Astrophotography questions
Starting to reeducate myself RE astrophotography and have a couple of questions: What should my individual images for stacking look like? I know stacking increases S/N. Assuming typical suburban light pollution, at say ISO 800 what is a typical exposure for eg M27, or M8 with f4 optics? How light/dark should the background be, i.e. how much skylight can I tolerate and subsequently "remove" by stacking? Confirm that for auto-tracking a short FL refractor is better than a longer FL one. The Solitaire tracker has trouble finding a suitable star, say near M27, with my f9 85mm refractor. Thanks for any help. Steve
Steve, The individual exposure times tend to be driven by the brightest objects in the field of view. The goal is to prevent saturation of anything except perhaps the brightest star or two. If you allow any object to saturate completely, you can never bring back any detail. It will just be white. A normal histogram for a single exposure has a few bright stars that are near saturation, and a whole bunch of stuff that is almost black. The processing phase targets enhancing the almost black parts of the image. With my equipment and filters, I most often use 10 minute subs, but often reduce that to 5 minutes for bright objects, and go to 20 or 30 minutes for really faint stuff. I don't use my DSLRs for imaging, so I really can't speak to exposure times with those cameras. David Rankin could probably help you with that, since he uses a DSLR. I like using a short focal length for my guide scope. Mine guide setup is a Mini Borg 45ED refractor with a focal length of 325mm, fitted with a Starfish autoguider. It gives me plenty of guide stars to choose from, no matter where I point it. I typically use 3 second exposure for the autoguider. I hope that helps a bit. Cheers, Tyler -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Stephen Peterson Sent: Wednesday, November 03, 2010 7:12 AM To: utah-astronomy@mailman.xmission.com Subject: [Utah-astronomy] Astrophotography questions Starting to reeducate myself RE astrophotography and have a couple of questions: What should my individual images for stacking look like? I know stacking increases S/N. Assuming typical suburban light pollution, at say ISO 800 what is a typical exposure for eg M27, or M8 with f4 optics? How light/dark should the background be, i.e. how much skylight can I tolerate and subsequently "remove" by stacking? Confirm that for auto-tracking a short FL refractor is better than a longer FL one. The Solitaire tracker has trouble finding a suitable star, say near M27, with my f9 85mm refractor. Thanks for any help. Steve _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php Visit the Wiki: http://www.utahastronomy.com
Steve, As Tyler pointed out, you want to avoid fully saturating your object, this is generally pretty hard to do with AP as most objects are so faint. M27 is a very bright object to shoot and will saturate quickly, M8 not so much. Shooting DSO's from the city just sucks, so much of the LP is in the same wavelengths as the object when you try to remove it with tools in photoshop like curves or levels, you end up chopping into your object data as well. Most of the time people end up fully clipping out the background trying to remove the light pollution from their shots. A good AP shot has a background with values from about 20, 20, 20, to 40, 40, 40 RGB. I'm not saying you cant get decent results from the city, it just makes the whole processing experience a lot harder. Stacking really wont remove any of the skyglow from light pollution as it ends up as part of the light signal. The reason that stacking removes random noise form the shots, is because the noise is truly random. When you take a bunch of shots, no two will be the same in regards to noise, but they will in regards to where the photons from the object hit in relation to one another. Averaging, or median combining these out, will remove any random effects. Hot pixels are noise that is not random, so if you don't shots darks, they will degrade the final result. F4 optics are very fast and beneficial for AP with a DSLR. Iso 800 is recommended for most canon DSLR's, but there is nothing wrong with going up to 1600. Stacking the shots does increase the S/N ratio, but if you don't do dark subtraction, this wont help a lot. Shooting darks is very important also. You should shoot darks in the same conditions the lights where shot in, as the sensor will perform differently in different temperatures and at different exposure lengths. I would recommend a good coma corrector for F4 optics if you don't already have one. Baader makes a very cheap one that works well, MPCC. I would estimate with F4 optics, Iso800, and M27, you would get decent results with exposures around just one minute. I use a very short refractor to auto-guide and ti works like a charm. I do this mainly because of the same reason you brought it up, finding a guide star can be a pain. It would work fine. A few more recommendations would be flats and bias, along with dithering. It doesn't take a whole lot of time and can really improve your final results as well. If you haven't seen this, I would also recommend reading it over well. http://deepskystacker.free.fr/english/theory.htm You can also find a lot of help from a lot of very good astrophotographers on cloudy nights DSLR forums. Here is a good tutorial on color balance, white balance, and processing out the background without clipping it (if you have photoshop). Cheers David On 11/3/2010 7:12 AM, Stephen Peterson wrote:
Starting to reeducate myself RE astrophotography and have a couple of questions: What should my individual images for stacking look like? I know stacking increases S/N. Assuming typical suburban light pollution, at say ISO 800 what is a typical exposure for eg M27, or M8 with f4 optics? How light/dark should the background be, i.e. how much skylight can I tolerate and subsequently "remove" by stacking? Confirm that for auto-tracking a short FL refractor is better than a longer FL one. The Solitaire tracker has trouble finding a suitable star, say near M27, with my f9 85mm refractor. Thanks for any help. Steve
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http://www.skyandtelescope.com/howto/astrophotography/Going_Deep_with_a_DSLR... There is the link I forgot to post in my reply Cheers David On 11/3/10 9:17 AM, David Rankin wrote:
Steve,
As Tyler pointed out, you want to avoid fully saturating your object, this is generally pretty hard to do with AP as most objects are so faint. M27 is a very bright object to shoot and will saturate quickly, M8 not so much.
Shooting DSO's from the city just sucks, so much of the LP is in the same wavelengths as the object when you try to remove it with tools in photoshop like curves or levels, you end up chopping into your object data as well. Most of the time people end up fully clipping out the background trying to remove the light pollution from their shots. A good AP shot has a background with values from about 20, 20, 20, to 40, 40, 40 RGB. I'm not saying you cant get decent results from the city, it just makes the whole processing experience a lot harder. Stacking really wont remove any of the skyglow from light pollution as it ends up as part of the light signal. The reason that stacking removes random noise form the shots, is because the noise is truly random. When you take a bunch of shots, no two will be the same in regards to noise, but they will in regards to where the photons from the object hit in relation to one another. Averaging, or median combining these out, will remove any random effects. Hot pixels are noise that is not random, so if you don't shots darks, they will degrade the final result.
F4 optics are very fast and beneficial for AP with a DSLR. Iso 800 is recommended for most canon DSLR's, but there is nothing wrong with going up to 1600. Stacking the shots does increase the S/N ratio, but if you don't do dark subtraction, this wont help a lot. Shooting darks is very important also. You should shoot darks in the same conditions the lights where shot in, as the sensor will perform differently in different temperatures and at different exposure lengths. I would recommend a good coma corrector for F4 optics if you don't already have one. Baader makes a very cheap one that works well, MPCC. I would estimate with F4 optics, Iso800, and M27, you would get decent results with exposures around just one minute.
I use a very short refractor to auto-guide and ti works like a charm. I do this mainly because of the same reason you brought it up, finding a guide star can be a pain. It would work fine.
A few more recommendations would be flats and bias, along with dithering. It doesn't take a whole lot of time and can really improve your final results as well.
If you haven't seen this, I would also recommend reading it over well.
http://deepskystacker.free.fr/english/theory.htm
You can also find a lot of help from a lot of very good astrophotographers on cloudy nights DSLR forums.
Here is a good tutorial on color balance, white balance, and processing out the background without clipping it (if you have photoshop).
Cheers
David
On 11/3/2010 7:12 AM, Stephen Peterson wrote:
Starting to reeducate myself RE astrophotography and have a couple of questions: What should my individual images for stacking look like? I know stacking increases S/N. Assuming typical suburban light pollution, at say ISO 800 what is a typical exposure for eg M27, or M8 with f4 optics? How light/dark should the background be, i.e. how much skylight can I tolerate and subsequently "remove" by stacking? Confirm that for auto-tracking a short FL refractor is better than a longer FL one. The Solitaire tracker has trouble finding a suitable star, say near M27, with my f9 85mm refractor. Thanks for any help. Steve
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participants (3)
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David Rankin -
Stephen Peterson -
Tyler Allred