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Troubleshooting FAQ | Problems? PM a Red or a Green Gu.... uh, User
jamida
super member
Reged: 11/17/04
Posts: 133
Loc: Washington (state)
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I'm an on-again/off-again DSLR imager, so my setup is not yet complete. And regarding a "complete" set up some thing have been bothering me and I'd like to get some expert feedback.
a) long exposure vs a bunch of short (but stacked) exposures. Is there a rule of thumb here? Seems like, if you have the ability to do the stacking, you'd want to expose to just enough past the SNR and then expose again (and again and again ...)
b) and this is related to (a) above... precise polar alignment vs guide-star guiding. I suppose if you're way off on your polar alignment you'd get some field rotation over a long period exposure but given (a) above (and I suppose the ability to untwist your images before stacking). The question is for those of you who guide-star guide, how precise do you need to make your polar alignment? If you know no single exposure is going to be longer than 1 minute, do you make due with less precise alignment than if you're gonna expose for 5 minutes?
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Mike Davis
8" LX 200 "classic"
Milburn wedge
Minolta Activa 7x50 Binoculars (2)
Canon Digital Rebel XT
Canon EF 70-200mm f/4 USM lens
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mewmartigan
member
Reged: 07/02/08
Posts: 77
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Hi Mike,
I am not an expert, but I'll offer my feedback anyway 
a and b are tied to each other. A lot of times the limit on a long exposure is due to either light pollution or tracking.
In my case, I do a precise polar alignment using software but I don't drift align. So when guiding, I get about 4 or 5 minutes max before field rotation/streaking starts to show. So I image 3-5 minute subs and get as many subs as possible to increase the SNR.
Others have posted on the forum that they can't image for more than 5 minutes before light pollution starts to ruin the exposure.
You basically want to get each exposure to be as long as you can get it without light pollution or tracking taking a toll on the image. This can vary depending on your circumstances.
If you want the exposure to be a minute you can probably get away with good polar alignment and no guiding. Longer than a minute though, you are better off guiding. Even if you are guiding, if the alignment is poor then the guider is doing way more work than it should to try and keep things together. The rule of thumb is that you want to get everything setup accurately so that your autoguider has to do as little work as possible (i.e. small and infrequent corrections).
Hope that helps a little, like I said, I'm no expert....but there are certainly some here on the forum!
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Marcus
Orion XX12
8" LX200R
ST80 guidescope
Atlas EQ-G w/EQMOD
Canon XSi unmodded
Astronomik CLS clip-in filter
Meade DSI Pro
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jamida
super member
Reged: 11/17/04
Posts: 133
Loc: Washington (state)
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It does help. Thanks. Although your answer was within my expectations, it was merely a guess of mine and ... I've had guesses be 90% correct but that last 10% caused me to waste some time/expense so I thought I'd check in.
For my part the best images I have so far are from my DSLR camera with a 200mm zoom lens on the camera sitting piggyback on my lx200. This is the simplest most forgiving setup I have. I'm interested in going up a notch but was unsure (till this post) of what that next notch would be. Now I know, specifically unguided prime focus "short" exposures. The notch after that would be guided.
Thanks!
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Mike Davis
8" LX 200 "classic"
Milburn wedge
Minolta Activa 7x50 Binoculars (2)
Canon Digital Rebel XT
Canon EF 70-200mm f/4 USM lens
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Arkalius
scholastic sledgehammer
Reged: 08/03/06
Posts: 878
Loc: Orange County, CA, USA
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Keep in mind that each exposure has a fixed amount of read noise that is not dependent on the signal. Therefore if you reduce subexposure time and increase subexposure count, the final product will have more noise than it would have otherwise because that read noise will be added in more times due to the higher subexposure count.
My general rule is expose as long as you can without overexposing highlights you want to preserve. More time is always better until you begin overexposing stuff.
Of course, sometimes exposing for a long time is not practical such as if you are unable to rely on the steadiness of your equipment for that long of a time or some other such thing. But, if you can go longer, then you probably should. It is always helpful.
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-Arkalius
11" Celestron SCT on Orion Atlas EQ-G
Celestron 100ED Refractor
8" Zhumell Dobsonian Reflector
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