Darks and Master Darks

[iceman (Mike)]

So how do you organise your darks?

What method do you use to combine your darks to create the master dark? (average, median, etc) How many darks do you take? 1 for each light or approx 10-20?

Do you categorise them by exposure time, ISO and ambient temperature?

If you've got a master dark from a previous session that matches your current session (ISO, exposure and ambient temp) then you don't need to take any more darks, right?

Thanks

[Doug]

Mike, I use a median combine to create a master dark due to the possibility of random cosmic ray strikes, Otherwise the combine method is largely unimportant. I use one master for each ISO, but I also use a set of bias frames so that the darks can be scaled for different exposure times.
10 darks is sufficient to make a good master dark, 30 would be better, 60 will gain you nothing except wasted time. I use 120 seconds for darks, to reduce the degree of scalling, and renew them monthly due to temperature changes as the seasons vary.
cheers,
Doug

[Lee]

I have always taken darks at the same time as lights each session, to try and keep temps constant (helps with CCD temp readout on DSI) - this is with fairly short exposures though, once I get guiding and increase exposure times, I don't know how keen I'll be to snap 10-20 at 2-5min per dark each time!
Maybe choose some fairly standard exposure times, renew darks each full moon???

Also - from the manual of Nebulosity on taking dark frames -

make sure that no light is getting to the camera (wrapping the front in tin foil is great for this – black plastic lenscaps are quite "clear" to near IR light).

Don't know what difference this would make, and have not done this, but could be worth a try....

[EzyStyles (Eric)]

Hey Mike, i take my darks after finish imaging the object. I don't really have a master dark. Pretty much i do my darks each imaging session. I use the combined method in deepskystacker.

[[1ponders] (Paul)]

I take mine during the imaging session normally. That is assuming that I'm not pushed for time or cloud moving in. Otherwise I do them while I'm packing up if the temperature was relatively stable.

I then set up a filing system for each object and method used to image with the top folder being the date eg 2007-10-24 then M31_135mm_600sec_ISO400, then in that folder I have one for Lights, ond one for darks. For Flats I use a single folder under the date folder at the same level as the object folder. I do this as I don't normally alter the orientation of my camera once I've started a nights imaging. If I do then I might place individual Flat folders in each object folder.

I find this is the simplest way for me to keep track of the images. Once I've processed each object in IP and have Fits masters and Tiff masters I will go through and delete the intermediate fits files that IP generates, just to keep free some space. A big imaging session can generate a lot of MB.

[seeker372011 (Narayan)]

Mike

Luc -who wrote DSS- says 20 darks is the minimum recommended number of darks-and he has some examples in the DSS help to show the impact of more darks
( dont know many people who actually take 20 darks_ the maximum I have taken myself is 6 x 10 minutes--we are all seduced into capturing more photons and not wasting the night taking darks

I take darks every session myself..but since you already have the discipline of recording temperature at each imaging session you could easily build up a library of darks identified by temperature..I know Gary Beal does that sort of thing

[jase (Jason)]

20 dark frames is rather excessive. Personally, I'd question the effectiveness this. As is shown in a flat field experiment, there comes a point where the quantity of flat field frames combined into a master makes little difference. The same rules would apply for darks, if fact I've tested this with a cooled astro cam (STL11k) and have seen little difference when blinking two masters (one of 12 subs and the other of 20 subs). I typically now use anywhere between 10-15 subs to produce a master (be it bias, flat or dark).

For the DSLR user, I'd recommend taking dark frames on the same night as your light frames. If you've modified your camera with active cooling, then calibration frames (such as darks and lights) becomes easier to maintain and are obviously reusable. Though I'd question the accuracy of these mods. They are different to the dedicated astro camera that can keep a chip cooled to .5 degrees of the camera's cooling set point (very accurate). If you're camera is capable of this, then you can easily reuse your calibration files for sometime. Though a chips characteristics change with time so it is wise to refresh your calibration library often. I typically refresh in two month cycles waiting for a cloudy or full moon night to record the darks.

It should also be noted that you don't need to take dark frames at the same duration of your lights. If your camera is actively cooled you scale your dark frames to match different light exposures. This is successfully achieved through the use of bias frames.

Either Sigma-Reject or Median combine is an effective method of creating the master. Just becareful of the sigma factor if you're using the this algorthm. Finally, if there is a calibration process that is going to stuff up your final image, it will be with your flat frames. If your flat frames don't accurately depict the telescope light path, you'll introduce gradients into your "calibrated" light frames.

[iceman (Mike)]

Thanks for all the info, i'm learning heaps.

[Alchemy (Clive)]

Originally Posted by plasmodium

.make sure that no light is getting to the camera (wrapping the front in tin foil is great for this – black plastic lenscaps are quite "clear" to near IR light).

how about that.. i wouldnt have believed it.... but i bought a modded camera recently and had a little light leakage through the lens but not the telescope, and could not work out why .... theres the answer!

I have been averaging my darks , it would appear that median is the go, thanks for the advice.