Darks

Hi all,

I am curious on your method doing darks. I was able to get 3x600sec frames of M1 tonight and shot 3x600sec darks all with the ST2000 XCM. I'm just curious if one dark is enough, if 3 is overkill? When I use the Starshoot, because I'm limited to about 60 second exposures, I shoot about 5:1 light to dark, but with 10 minutes exposures, one doesn't do all that many lights. I am feeling the burn from climbing this learning curve.

Thanks!!
Paul

You can get by with one. But a Median combine of three is better because it takes care of any variations in the chip. With your ST2k you can make a dark frame library. On those rainy nights set the cooler to a temp that you know the camera can reach on any given night and make your darks. I always did 3 at different exposures lengths ( 1,2,5,10 and 15 minutes). When its time to image, no darks needed as long as you image at the same temp as the darks. A big time saver!

Hi Paul, great to see you jumping in with both feet! As far as darks are concerned its generally accepted practice to take at least 10 darks for your exposure time and camera temp then do a median combine to make the master dark for that particular exposure time/camera temp. As mentioned this can be done before you go out and image, just put the camera in the fridge and you can take a whole library of darks for diferent exposures/camera temps in one evening or while you sleep. Once you take the camera out of the fridge make sure you immediatly put it back in the case and close it up so you dont get any condensation on the camera. I genereally make new darks each month before I go out but on occasion have used darks for several months.

With my setup I make darks for 300 sec, 600 sec, and 1200 sec for Ha exposures. I do it at -15C for summer and -20C winter, of course YMMV on operating temps to choose.

Also I have found that you really dont need to image with a camera temp cooler than -20C. I have tried -30C and could not tell a difference in the dark current. On warm nights (75F or so) I can still get down to -15C easily with water assisted cooling. So I generally use either -15C or -20C depending on the season.

Also instead of a median combine for darks and flats I use Ray Gralak's sigma program to make the master frames with outstanding results, and its free.

http://gralak.com/Sigma/index.html

I also use this to combine my light frames with the standard deviation mask as it provides a much smoother image than if using simple median or sum combine. The difference is quite evident when you start stretching the image. Ray has detailed instructions for using this program on his site. Highly recommended.

Also another good resource for figuring how many dark you should take to make your master frames, and a calculator for figuring optimal sub-exposure time for light frames can be found here:

http://ccdware.com/resources/subexposure.cfm

Good luck and look forward to more images from you. Vince