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Main
Date: 10 Jul 2007 06:04:07
From: Davoud
Subject: Fast Scope, Mediocre Skies?
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This question may be best suited for people living on the East Coast,
and particularly those crazy enough to try deep-sky imaging from the
Mid-Atlantic region. If you observe from 15,000 meters in the Southwest
you probably aren't going to get any of this, especially technical
jargon like "gray sky" :-)
I'm (still) searching for an astrograph that can cover the 24 x 36 mm
chip on my STL-11000. I'm willing to spend maybe $5k. There are many
choices in that price range; it will come down to suitability for my
sky conditions, physical size, and availability within a reasonable
time. I've been waffling on this for too long already; I'll wait months
for a 'scope if need be, but I'm not going to get in a years-long line
for an AP! The telescope that I buy will be mounted in my tiny
observatory <http://www.davidillig.com/observatory15.shtml >, so it
won't be physically large. I'm using a Tele Vue 76 at the moment, and
I'm very happy with it, except that it can't cover my CCD, even with a
reducer-flatener. (Please see
http://www.davidillig.com/ast-tv76drawtube.shtml > and
<http://www.davidillig.com/ast-stl-firstlight061125.shtml >.) The
astrograph that I buy will probably never have an eyepiece on it;
versatility is not important to me.
I never see a black sky. I may have a very dark gray sky in the wee
hours of a winter morning, but there is likely to be some high-altitude
turbulence. Conversely, during the current heat wave (100 today) the
sky has been no better than medium-dark gray (mag 3-4, maybe) in the
wee hours, but as steady as a rock; the all-seeing CCD can render dim
stars as sharp as needles (and I have sufficient experience in image
processing to take care of the gray skies.)
I can think of a number of refractors that would do the job for me,
with the Takahashi FSQ-106 "New Q" at or near the top of the list based
on its eminent suitability as an astrograph and its small size -- only
520 mm long with the lens shade extended and 6.8 kg. (I did say /tiny/
observatory.) This is a 530 mm f5 'scope @ prime focus. The New Q is in
short supply, but I could have one in two to three months.
And then there is the Takahashi Epsilon 180ED. I believe that this is
essentially a very fast Newtonian -- 500 mm f2.8 @ prime focus --
modified by the addition of correctors in the focuser. It could hardly
be more different from the FSQ-106 in design, but the bottom line is
pretty much the same -- I've seen raw, unprocessed FITS images from
from both 'scopes and they were superb.
At 500 mm (180 ED) and 530 mm (New Q) there is no significant
difference in image scale (3.7 and 3.5 arcsec/pixel, respectively.) The
180ED is also reasonably compact -- 570 mm in physical length and 10
kg. The 180ED is available today. Does such a 'scope as the 180ED --
and the short exposures it promises -- offer an advantage under gray
skies? Short exposures, long exposures -- aside from guiding
requirements, does it matter? Or, conversely, do non-Southwest skies
militate against the use of a very fast system like the 180ED?
TIA.
Davoud
--
usenet *at* davidillig dawt com
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Date: 12 Jul 2007 06:09:46
From: Pierre Vandevenne
Subject: Re: Fast Scope, Mediocre Skies?
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On Jul 12, 12:04 pm, "Roger Hamlett"
<rogerspamigno...@ttelmah.demon.co.uk > wrote:
> 'down' to a really fast combination. My own TMB115, on the same camera,
> with just the flattener (only tried on a couple of occasions, the camera
BTW, which flattener are you using?
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Date: 12 Jul 2007 18:45:28
From: Roger Hamlett
Subject: Re: Fast Scope, Mediocre Skies?
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"Pierre Vandevenne" <pierre@datarescue.com > wrote in message
news:1184245786.110696.40380@m3g2000hsh.googlegroups.com...
> On Jul 12, 12:04 pm, "Roger Hamlett"
> <rogerspamigno...@ttelmah.demon.co.uk> wrote:
>
>> 'down' to a really fast combination. My own TMB115, on the same camera,
>> with just the flattener (only tried on a couple of occasions, the
>> camera
>
> BTW, which flattener are you using?
The TMB one (designed for the Pentax 6*7). Homemade fittings.
Best Wishes
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Date: 12 Jul 2007 06:06:56
From: Pierre Vandevenne
Subject: Re: Fast Scope, Mediocre Skies?
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On Jul 12, 5:20 am, "M104gal...@gmail.com" <M104gal...@gmail.com >
wrote:
> On Jul 11, 7:39 pm, Pierre Vandevenne <pie...@datarescue.com> wrote:
>
> > On Jul 12, 12:50 am, "M104gal...@gmail.com" <M104gal...@gmail.com>
> > wrote:
>
> > > A, gasp, Meade RCX400 does extremely well in polluted city skies--with
> > > filters as well. See the work at http://www.galaxyphoto.com/Andit
> > > doesn't cost a fortune.
>
> > Seems you missed the fact that the question was about _fast_ scopes.
> > f8 isn't really "fast". 2.8 is.
>
> A good Meade reducer-corrector is relatively cheap--f3.3--I'd call
> that fast but keep in mind these are not film cameras--"fast" doesn't
> mean the same thing rather it has more to do with field of view.
Yes, I have read Stan Moore's excellent demonstration. This being said
- SCTs and even the RCX really don't cut it with a focal reducer. I
have one on a C11 and was shocked when I saw the awful tunnel effect
it produced. It was great a few years ago on ST7, and some people have
made very nice things with them, but they are imho, a thing of the
past given the other options amateurs have today and given the sizes
of the CCDs.
> And, of course, to set up impossible specs ( money vs capabilities )
> and then also expect them cheap is, well, counterproductive, isn't it?
> All astrophotography gear is a compromise as you and Mattil have
> demonstrated.
Agreed.
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Date: 12 Jul 2007 06:01:43
From: Pierre Vandevenne
Subject: Re: Fast Scope, Mediocre Skies?
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On Jul 12, 3:18 am, Chris L Peterson <c...@alumni.caltech.edu > wrote:
> On Wed, 11 Jul 2007 17:50:08 -0700, Pierre Vandevenne
>
> <pie...@datarescue.com> wrote:
> >I was surprised to discover that it actually wasn't that bad with the
> >5D (24x36mm). Not nearly as bas as with other more expensive
> >instruments, probably because the field isn't as flat. I am under the
> >impression that it is not possible to avoid vignetting in a reasonably
> >obstructed 8 inch reflector.
>
> I'm not sure that's true. Most good optics don't show any vignetting
> over the field they are designed to cover. However, all optics show cos4
> light falloff, which mimics vignetting.
Thanks for the additional info. I can probably restate my point more
correctly by stating that I am under the impression that it is not
possible to design an 8 inch obstructed reflector that does not show a
significant amount of light falloff on a 36mm circle. I am not as
certain when it comes to refractors. (but I don't claim any kind of
expertise - I was just disappointed by manufacturer's advertisements)
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Date: 12 Jul 2007 03:20:37
From: M104galaxy@gmail.com
Subject: Re: Fast Scope, Mediocre Skies?
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On Jul 11, 7:39 pm, Pierre Vandevenne <pie...@datarescue.com > wrote:
> On Jul 12, 12:50 am, "M104gal...@gmail.com" <M104gal...@gmail.com>
> wrote:
>
> > A, gasp, Meade RCX400 does extremely well in polluted city skies--with
> > filters as well. See the work at http://www.galaxyphoto.com/Andit
> > doesn't cost a fortune.
>
> Seems you missed the fact that the question was about _fast_ scopes.
> f8 isn't really "fast". 2.8 is.
A good Meade reducer-corrector is relatively cheap--f3.3--I'd call
that fast but keep in mind these are not film cameras--"fast" doesn't
mean the same thing rather it has more to do with field of view.
And, of course, to set up impossible specs ( money vs capabilities )
and then also expect them cheap is, well, counterproductive, isn't it?
All astrophotography gear is a compromise as you and Mattil have
demonstrated.
Perhaps you missed the price point the original poster mentioned??
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Date: 12 Jul 2007 10:04:15
From: Roger Hamlett
Subject: Re: Fast Scope, Mediocre Skies?
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<M104galaxy@gmail.com > wrote in message
news:1184210437.486384.281050@n2g2000hse.googlegroups.com...
> On Jul 11, 7:39 pm, Pierre Vandevenne <pie...@datarescue.com> wrote:
>> On Jul 12, 12:50 am, "M104gal...@gmail.com" <M104gal...@gmail.com>
>> wrote:
>>
>> > A, gasp, Meade RCX400 does extremely well in polluted city
>> > skies--with
>> > filters as well. See the work at http://www.galaxyphoto.com/Andit
>> > doesn't cost a fortune.
>>
>> Seems you missed the fact that the question was about _fast_ scopes.
>> f8 isn't really "fast". 2.8 is.
>
> A good Meade reducer-corrector is relatively cheap--f3.3--I'd call
> that fast but keep in mind these are not film cameras--"fast" doesn't
> mean the same thing rather it has more to do with field of view.
>
> And, of course, to set up impossible specs ( money vs capabilities )
> and then also expect them cheap is, well, counterproductive, isn't it?
> All astrophotography gear is a compromise as you and Mattil have
> demonstrated.
>
> Perhaps you missed the price point the original poster mentioned??
The problem here is that the scope itself, has a maximum useable FOV,
under 1.5 degrees. The F*.33 reducer, will only cover about the central
1/4 of the chip involved, before vignetting becomes awful. Also, the SCT
reducer corrector, generates coma, if used with the coma corrected RCX
design...
The Epsilon-180 already mentioned, is at 'heart', a Newtonian, but using a
hyperbolic primary, and a corrector to give a large flat field. It is an
impressive scope for wide field views.
However, I'd suggest thinking slightly the 'other way'.
In astronomy, the pixel 'scale', is really more important than the 'speed'
of the scope. In fact the photographic concept of 'speed', has to bend
significantly when dealing with point objects.
Given the pixel size of the STL11000, what image scale do you want?. The
500mm focal length of the Epsilon, while great for a wide FOV, does imply
an image scale on this camera, of 3.7 arc seconds/pixel, and very real
risk of finding square corners appearing on the stars...
That having been said, a focal length perhaps closer to 1m, becomes
possibly the sort of area that should be being looked at, rather than the
very 'fast' Epsilon unit. For a 200mm aperture, this implies something
around F/4 to F/5.
'Speed', does bring a problem in poor skies. Unfortunately, because the
skyglow is a 'diffuse' object (and hence behaves in the normal
photographic manner of being recorded quicker, with the 'faster' scope),
while the stars do not behave in the same way, higher effective
magnifications (and 'slower' scopes), can tend to be better performers in
these circumstances.
The Vixen already mentioned by other posters is a possibility, as
(unfortunately at a higher price), is something like a TMB130 F/6, with
the matching field flattener. This while relatively 'slow' in terms of
this discussion, is a combination that is likely to perform excellently
with this camera. Several other similar refractor combinations exist, as
do things like the AP F*.75 reducer, which could take such a refractor
'down' to a really fast combination. My own TMB115, on the same camera,
with just the flattener (only tried on a couple of occasions, the camera
is normally on my RC), performs well, and gives the sort of satisfying
fields that are really enjoyable.
However then comes the 'caveat' of the mount. It is suprising what this
sort of scope weighs, and unless the mount is comfortable with the weight,
imaging will not be a pleasure...
Best Wishes
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Date: 11 Jul 2007 17:50:08
From: Pierre Vandevenne
Subject: Re: Fast Scope, Mediocre Skies?
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On Jul 12, 1:12 am, "William R. Mattil" <wrmat...@ix.netcom.com >
wrote:
> While this is not to malign the Vixen f/4 Newtonian which is a fine
> instrument there are going to be significant challenges to adapting one
> to use with the STL11000M. Not the least of which you already pointed
> out being the focuser. Additionally and this isn't trivial at all is
> trying to mount the camera and keep the correct spacing from the coma
> corrector to the detector. And after all that is said and done the
Agreed.
> vignetting will be significant. Which is not to say that some of the
I was surprised to discover that it actually wasn't that bad with the
5D (24x36mm). Not nearly as bas as with other more expensive
instruments, probably because the field isn't as flat. I am under the
impression that it is not possible to avoid vignetting in a reasonably
obstructed 8 inch reflector.
> guidescope will not work well. Trying to add in an Off Axis guider which
> is the only solution guaranteed to work is a very non-trivial task.
Totally agree. But the STL 11000 M is self guiding. Never had any
problem, with the ST2000XM guider and a H-Alpha filter, to find a
guide star. OTOH, the refractor is a major pain with the same setup.
> A better solution might be to build a 253mm f/5 Newtonian incorporating
> a 3 inch focuser and a Wynne Corrector. This gives the biggest possible
> opening with which to collect light and at the same time can correct the
> coma from such a system. Then the added aperture doesn't hurt either.
> This would fit within a 6 foot dome which was another requirement.
That's certainly a good solution, but the Wynne corrector aone is
going to cost the price of the Vixen ;-)
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Date: 12 Jul 2007 01:18:53
From: Chris L Peterson
Subject: Re: Fast Scope, Mediocre Skies?
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On Wed, 11 Jul 2007 17:50:08 -0700, Pierre Vandevenne
<pierre@datarescue.com > wrote:
>I was surprised to discover that it actually wasn't that bad with the
>5D (24x36mm). Not nearly as bas as with other more expensive
>instruments, probably because the field isn't as flat. I am under the
>impression that it is not possible to avoid vignetting in a reasonably
>obstructed 8 inch reflector.
I'm not sure that's true. Most good optics don't show any vignetting
over the field they are designed to cover. However, all optics show cos4
light falloff, which mimics vignetting.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 11 Jul 2007 21:50:44
From: William R. Mattil
Subject: Re: Fast Scope, Mediocre Skies?
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Chris L Peterson wrote:
> On Wed, 11 Jul 2007 17:50:08 -0700, Pierre Vandevenne
> <pierre@datarescue.com> wrote:
>
>> I was surprised to discover that it actually wasn't that bad with the
>> 5D (24x36mm). Not nearly as bas as with other more expensive
>> instruments, probably because the field isn't as flat. I am under the
>> impression that it is not possible to avoid vignetting in a reasonably
>> obstructed 8 inch reflector.
>
> I'm not sure that's true. Most good optics don't show any vignetting
> over the field they are designed to cover. However, all optics show cos4
> light falloff, which mimics vignetting.
Hi Chris,
You are certainly correct that in a perfect system (from a design
standpoint) the light falloff would overshadow any vignetting. The
problem is that, in my opinion, the fast light cone coupled with an
imperfect detector and the corrector lens itself some vignetting cannot
be avoided. Even at f/6 an STL11000M and Paracorr get pretty darn dark
in the corners <g > I'm not certain what the biggest cause for this
actually is but I suspect the filter wheel and the nosepiece of the
camera. At least it appears to be so in test images that I have taken.
At f/4 the coma corrector starts to be a larger factor because of the
smallish diameter of the supporting tube. But I will admit that it's
hard to quantify exactly. But, opening up the coma corrector to 3 inches
ID puts the onus back on the camera/Filter wheel.
Naturally this design demands a secondary that will fully illuminate the
detector as well. Otherwise there will be a *lot* of falloff <g >
Regards
Bill
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Date: 12 Jul 2007 03:13:55
From: Chris L Peterson
Subject: Re: Fast Scope, Mediocre Skies?
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On Wed, 11 Jul 2007 21:50:44 -0500, "William R. Mattil"
<wrmattil@ix.netcom.com > wrote:
>Hi Chris,
>
>You are certainly correct that in a perfect system (from a design
>standpoint) the light falloff would overshadow any vignetting. The
>problem is that, in my opinion, the fast light cone coupled with an
>imperfect detector and the corrector lens itself some vignetting cannot
>be avoided. Even at f/6 an STL11000M and Paracorr get pretty darn dark
>in the corners <g> I'm not certain what the biggest cause for this
>actually is but I suspect the filter wheel and the nosepiece of the
>camera. At least it appears to be so in test images that I have taken.
Yeah, with a fast system and accessories added on to correct other
problems, true vignetting likely becomes an issue. I was referring
mainly to systems designed from scratch to cover a large field, like the
FSQ. I expect that all the intensity falloff there over a 35mm frame is
cos4, not vignetting.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 11 Jul 2007 17:39:53
From: Pierre Vandevenne
Subject: Re: Fast Scope, Mediocre Skies?
|
On Jul 12, 12:50 am, "M104gal...@gmail.com" <M104gal...@gmail.com >
wrote:
> A, gasp, Meade RCX400 does extremely well in polluted city skies--with
> filters as well. See the work at http://www.galaxyphoto.com/And it
> doesn't cost a fortune.
Seems you missed the fact that the question was about _fast_ scopes.
f8 isn't really "fast". 2.8 is.
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Date: 11 Jul 2007 22:50:35
From: M104galaxy@gmail.com
Subject: Re: Fast Scope, Mediocre Skies?
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On Jul 11, 5:13 pm, Pierre Vandevenne <pie...@datarescue.com > wrote:
> Pierre Vandevenne <pie...@datarescue.com> wrote innews:Xns996BAC015B0datarescue@216.196.109.144:
>
> > Where did I read very interesting stuff about filters and fast F ratio
> > scopes? Ah yes, Sky Insight - see fast astrograph considerations by
> > David Erikson.
>
> OK, and since I had to Google, I of course lost track. Davoud's main
> question was, I believe, whether sky fog in light polluted skies was a
> major issue for fast f-ratio instruments. Based on my experience (Belgium,
> bad bad skies), it is. It can be checked easily with a 2.8 photographic
> lens. Results with a 2.8 scope aren't going to be much better, except if
> you have parts of the skies markedly better than other.
>
> Filters do not help as much as expected and may introduce artefacts (See
> David Erickson's article for more information).
>
> I'd suggest you test your skies with a 2.8 tele lens before spending
> sigificant amount of cash.
>
> --
> Pierre Vandevenne - DataRescue sa/nv -www.datarescue.com
A, gasp, Meade RCX400 does extremely well in polluted city skies--with
filters as well. See the work at http://www.galaxyphoto.com/ And it
doesn't cost a fortune.
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Date: 10 Jul 2007 07:16:54
From: William R. Mattil
Subject: Re: Fast Scope, Mediocre Skies?
|
Davoud wrote:
> This question may be best suited for people living on the East Coast,
> and particularly those crazy enough to try deep-sky imaging from the
> Mid-Atlantic region. If you observe from 15,000 meters in the Southwest
> you probably aren't going to get any of this, especially technical
> jargon like "gray sky" :-)
[ignoring the 15,00 meters gaffe]
What makes you think that imaging from the East Coast (Mid Atlantic
Region) cannot be done ? There are plenty of people, myself included,
that image regularly from a mag 3.x environment. Geography has little to
do with it unless you are buried beneath the jet stream.
>
> I'm (still) searching for an astrograph that can cover the 24 x 36 mm
> chip on my STL-11000. I'm willing to spend maybe $5k.
That's going to be a problem. At it will probably limit you to a small
refractor.
[snip]
>
> I can think of a number of refractors that would do the job for me,
> with the Takahashi FSQ-106 "New Q" at or near the top of the list based
> on its eminent suitability as an astrograph and its small size -- only
> 520 mm long with the lens shade extended and 6.8 kg. (I did say /tiny/
> observatory.) This is a 530 mm f5 'scope @ prime focus. The New Q is in
> short supply, but I could have one in two to three months.
The Old Q is also a solution. But ...... most mounting schemes that I've
seen for the FSQ/STL still vignettes the corners. The problem occurs
from two locations. Only one of which you can do anything about.
1) The internal filter wheel does vignette. It's not a lot though and
flat fielding will fix this easily. It's worse on faster systems.
2) The usual mounting method for the Q is to screw in a "Feldstein"
Adapter into the threads on the front of the camera. This also
introduces vignetting. You'd be better off in my opinion with a solution
that uses the nosepiece mounting holes instead. This does not prevent
images from being taken mind you ... but you will need to flat field
your images.
>
> And then there is the Takahashi Epsilon 180ED. I believe that this is
> essentially a very fast Newtonian -- 500 mm f2.8 @ prime focus --
> modified by the addition of correctors in the focuser. It could hardly
> be more different from the FSQ-106 in design, but the bottom line is
> pretty much the same -- I've seen raw, unprocessed FITS images from
> from both 'scopes and they were superb.
>
> At 500 mm (180 ED) and 530 mm (New Q) there is no significant
> difference in image scale (3.7 and 3.5 arcsec/pixel, respectively.) The
> 180ED is also reasonably compact -- 570 mm in physical length and 10
> kg. The 180ED is available today. Does such a 'scope as the 180ED --
> and the short exposures it promises -- offer an advantage under gray
> skies? Short exposures, long exposures -- aside from guiding
> requirements, does it matter?
No. Assuming that your guiding and mount are up to snuff. It will affect
your sub exposures possibly. Sky Fog. But that's easily gotten around.
Can you say Ha filter ?
> Or, conversely, do non-Southwest skies
> militate against the use of a very fast system like the 180ED?
No. But keep your expectations reasonable. Certain targets will be
preferred over others with such a short focal length. And removing
gradients caused by light pollution can be a challenge.
Personally I haven't done much with my FSQ from in town. At least not
with the STL. I don't have a good reason for not doing so other than
opportunities this spring have been extremely limited anyway and
springtime skies are not a great time to be using an FSQ.
Bill
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Date: 12 Jul 2007 05:26:03
From: Rick Evans
Subject: Re: Fast Scope, Mediocre Skies?
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"William R. Mattil" <wrmattil@ix.netcom.com > wrote in message
news:sMKki.4702$rL1.4621@newssvr19.news.prodigy.net...
> Davoud wrote:
>> This question may be best suited for people living on the
>> East Coast,
>> and particularly those crazy enough to try deep-sky imaging
>> from the
>> Mid-Atlantic region.
> [ignoring the 15,00 meters gaffe]
>
> What makes you think that imaging from the East Coast (Mid
> Atlantic Region) cannot be done ?
Since he's imaging from Maryland I don't think that
Davoud thinks it cannot be done. I suspect his "crazy
enough" comment
was more hyperbole ;)
http://www.davidillig.com/ast-stl-firstlight061125.shtml
--
Rick Evans
---------------------------------------------------------------
Lon -71° 04' 35.3"
Lat +42° 11' 06.7"
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Date: 12 Jul 2007 12:59:52
From: Davoud
Subject: Re: Fast Scope, Mediocre Skies?
|
Davoud:
> >> This question may be best suited for people living on the
> >> East Coast, and particularly those crazy enough to try deep-sky imaging
> >> from the Mid-Atlantic region. If you observe from 15,000 meters
> >> in the Southwest you probably aren't going to get any of this,
> >> especially technical jargon like "gray sky" :-)
William R. Mattil:
> > [ignoring the 15,00 meters gaffe]
> >
> > What makes you think that imaging from the East Coast (Mid
> > Atlantic Region) cannot be done ?
Rick Evans:
> Since he's imaging from Maryland I don't think that
> Davoud thinks it cannot be done. I suspect his "crazy
> enough" comment was more hyperbole ;)
> http://www.davidillig.com/ast-stl-firstlight061125.shtml
Thank you. In my response to Mr. Croulet early in this thread I wrote:
"hyperbole (n): exaggerated statements or claims not meant to be taken
literally. Very common in ordinary speech (everyone knows... I've
searched everywhere) but also employed humorously. In the latter case
the exaggeration is often gross, e.g., a "15,000 meter" mountain."
Of course, there /are/ a few 15,000-meter mountains in Arizona and New
Mexico, the precise locations of which are known only to a tiny group
of very elite amateurs who are conspiring to keep the locations secret
from the rest of us. People like Chris Peterson, who actually owns such
a mountain :-)
(My news reader does not show me Mr. Mattil's posts directly, but only
if he is quoted. For me, persistent failure to read and understand what
the OP said and/or asked, plus the inability to recognize figurative,
ironic, or hyperbolic speech may lead to replies that are less useful
than they might otherwise be.)
*****
While I'm passing out the thanks, thanks to Mr. Vandervenne and to all
who contributed useful information.
My observations to date: While I am aware of the possibility of a
pixel-scale mismatch giving square stars, I note that my Tele Vue 76
(FL 480 mm, f6.3) gives a scale of 3.9, not greatly different from the
3.7 the Epsilon would give. I get nice stars and I am very happy with
my Tele Vue 76 on this big camera except for the fact that the Tele Vue
can't begin to cover the CCD chip. Flat fielding doesn't help. The TV
76 is fairly easy to focus by hand with a FeatherTouch, and it is
remarkable how well it keeps its focus. I suppose that is due partly to
the fact that everything is in an observatory, and there are no rapid
temperature swings during a photography session.
Cost is less an issue than size. I could buy a nice used 130 mm f6
STARFIRE EDF, but the OTA is nearly a meter long with the dewcap
extended; mock-ups show that this would be a bit much in my 2-meter
dome, particularly when the camera is tacked on. The FSQ 106, at just
over half a meter with lens shade extended, and the Epsilon 180ED, at
570 mm, no lens shade, are much better fits.
The problem with the Epsilon 180ED is the focuser, according to a
number of users. Some report no problems, but Richard Crisp may be
more typical as he describes the focuser as "...a bit too lightweight
to handle a heavy Astro-ccd camera load." He offers a solution at
<http://www.narrowbandimaging.com/pdf_e180ed_page.htm >. It's not a
cheap solution, but this beast is going to require some sort of digital
focusing aid in any case. The 180's ED corrector elements give a very
flat field at the cost of allowing virtually no error in orthogonality
between the optical axes of the telescope and the camera.
Bottom line (for now): studying the matter, but to reiterate, "I've
been waffling on this for too long already..." I am well acquainted
with the danger of studying a subject to death without ever taking
action. Whenever a clear night is wasted an Angel cries.
Thanks,
Davoud
--
usenet *at* davidillig dawt com
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Date: 11 Jul 2007 17:04:05
From: Pierre Vandevenne
Subject: Re: Fast Scope, Mediocre Skies?
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"William R. Mattil" <wrmattil@ix.netcom.com > wrote in news:sMKki.4702
$rL1.4621@newssvr19.news.prodigy.net:
>> I'm (still) searching for an astrograph that can cover the 24 x 36 mm
>> chip on my STL-11000. I'm willing to spend maybe $5k.
>
> That's going to be a problem.
I tend to agree. <evil G >
>> And then there is the Takahashi Epsilon 180ED. I believe that this is
>> essentially a very fast Newtonian -- 500 mm f2.8 @ prime focus --
I purchased a Lichtenknecker 200/540 Flat field camera last July for the
very same purpose. It is, in a way, a more "advanced" solution, but it
definitely won't fit the $5K condition (especially with the record lows for
the USD/EUR exchange rate). I also have what is supposed to be a fairly
decent apo triplet.
A few observations
- you will suffer from vignetting, regardless of what the manufacturer
says. Be prepared to spend some significant time on image processing.
- the flatter the field is, the more vignetting you'll have (it's logical,
the same amount of light is basically stretched in a flat field instrument)
- "non" flat field fast instruments that I have had the chance to play with
are unusable for some purposes (mosaics for example).
- I can testify the Lichtenknecker FFC is temperamental. The invar model is
relatively stable with reasonable delta Ts, but distance of the focal plane
seems to be critical.
- the Takahashi is a bitch to collimate, according to friends who have
used, and then sold, one. Perfect for people who have a fixed installation,
but not that cool if you have to move the stuff around.
- the Lichtenknecker is a bitch to collimate/align. I wouldn't have bought
it if I didn't live 20 miles from the producer. And even considering that,
I still don't know if that was a good idea....
> your sub exposures possibly. Sky Fog. But that's easily gotten around.
> Can you say Ha filter ?
Where did I read very interesting stuff about filters and fast F ratio
scopes? Ah yes, Sky Insight - see fast astrograph considerations by David
Erikson.
This is what made me understand why I got awful results (circles) with
filters and the 1.2 85mm Canon lens and results not as good as expected
with the Lichtenknecker.
Davoud, have you considered the Vixen 200/800mm (F4) Newtonian? Focuser
issues aside (the stock focuser is not nearly as good as it should for a F4
instrument, backfocus is borderline) this is a really good instrument with
the version II coma corrector... And it is extremely affordable.
--
Pierre Vandevenne - DataRescue sa/nv - www.datarescue.com
The IDA Pro Disassembler & Debugger - world leader in hostile code analysis
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Date: 11 Jul 2007 18:12:57
From: William R. Mattil
Subject: Re: Fast Scope, Mediocre Skies?
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Pierre Vandevenne wrote:
>
> Davoud, have you considered the Vixen 200/800mm (F4) Newtonian? Focuser
> issues aside (the stock focuser is not nearly as good as it should for a F4
> instrument, backfocus is borderline) this is a really good instrument with
> the version II coma corrector... And it is extremely affordable.
>
Pierre,
While this is not to malign the Vixen f/4 Newtonian which is a fine
instrument there are going to be significant challenges to adapting one
to use with the STL11000M. Not the least of which you already pointed
out being the focuser. Additionally and this isn't trivial at all is
trying to mount the camera and keep the correct spacing from the coma
corrector to the detector. And after all that is said and done the
vignetting will be significant. Which is not to say that some of the
mechanical issues couldn't be worked through by someone with enough
resources. But it wouldn't be a turnkey operation. And while I am a huge
fan of the Newtonian Design they also introduce additional problems when
trying to do narrow band imaging. Guidestar acquisition can be a real
problem and unless the integrations are kept short an external
guidescope will not work well. Trying to add in an Off Axis guider which
is the only solution guaranteed to work is a very non-trivial task.
A better solution might be to build a 253mm f/5 Newtonian incorporating
a 3 inch focuser and a Wynne Corrector. This gives the biggest possible
opening with which to collect light and at the same time can correct the
coma from such a system. Then the added aperture doesn't hurt either.
This would fit within a 6 foot dome which was another requirement.
Regards
Bill
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Date: 11 Jul 2007 17:13:55
From: Pierre Vandevenne
Subject: Re: Fast Scope, Mediocre Skies?
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Pierre Vandevenne <pierre@datarescue.com > wrote in
news:Xns996BAC015B0datarescue@216.196.109.144:
> Where did I read very interesting stuff about filters and fast F ratio
> scopes? Ah yes, Sky Insight - see fast astrograph considerations by
> David Erikson.
OK, and since I had to Google, I of course lost track. Davoud's main
question was, I believe, whether sky fog in light polluted skies was a
major issue for fast f-ratio instruments. Based on my experience (Belgium,
bad bad skies), it is. It can be checked easily with a 2.8 photographic
lens. Results with a 2.8 scope aren't going to be much better, except if
you have parts of the skies markedly better than other.
Filters do not help as much as expected and may introduce artefacts (See
David Erickson's article for more information).
I'd suggest you test your skies with a 2.8 tele lens before spending
sigificant amount of cash.
--
Pierre Vandevenne - DataRescue sa/nv - www.datarescue.com
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Date: 10 Jul 2007 06:13:19
From: Curtis Croulet
Subject: Re: Fast Scope, Mediocre Skies?
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> If you observe from 15,000 meters in the Southwest
> you probably aren't going to get any of this, especially technical
> jargon like "gray sky" :-)
15,000 meters???!!!
--
Curtis Croulet
Temecula, California
33°27'59"N, 117°05'53"W
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Date: 10 Jul 2007 12:59:23
From: Davoud
Subject: Re: Fast Scope, Mediocre Skies?
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Davoud:
> > If you observe from 15,000 meters in the Southwest
> > you probably aren't going to get any of this, especially technical
> > jargon like "gray sky" :-)
Curtis Croulet:
> 15,000 meters???!!!
hyperbole (n): exaggerated statements or claims not meant to be taken
literally. Very common in ordinary speech (everyone knows... I've
searched everywhere) but also employed humorously. In the latter case
the exaggeration is often gross, e.g., a "15,000 meter" mountain.
Now, what about my telescope question?
Thanks,
Davoud
--
usenet *at* davidillig dawt com
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Date: 14 Jul 2007 17:10:33
From: Brian Tung
Subject: Re: Fast Scope, Mediocre Skies?
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Davoud wrote:
> hyperbole (n): exaggerated statements or claims not meant to be taken
> literally. Very common in ordinary speech (everyone knows... I've
> searched everywhere) but also employed humorously. In the latter case
> the exaggeration is often gross, e.g., a "15,000 meter" mountain.
<nitpick >
Less than a factor of two from Everest, though, so perhaps it was not
gross enough for this crowd. :)
</nitpick >
> Now, what about my telescope question?
Sorry--I'm a terrible astrophotographer. I can offer general advice,
but probably not specific enough for your situation, I'm afraid.
--
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: 10 Jul 2007 06:47:08
From: Joe S.
Subject: Re: Fast Scope, Mediocre Skies?
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"Curtis Croulet" <calypte@_NO_SPAM_verizon.net > wrote in message
news:3sFki.7294$MV6.6453@trnddc01...
>> If you observe from 15,000 meters in the Southwest
>> you probably aren't going to get any of this, especially technical
>> jargon like "gray sky" :-)
>
> 15,000 meters???!!!
Peyote -- readily available in the SW -- increases the altitude of the
observer.
> --
> Curtis Croulet
> Temecula, California
> 33°27'59"N, 117°05'53"W
>
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Date: 12 Jul 2007 05:17:52
From: Rick Evans
Subject: Re: Fast Scope, Mediocre Skies?
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"Joe S." <noname@nosuch.net > wrote in message
news:f6vo3e018ve@news1.newsguy.com...
>
> "Curtis Croulet" <calypte@_NO_SPAM_verizon.net> wrote in
> message news:3sFki.7294$MV6.6453@trnddc01...
>>> If you observe from 15,000 meters in the Southwest
>>> you probably aren't going to get any of this, especially
>>> technical
>>> jargon like "gray sky" :-)
>>
>> 15,000 meters???!!!
>
> Peyote -- readily available in the SW -- increases the
> altitude of the observer.
LOL!
--
Rick Evans
---------------------------------------------------------------
Lon -71° 04' 35.3"
Lat +42° 11' 06.7"
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