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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: 988
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: 988
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: 988
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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