Dark Frames over multiple nights

[JohnDeathridge]

Hi all if I may I would like your expert opinions please.
Scenario I have build myself a little Dark Frame library using my OSC cooled consisting of various sub lengths.

Now if I wanted to shoot same image say the Rosette Nebula over 3 nights

I take the lights (say 3 mins x 30) and flats over these nights.

I obviously need to obtain the darks to match the number exposure and gain temp settings for these but I am unclear if I have to have 90 x 3 mins darks or I can use my dark library of 30 x 3 mins.

I have read a bit in Pixinsight about creating Master Darks but I'm unclear.

Would really appreciate your help

[Carole]

John, I don't use a CMOS camera, but all I can say is the darks should match the lights in terms of length of exposure and temperature, so presumably gain settings as well (I don't have gain with a CCD camera) but you don't need to have the same number of exposures.

Don;t use Pixinsight either.  Hopefully one of the CMOS users can clarify.  In the meantime you could give it a go and see how it turns out. 

HTH

Carole

[The Thing]

You can use darks of matching time, temp, gain and offset. With CMOS don't use any sort of dark scaling which worked with CCD/DSLR and allowed mismatched darks to be used. You only need 10 darks though I use 20. It's a statistaical thing to do with noise and diminishing returns (same for flats and dark flats), nothing to do with the number of lights of which you want as many as you can get.

I use WBPP Weighted Batch PreProcessing script in Pixinsight and it takes care of producing master darks and using them in subsequent runs. I used to do the manual route but it's a real phaff.
 
I have one folder I use as the base WBPP folder on a drive separate to the subs. The masters are created in a Master subfolder along with all the other intermediate files in there appropriate folders. I move the Master folder to the same folder tree as the subs for reuse. I use NINA and have things set so it might be so it might be:

D:\NINA\M 82\TS1506UNC\ASI294MC PRO\2023-04-15\(subs, flats, darkflats)
D:\NINA\M 82\TS1506UNC\ASI294MC PRO\2023-04-16\(subs)
D:\NINA\M 82\TS1506UNC\ASI294MC PRO\master\(stack results of all subs, master calibration files)
D:\NINA\M 82\CC8\QHY294PROC\2023-03-12\(subs, flats, darkflats)
D:\NINA\M 82\CC8\QHY294PROC\master\(stack results, master calibration files)

so I have all the data for each target grouped together by telescope, camera and date.

I use NINA to create dark libraries which end up in a similar tree under a D:\NINA\Darks folder.

[JohnDeathridge]

Hi many thanks for the advice both. So if I understand correctly if i wish to shoot same object over several nights I can use the 30 Dark Frames that I have as long as the sub length gain and temp are good and effectively use Master Dark rather than having to shoot more Darks to match the number of lights that I will take.

The other challenge of course is matching the object position as close as I can so not offset but I found that ASIAIR has a nice feature that allows you to plate solve to a prior image which should help I hope !

Hope to see some clear skies soon everyone.

[Carole]

Absolutely John. 

The problem with multi night imaging is if you have to strip down the kit between nights.  Trying to get the camera orientated exactly the same is not easy and you need separate flats for each time you reinsert the camera. 

If you are leaving the rig up without moving it then that won't apply. 

Yes can re-use darks. 

[The Thing]

rather than having to shoot more Darks to match the number of lights that I will take.

John, you DO NOT NEED more darks. EVER. 30 is plenty, 10 is enough. It does not need to match the number of lights. I usually have 80-100 240s lights from a winter session, that many darks would be daft and result in a minute miniscule improvement over 10 or 20 matching darks.
 
Just update your darks library with ten/twenty of each combination of time, gain, temp (I always use -5c), offset (if you change this) you are likely to use every month or so to take account of CMOS changes and your good to go.

I second what Carole says about flats. I have a permanent setup (sometimes months between changes of OTA, filter etc) but I take flats every session as dust moves over time, if it does your subs are useless as the old flats won't do the business on them dust bunnies.

Hope that helps!

[Carole]

I have to say l have had the same rig unmoved for a number of multi sessions and different targets and have reused flats and it has been fine.  Obviously l wouldn't move the rig/camera until l have processed an image giving me the option of taking further flats if need be.  But l have never needed to.