Darks/bias library

Astrophotography: share your photos & discuss techniques
User avatar
bluemax
10+ Years Member
Posts: 817
Joined: Thu Aug 13, 2009 5:00 pm

Darks/bias library

Unread post by bluemax »

I understand that flats need to be done at the same time as the lights but how about darks and bias? I was wondering if the sensor temperature or something else would require darks be taken just after lights or, if I can take them anytime. I have started the first night of a long series of subs and saved some darks and bias using the same exposure time (for the darks of course) as the lights. I have saved these calibration frames in a library for future use. Can I now expose some more darks/bias at my workbench (the kitchen table) and expect them to be useable even though they were not taken at the same time as the lights?
Frank N

Stellarvue 80mm APO, Skyguider Pro, Celestron AVX

"I'm a seeker too. But my dreams aren't like yours. I can't help thinking that somewhere in the universe there has to be something better than man. Has to be"

Taylor in "Planet of the Apes" 1968
User avatar
Pete
Astro Day Coordinator
Posts: 3999
Joined: Fri Jun 13, 2003 9:03 pm

Re: Darks/bias library

Unread post by Pete »

Hi Frank,

Flats do not need to be done at the same time as lights, nor at the same temperature. But flats need to be done with the camera in focus and done each time the camera's repositioned or moved. Dark frames and Bias frames should be taken at the same temperature as lights but necessarily at the same time. If your camera doesn't have thermostatically controlled cooling you need to get a few dark and bias frames mixed in with your exposures so that everything's at approximately the same temperature. But I believe that you're using a temperature regulated camera - right?

My observatory computer files are set up similar to below:
SBIG
2013
2014
observations night of June 1
observations night of June 6
Flats taken June 6
5 second Darks for flats (usually a total of 15)
Flats (usually a total of 15)
observations night of June 24
Reductions
Bias Frames
1X1 binned -15C bias set of 15
2X2 binned -15C bias set of 15
2X2 binned -20C bias set of 15
2X2 binned -25C bias set of 15
2X2 binned -30C bias set of 15
Dark Frames
1X1 darks
2X2 darks
1 min 2X2 -15C dark set of 15
2 min 2X2 -15C dark set of 15
3 min 2X2 -15C dark set of 15

When imaging I set my camera software to do reductions on individual frames as they are captured. The Bias, Dark, and Flat (with separate flat dark applied) are automatically run to clean up the image. 2 images are actually stored - the reduced image and the raw unreduced image.

Hope this is helpful.

Pete
Pete P.
User avatar
bluemax
10+ Years Member
Posts: 817
Joined: Thu Aug 13, 2009 5:00 pm

Re: Darks/bias library

Unread post by bluemax »

Thanks Bill & Pete......yes, it's a cooled camera the delta T of which I set in my capture software so, if I take a series of darks in the house, the temp should not be an issue. I generally take light frames at 30-45 seconds with Hyperstar and 180-240 with an F6 refractor thus, I want to fill a library at the various exposure settings I use for lights. Once I get better polar alignment I hope to get up to 5 or 6 minutes exposure.
Frank N

Stellarvue 80mm APO, Skyguider Pro, Celestron AVX

"I'm a seeker too. But my dreams aren't like yours. I can't help thinking that somewhere in the universe there has to be something better than man. Has to be"

Taylor in "Planet of the Apes" 1968
User avatar
AndyG
Network Admin
Posts: 1160
Joined: Wed Nov 30, 2011 10:18 pm

Re: Darks/bias library

Unread post by AndyG »

bluemax wrote:yes, it's a cooled camera the delta T of which I set in my capture software
With your ATIK it's the actual temperature you are setting, not the delta. So if you set -15C, the CCD temperature is -15C (not 15C below ambient). Since it is the actual temp you can, as Bill and Pete said, take your bias and darks once and reuse them. I pick a temperature my camera can reach all year round and build a library of darks and biases that I use for about a year, then always take my lights at that same temperature. Flats are most annoying as they have to be taken any time the imaging train is changed.
bluemax wrote:Once I get better polar alignment I hope to get up to 5 or 6 minutes exposure.
I recommend the Drift Align tool in PHD2!
Andy
User avatar
bluemax
10+ Years Member
Posts: 817
Joined: Thu Aug 13, 2009 5:00 pm

Re: Darks/bias library

Unread post by bluemax »

Thanks Andy, for the clarification on the Atik....I will likely set it at -15 and try it for now and go forward with making the dark/bias library.
Frank N

Stellarvue 80mm APO, Skyguider Pro, Celestron AVX

"I'm a seeker too. But my dreams aren't like yours. I can't help thinking that somewhere in the universe there has to be something better than man. Has to be"

Taylor in "Planet of the Apes" 1968
User avatar
AndyG
Network Admin
Posts: 1160
Joined: Wed Nov 30, 2011 10:18 pm

Re: Darks/bias library

Unread post by AndyG »

Bill Lee wrote:
AndyG wrote:I recommend the Drift Align tool in PHD2!
Wait! Aren't you somehow involved with PHD2? :lol:
Yeah, a somewhat biased recommendation :lol:
Andy
Post Reply