Dark Library

[Mav359]

Hi Guys

Do/Have you guys put together dark libraries? Steves book mentions that you can use Dark libraries with cooled CCD's and someone else just reminded me of it, this seems to me to be a great idea mainly becuase i hate gathering Darks i feel like i'm wasting time.

I was thinking sets of......

30x1,2,3,5,10,15,20,25,30mins

does the orientaion of the CCD in the scope matter with darks like it does with flats?, ie do i have to take 0deg, 90deg, 180deg etc?

Cheers

[old_eyes]

My understanding is that you don't have to worry about orientation, as there is no light going through the optical train - hence dark frames. Dark frames are a measure of the noise pattern on the chip itself.

I have set up a few libraries at different set point temperatures and exposures (basically the ones I use most regularly), and find it works very well. Mind you given my general imaging skills I probably couldn't tell if it wasn't working properly.

Depending on the set point temperatures you usually work at you can capture dark frames in the daytime and in the house. If you need lower ambent temperatures to reach your normal working set points, put the camera in the fridge and run the wires out through the door seal to a convenient laptop. Works a treat. Mind you it annoys the hell out of the rest of the family, but since my partner tends to keep live moths in the fridge I always have a comeback!

Haven't used the freezer, but can see no reason why it would not work if your typical ambient temperatures are that low.

old_eyes

[Mav359]

Everything lives in the obs, i reckon the ambient temp is on avarage between 2-4c, its a cooled CCD and you do have a little control over the temp as well so i reckon there be little or no swing in temp.

Tomorrow is suppossed to be about that so this is getting done.

[kirkster501]

Hopefully an expert will confirm. But I think you also need to get the darks at different ISO's and different temperatures as well. So your seelction above would need to be at intervals of a couple of degrees Centigrade (i.e. -4,-2,0,2,4,6 etc) AND at the common ISO's you use for each one.

In other words you'd have a whole shed full of darks.

I have thought of doing it too.....

[rfdesigner]

Dark current varys exponentially with temperature, doubling with every 6 to 8C rise in temperature.

To get the benefit of dark frames you really need them to be very closely aligned with the temperature at which the image is taken, ideally within 1C.

HOWEVER

You can isolate just the dark current from the bias frame and then adjust one set of dark current readings for a number of shot lengths, using a correlation calculation to work out how much dark current to remove from any one frame so to some extent correcting for temperature misalignments.. there is a limiting condition however, in that hot pixels do not vary in magnitude vs temperature as other pixels, so it is a good idea to make up a 'hot pixel map' and then use that to make corrections to sub frames.

Your No.1 limitation is that you would need to repeat your process periodically anyway as sensors age, so keep it simple.

Personally I think your best bet is to temperature controll the camera as best you can, then stick with only two or three image lengths quite widely spaced. i.e. 4 minutes, 30 seconds, 4 seconds... which for dark frames probably just means dealing with 4 minute frames.

Derek