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Date: 28 Apr 2007 23:59:28
From: Bullseye
Subject: 4 inch relector or bigger for the moon?
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I have a 4 inch Nexstar 114GT. The moon looks impressive with this
scope but I was wondering how much better does an 8 incher make it? It
seems like overkill but I don't know.
Also is a refractor any good for the moon? And which is better for the
planets, an 8 inch reflector or a 3.5" refractor?
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Date: 30 Apr 2007 23:35:07
From: canopus56
Subject: Re: 4 inch relector or bigger for the moon?
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On Apr 28, 5:59 pm, Bullseye <b...@ei.com > wrote:
> I have a 4 inch Nexstar 114GT. The moon looks impressive with this
> scope but I was wondering how much better does an 8 incher make it? It
> seems like overkill but I don't know.
There are three factors: aperture, focal length and tracking.
Because the Moon is relatively bright, there is no need to increase
aperture from 4" to 10" in order to brighten features.
I. Aperture
But up to 10" you will continue to get increased detailed because the
Airy disk becomes smaller. The limiting factor is general 1 arcsec
atmospheric turbulence. Absent adaptive optics, after 10" your scope
is on the same resolution footing as 100" scope.
II. Focal length
The Moon is best viewed at the highest feasible focal length and
highest magnifications. So, let's say I drop a 6mm e.p. into my 5
1/4" Apogee refractor, f/l 1140mm. That gives about 190x and a TFOV
of 17 arcmins or 985 arcsecs. With the Moon at an average of 384400
kilometers, that gives a huge linear size of almost 1800 kilometers.
That's a huge amount of terrain on the Moon. Features are small.
Now let's say I put the 6mm ep in a 16" scope with a focal length of
5080mm. Then the 6mm gives about 845 power and the TFOV on the lunar
surface is about 410 kilometers. Now you are down to the apparent
size of terrain that you see in popular LPOD images. http://www.lpod.org
III. Tracking
A higher magnifications, the usefulness of magnification and apeture
depends on good tracking. So for example, on last Friday night, I
watched the Moon at a local club star party using a friend's 16" Dob
light bucket. It had tremendous resolution and could even pull a
wonderful view of M13 out of near full Moon sky. But when viewing
lunar features, the highest resolution the scope could be used at was
about 150x - something your 4"' Nexstar can do. A higher
magnifications, features would zip past the eyepiece if you turned
away for only a second.
On Saturday night, I watched the Moon using my 5 1/4" refractor on a
heavy EQ-6 mount with solid tracking. At 200x with stops in the e.p.
I was getting 400-600km mare spanning views, but the view was more
enjoyable as I could sit in a relaxed manner as the image remained in
the center of the eyepiece and concentrate on perceiving details on
the surface. Tracking adjustments were only required every 10
minutes. So I could perceive far more in my 5 1/4" refractor than I
could in the 16" Dob.
The real - knock your socks off - transition comes in when I use an
old 16" equatorial mounted teaching telescope from the 1960s that my
local club owns. Then you get the sensation you are in an orbiter
about 1200-1500km above the lunar surface - looking out a portal.
As others have noted, the best way to experience these differences is
to go to a local star party and tour a variety of scopes looking at
the Moon.
- Canopus56
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Date: 30 Apr 2007 20:24:52
From: Grumman
Subject: Re: 4 inch relector or bigger for the moon?
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Bullseye wrote:
> And which is better for the planets, an 8 inch reflector or a 3.5" refractor?
Theoretically, the 8" will perform far better. It'll be brighter, handle
higher magnification better. And since the planets look *really* tiny
from here, that's all good.
Practically, if that 8" isn't mounted on some form of tracking mount
(equatorial, motorized alt-az, etc) then its going to be a miserable
experience trying to view anything at high power for more than a few
seconds at a time.
IMO, hand 'guiding' a dob at high mags is off-putting enough to make me
wonder why I bought my scope in the first place. But I keep it since
there really aren't that many things visible in an 8" scope that benefit
much from high mags in the first place.
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Date: 30 Apr 2007 23:44:15
From: Brian Tung
Subject: Re: 4 inch relector or bigger for the moon?
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Grumman wrote:
> Practically, if that 8" isn't mounted on some form of tracking mount
> (equatorial, motorized alt-az, etc) then its going to be a miserable
> experience trying to view anything at high power for more than a few
> seconds at a time.
It's something you have to account for, but in my opinion, it's not
quite as bad as all that. At 300x, as high a power as is practical on
most nights, the field of view is about 10 arcminutes for a 50-degree
AFOV eyepiece, or about 40 seconds of time at the celestial equator
(somewhat more at higher declinations). The planet is in the central
half of the field of view, and therefore reasonable to see in detail,
for about half that time, of course.
> IMO, hand 'guiding' a dob at high mags is off-putting enough to make me
> wonder why I bought my scope in the first place. But I keep it since
> there really aren't that many things visible in an 8" scope that benefit
> much from high mags in the first place.
???
The things that demand high magnifications the most are the planets, and
they are visible in a scope of any size. What's more, 8 inches is right
about where the planets are really getting exciting.
--
Brian Tung <brian@isi.edu >
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
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Date: 30 Apr 2007 20:56:39
From: Dennis Woos
Subject: Re: 4 inch relector or bigger for the moon?
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> IMO, hand 'guiding' a dob at high mags is off-putting enough to make me
> wonder why I bought my scope in the first place. But I keep it since there
> really aren't that many things visible in an 8" scope that benefit much
> from high mags in the first place.
Six planets and their moons, features on Luna, many planetary nebulae, and I
don't know how many double/multiple stars immediately come to mind as stuff
we have profitably observed using high power (around 200x) in the past year
in our 6" and 10" dobs. Lowering the power a bit to 150x adds a bunch of
globular clusters, etc. I think that there are enough good high-power
targets for a 8" dob to last a lifetime.
So far as hand guiding a dob goes, I can believe that it is not everyone's
cup of tea. However, it is possible that the alt-az bearings on your dob
aren't any good, and make it impossible for you to effortlessly guide. This
is especially likely if it is one of the ubiquitous Chinese dobs, or the
Meade LightBridge. This is simply fixable, and I have assisted a friend of
ours upgrade his Orion 8" dob with Ebony Star and Teflon. It made a world
of difference.
Dennis
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Date: 29 Apr 2007 11:10:14
From: Pat O'Connell
Subject: Re: what has happened to people
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AustinMN wrote:
> On Apr 28, 4:45 pm, Chris L Peterson <c...@alumni.caltech.edu> wrote:
>> On Sun, 29 Apr 2007 00:35:24 +0300, Anthony Ayiomamitis
>>
>> <anth...@perseus.no2spam.gr> wrote:
>> >Thanks for the quote! I saved it.
>>
>> You might also appreciate this bit of wisdom from Richard Feynman:
>>
>> "It doesn't seem to me that this fantastically marvelous universe, this
>> tremendous range of time and space and different kinds of animals, and
>> all the different planets, and all these atoms with all their motions
>> and so on, all this complicated thing can merely be a stage so that God
>> can watch human beings struggle for good and evil, which is the view
>> that religion has.
>>
>> "The stage is too big for the drama."
>
> I find the very concept of an astronomer trying to make authoritarian
> comments on religion as unsound as a priest trying to make
> authoritarian comments on astronomy.
Almost by definition, no one can make authoritarian comments about
religion, especially a priest, mullah, monk or minister. All we have
about any religion is books, and there's no proof that any of the
religious events in the various texts ever occurred. Jesus, Buddha,
Mohammed etc., or their followers may have indeed put beliefs down on
paper but something written in a book may be fact (if there's evidence),
opnion, or fiction.
--
Pat O'Connell
[note munged EMail address]
Take nothing but pictures, Leave nothing but footprints,
Kill nothing but vandals...
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Date: 29 Apr 2007 20:12:47
From: Paul Schlyter
Subject: Re: what has happened to people
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In article <lvmdnYkAvZ73TKnbnZ2dnUVZ_gOdnZ2d@comcast.com >,
Pat O'Connell <gypkap.figureitout@gmail.com > wrote:
> AustinMN wrote:
>> On Apr 28, 4:45 pm, Chris L Peterson <c...@alumni.caltech.edu> wrote:
>>> On Sun, 29 Apr 2007 00:35:24 +0300, Anthony Ayiomamitis
>>>
>>> <anth...@perseus.no2spam.gr> wrote:
>>>>Thanks for the quote! I saved it.
>>>
>>> You might also appreciate this bit of wisdom from Richard Feynman:
>>>
>>> "It doesn't seem to me that this fantastically marvelous universe, this
>>> tremendous range of time and space and different kinds of animals, and
>>> all the different planets, and all these atoms with all their motions
>>> and so on, all this complicated thing can merely be a stage so that God
>>> can watch human beings struggle for good and evil, which is the view
>>> that religion has.
>>>
>>> "The stage is too big for the drama."
>>
>> I find the very concept of an astronomer trying to make authoritarian
>> comments on religion as unsound as a priest trying to make
>> authoritarian comments on astronomy.
>
> Almost by definition, no one can make authoritarian comments about
> religion, especially a priest, mullah, monk or minister.
...in particular when these people frequently contradict one another.
And on top of that, they're unwilling to try to find empirical
evidence of whom is right and whom is wrong...
> All we have
> about any religion is books, and there's no proof that any of the
> religious events in the various texts ever occurred. Jesus, Buddha,
> Mohammed etc., or their followers may have indeed put beliefs down on
> paper but something written in a book may be fact (if there's evidence),
> opnion, or fiction.
--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://stjarnhimlen.se/
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Date: 29 Apr 2007 16:43:26
From: Mick
Subject: Re: 4 inch relector or bigger for the moon?
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"Bullseye" <bulls@ei.com > wrote in message
news:a3n7331i0hajrbm8m81cp6p2lrordamhag@4ax.com...
>I have a 4 inch Nexstar 114GT. The moon looks impressive with this
> scope but I was wondering how much better does an 8 incher make it? It
> seems like overkill but I don't know.
>
> Also is a refractor any good for the moon? And which is better for the
> planets, an 8 inch reflector or a 3.5" refractor?
8 inches will blind you looking at a moon that is bright. You'll need
filters.
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Date: 29 Apr 2007 09:23:21
From: W. H. Greer
Subject: Re: 4 inch relector or bigger for the moon?
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On Sat, 28 Apr 2007, Bullseye wrote:
>I have a 4 inch Nexstar 114GT. The moon looks impressive with this
>scope but I was wondering how much better does an 8 incher make it? It
>seems like overkill but I don't know.
It's not uncommon to set up and compare two telescopes of equal
aperture and discover that one is clearly superior to the other.
Similarly, sometimes a smaller telescope will prove to be superior to
a larger telescope.
In other words, we don't live in a perfect world. The answer to your
question would vary depending upon a wide variety of factors specific
to each telescope.
That being said, in an ideal world an 8-incher would have four times
the light grasp and double the resolving power of a 4-incher. Under
the best of conditions the 8-incher would be capable of double the
magnification of the 4-incher. In other words, there would be a very
noticeable difference in what the observer would see when looking
through the two telescopes.
An 8-inch telescope is not over-kill for lunar observing; neither is a
20-inch telescope . . .
>Also is a refractor any good for the moon? And which is better for the
>planets, an 8 inch reflector or a 3.5" refractor?
A decent refractor makes a very good telescope for lunar work. For
the faster achromats, filters can be used to substantially reduce
chromatic aberration. (I was recently impressed at the performance of
an inexpensive, little, f/5 ST-80 on the moon. At over 100x with a
yellow filter in place the view was remarkably good!)
An average 8-inch reflector should be *significantly* better than an
average 3.5-inch refractor; but I'm assuming that the owner of the
reflector knows how to get the most performance out of a reflecting
telescope (something that's easier to achieve with most refractors).
--
Bill
Celestial Journeys
http://cejour.blogspot.com
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Date: 29 Apr 2007 10:16:05
From: John
Subject: Re: 4 inch relector or bigger for the moon?
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Bullseye wrote:
> I have a 4 inch Nexstar 114GT. The moon looks impressive with this
> scope but I was wondering how much better does an 8 incher make it? It
> seems like overkill but I don't know.
>
> Also is a refractor any good for the moon? And which is better for the
> planets, an 8 inch reflector or a 3.5" refractor?
It will also depend on what mount/tripod set-up you will be using, as
the 8" scope will require more stability than the 4" to be of any gain
in usable performance.
John.
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Date: 28 Apr 2007 21:15:11
From: laura halliday
Subject: Re: 4 inch relector or bigger for the moon?
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On Apr 28, 4:59 pm, Bullseye <b...@ei.com > wrote:
> I have a 4 inch Nexstar 114GT. The moon looks impressive with this
> scope but I was wondering how much better does an 8 incher make it? It
> seems like overkill but I don't know.
>
> Also is a refractor any good for the moon? And which is better for the
> planets, an 8 inch reflector or a 3.5" refractor?
The usual suggestion stands: have a look through one
and decide for yourself. At least read some of the introductory
material that's out there.
I own both, an 8" Celestron SCT and a 5" Takahashi
TOA-130 refractor. Both provide excellent views of the
sky. Both show lots of goodies. A few (galaxies, mainly)
show up better in the C8; others in the Tak. Both are a
pleasure to use.
Yes, theoretically, an 8" telescope will have better
resolving power than a 5". That assumes equal optical
quality (not with a 10:1 price difference!), similar aperture
obstruction (nope...), and seeing that lets each telescope
to its full potential (1 or 2 nights a year).
Whether it's an 8" reflector or a 5" refractor, your first
view of the Moon in a good scope will be one you will
remember for a long time.
Laura Halliday VE7LDH "Que les nuages soient notre
Grid: CN89lg pied a terre..."
ICBM: 49 16.57 N 123 0.24 W - Hospital/Shafte
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Date: 28 Apr 2007 22:24:48
From: Starlord
Subject: Re: 4 inch relector or bigger for the moon?
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I like using my 4.25 inch F9 Stargazer Steve Dob as I can add the moonfilter
cobo I have and the moon is very nice then.
--
The Lone Sidewalk Astronomer of Rosamond
Telescope Buyers FAQ
http://home.inreach.com/starlord
Sidewalk Astronomy
www.sidewalkastronomy.info
The Church of Eternity
http://home.inreach.com/starlord/church/Eternity.html
AD World
http://www.adworld.netfirms.com/
"laura halliday" <marsgal42@hotmail.com > wrote in message
news:1177820111.527874.257170@u30g2000hsc.googlegroups.com...
> On Apr 28, 4:59 pm, Bullseye <b...@ei.com> wrote:
>> I have a 4 inch Nexstar 114GT. The moon looks impressive with this
>> scope but I was wondering how much better does an 8 incher make it? It
>> seems like overkill but I don't know.
>>
>> Also is a refractor any good for the moon? And which is better for the
>> planets, an 8 inch reflector or a 3.5" refractor?
>
> The usual suggestion stands: have a look through one
> and decide for yourself. At least read some of the introductory
> material that's out there.
>
> I own both, an 8" Celestron SCT and a 5" Takahashi
> TOA-130 refractor. Both provide excellent views of the
> sky. Both show lots of goodies. A few (galaxies, mainly)
> show up better in the C8; others in the Tak. Both are a
> pleasure to use.
>
> Yes, theoretically, an 8" telescope will have better
> resolving power than a 5". That assumes equal optical
> quality (not with a 10:1 price difference!), similar aperture
> obstruction (nope...), and seeing that lets each telescope
> to its full potential (1 or 2 nights a year).
>
> Whether it's an 8" reflector or a 5" refractor, your first
> view of the Moon in a good scope will be one you will
> remember for a long time.
>
> Laura Halliday VE7LDH "Que les nuages soient notre
> Grid: CN89lg pied a terre..."
> ICBM: 49 16.57 N 123 0.24 W - Hospital/Shafte
>
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Date: 29 Apr 2007 00:12:18
From: Chris L Peterson
Subject: Re: 4 inch relector or bigger for the moon?
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On Sat, 28 Apr 2007 23:59:28 GMT, Bullseye <bulls@ei.com > wrote:
>I have a 4 inch Nexstar 114GT. The moon looks impressive with this
>scope but I was wondering how much better does an 8 incher make it? It
>seems like overkill but I don't know.
The Moon is a target that often begs for high magnification, and the 8"
aperture will let you push the magnification quite a bit higher than the
4". It certainly isn't overkill.
>Also is a refractor any good for the moon? And which is better for the
>planets, an 8 inch reflector or a 3.5" refractor?
Sure, a refractor is fine (some would argue better). For the planets, or
anything else, a good 8" reflector will massively outperform any 3.5"
refractor. You are better off comparing apertures than comparing
specific optical designs.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 28 Apr 2007 21:43:28
From: Dennis Woos
Subject: Re: 4 inch relector or bigger for the moon?
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> The Moon is a target that often begs for high magnification, and the 8"
> aperture will let you push the magnification quite a bit higher than the
> 4". It certainly isn't overkill.
>
>>Also is a refractor any good for the moon? And which is better for the
>>planets, an 8 inch reflector or a 3.5" refractor?
>
> Sure, a refractor is fine (some would argue better). For the planets, or
> anything else, a good 8" reflector will massively outperform any 3.5"
> refractor. You are better off comparing apertures than comparing
> specific optical designs.
>
How about a 5" refractor? I am a big reflector fan, and in fact I don't own
a refractor > 75mm. However, a friend of ours has a 5" Tak TOA, and from
experience I can say that the views are outrageously good. The hypothetical
8" had better have excellent optics in order to compare favorably.
Furthermore, so far as magnification goes, I think many folks live in areas
where the seeing will usually limit the magnification to below 400x, which
is within the capabilities of my friend's 5" TOA. I think the advantage of
the 8" is more in the area of resolving fine detail, though again the 5"
TOA's sub-arcsecond capability is not much of a limitation. If you told me
that I had the chance to spend the night observing planets and/or the Moon
either with a 5" high quality refractor or a good qualtiy (1/4 wave?) 8"
reflector, I would certainly choose the refractor, and leave the reflector
to you! That said, the TOA is $4k, while the reflector is $400, making the
reflector a good value. I think that a high-quality (1/8 wave) reflector is
an outstanding value, though I wonder whether many low-end/Chinese
commercial dobs achieve this quality.
Dennis
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Date: 29 Apr 2007 04:28:04
From: Chris L Peterson
Subject: Re: 4 inch relector or bigger for the moon?
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On Sat, 28 Apr 2007 21:43:28 -0400, "Dennis Woos" <dpwoos@gmavt.net >
wrote:
>How about a 5" refractor? ...
Obviously, there's some crossover point (IMO at about equal apertures,
but opinion varies). But the comparison being asked for is an 8"
reflector (I'm assuming a Newt) with a 3.5" refractor, and these are
clearly in different classes.
Personally, I'd choose a good 8" Newt over any 5" refractor for high
magnification observing, and possibly the refractor for lower power,
wider field observing (depending on the target). But everyone has their
own preferences.
Seeing is an important factor for high magnification observing, of
course. But as somebody who lives in an area well known for poor seeing,
I can say that even under the worst of conditions, a large aperture
brings out more detail. But it's wasted on observers who don't have the
patience to wait for moments of clarity. If he just wants a quick view,
most of the time an uncritical observer probably won't see much
difference between an 8" and a 4" when looking at the Moon.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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