Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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I am (slowly) putting together my imaging rig. I have a new CG-5 mount and am now looking at an imaging scope and guidescope setup.
Whatever scopes I end up with I will be using a canon DSLR for the imaging camera and almost certainly an Orion StarShoot Auto Guider for the guide cam. There are various imaging scopes in consideration, but I am less sure about what I should be looking at for a guidescope and have a few questions:
- Under ideal circumstances what should the relationship of the focal length of the imaging scope to the guide scope be? Should the guidescope be a similar focal length? Is it a problem if the guidescope is a significantly longer focal length? How about significantly shorter?
- How much does it matter if the guidescope has optical issues? Spherical aberration? Chromatic?
- I assume using something like an f/12 guidescope with an f/5 imaging scope is fine so long as the SSAG can see a star of sufficient brightness. Am I correct?
I have a B&L Criterion 4000 that, alas, had it's corrector plate broken. It is being converted into a classical cas by suspending the secondary with a spider, but given the corrector plate was there for a reason I do not expect optical wonderfulness. I am *hoping* I can turn this into my guidescope, so the second question about optically imperfect guidescopes certainly applies here.
What else, other than total system weight, would you recommend be considered when looking for a guidescope?
Thanks 
- Sean
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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David Pavlich
Postmaster
Reged: 05/18/05
Posts: 8687
Loc: Mandeville, LA USA 30.22 X 90....
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With the advent of guiding software like PHD Guiding, the old adage about focal length isn't nearly as critical. I was guiding a 12" Meade LX200 with a 6.3 focal reducer with a 50mm Borg guider which is all of 250mm of focal length and it did just fine.
One of the nice setups of late has been the Orion ST80 achromat and the Orion SSAG. Light weight and inexpensive.
David
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Proud Member; PAS NOLA,
"If we knew what we were doing, it wouldn't be called research..."
A. Einstein
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Samir Kharusi
scholastic sledgehammer
Reged: 06/14/05
Posts: 989
Loc: Oman
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You should look at the arc-sec per pixel ratio rather than the focal lengths. The latest DSLRs have quite small pixels, under 5microns square. The ratio between the guide and the imaging trains can be quite significant, eg an imaging train at 2 arc-sec per pixel can be guided quite well with a guider at 5 or even 6 arc-sec per pixel but why push things that far? It can be much easier with a ratio under two. Around one (eg an off-axis guider) may work even better if you are using rather crude guiding software. So I would aim for between one and two in arc-sec per pixel. Generally not critical.
Long focal lengths are often disastrous on a guider. Good quality guiding demands a bright guidestar that enables a correction, say, in one to three seconds (lousy mount = more frequent corrections). Consequently the ideal guidescope would be short focal length, large aperture, i.e. fast focal ratio. If the focal ratio is fast enough and the aperture large enough then it is almost always possible to find a bright enough guidestar within the FoV. Narrow FoV will make you tear your hair out, except if it is a very large aperture. Eg a C14 will always find a suitably bright guidestar, even though it is at f11. This is what makes a large SCT a good guide scope (despite mirror flop) for a small imaging APO, but generally not the reverse (again because of mirror flop and parhaps the arc-sec per pixel differences being too ambitious).
Chromatic and similar aberrations in a guidescope should not matter.
Ideal guidescope: Rigidity, 80mm or larger aperture (to find a bright enough guidestar for one to 2sec exposures anywhere in the sky), lightweight, f5 or faster (to have a reasonable FoV). Do NOT under-estimate the difficulty of finding a bright enough guidestar if you have a cheap mount. With an AP mount you generally use guide exposures of 3 to 5sec (plenty of stars bright enough), with eg a CGE you need one sec guide exposures (can be horrendously frustrating with a small aperture or a narrow FoV guidescope).
What arc-sec/pixel are you trying to image at? Too ambitious for a CG-5? With a judicious choice of imaging OTA (fast focal ratio) and an image scale of 2 to 4 arc-sec per pixel, you may not need any guiding at all. You do not need 100% yield, 70+% yield of well-tracked unguided subs is much more pleasant than setting up autoguiding, just my view. Estimate what sub-exposure lengths you are trying to achieve with your imaging OTA. Here is some guidance on the minimal length subs for any set-up. Multiply the minimal by 3 for narrowband. If your CG-5 can give you a 70+ percent yield on those length subs then just skip autoguiding and be happy. If not then pay attention to arc-sec/pixel. 1.5 arc-sec per pixel is more challenge than most people (and seeing conditions) enjoy coping with. Not much point in being over ambitious for your seeing and equipment.
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Bored? Peruse my website:
http://www.samirkharusi.net/
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cdndob
professor emeritus
Reged: 07/28/06
Posts: 665
Loc: The Great White North
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As David already pointed out the guidescope focal length can be much shorter than the imaging focal length. Many are using their modified 50mm finder scopes w/ PHD and not havng any issues.
An 400mm f/5 ST80 is a great choice for guiding and the real bonus is, its cheap.
Steve
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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David:
wow, if I understand how that focal reducer works correctly that would have you guiding a 1920mm scope with a 250mm scope! Quite a mismatch.
I was looking at the SSAG/ST80 combo, I had HOPED to use my existing scope instead of the ST80 to save a bit and get me just that much closer to a proper imaging scope....
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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Samir - you are a wealth of information - thanks!
Quote:
You should look at the arc-sec per pixel ratio rather than the focal lengths.... ....Ideal guidescope: Rigidity, 80mm or larger aperture (to find a bright enough guidestar for one to 2sec exposures anywhere in the sky), lightweight, f5 or faster (to have a reasonable FoV). Do NOT under-estimate the difficulty of finding a bright enough guidestar if you have a cheap mount.
ok, so my hoped-for setup is an Orion 6" Imaging Newtonian. and I have a Rebel XT (350D) now. As mentioned I anticipate purchasing a SSAG for the guidecam.
The XT on the 6"newt results in 1.77 arc-sec/per pixel.
The B&L Criterion 4000, being a 100mm aperture f/12 scope has quite a bit longer focal ratio then the newt. The SSAG on the C4000 results in 0.89 arc-sec/per pixel for a ratio of 0.5!
The SSAG on the Orion ST80 results in 2.68 ars-sec/per pixel for a ratio of 1.5.
Just for fun, the SSAG on the Galileoscope (50mm f/10) results in 2.37 arc-sec/per pixel for a ratio of 1.38.
My budget is sadly lacking unfortunately so I had really hoped the C4000 could be used as a guidescope. Doing so would let me afford the imaging scope a couple months earlier then I would have otherwise. From your advice, if I understand you correctly, the C4000's 4" aperture is a plus, but that extra light-gathering power is offset completely by the f/12 (1200mm) focal length, meaning I should consider other scopes.
It would be funny to use the Galileoscope, but of course it fails in aperture, focal-length, and REALLY fails in rigidity!
Quote:
What arc-sec/pixel are you trying to image at? Too ambitious for a CG-5?
I am hoping not. As I said, I was looking at the Orion 6" imaging newt. That is a 750mm focal length scope, a value that I felt would not be pushing TOO far on the CG-5, yet would let me get quite a bit closer then a 300mm lens or even 400-500mm small ED refractor. The f/5 ratio should also help keep exposures shorter. I had assumed though that since I was quite a bit heavier and with a much longer focal length then my just-a-camera current setup I would indeed need to guide that scope. Oh, one other major point in favour of that scope.... I can afford it (or will be able to with a bit of saving up).
Quote:
With a judicious choice of imaging OTA (fast focal ratio) and an image scale of 2 to 4 arc-sec per pixel, you may not need any guiding at all.
As for unguided shooting.... Right now I shoot with camera lenses ranging from 28mm to 300mm, and indeed I have been able to get 5-minute unguided exposures, so I can see not doing guiding at all with these lenses (unless I wanted to attempt narrowband). But I *would* like to be able to get closer in on some objects then just those lenses allow, and there I suspect I WOULD need guiding. Do you agree?
Thanks again for the advice!
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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bardo
member
Reged: 09/13/09
Posts: 54
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why not look at a generic 5" f/5 newt that you can find everywhere for about $150? cheaper and lighter and will give you room to get the guide scope setup. the focuser is much less than the orion but you could just get an electric unit to put on it.
im in the same boat as you (almost exactly actually) so thats what has been running through my mind.
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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bardo: Orion *claims* the imaging-optimized newt produces a better image (little coma, flat field, better contrast). I know people seem to have good results with the Astro-tech AT8IN (I looked at that first, but it is a bit heavy for my CG-5) and I am making the *assumption* that the Orion 6", based on Orion's claims, will be similarly nice.
Also, I do have that old Tasco 4.5" f/8, and I got myself a 1.25" focuser for it and am going to experiment with it as a leaning exercise. I do not expect optical wonderfulness out of it's spherical mirror, but it will get me started trying to image at longer focal lengths and, possibly, with guiding if I can get that part of the setup.
However.... can you point me to a specific example of a $150 5" f/5 newt?
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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bardo
member
Reged: 09/13/09
Posts: 54
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http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&Item=350263102948&Category=28181&_trkparms=algo%3DLVI%26its%3DI%26otn%3D2
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adamsp123
sage
Reged: 11/20/08
Posts: 444
Loc: welshpool mid wales UK
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A 50mm finder modified to accomodate a cam, in my case a QHY5 and PHD works just fine (on either 120ED or 200 F/4 newt) very light, cheap, large FOV and I just swap over my visual 50mm finder for the finderguider an a matter of seconds with no real extra weight.
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SkyWatcher 120ED PRO, GSO 200mm F4 Astrograph, Meade 10" SNT, WO 72ED & 66ED
Vixen Sphinx and Vixen Atlux with starbook,
QHY5 guider, Modded Canon 1000D, Baader MPCC.
WO Flattener III
Don't you wish there were a knob on the TV to turn up the intelligence? There's one marked 'Brightness,' but it doesn't work."
TV is called a "medium" because it is neither rare nor well done.
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guyroch
sage
Reged: 01/22/08
Posts: 255
Loc: Ottawa, Canada
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Quote:
Orion *claims* the imaging-optimized newt produces a better image (little coma, flat field, better contrast). I know people seem to have good results with the Astro-tech AT8IN (I looked at that first, but it is a bit heavy for my CG-5) and I am making the *assumption* that the Orion 6", based on Orion's claims, will be similarly nice.
Flacon, I'm not sure I would trust such a claim without _real_ feedback form _real_ users. A reflector is still a reflector at the end of the day - not much more than a primary and secondary mirrors with no optics to enhance the image/contrast. I have a basic 6" reflector from SkyWatcher and the day I have enough $$$ to buy myself a good APO can't be soon enough. Orion is indeed making a strong claim on this one but I would be very skeptical. For just an extra $100 you could get an exquisite Orion EON 72mm which has been tried, tested and true. Sure, the aperature is twice as much on your reflector, but the EON has made several happy campers out there.
As for the autoguider, I just purchased the SSAG and Orion ST 400mm f/5 combo and I'm already in love - this is a steel at $410. This is the best price in Canada I was able to find http://www.scopeoptics.com/orawaurepa.html
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Skywatcher 750mm 6" Reflector f/5
Celestron CG5-GT mount
Orion ST80 + SSAG autoguider + PHD
Canon 40D (unmod) + TC-80N3 remote timer
Astronomik CLS clip filter
Orion SkyGlow light polution filter
Orion AccuFocus + homemade Bahtinov mask
Bushnell 8x42 h2o® Waterproof Binoculars
Sky Atlas 2000.0 Field Version (Laminated)
Plenty of patience and a wife that doesn’t quite understand my love for clear skies, but let’s me go out anyway ~ thanks honey bunny 
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cdndob
professor emeritus
Reged: 07/28/06
Posts: 665
Loc: The Great White North
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Falcon,
Having a fast and small(ish) guide scope gives you the following benefits,
1> Minimizes flexure and overall weight
2> Wider FOV to find guidestars
3> Better signal to noise ratio on guidestars
While programs like PHD guide using "sub-pixel" accuracy they are still limited to "seeing" issues which normally pushes your guiding exposures in the 1 to 3 second range.
There's some articles on Craig Stark's (the PHD Author) site that I found were a good read, check out the "2007 Autoguiding" and "2008 Guider roundup".
Articles Link
I'm current using an ST80/SPC900 webcam for guiding which works well. I have had some issues with the webcam at 1/5 sec exposures and not getting a good enough SNR so I have to find another star. This shouldn't be a problem with a more sensitive astro CCD setup which will probably be my next purchase.
Steve
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David Pavlich
Postmaster
Reged: 05/18/05
Posts: 8687
Loc: Mandeville, LA USA 30.22 X 90....
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I attended Craig Stark's Autoguiding lecture at the AIC. He discussed a bunch of stuff, but didn't mention arc seconds, pixel size and so on. Yes they are very important, however, the big issues were Flexure...that's the big one and right along with that is go as light as possible.
One thing that really stuck out was a graph he posted showing the gain in signal to noise ratio relative to exposure time. I must say, I was a bit stunned. The graph clearly shows that the typical backyarder makes the largest gains in SNR beyond the 3 minute exposure length. But what really opened my eyes was once the exposure length hits about 10 minutes, the graph flattens DRAMATICALLY!
So...for the typical scope to get good, deep data, you've got to go beyond 3 minutes. However, after 10 minutes, the gains in SNR drop dramatically. You still get gains, but they are small relative to the time spent.
Interesting data, to say the least.
David
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Proud Member; PAS NOLA,
"If we knew what we were doing, it wouldn't be called research..."
A. Einstein
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Samir Kharusi
scholastic sledgehammer
Reged: 06/14/05
Posts: 989
Loc: Oman
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Quote:
I attended Craig Stark's Autoguiding lecture at the AIC. He discussed a bunch of stuff, but didn't mention arc seconds, pixel size and so on. Yes they are very important, however, the big issues were Flexure...that's the big one and right along with that is go as light as possible.
One thing that really stuck out was a graph he posted showing the gain in signal to noise ratio relative to exposure time. I must say, I was a bit stunned. The graph clearly shows that the typical backyarder makes the largest gains in SNR beyond the 3 minute exposure length. But what really opened my eyes was once the exposure length hits about 10 minutes, the graph flattens DRAMATICALLY!
So...for the typical scope to get good, deep data, you've got to go beyond 3 minutes. However, after 10 minutes, the gains in SNR drop dramatically. You still get gains, but they are small relative to the time spent.
Interesting data, to say the least.
David
3-minute sub-exposure length means absolutely nothing without specifying the focal ratio of the OTA and the level of skyfog. A 3-minute sub on an f10 OTA is absurdly short, and on a Hyperstar at f2 unnecessarily long. On the other hand, a 3-minute sub is about the minimum you can get away with using a current-model DSLR and an f5 OTA at a dark site. Was his example using equipment similar to those specs? Most one-shot-color astroCCDs would require a doubling of the length of subs compared to a current-model DSLR because they tend to have twice the Read Noise. Your back-of-camera histogram is the best, simplest optimiser as to how short you can make your subs and still have good stacking efficiency (presumably the graph he was showing?). Presumably a graph like this from Steve Cannistra?
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Bored? Peruse my website:
http://www.samirkharusi.net/
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Samir Kharusi
scholastic sledgehammer
Reged: 06/14/05
Posts: 989
Loc: Oman
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Quote:
Samir - you are a wealth of information - thanks!
Quote:
You should look at the arc-sec per pixel ratio rather than the focal lengths.... ....Ideal guidescope: Rigidity, 80mm or larger aperture (to find a bright enough guidestar for one to 2sec exposures anywhere in the sky), lightweight, f5 or faster (to have a reasonable FoV). Do NOT under-estimate the difficulty of finding a bright enough guidestar if you have a cheap mount.
ok, so my hoped-for setup is an Orion 6" Imaging Newtonian. and I have a Rebel XT (350D) now. As mentioned I anticipate purchasing a SSAG for the guidecam.
The XT on the 6"newt results in 1.77 arc-sec/per pixel.
The B&L Criterion 4000, being a 100mm aperture f/12 scope has quite a bit longer focal ratio then the newt. The SSAG on the C4000 results in 0.89 arc-sec/per pixel for a ratio of 0.5!
The SSAG on the Orion ST80 results in 2.68 ars-sec/per pixel for a ratio of 1.5.
Just for fun, the SSAG on the Galileoscope (50mm f/10) results in 2.37 arc-sec/per pixel for a ratio of 1.38.
My budget is sadly lacking unfortunately so I had really hoped the C4000 could be used as a guidescope. Doing so would let me afford the imaging scope a couple months earlier then I would have otherwise. From your advice, if I understand you correctly, the C4000's 4" aperture is a plus, but that extra light-gathering power is offset completely by the f/12 (1200mm) focal length, meaning I should consider other scopes.
It would be funny to use the Galileoscope, but of course it fails in aperture, focal-length, and REALLY fails in rigidity!
Quote:
What arc-sec/pixel are you trying to image at? Too ambitious for a CG-5?
I am hoping not. As I said, I was looking at the Orion 6" imaging newt. That is a 750mm focal length scope, a value that I felt would not be pushing TOO far on the CG-5, yet would let me get quite a bit closer then a 300mm lens or even 400-500mm small ED refractor. The f/5 ratio should also help keep exposures shorter. I had assumed though that since I was quite a bit heavier and with a much longer focal length then my just-a-camera current setup I would indeed need to guide that scope. Oh, one other major point in favour of that scope.... I can afford it (or will be able to with a bit of saving up).
Quote:
With a judicious choice of imaging OTA (fast focal ratio) and an image scale of 2 to 4 arc-sec per pixel, you may not need any guiding at all.
As for unguided shooting.... Right now I shoot with camera lenses ranging from 28mm to 300mm, and indeed I have been able to get 5-minute unguided exposures, so I can see not doing guiding at all with these lenses (unless I wanted to attempt narrowband). But I *would* like to be able to get closer in on some objects then just those lenses allow, and there I suspect I WOULD need guiding. Do you agree?
Thanks again for the advice!
The SSAG on the ST80 sounds close to perfect. By the way, rather than purchasing new equipment I'd suggest that you first establish what is the longest sub you can use the 300mm lens with and still get 70+% well tracked frames. Let's assume 4 minutes. This will imply that you can also shoot at 600mm using 2minute subs unguided. But that will mean using a 600mm lens at f4. And yes, 2 minute-subs at f4 do work well, even with my now ancient unmodded Canon 1Ds:
M88 is shown enlarged in the corner crop. I was trying to evaluate it as an astroimaging OTA, and it does as well as most premium APOs, possibly better than most on full 35mm format. OK, purchasing an $8000 lens to avoid autoguiding is not a serious suggestion but a C11 Hyperstar can allow anyone to skip all autoguiding, and to sell off all his other OTAs...
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Bored? Peruse my website:
http://www.samirkharusi.net/
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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Bardo: ahh - skywatcher 'eh? interesting option... I would likely have to upgrade things like the focuser on such a scope, but the price is certainly nice! I wonder if my old clock drive would work on that with that EQ mount... certainly looks similar.
Adamsp123: so you are using something similar to the KWIQ Guider setup? That does look appealing, especially considering weight issues since I have a CG-5 mount.
guyroch: There are quite a few feedback posts left on orion's own site, and they do indeed sound good. I have not yet had luck finding any feedback/reviews from other sites though. The 72ED is interesting - I wonder why Orion does not list it in their Astrophotography section (that is why I missed it when I was looking). Of course if you are going to play the "for only $$ more" game you can keep going..... like this WO Zenithstar II 80mm for that extra $28 above the EON 72mm. Thanks for the Canadian source link. There is a shipping agent in washington state I use so I am lucky enough to be able to consider US sources without worrying about brokerage or the like, but always good to find sources where here in country!
Steve: Yet another vote for the ST80 (or similar). PHD is indeed what I planned to use. It is starting to look like a tossup between a ST80 and a 50mm finder-mod as the guidescope.
David: Flexure - ya, that is one of those things that, until I started reading things here, I would simply never have considered! Sure is nice to have places like CN! From the sounds of it the ST80/finder type setups should be good for flexure prevention, given that how they are mounted to the main OTA is solid.
Samir: Unfortunately the weather here at this time of year does not allow for much experimenting, but given what I saw from the meagre 3-frame 5 minute series I tried last time I had a clear night I think 4 minutes is about right for a 70% acceptable rate. I MIGHT be able to do 70% at 5-minutes, but I do not think so at 6-minutes. Given I can not afford a hyperstar setup or that $8000 lens (if only!) it looks like nearly all the options for imaging OTA are going to be *at least* one full stop slower then f/4. Makes me think that I will indeed want to be doing guiding for just about anything dimmer then Andromeda. Nice shot of those small galaxies! I read your article on minimal exposures - handy info, I had not known to look for the skyfox peak specifically. I better make that a separate reply in the thread though.
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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Samir:
Quote:
Here is some guidance on the minimal length subs for any set-up. Multiply the minimal by 3 for narrowband.
Quote:
By the way, rather than purchasing new equipment I'd suggest that you first establish what is the longest sub you can use the 300mm lens with and still get 70+% well tracked frames. Let's assume 4 minutes. This will imply that you can also shoot at 600mm using 2minute subs unguided. But that will mean using a 600mm lens at f4.
As I said earlier 4-minutes does seem the reasonable value for tracking at 300mm with a good polar align with my CG-5. So then it comes to appropriate exposures. I used the series of exposures I took for this Orion shot from the last time the clouds cleared. At that time I took 50 each of 10-second shots, 30-second shots, and 60-second shots. I also took one 120-second shot and one 300-second shot. The comparison is not exactly perfect because at the start of the 10-second shots the moon had just set and the target was lower in the sky in the direction of the nearest city (Vancouver), while by the time the 5 minute shot was taken the sky was quite a bit darker and the target was high in the sky (away from the city-glow). So take the 10 second shot as worst-case and 300-second as best-case (or close enough).
It seems that the skyfog mountain is already detached in those 10 second images! However the start of the skyfog mountain is not more then 10% from the zero point until the 120 second exposures. The 300 second exposure, by your guidelines for this target on that night, is too long I think. Do you concur that for that night and the 300mm lens at f/4 a two minute exposure is the one meets your minimum-exposure guidelines? Would a three minute have been slightly better perhaps, or is that good enough as is?
So if two-minutes @f/4 is about right, then if we use the orion EON 80ED as example we move to f/6.2 & 500mm so more then double the exposure time to someplace near 5 minutes, and given the longer focal length we can probably only count on half of that as a reasonable unguided exposure. So guiding needed. The f/5 & 750mm 6" newt is much faster, I am guessing 3-minute exposures, however it is more then double the focal length so again, the unguided exposures limit is probably going to be 1.5-2 minutes with that setup - so guiding again is needed! Does all this figuring make sense to you?
Either way (assuming I can keep borrowing this nice L lens, as my own inexpensive 70-300 is not very good for AP) it sounds like the 300mm lens plus the addition of an LPS filter should be able to do quite nice without the addition of any guiding at all. At least until I get the longer OTA to go after some of the smaller targets! 
Thanks for the help Samir - and for posting such a wealth of information on your site!
Below are the histograms for the 5 exposures of orion.
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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bardo
member
Reged: 09/13/09
Posts: 54
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falcon, did you notice that orion has come out with its own 8" f/4 newt as well? they list theirs at 16lbs. its the exact same as the astro tech so i dont know how that is or who is right. but hey at $450 that might be doable for you. ive seen some great pics from people using similar loads. and another option is to modify a 50mm finder for a guide scope and comb the classifieds for an autoguider on the cheap and light. my guide scope is a generic 80mm f/5 like the orion but i got it for $30. its just as decent and works fine. so if you search you can definitely do it cheaper for sure.
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guyroch
sage
Reged: 01/22/08
Posts: 255
Loc: Ottawa, Canada
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Quote:
the unguided exposures limit is probably going to be 1.5-2 minutes with that setup - so guiding again is needed! Does all this figuring make sense to you?
Falcon, not all CG5 are created equal but mine limited me to just that, exposures of no more that 90 to 120 seconds and it was highly dependable to where in the sky I was imaging. This seems all fine, but the warm gears in those mounts aren't reliable... to the point that even at 90 to 120 seconds unguided, a good 40% to 50% of my subs were unusable. I ended up throwing photons in the trash can  . Guiding is a MUST with a CG5. Glad you already realized that, I lost several nights trying without one.
Now the only thing left to do is to find who the hell I need to call to cancel my subscription to the clouds 
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Skywatcher 750mm 6" Reflector f/5
Celestron CG5-GT mount
Orion ST80 + SSAG autoguider + PHD
Canon 40D (unmod) + TC-80N3 remote timer
Astronomik CLS clip filter
Orion SkyGlow light polution filter
Orion AccuFocus + homemade Bahtinov mask
Bushnell 8x42 h2o® Waterproof Binoculars
Sky Atlas 2000.0 Field Version (Laminated)
Plenty of patience and a wife that doesn’t quite understand my love for clear skies, but let’s me go out anyway ~ thanks honey bunny
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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Quote:
Now the only thing left to do is to find who the hell I need to call to cancel my subscription to the clouds
Doesn't work.... I have cancelled my subscription several times, but they just keep giving me "Free Trials!" to get me back on their distribution list!
With your CG-5, what scope/lens where you imaging through?
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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guyroch
sage
Reged: 01/22/08
Posts: 255
Loc: Ottawa, Canada
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It's a basic SkyWatcher 6" reflector at f/5.
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Skywatcher 750mm 6" Reflector f/5
Celestron CG5-GT mount
Orion ST80 + SSAG autoguider + PHD
Canon 40D (unmod) + TC-80N3 remote timer
Astronomik CLS clip filter
Orion SkyGlow light polution filter
Orion AccuFocus + homemade Bahtinov mask
Bushnell 8x42 h2o® Waterproof Binoculars
Sky Atlas 2000.0 Field Version (Laminated)
Plenty of patience and a wife that doesn’t quite understand my love for clear skies, but let’s me go out anyway ~ thanks honey bunny
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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Quote:
did you notice that orion has come out with its own 8" f/4 newt as well? they list theirs at 16lbs. its the exact same as the astro tech so i dont know how that is or who is right.
Well that is new! I am sure it *is* made by the same factory as the AT8IN. I had been warned away from the AT8IN because of the extra weight for the CG-5 mount - 50% rule and all that. The Orion 8" is 2lb lighter then the AT, probably because the AT model has the extended tube and internal baffles... So that leaves me with 1.5 lb for the DSLR and guidecam (SSAG?) if I use a 50mm finder as the guidescope. Kinda tight, but perhaps if I made it a 55% rule.... 
I *really* want to think this will be a good setup, it does exactly what I want (f/4, but a nice 800mm focal length). With the 300mm lens as the "wide" OTA as the alternative scope it SOUDNS just about right for my CG-5! Anything more and *certainly* a larger mount would be needed.
Quote:
ive seen some great pics from people using similar loads
A similar load on CG-5 mounts?
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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Quote:
It's a basic SkyWatcher 6" reflector at f/5.
Ok, so in exactly the same range as I am going to try for then!
So I have to ask - how *IS* your experience imaging with a guided newt on a CG-5? I had been warned a newt would be harder to guide to do it's longer moment-arm compared to a similar-weight refractor or SCT so hearing real-world experience would be good!
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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waassaabee
Carpal Tunnel
Reged: 11/26/07
Posts: 2710
Loc: Central California Coast
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My CG5 was pretty good to me. I used a William Optics Megrez 90FD (621mm @ f6.9 and with a TV .8x FR/FF, 500mm f5.5) and @ 2 minutes unguided, could keep 70% no problem. I added a miniBorg 50 w/Qguider (predecessor to the SSAG) and could go 10 minutes if I wanted to, but now had 80-90% at my target 3-5 minute exposures.
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Gary
34N 120W
-My kingdom for blue squares!-
WO Megrez 90FD/TV 0.8x FR/FF
AT8RC
mini Borg 50/Q-Guide/PHD
CGEM
Canon 350D Hap Griffin Baader mod - o.o
My Friend Flickr
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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Waassaabee - thanks, looks like you have about 1/2 the weight I am looking at, but very encouraging you are able to get 10-minutes exposures! Now if a 6 or 8" newt can just get that 70% at 5 minutes....
Would be nice to have one of those miniBorgs!
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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bardo
member
Reged: 09/13/09
Posts: 54
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Quote:
A similar load on CG-5 mounts?
yes. just from clicking on everyone personal sites i seen some really great shots coming from cg-5's/lxd's/GP[D] with 8" f/5 newts, 8" sct's, etc. probablly more of an exception though.
for instance
http://www.saratogaskies.com/category.pl?c=equip&sub=Scope1
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waassaabee
Carpal Tunnel
Reged: 11/26/07
Posts: 2710
Loc: Central California Coast
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I'm looking at your location... How much wind do you get? An open tube and wind make for frustrating adventures. The CG5's have known slop in the gears, and when I used to view with my 5" Newt it was very noticeable on windy nights.
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Gary
34N 120W
-My kingdom for blue squares!-
WO Megrez 90FD/TV 0.8x FR/FF
AT8RC
mini Borg 50/Q-Guide/PHD
CGEM
Canon 350D Hap Griffin Baader mod - o.o
My Friend Flickr
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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Where I am tends to be quite calm, unless there is a storm blowing in from the south. Durring the summer it is common to have thermals blowing throughout the day, but they die down quick come dusk, and during the winter there is almost ALWAYS a north wind blowout down the sound (a local condition known as the Squamish winds). The Squamishs will be blowing 10-25 knots much of the fall/winter/spring, but where *I* am on the south end of Gambier is sheltered from them unless it is blowing in the 35knot range (not too often thankfully as those winds also strand me on the island - too rough to cross!).
So to sum up the overly complicated answer, wind is not often an issue on nights where imaging is possible.
I have much more of a problem with those pesky clouds...... 
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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Quote:
for instance
http://www.saratogaskies.com/category.pl?c=equip&sub=Scope1
Quite a productive imager! With 8-minute exposures no less! The C8-N + CG5 combo on celestron's site is listed at 67lb... take away 11 for the extra counterweight, and 42 more for the mount itself and you are left with 14lb. slightly lighter, but at 37" MUCH longer then the 16lb 27" Orion 8" f/4 newt. Encouraging indeed!
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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waassaabee
Carpal Tunnel
Reged: 11/26/07
Posts: 2710
Loc: Central California Coast
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Then if you like diffraction spikes (and I do...), then the 6" or 8" would be great! And instead of the miniBorg, look at some of the threads on modifying a 50mm finder scope.
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Gary
34N 120W
-My kingdom for blue squares!-
WO Megrez 90FD/TV 0.8x FR/FF
AT8RC
mini Borg 50/Q-Guide/PHD
CGEM
Canon 350D Hap Griffin Baader mod - o.o
My Friend Flickr
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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I also like the spikes Ya, the modified 50mm finder is my current plan for my finder - or it is now after getting this far into this thread! A 50mm finder based one also works perfect given the 8" comes with one!
But, you know, if I won the lottery or something, a miniBorg WOULD be nice to have even if I did not use it for a finder! :P
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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guyroch
sage
Reged: 01/22/08
Posts: 255
Loc: Ottawa, Canada
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Quote:
Quote:
It's a basic SkyWatcher 6" reflector at f/5.
Ok, so in exactly the same range as I am going to try for then! So I have to ask - how *IS* your experience imaging with a guided newt on a CG-5? I had been warned a newt would be harder to guide to do it's longer moment-arm compared to a similar-weight refractor or SCT so hearing real-world experience would be good!
I bought my SSAG and Orion ST80 in September 2009 and only had a few clear nights so far. I was able to get 5 and 6 minutes shot with pin point stars right from the get go with my setup. I was also successful with a few test shots at 10 minutes with pin point stars. But I was not able to get consistent shots in different parts of the sky. However, my setup was a tab bit off balance as I did not have enough counter weight to offset the guide scope. I have since bought additional counter weight but the clouds are not cooperating but my guess is that I will be successful with 6 minutes exposures 80% to 90% of the time. If not, I see an NEQ6 Pro mount and a 120mm EQUINOX APO in the crystal ball.
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Skywatcher 750mm 6" Reflector f/5
Celestron CG5-GT mount
Orion ST80 + SSAG autoguider + PHD
Canon 40D (unmod) + TC-80N3 remote timer
Astronomik CLS clip filter
Orion SkyGlow light polution filter
Orion AccuFocus + homemade Bahtinov mask
Bushnell 8x42 h2o® Waterproof Binoculars
Sky Atlas 2000.0 Field Version (Laminated)
Plenty of patience and a wife that doesn’t quite understand my love for clear skies, but let’s me go out anyway ~ thanks honey bunny
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Samir Kharusi
scholastic sledgehammer
Reged: 06/14/05
Posts: 989
Loc: Oman
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I think your conclusions are fine. These days I do not even bother to re-think all this stuff each time. For white light I use 1 minute subs at f2.8, 2min at f4, 4min at f5.6, etc. Triple length for narrowband. ISO 1600 at a dark site, lower ISO if there is appreciable light pollution. I suspect too many people have not yet caught onto the fantastic ease of everything when using a fast focal ratio OTA. The mentioned Orion 8" f4 Newt sounds intriguing! If one can find a suitable flattener for it (Parracor?).
Another thing that changes all the parameters fundamentally is that the new Canon DSLRs have such tiny pixels; eg the 500d (T1i) has a 4.7 micron pixel pitch. People used to be very happy to have a large, super-priced APO ($10,000+) and a $10,000 35mm format astroCCD to image at 1.5 to 2 arc-sec per pixel, to end up with a decent A3 sized print. Well, with the 500D you can do similarly with a focal length of only 500mm to 650mm. Basically, for average seeing (around 3 arc-sec) you never need a focal length longer than 650mm to shoot DSOs with the 500D. This reduces the cost of the OTAs tremendously, and you often end up with a faster focal ratio. The OTA being much lighter also enables the use of less expensive mounts. Of course, because of the fine image scale, guiding may still be necessary. You cannot have high resolution without paying a price somewhere  But the total package price is much, much lower. The prices in these things go up exponentially, so they have now also come down exponentially. If only people took the time to comprehend what impacts their budgets and how.
Camera? Tiny pixels, modded. OTA? Fast focal ratio, max focal length of 650mm. Mount: best you can afford for the load. Autoguiding? Depends on previous selections.
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Bored? Peruse my website:
http://www.samirkharusi.net/
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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Interesting that you do not think going past 650mm is needed... It does indeed makes sense to match your pixel-resolution to seeing-limited resolution. If 3 arc-sec is "average" seeing then that 8" newt will be seeing limited unless I get an exceptional night! With an XT or 20D I would have 1.65 arc-sec/per pixel.
Looks like someone needs to make us a 6.5" f/4 660mm schmidt-newtonian imager!  That should hit the spot! Or while I am dreaming of the impossible an 8" f/3.25 650mm or 9.25" f/2.8 658mm would do quite nicely. Oh, and $99 is an excellent price BTW....
Back to the real world... You mention using a lower ISO in light-poluted areas, what is the reasoning behind this? So far I have been shooting basically all ISO-1600 here.
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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Small bit of extra info - from watching ClearSkyChart's graphs of seeing for my area and looking at what Environment Canada's definitions are for those levels I generally have seeing in the ~1.0-2.0 arc-sec range, so with the XT and the 8" newt it seems I SHOULD be in about the right resolution to be at the seeing limit, but not beyond, on most nights.
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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freestar8n
sage
Reged: 10/12/07
Posts: 301
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I will summarize my take on this issue, which is very different from what is found on the web. The claim that guiders are now very accurate because they do sub-pixel centroid calculations is based on assumed precision in the centroid estimation, and not the actual accuracy. There are many ccd imperfections that cause a small star spot to have a poorly defined centroid, so I claim that as long as the star is bright enough, you want to make the focal length as long as possible so these pixel-scale artifacts have less impact.
A key measure is the actual fwhm of the guidestars on your guide camera. You may have 2" per pixel on the guide camera, but with a short refractor the actual stars may be bloated to 12" fwhm, which makes it hard to get 2" fwhm stars on the image. It is like an archer not wearing glasses - they can still see the target and estimate its center, but the actual aim will improve with a clearer view.
This is why I promote OAG and dual-chip guiding wherever possible. If you want to do long exposures (you mentioned narrow band) with a reflector, it will result in much tighter stars and more consistently due to flexure and centroid inaccuracy. It is how most high-end images at long focal length are taken.
On the other hand, if you are just getting into imaging and want to keep it simple - the best way may be to guide a short refractor with a short guidescope. This will be much more forgiving, and the lack of mirrors will reduce the impact of internal flexure.
Finally - I recommend examining the results and techniques of other people with similar equipment to what you plan to use. The best practice with your mount may be very different from what works on another mount.
I aim for small, round stars in the 2" fwhm realm with a C11 on CGE guided with OAG and MetaGuide. Some of my images are here.
If you want to work with your Newtonian, it may be possible to do short exposures with a guidescope, but narrow band may be too difficult. Some people do well with Vixen R200ss with a guidescope, so if your OTA is similarly tight, you may be able to get good results.
Frank
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bardo
member
Reged: 09/13/09
Posts: 54
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a tad off topic here as well but has anyone seen orions new adaptive optics that works with dslrs? its pretty pricey but i wonder how well it will work.
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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Frank
Interesting - this was a bit closer to my assumption beforehand, that long focal length is good. (though high f/ratio is bad). What exactly do you mean by dual-chip guiding?
As for narrowband - I do not have plans to do narrowband myself at this time, Samir was just pointing out the extra exposure times required if I ever did.
Bardo: pricy indeed! Looks sorta like IS for terrestrial photography built into an off-axis guider adaptor.
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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bardo
member
Reged: 09/13/09
Posts: 54
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sbig, starlightxpress, etc ccd cameras use two chips in one unit. or one chip where a portion is allocated to guiding.
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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Ahh - interesting, I was unaware of those type of cameras! Starting with a DSLR as part of my equipment blinded me a bit to that aspect of potential gear. Ya, I can see why having a guide cam integrated into the main cam would be nice - quite a bit nicer then off-axis guiding I would imagine!
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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Samir Kharusi
scholastic sledgehammer
Reged: 06/14/05
Posts: 989
Loc: Oman
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Quote:
Ahh - interesting, I was unaware of those type of cameras! Starting with a DSLR as part of my equipment blinded me a bit to that aspect of potential gear. Ya, I can see why having a guide cam integrated into the main cam would be nice - quite a bit nicer then off-axis guiding I would imagine!
Ummmm. Dual chip is actually quite a lot more inconvenient than off-axis guiding. You see, in OAG you can independently rotate your guide chip to find a bright-enough guidestar and normally the OAG guidechip is before any filter you may be using, so the stars look much brighter to it. With a dual-chip, when you rotate to search for a guidestar you are also rotating your carefully-framed composition on your imaging chip. People have also tried to work around the filter business by having filters that are smaller and more rectangular that they do not cover the guidechip, but with most dual chips you end up covering the guidechip simultaneously. There is always some fly in the ointment. That's why my huge preference is no guiding at all. The mount is where most $ should be spent.
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Bored? Peruse my website:
http://www.samirkharusi.net/
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Samir Kharusi
scholastic sledgehammer
Reged: 06/14/05
Posts: 989
Loc: Oman
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Quote:
Interesting that you do not think going past 650mm is needed... It does indeed makes sense to match your pixel-resolution to seeing-limited resolution. If 3 arc-sec is "average" seeing then that 8" newt will be seeing limited unless I get an exceptional night! With an XT or 20D I would have 1.65 arc-sec/per pixel.
Looks like someone needs to make us a 6.5" f/4 660mm schmidt-newtonian imager! That should hit the spot! Or while I am dreaming of the impossible an 8" f/3.25 650mm or 9.25" f/2.8 658mm would do quite nicely. Oh, and $99 is an excellent price BTW....
Back to the real world... You mention using a lower ISO in light-poluted areas, what is the reasoning behind this? So far I have been shooting basically all ISO-1600 here.
Basically if the seeing is 2 arc-sec per pixel (and that is quite rare!) then the max useful image scale is around 1 arc-sec per pixel. People who are chasing the ultimate resolution go for a factor one-third instead of one-half. But frankly anything finer than 1.5arc-sec per pixel is most likely to be futile for most of us at average locations. A C14 Hyperstar delivers 1.4arc-sec per pixel on a Canon 500D, close to perfection. 30sec sub-exposures and no autoguiding is required.
The reason why I use a lower ISO at light-polluted sites is mainly because I want to keep the total number of subs under, say, 100. At a light-polluted site you need to increase the integration time (same factor as the skyfog is brighter). Also you end up eating up the dynamic range available in each sub if the skyfog hump is way up on the histogram. Between ISO 1600 and ISO 200 the Read Noise in Canon DSLRs does increase, but not exponentially. Eg there is little difference between 1600 and 800, so you do not do much harm in using 800 (or even 400) when the skyfog is brighter. At my home there is so much light pollution that with ISO 1600 my subs using a C14 Hyperstar are total wash-outs with 10sec exposures! To get 5 hours' integration time (quite reasonable but not very deep for the amount of skyfog) I'll have to shoot several thousand subs each 10sec long! Indeed no autoguiding required but just the logistics of processing that lot...
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Bored? Peruse my website:
http://www.samirkharusi.net/
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freestar8n
sage
Reged: 10/12/07
Posts: 301
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Quote:
Basically if the seeing is 2 arc-sec per pixel (and that is quite rare!) then the max useful image scale is around 1 arc-sec per pixel. People who are chasing the ultimate resolution go for a factor one-third instead of one-half. But frankly anything finer than 1.5arc-sec per pixel is most likely to be futile for most of us at average locations. A C14 Hyperstar delivers 1.4arc-sec per pixel on a Canon 500D, close to perfection. 30sec sub-exposures and no autoguiding is required.
Many of us are guiding SCT's on mid-range mounts with 0.5-0.8" per pixel and getting 2" fwhm under non-exceptional skies. With OAG and good guiding technique you can make a mid-range mount yield results comparable to a much more expensive setup. It is easier to use short focal length and not guide at all - but with proper technique and software, high resolution results are obtainable with minimal effort and expense.
My examples, with C11 and CGE totalling $4k combined, which is much less than a high end mount alone, can be seen here.
Some of the images have details related to my OAG setup and its use with a field of view indicator (FOVI).
Frank
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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Quote:
Ummmm. Dual chip is actually quite a lot more inconvenient than off-axis guiding. You see, in OAG you can independently rotate your guide chip to find a bright-enough guidestar and normally the OAG guidechip is before any filter you may be using, so the stars look much brighter to it.
Ahh - and again my inexperience shows itself. I was unaware you could rotate the OAG to find your guidestar. Obvious once pointed out though! Does make me wonder about an OAG option for something like that f/4 newt. Lightweight compared to something like a ST80 and higher resolution then a 50mm finder, and given that the newt is an f/4 it is more likely to be able to find a guidestar then something like an f/10 SCT. I wonder if the newt would have issues coming to focus with an OAG in the path....
Quote:
The reason why I use a lower ISO at light-polluted sites is mainly because I want to keep the total number of subs under, say, 100.... ......At my home there is so much light pollution that with ISO 1600 my subs using a C14 Hyperstar are total wash-outs with 10sec exposures!
Yikes - 10-seconds only! From my local (and my lack of a C14/hyperstar! ) I suspect I will not be bumping below iso800 often. Certainly iso1600 seems to be the way to go with the 300mm camera lens. Then again... the sky was REALLY REALLY bright with that full moon out!
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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adamsp123
sage
Reged: 11/20/08
Posts: 444
Loc: welshpool mid wales UK
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Quote:
Adamsp123: so you are using something similar to the KWIQ Guider setup? That does look appealing, especially considering weight issues since I have a CG-5 mount.
JUst like that but self assembled from a spare finder and QHY5 link
http://stargazerslounge.com/diy-astronomer/84726-diy-finder-guider.html
It works with no weight adder to the system and I can get nice round stars at 8 mins on my F/4 8" Newt+MPCC. BTW having a fast scope makes life so much easier.
The only major problem I had was an inbalance I had managed to induce in the DEC axis by the position of the camera on the Newt, took a long time to suss but rotating the scope in the rings fixed it 100%.
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SkyWatcher 120ED PRO, GSO 200mm F4 Astrograph, Meade 10" SNT, WO 72ED & 66ED
Vixen Sphinx and Vixen Atlux with starbook,
QHY5 guider, Modded Canon 1000D, Baader MPCC.
WO Flattener III
Don't you wish there were a knob on the TV to turn up the intelligence? There's one marked 'Brightness,' but it doesn't work."
TV is called a "medium" because it is neither rare nor well done.
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freestar8n
sage
Reged: 10/12/07
Posts: 301
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Hi-
Newtonians are not as easily adapted to OAG because of limited backfocus. But for that matter, they may also have trouble focusing on a DSLR. Some Newtonians, like the Vixen and the Tak 180, have imaging in mind, so they have lots of backfocus.
So - for your particular OTA, OAG may not be an option with your DSLR and you may need to use a guidescope. If you do, be prepared for signs of flexure in long exposures, and beware that flexure can happen not only in the guidescope but within the OTA itself at the mirrors, which is harder to fix.
Since you are working on a budget, it may make most sense to make your own small guidescope with available components and mount it tightly. If flexure dominates, then the guidescope quality doesn't matter much - but if you don't have much flexure and you want small stars in the image, you may need a higher quality guidescope.
Frank
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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Quote:
The only major problem I had was an inbalance I had managed to induce in the DEC axis by the position of the camera on the Newt, took a long time to suss but rotating the scope in the rings fixed it 100%.
I am guessing that you rotated the tube so the camera is above the tube rather then to one side or the other - is that correct?
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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Quote:
Newtonians are not as easily adapted to OAG because of limited backfocus. But for that matter, they may also have trouble focusing on a DSLR.
The 8" f/4 I was looking at is the new Orion Imaging Newt - so it is set up with imaging in mind. Looks to be the same sort of setup as the Astrotech AT8IN. Still unknown just how much extra back focus there is though.
You are right that on my 4.5" f/8 scope there *ARE* backfocus issues getting the camera to focus, let alone with an OAG inline. I am going to experiment with the f/8 newt anyway, I got a cheap 1.25" focuser that I will try and modify till I get enough backfocus to bring my DSLR or a webcam into focus. If that does not work there is no real loss other then the plastic focuser I "modified" (I have the original 0.965 focuser stored away so I can restore the scope to original specs very easy).
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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Samir Kharusi
scholastic sledgehammer
Reged: 06/14/05
Posts: 989
Loc: Oman
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Quote:
Quote:
Basically if the seeing is 2 arc-sec per pixel (and that is quite rare!) then the max useful image scale is around 1 arc-sec per pixel. People who are chasing the ultimate resolution go for a factor one-third instead of one-half. But frankly anything finer than 1.5arc-sec per pixel is most likely to be futile for most of us at average locations. A C14 Hyperstar delivers 1.4arc-sec per pixel on a Canon 500D, close to perfection. 30sec sub-exposures and no autoguiding is required.
Many of us are guiding SCT's on mid-range mounts with 0.5-0.8" per pixel and getting 2" fwhm under non-exceptional skies. With OAG and good guiding technique you can make a mid-range mount yield results comparable to a much more expensive setup. It is easier to use short focal length and not guide at all - but with proper technique and software, high resolution results are obtainable with minimal effort and expense.
My examples, with C11 and CGE totalling $4k combined, which is much less than a high end mount alone, can be seen here.
Some of the images have details related to my OAG setup and its use with a field of view indicator (FOVI).
Frank
I suppose your non-exceptional skies have better seeing than my non-exceptional skies Basically the message I was trying to put across is that very long focal lengths require rather heroic efforts at guiding, and are unlikely to yield a commensurate response for the amount of diligence. Of course, e.g. your Bubble does show very nice detail and looks well worthy of the level of effort you seem to be putting into it. For my highest resolution attempts I just go for ultra short subs and ultra bright targets (star clusters), like 4 to 15 seconds for which guiding is often superfluous, even at 4000mm focal length. Of course one runs into a whole slew of other problems, like even after mirror pre-release I still get shutter shudder for exposures between 1/30th sec and 1 sec. Anyway this below is the highest res DSO image I managed to get so far, the Trapezium, but only 20sec integration time. One night in the indefinite future I may yet try for more, if and when the seeing ever cooperates:
That H pair of stars are only 1.6 arc-sec apart. Autoguiding longer exposures may work (OAG) but then the seeing is unlikely to cooperate for several minutes per sub. Ah well, I suppose we all agree that OAG is the superior method but nothing beats unguided for sheer simplicity...
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Bored? Peruse my website:
http://www.samirkharusi.net/
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adamsp123
sage
Reged: 11/20/08
Posts: 444
Loc: welshpool mid wales UK
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Quote:
Quote:
The only major problem I had was an inbalance I had managed to induce in the DEC axis by the position of the camera on the Newt, took a long time to suss but rotating the scope in the rings fixed it 100%.
I am guessing that you rotated the tube so the camera is above the tube rather then to one side or the other - is that correct?
I had the camera to "one side causing a strong DEC inbalance - top heavy, so I rotated the scope/camera in the rings so the CofG of the camera and finderguider were opposite the counter weight no DEC inbalance and CW sorts out the RA balance.
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SkyWatcher 120ED PRO, GSO 200mm F4 Astrograph, Meade 10" SNT, WO 72ED & 66ED
Vixen Sphinx and Vixen Atlux with starbook,
QHY5 guider, Modded Canon 1000D, Baader MPCC.
WO Flattener III
Don't you wish there were a knob on the TV to turn up the intelligence? There's one marked 'Brightness,' but it doesn't work."
TV is called a "medium" because it is neither rare nor well done.
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adamsp123
sage
Reged: 11/20/08
Posts: 444
Loc: welshpool mid wales UK
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Quote:
Quote:
Newtonians are not as easily adapted to OAG because of limited backfocus. But for that matter, they may also have trouble focusing on a DSLR.
The 8" f/4 I was looking at is the new Orion Imaging Newt - so it is set up with imaging in mind. Looks to be the same sort of setup as the Astrotech AT8IN. Still unknown just how much extra back focus there is though.
You are right that on my 4.5" f/8 scope there *ARE* backfocus issues getting the camera to focus, let alone with an OAG inline. I am going to experiment with the f/8 newt anyway, I got a cheap 1.25" focuser that I will try and modify till I get enough backfocus to bring my DSLR or a webcam into focus. If that does not work there is no real loss other then the plastic focuser I "modified" (I have the original 0.965 focuser stored away so I can restore the scope to original specs very easy).
The GSO F/4 I have has an 2" extension piece as standard on the focuser, you need it for EPs normally and remove it for the Camera, with no problems focusing the camera, a 1000D with MPCC attached. The secondary is the large size for imaging and I am pleased with the scope indeed.
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SkyWatcher 120ED PRO, GSO 200mm F4 Astrograph, Meade 10" SNT, WO 72ED & 66ED
Vixen Sphinx and Vixen Atlux with starbook,
QHY5 guider, Modded Canon 1000D, Baader MPCC.
WO Flattener III
Don't you wish there were a knob on the TV to turn up the intelligence? There's one marked 'Brightness,' but it doesn't work."
TV is called a "medium" because it is neither rare nor well done.
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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adamsp123: where did you pick up your GSO newt in the UK? I have been trying to find a source for them in the US or Canada but not found any yet....
The GSO scope *looks* rather like it may be the source for the AT8IN and Orion 8" f/4 imaging scope - would be interesting to see how they compare in price.
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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adamsp123
sage
Reged: 11/20/08
Posts: 444
Loc: welshpool mid wales UK
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Quote:
adamsp123: where did you pick up your GSO newt in the UK? I have been trying to find a source for them in the US or Canada but not found any yet....
The GSO scope *looks* rather like it may be the source for the AT8IN and Orion 8" f/4 imaging scope - would be interesting to see how they compare in price.
Yes got mine from Modernastronomy UK also Telescope-service.com - Germany do them (I think they are the main european suppliers). I believe the AT version has the same optics but longer tube, I just added a home made dew/light shield made out of a camping mat, dead cheap and lightweight.....works to!
link for Teleservice http://www.teleskop-express.de/shop/product_info.php/info/p52_GSO-Newtonian-200-800mm---2--Crayford-Focuser---optical-tube.html
Pete
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SkyWatcher 120ED PRO, GSO 200mm F4 Astrograph, Meade 10" SNT, WO 72ED & 66ED
Vixen Sphinx and Vixen Atlux with starbook,
QHY5 guider, Modded Canon 1000D, Baader MPCC.
WO Flattener III
Don't you wish there were a knob on the TV to turn up the intelligence? There's one marked 'Brightness,' but it doesn't work."
TV is called a "medium" because it is neither rare nor well done.
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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*nod* that longer tube & internal baffles on the AT8IN do seem to be the only difference. The Orion version just like it is just a black painted version of the GSO original. Since the GSO at those shops is 449 pounds or 478eur ($755.77cdn) vs $449usd ($477.74cdn) for the Orion and AT versions there does not seem to be much point in going for the GSO version from my location. Odd that the standard telescope carriers here do not sell the GSO brand directly. *shrug*
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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This is an interesting product for low-backfocus OAG:
http://www.teleskop-express.de/shop/product_info.php/info/p1188_Off-Axis-Guider---only-9mm-length---w--Canon-EOS-adaptation.html
Costs more then the SSAG, nearly as much as the scope itself, but it DOES look like the right tool for the job!
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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bardo
member
Reged: 09/13/09
Posts: 54
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wonder if any north american retailers have anything like that?
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guyroch
sage
Reged: 01/22/08
Posts: 255
Loc: Ottawa, Canada
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Quote:
Quote:
Quote:
The only major problem I had was an inbalance I had managed to induce in the DEC axis by the position of the camera on the Newt, took a long time to suss but rotating the scope in the rings fixed it 100%.
I am guessing that you rotated the tube so the camera is above the tube rather then to one side or the other - is that correct?
I had the camera to "one side causing a strong DEC inbalance - top heavy, so I rotated the scope/camera in the rings so the CofG of the camera and finderguider were opposite the counter weight no DEC inbalance and CW sorts out the RA balance.
But wouldn't this setup create an obstructed view for you guide scope which is aiming dead smack in between your camera and finder scope? Is there enough clearance? I don't think there is enough for my setup. Furthermore, this would make my finder scope almost useless as the guide scope would be in _my_ way for when I would try to peek in the finder scope. I am reading this correctly?
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Skywatcher 750mm 6" Reflector f/5
Celestron CG5-GT mount
Orion ST80 + SSAG autoguider + PHD
Canon 40D (unmod) + TC-80N3 remote timer
Astronomik CLS clip filter
Orion SkyGlow light polution filter
Orion AccuFocus + homemade Bahtinov mask
Bushnell 8x42 h2o® Waterproof Binoculars
Sky Atlas 2000.0 Field Version (Laminated)
Plenty of patience and a wife that doesn’t quite understand my love for clear skies, but let’s me go out anyway ~ thanks honey bunny
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Falcon-
sage
Reged: 09/11/09
Posts: 242
Loc: Gambier Island, BC, Canada
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Quote:
But wouldn't this setup create an obstructed view for you guide scope which is aiming dead smack in between your camera and finder scope? Is there enough clearance? I don't think there is enough for my setup. Furthermore, this would make my finder scope almost useless as the guide scope would be in _my_ way for when I would try to peek in the finder scope. I am reading this correctly?
I am guessing that his guide scope is installed in the finder bracket attached to the tube rather then attached to the top of the rings.... In that configuration you would not be using a visual finder, but rather just watching the output of the guidecam to act as your finder.
That is how I get on target when using the camera & standard lens, use the Frame and Focus mode (keeps taking shots as fast as possible) in nebulosity to find my location on the sky as looking through the camera is useless and there is no finder scope at all.
Pete - any chance you could post a picture of the configuration you are talking about?
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Tasco 11TE-5 'Lunagrosso': 4.5" Newtonian, 900mm f/7.9
Meade DS-2114S: 4.5" Newtonian, 1000mm f/8.8
Galileoscope: 50mm Achromatic Refractor, 500mm f/10
Tasco EQ-2-like mount w/ clock drive
Celestron CG-5GT mount
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freestar8n
sage
Reged: 10/12/07
Posts: 301
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Quote:
I suppose your non-exceptional skies have better seeing than my non-exceptional skies
Hi Samir-
It's true I don't know much about the skies in Oman - but your trapezium image does look like it can be pretty steady. My skies in the northeast U.S. seem pretty well correlated with the jet stream. I don't know if you have other people in your area doing high res. imaging with OAG on good mounts, but that is one way to know what is possible. It's not clear to me you have tried it, so it may be possible at your site to get good results at long focal length and long exposure.
Here is an image of NGC 6946, and beneath it is the luminance version, plus a close up of a single 10m exposure with no processing showing how small the stars are on the scale of the 0.8" pixels. The seeing was pretty good that night, but such nights do happen, and with OAG and rapid corrections I don't find it too difficult to get results like these regularly at long exposure with a relatively humble CGE.
There is no question that unguided imaging is easier, but high res. work with mid-range equipment can be very rewarding without too much effort.
Frank
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adamsp123
sage
Reged: 11/20/08
Posts: 444
Loc: welshpool mid wales UK
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I will try to post a pic of my setup, but Falcon you are right, the finderguider swaps with the finder and simply goes into the standard finder shoe. Once I am on target I don't need the std finder and if I want to change to a different target off goes the finderguider on goes the finder, takes but a few seconds each swap. The one thing I do have is a dew strap on the finderguider as they will dew up at a drop of the hat!
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SkyWatcher 120ED PRO, GSO 200mm F4 Astrograph, Meade 10" SNT, WO 72ED & 66ED
Vixen Sphinx and Vixen Atlux with starbook,
QHY5 guider, Modded Canon 1000D, Baader MPCC.
WO Flattener III
Don't you wish there were a knob on the TV to turn up the intelligence? There's one marked 'Brightness,' but it doesn't work."
TV is called a "medium" because it is neither rare nor well done.
|
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