" emission nebulae? I have Sky
Atlas 2000.0 but it's in black and white. Thanks.
-John
"David Knisely" <KA0CZC@navix.net > wrote in message
news:70bfc$44e7f946$471d4219$22698@ALLTEL.NET...
The Veil and Helix Nebulae look like photographs in a 10"
> with the OIII filter, and some of the "green box" emission nebulae in SKY
> ATLAS 2000.0 jump out at you.
|
| | | |
Date: 21 Aug 2006 01:23:49
From: David Knisely
Subject: Re: Will an O III help ?
|
John Banister posted:
> Pardon my ignorance, but what are "green box" emission nebulae? I have Sky
> Atlas 2000.0 but it's in black and white. Thanks.
These are on the color "Deluxe" edition of Sky Atlas 2000, and represent
emission nebulae which are too small (less than 10' arc in size) to be
plotted with their true outlines. S.A. 2000 plots them as small green
boxes (on the black and white, they are just just boxes). Clear skies
to you.
--
David W. Knisely KA0CZC@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/
**********************************************
* Attend the 13th Annual NEBRASKA STAR PARTY *
* July 23-28, 2006, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************
|
| |
Date: 20 Aug 2006 15:35:06
From: George Hein
Subject: Re: Will an O III help ?
|
Some 20 years ago I tried one and was unsatisfied on a Celestron-8 refl.
Last month, likewise, on a TeleVue-85mm with a new filter.
Only visual from suburban-NYC, never tried with photo.
Don't waste your $ unless you really know what you are doing.
|
| | |
Date: 23 Aug 2006 18:36:30
From: AstroApp
Subject: Re: Will an O III help ?
|
On Sun, 20 Aug 2006 15:35:06 -0400, George Hein
<zweistein@optonline.net > wrote:
>Some 20 years ago I tried one and was unsatisfied on a Celestron-8 refl.
>>Last month, likewise, on a TeleVue-85mm with a new filter.
>>Only visual from suburban-NYC, never tried with photo.
>>Don't waste your $ unless you really know what you are doing.
I have no objection to George's reported experience; the filter did
not work for him and he learned from that.
The problem, is -- however -- that the CONTEXT is rather lacking, for
those other people who would want to benefit from this report.
This reminds me of an experience I had, working as a salesman at
Orion's Cupertino store in the 1980s (and also as the person who wrote
catalog articles, particularly the ones about the original Orion
models of filters.) Right after the company introduced the UltraBlock
filter, a customer came in and bought one. The *next day* he brought
it back and was very dismissive, and a little indignant about it.
I asked him to describe his experience so that we'd know why he was so
disappointed. He said that he took his 8" Celestron Schmidt-
Cassegrain scope outside that night and looked at M57, with and
without the filter, and that "it didn't help at all". DONE. That was
his "test".
I asked him what eyepiece he used, and it turned out that it gave
nearly 300x of magnification.
This was the problem with marketing the filter *without* specific
recommendations of exit pupil (comparing Orion's very basic
instructions with the more complex and explicit ones of Marling for
the Lumicon models.) At the exit pupil the fellow was using, the
filter would cause mostly extinction of the light.
Furthermore, M57 is a pretty high-surface-brightness object, visible
even in the central San Jose light pollution with a 2" aperture scope.
The test of the filter that I made, on the other hand, was to use an
Astroscan (4" aperture) -- a "fast" rich field scope that is optimal
at low magnificatiom, giving a very bright light background and
sensitive to light pollution -- to find the Owl Nebula in the middle
of San Jose. The nebula -- a much lower surface brightness one than
M57 -- was absolutely, totally invisible in the Astroscan without the
filter. Adding the filter caused it to be visible, if not really
looking anything like a photo.
If the object is already completely visible so that you can see it in
essential detail, such as M57 looks to me in my front yard using my
Orion ShortTube 80 mm refractor, such filters will not yield any sort
of result that will quicken your pulse and make you delighted that you
spent a hundred bucks for it.
And, if you use such high magnification that the light is greatly
dimmed, the filter is outside of its effective range; after all, it
works by DIMMING light further: the light you do NOT want to see; and
depending on the object, it may even dim some of the object's light as
well, while passing a relatively narrow band of frequencies without
much attenuation. If you use such high magnification that the
background is already very dark, the filter will just cause almost
everything to "go black".
Under such circumstances, a filter can still be very useful, IF the
object being sought is small and has a high surface brightness, such
as a "tiny" planetary nebula. Everything ELSE goes nearly away; only
that nebula remains -- somewhat dimmer, but now unmistakable.
A larger diameter, diffuse object (such as the Owl Nebula) will be
very dim at extremely high magnifications. You need moderate power so
that the filter enhances the overall contrast just right: it leaves
most of the nebula visible, while taking away a lot of the overall
background light. In some cases it is actually helpful if the filter
DOES diminish stars (especially if you are trying to pinpoint a small
planetary in a very starry field, such as IC-1295, which I talk about
here:
http://home.earthlink.net/~astro-app/essays/faint-fuzzies.htm#41-IC1295
Beginners must remember these factors:
1. Filters make things DARKER. The idea is to leave a narrow region
of wavelengths more-or-less unattenuated, and to drop the level of all
other wavelengths. But, many objects radiate a wide bandwidth of
light, especially galaxies. So, even the LPR type (broadband light
pollution rejection) filter will make them dimmer, too. (But, some of
the details you might *want* to see, such as dark lanes in the spiral
arms, get darker still: and stand out.)
2. Filters generally help reveal fine details in brighter object, or
make visible obscure, faint objects that in a given scope and sky are
at the threshold of perception. Bright objects don't *always* need
them, or benefit from them. But SOME aspects of those objects will be
helped. In a dark sky I can see the Owl Nebula very clearly with my 8
inch or larger scopes; but the small darker central regions that seem
to look like "eyes" (giving the nebula its nickname) aren't
necessarily very clearly visible, if at all. Add the filter; you then
see them, because the "eyes" get a bit darker (as explained in another
section of the same article referenced above.) Or, in the case of a
planetary nebula with a large diffuse shell, that region becomes more
easily traced out by eye because the overall diffused background light
in the eyepiece field is diminished. I talk about that in the URL
above, in my examination of the shell of IC-1295 using an O-III
filter. I had no trouble at all finding the object WITHOUT a filter;
adding the O-III made the outline of the nebula much more obvious in
the rich star fields.
3. Filters tend not to work well at extremely high powers of
magnificatioin UNLESS the object is suited. In some cases, they don't
work effectively at extremely LOW powers either. And as Jack Marling
points out in the article at --
http://home.earthlink.net/~astro-app/horsehead/Marling.htm
-- "...the performance of the interference filter will depend on the
f-ratio of the optical system it is used with...this discussion is
only important for nebula filters with rapidly changing transmission
peaks." So you may get varying performance when comparing results with
scopes of different focal ratios. The overall result is a complex
integration of many factors: this is one reason that opinions vary so
widely.
It's possible to be simplistic, and just answer a question about the
potential value of filters by saying, "You wanna see galaxies better?
Don't get filters; go to a darker site!" This is ultimately true, but
it may not be possible or practical for a specific observer.
Maybe galaxies, per se, are hard to see in some urban areas. But, if
the observer would like to use her scope even in a city sky and see
real deep-sky objects satisfactorily, why not look for planetary
nebulae?!
Years ago an article was published in Sky & Tel by a Texas amateur,
Don Ferguson, who used his 3.5" aperture Questar to view planetaries
in his home in Dallas, I believe. Lots of light pollution there! I
took the list of objects he observed and added it to my computer
program "Eyepiece", giving drawings of how the objects looked in my 8
inch scope and suggestions for viewing them, as one of the files in my
"observing" section of the program. I can see all of these
planetaries beautifully in my yard in San Jose, with my 8" or 10" or
11" scopes. In most every case, either the UltraBlock/UHC or the
O-III filter will help dramatically.
So, the beginner who is frustrated that the new scope doesn't show
much more than "faint blobs" when he or she tries to see galaxies,
might instead try different kinds of objects. Double stars don't
require extremely dark skies. Planetary nebulae with high surface
brightness are much more easily seen than galaxies. Bright globulars
and even some high surface brightness diffuse nebulae -- M42, M8, M17
-- can be studied. Reserve you faint galaxy-hunting for the great
opportunities you get when you take the scope away from city lights.
But, filters can certainly help you see things even under adverse
conditions -- if the objects are suitable, and the magnification is in
the proper range.
I have been on the marketing side of this issue, having done original
field testing and catalog descriptions of the Orion models in the
1980s, and having written a computer program for Lumicon to provide
that calculated the correct use of its eyepiece filters ("Lumiview",
now old and discontinued.) But, ultimately, I was a *user* of filters
and an amateur telescopic observer. I was glad to buy filters, even
after I was no longer working for optical dealers, simply because they
were an extremely valuable tool; without them, my observing would be
seriously crippled and limited. Within the last year I have purchased
-- as a retired observer, not connected in ANY way with a dealer or
maker -- an Orion SkyGlow, UltraBlock, and O-III filter, and a Lumicon
H-Beta filter, to replace ones that I sold in 2000 along with some old
scopes that I gave up using. Even when I go to the darkest skies, I
use some of them with every one of my scopes (ranging from 3" to 11"
aperture.)
I have used them on a limited basis for certain galaxies -- primarily
to see faint edge details, H-II regions, or dark lanes -- as well as
very faint, distant globulars; I use them on almost every nebular
object, at least to TEST the view and see if it is improved in some
way or another. If somebody took away my filters, I'd be devastated!
To pull a number out of a hat, I'd say that my ability to see sky
objects had been reduced by 50% -- or greater! -- without my filters.
On the other hand, I do much less planetary and double star observing
than chasing deep sky objects. If I were a planet or lunar guy, I
would not care LESS about nebular filters!
But I have equipped myself to be able to do all kinds of observing, to
make the best of conditions. If this is important to you, and if it
will mean that you get much more use from your scope, then the cost of
the filter is not only reasonable, but also almost inconsequential!
So, rather than respond either one of two ways -- "nah, don't bother
with filters" or "ya can't live without 'em!" -- I think it is better
to arm the user with the context in which the filter MIGHT be useful;
let him or her decide their personal priorities.
AstroApp
|
| | | |
Date: 24 Aug 2006 05:53:26
From: David Nakamoto
Subject: Re: Will an O III help ?
|
Although a bit long, thanks for clarifying some aspects of filter use,
what they can AND cannot do, and what to expect. As with any
technically oriented hobby, astronomer benefits from both knowledge AND
using your head.
As for George and others like him, always try out a telescope before you
buy if possible; public star parties and other such gatherings are good
venues. If you cannot do this, then be prepared to patiently do some
research (and going through brochures is not research, since advertisers
and companies seldom tell you what you really need to know about their
equipment), and prepare to send equipment back.
--- Dave
AstroApp wrote:
> On Sun, 20 Aug 2006 15:35:06 -0400, George Hein
> <zweistein@optonline.net> wrote:
>
>> Some 20 years ago I tried one and was unsatisfied on a Celestron-8 refl.
>>> Last month, likewise, on a TeleVue-85mm with a new filter.
>>> Only visual from suburban-NYC, never tried with photo.
>>> Don't waste your $ unless you really know what you are doing.
>
> I have no objection to George's reported experience; the filter did
> not work for him and he learned from that.
>
> The problem, is -- however -- that the CONTEXT is rather lacking, for
> those other people who would want to benefit from this report.
>
> This reminds me of an experience I had, working as a salesman at
> Orion's Cupertino store in the 1980s (and also as the person who wrote
> catalog articles, particularly the ones about the original Orion
> models of filters.) Right after the company introduced the UltraBlock
> filter, a customer came in and bought one. The *next day* he brought
> it back and was very dismissive, and a little indignant about it.
>
> I asked him to describe his experience so that we'd know why he was so
> disappointed. He said that he took his 8" Celestron Schmidt-
> Cassegrain scope outside that night and looked at M57, with and
> without the filter, and that "it didn't help at all". DONE. That was
> his "test".
>
> I asked him what eyepiece he used, and it turned out that it gave
> nearly 300x of magnification.
>
> This was the problem with marketing the filter *without* specific
> recommendations of exit pupil (comparing Orion's very basic
> instructions with the more complex and explicit ones of Marling for
> the Lumicon models.) At the exit pupil the fellow was using, the
> filter would cause mostly extinction of the light.
>
> Furthermore, M57 is a pretty high-surface-brightness object, visible
> even in the central San Jose light pollution with a 2" aperture scope.
>
> The test of the filter that I made, on the other hand, was to use an
> Astroscan (4" aperture) -- a "fast" rich field scope that is optimal
> at low magnificatiom, giving a very bright light background and
> sensitive to light pollution -- to find the Owl Nebula in the middle
> of San Jose. The nebula -- a much lower surface brightness one than
> M57 -- was absolutely, totally invisible in the Astroscan without the
> filter. Adding the filter caused it to be visible, if not really
> looking anything like a photo.
>
> If the object is already completely visible so that you can see it in
> essential detail, such as M57 looks to me in my front yard using my
> Orion ShortTube 80 mm refractor, such filters will not yield any sort
> of result that will quicken your pulse and make you delighted that you
> spent a hundred bucks for it.
>
> And, if you use such high magnification that the light is greatly
> dimmed, the filter is outside of its effective range; after all, it
> works by DIMMING light further: the light you do NOT want to see; and
> depending on the object, it may even dim some of the object's light as
> well, while passing a relatively narrow band of frequencies without
> much attenuation. If you use such high magnification that the
> background is already very dark, the filter will just cause almost
> everything to "go black".
>
> Under such circumstances, a filter can still be very useful, IF the
> object being sought is small and has a high surface brightness, such
> as a "tiny" planetary nebula. Everything ELSE goes nearly away; only
> that nebula remains -- somewhat dimmer, but now unmistakable.
>
> A larger diameter, diffuse object (such as the Owl Nebula) will be
> very dim at extremely high magnifications. You need moderate power so
> that the filter enhances the overall contrast just right: it leaves
> most of the nebula visible, while taking away a lot of the overall
> background light. In some cases it is actually helpful if the filter
> DOES diminish stars (especially if you are trying to pinpoint a small
> planetary in a very starry field, such as IC-1295, which I talk about
> here:
> http://home.earthlink.net/~astro-app/essays/faint-fuzzies.htm#41-IC1295
>
> Beginners must remember these factors:
>
> 1. Filters make things DARKER. The idea is to leave a narrow region
> of wavelengths more-or-less unattenuated, and to drop the level of all
> other wavelengths. But, many objects radiate a wide bandwidth of
> light, especially galaxies. So, even the LPR type (broadband light
> pollution rejection) filter will make them dimmer, too. (But, some of
> the details you might *want* to see, such as dark lanes in the spiral
> arms, get darker still: and stand out.)
>
> 2. Filters generally help reveal fine details in brighter object, or
> make visible obscure, faint objects that in a given scope and sky are
> at the threshold of perception. Bright objects don't *always* need
> them, or benefit from them. But SOME aspects of those objects will be
> helped. In a dark sky I can see the Owl Nebula very clearly with my 8
> inch or larger scopes; but the small darker central regions that seem
> to look like "eyes" (giving the nebula its nickname) aren't
> necessarily very clearly visible, if at all. Add the filter; you then
> see them, because the "eyes" get a bit darker (as explained in another
> section of the same article referenced above.) Or, in the case of a
> planetary nebula with a large diffuse shell, that region becomes more
> easily traced out by eye because the overall diffused background light
> in the eyepiece field is diminished. I talk about that in the URL
> above, in my examination of the shell of IC-1295 using an O-III
> filter. I had no trouble at all finding the object WITHOUT a filter;
> adding the O-III made the outline of the nebula much more obvious in
> the rich star fields.
>
> 3. Filters tend not to work well at extremely high powers of
> magnificatioin UNLESS the object is suited. In some cases, they don't
> work effectively at extremely LOW powers either. And as Jack Marling
> points out in the article at --
> http://home.earthlink.net/~astro-app/horsehead/Marling.htm
> -- "...the performance of the interference filter will depend on the
> f-ratio of the optical system it is used with...this discussion is
> only important for nebula filters with rapidly changing transmission
> peaks." So you may get varying performance when comparing results with
> scopes of different focal ratios. The overall result is a complex
> integration of many factors: this is one reason that opinions vary so
> widely.
>
> It's possible to be simplistic, and just answer a question about the
> potential value of filters by saying, "You wanna see galaxies better?
> Don't get filters; go to a darker site!" This is ultimately true, but
> it may not be possible or practical for a specific observer.
>
> Maybe galaxies, per se, are hard to see in some urban areas. But, if
> the observer would like to use her scope even in a city sky and see
> real deep-sky objects satisfactorily, why not look for planetary
> nebulae?!
>
> Years ago an article was published in Sky & Tel by a Texas amateur,
> Don Ferguson, who used his 3.5" aperture Questar to view planetaries
> in his home in Dallas, I believe. Lots of light pollution there! I
> took the list of objects he observed and added it to my computer
> program "Eyepiece", giving drawings of how the objects looked in my 8
> inch scope and suggestions for viewing them, as one of the files in my
> "observing" section of the program. I can see all of these
> planetaries beautifully in my yard in San Jose, with my 8" or 10" or
> 11" scopes. In most every case, either the UltraBlock/UHC or the
> O-III filter will help dramatically.
>
> So, the beginner who is frustrated that the new scope doesn't show
> much more than "faint blobs" when he or she tries to see galaxies,
> might instead try different kinds of objects. Double stars don't
> require extremely dark skies. Planetary nebulae with high surface
> brightness are much more easily seen than galaxies. Bright globulars
> and even some high surface brightness diffuse nebulae -- M42, M8, M17
> -- can be studied. Reserve you faint galaxy-hunting for the great
> opportunities you get when you take the scope away from city lights.
> But, filters can certainly help you see things even under adverse
> conditions -- if the objects are suitable, and the magnification is in
> the proper range.
>
> I have been on the marketing side of this issue, having done original
> field testing and catalog descriptions of the Orion models in the
> 1980s, and having written a computer program for Lumicon to provide
> that calculated the correct use of its eyepiece filters ("Lumiview",
> now old and discontinued.) But, ultimately, I was a *user* of filters
> and an amateur telescopic observer. I was glad to buy filters, even
> after I was no longer working for optical dealers, simply because they
> were an extremely valuable tool; without them, my observing would be
> seriously crippled and limited. Within the last year I have purchased
> -- as a retired observer, not connected in ANY way with a dealer or
> maker -- an Orion SkyGlow, UltraBlock, and O-III filter, and a Lumicon
> H-Beta filter, to replace ones that I sold in 2000 along with some old
> scopes that I gave up using. Even when I go to the darkest skies, I
> use some of them with every one of my scopes (ranging from 3" to 11"
> aperture.)
>
> I have used them on a limited basis for certain galaxies -- primarily
> to see faint edge details, H-II regions, or dark lanes -- as well as
> very faint, distant globulars; I use them on almost every nebular
> object, at least to TEST the view and see if it is improved in some
> way or another. If somebody took away my filters, I'd be devastated!
> To pull a number out of a hat, I'd say that my ability to see sky
> objects had been reduced by 50% -- or greater! -- without my filters.
>
> On the other hand, I do much less planetary and double star observing
> than chasing deep sky objects. If I were a planet or lunar guy, I
> would not care LESS about nebular filters!
>
> But I have equipped myself to be able to do all kinds of observing, to
> make the best of conditions. If this is important to you, and if it
> will mean that you get much more use from your scope, then the cost of
> the filter is not only reasonable, but also almost inconsequential!
>
> So, rather than respond either one of two ways -- "nah, don't bother
> with filters" or "ya can't live without 'em!" -- I think it is better
> to arm the user with the context in which the filter MIGHT be useful;
> let him or her decide their personal priorities.
>
> AstroApp
>
>
>
>
>
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Date: 21 Aug 2006 05:50:34
From:
Subject: Re: Will an O III help ?
|
See if you caan find http:\\sites.netscape.net/raycash715/filters.htm
RMOLLISE wrote:
> Stephen Paul wrote:
>
> >
> > The OIII is primarily for Planetary Nebulae.
> > The UHC is primarily for reflection and emission nebulae, but also helps
> > with PNs.
> >
>
>
> Hi:
>
> Quite true. However...there are other nebulae that respond incredibly
> well to the OIII, too. Mainly supernova remnants like the Veil and the
> Crab. But also, for example, things like the Crescent Nebula (NGC 6888,
> associated with a Wolf-Rayet star), and even some "normal" emission
> nebulae.
>
> Peace,
> Rod Mollise
> Author of:
> Choosing and Using a Schmidt Cassegrain Telescope
> and
> The Urban Astronomer's Guide
> <http://skywatch.brainiac.com/astroland>
> The Annual SCT User Imaging Contest is Underway!
> <http://www.rothritter.com/contest/2006/>
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Date: 21 Aug 2006 13:52:02
From: Jim
Subject: Re: Will an O III help ?
|
In article <1156164634.118056.41740@i42g2000cwa.googlegroups.com >, Havriliak@aol.com wrote:
> See if you caan find http:\\sites.netscape.net/raycash715/filters.htm
Doesn't resolve, but I think this is the same:
<http://pages.sbcglobal.net/raycash/filters.htm >
Jim
--
Find me at http://www.ursaMinorBeta.co.uk
JediGeeks http://www.jedigeeks.com
"Ah, gentle dames, it gars me greet, To think how monie councels sweet,
How monie lengthen'd, sage advices, The Husband frae the wife despises!"
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