ClownFish
Post Laureate
Reged: 04/26/05
Posts: 6052
Loc: Islamabad, Pakistan
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Many beginners often ask why you need to guide on a star so close to the subject. And many ask why is it so difficult to get good long exposures with an ALT/AZ mount - even when it is tracking on a star so well. The answer I have always given was to try and describe how the sky moves - but nothing I have written can compare to this excellent photography by Dick Locke. It truly demonstrates the complex movement of the sky, and why an equatorial mount alone is not the answer - careful selection of a guidestar is just as important!
Dick’s Excellent Photo!
Imagine (as an extreme example) guiding on a star left of center, and while the camera is trying to capture an object towards the right. While this is extreme - the same effect can happen at a smaller scale in any part of the sky - when the guidestar is not near the same declination as the subject. A picture is worth a thousand words here.
CF
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Learn all about Polar Alignment and Manual Guiding on my website at www.PetesAstrophotography.com! Or visit my Foreign Service Blog!
Edited by ClownFish (12/04/07 04:34 AM)
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novbabies
Postmaster
Reged: 06/05/05
Posts: 15678
Loc: Northern Georgia!
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Good Seeing!
Mark
Orion 12" XTi f/4.9
VERY old Edmund 6" f/8 reflector
Assorted binoculars
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tommyhawk13
professor emeritus
Reged: 09/28/07
Posts: 585
Loc: Jacksonville, Fl
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Thanks for the link. It makes perfect sense.
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Meade Starfinder 8,Meade SN-8 OTA, Orion Atlas, and a handfull of film cameras
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Chuck Anstey
scholastic sledgehammer
Reged: 01/17/05
Posts: 834
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There is a problem here. The lens he used is causing the distortion in star trails showing stars on the left "separating" from stars on the right. Stars always maintain their angular distance from each other (ignoring proper motion) as the Earth rotates. There are issues with camera rotation using a guide star at a different declination if your alignment is even slightly off, getting worse the closer the the poles you get but that link doesn't show that.
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Carlos Milovic
sage
Reged: 09/07/04
Posts: 272
Loc: Santiago, Chile
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Chuck, the effect CF tried to show is right. Stars do separate in that "proyection". This is not an issue of lens distortion, but to seeing at a flat image instead of a thee dimensional movement.
The real problem with using guiding stars far from the imaging target is field rotation. This is due for a less than perfect polar aligment. If our aligment was "perfect" the only problem that we'll have with far guiding stars is the guiding rate, becouse near the celestial ecuador they move faster.
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Regards,
Carlos Milovic F.
// Astro & Photo - CMF
// http://www.astrophoto.vze.com
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ClownFish
Post Laureate
Reged: 04/26/05
Posts: 6052
Loc: Islamabad, Pakistan
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This also shows atmopspheric refraction. All these troubles are why guiding an equatorial mount is the best way to get decent long exposure astrophotography.
CF
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Learn all about Polar Alignment and Manual Guiding on my website at www.PetesAstrophotography.com! Or visit my Foreign Service Blog!
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Mike D
scholastic sledgehammer
Reged: 08/15/07
Posts: 864
Loc: South GA
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So what then is your advice for selecting a guidestar?
I assume all this means to find a star as close to the subject as possible, while realizing that it is more important to be closer in DEC than in RA?
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Vixen SXD
Vixen ED80Sf
Hutech 350D
STI Stiletto
Meade 2080
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russ_watters
Pooh-Bah
Reged: 11/24/04
Posts: 1262
Loc: Trappe, PA
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Quote:
Many beginners often ask why you need to guide on a star so close to the subject.
Next time I get clear skies and can actually image, I'll save Envisage's output. I don't know if it is the sky's movement, the mirror's or scope/mount/guidescope flex, but around the meridian I get some image shift (not sure if it is rotation that I see).
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Equipment: Orion Atlas 11, ED80, DSI-C, DSI II Pro, Dell Inspiron Laptop.
www.russsscope.net
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ClownFish
Post Laureate
Reged: 04/26/05
Posts: 6052
Loc: Islamabad, Pakistan
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Whenever you have trouble understanding exactly what is causing an autoguide error, I suggest you try manual guiding. This will put you in complete control of the scope and help you quickly spot what is causing the error.
Mike - yes, you should center your target in the camera, and then move the guidescope (or OAG) to select a star very close, and if you have to move away from it, do so in DEC direction mostly. We're not talking far - you can always find a suitable star close by. Where you get into trouble is when you are either using a very long focal length camera, very long exposures (30 minutes per shot), a target near the horizon or all three!
CF
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Learn all about Polar Alignment and Manual Guiding on my website at www.PetesAstrophotography.com! Or visit my Foreign Service Blog!
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russ_watters
Pooh-Bah
Reged: 11/24/04
Posts: 1262
Loc: Trappe, PA
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Actually, I was thinking of doing a test run, autoguiding on the same star as I have my imager set on. That'll show if I'm getting rotation or flex - it's just that I'd waste a night of imaging doing that. Now if I could find a target that had a guidestar in the target.... (perhaps when I do a widefield of the flame/horsehead)
It isn't a bad enough problem that I'm willing to spend 6 hours out in the cold to try to diagnose. Being young and into technology, there's no way in ..... that I'm ever going to manual guide.
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Equipment: Orion Atlas 11, ED80, DSI-C, DSI II Pro, Dell Inspiron Laptop.
www.russsscope.net
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ClownFish
Post Laureate
Reged: 04/26/05
Posts: 6052
Loc: Islamabad, Pakistan
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Take it easy man - it's just a diagnostic suggestion!
CF
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Learn all about Polar Alignment and Manual Guiding on my website at www.PetesAstrophotography.com! Or visit my Foreign Service Blog!
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