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Flats, Darks and the rest.


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Very new  to imaging. Using a very simple rig to play around, see below.

Question is, if I take data from same object over several nights, what do i do about flats and darks plus bias. 

Do I do all at end of each session or one job lot at my overall end of data capture.

cheers

steve

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Your imaging camera is cooled so you can create a darks library to cover your exposure time, gain and temperature any time of the day. 

Flats are best taken when you finish up your imaging session for the night. 

I don't know about bias as I dont use them but I think taken at the end of your imaging session too. 

Flat darks are taken at the end of an imaging session too. 

Rightly or wrongly this is what I do 👍

Cheers 

Lee 

Edited by AstroNebulee
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Is that a DIY cooler? Looks interesting.

If you can make the DIY cooler work at consistent temperatures night after night, you can take one set of darks, biases and dark flats and use those for every session and dont need to take them each night. If your flats exposure time is very short you can get away with not using dark flats and instead just use bias to calibrate the flats. If its maybe not so controllable you could take several sets of calibration frames at different temperatures and then use whichever set is closest to the actual imaging temperature that night. Flats will have to be taken in the exact condition the optics were in when imaging, so before you remove the camera from the scope and each night with their own flats. If you can leave the setup as is each night and dont take anything apart, its possible your flats will work just fine reused but i would take them every time. Using a light panel of some kind is the simplest method, but since you have 50mm of aperture you can use a decent sized phone screen, tablet or laptop screen as a light source for the flats. and so probably dont need an extra tool for this. Very quick to take and so not much in the way of excuses to not do this each time you're out.

As for the multi session thing, you need to calibrate your session with matching calibration frames. So if you can take a library of darks, bias (and dark flats, if bothered) at matching temperatures where they can be reused indefinitely, the only calibration frame that is session sensitive would be the flats. Once you have calibrated the subs well it doesn't matter from which session it was anymore and you can stack any number of them together without issue and disregarding the session it was from. If you dont want to go through the trouble of storing calibrated frames you can use DeepSkyStacker with the "groups" function to drop each session to their own group. This feature calibrates each set with their own calibration frames and then stacks everything together in the end.

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Posted (edited)
13 hours ago, ONIKKINEN said:

Is that a DIY cooler? Looks interesting.

If you can make the DIY cooler work at consistent temperatures night after night, you can take one set of darks, biases and dark flats and use those for every session and dont need to take them each night. If your flats exposure time is very short you can get away with not using dark flats and instead just use bias to calibrate the flats. If its maybe not so controllable you could take several sets of calibration frames at different temperatures and then use whichever set is closest to the actual imaging temperature that night. Flats will have to be taken in the exact condition the optics were in when imaging, so before you remove the camera from the scope and each night with their own flats. If you can leave the setup as is each night and dont take anything apart, its possible your flats will work just fine reused but i would take them every time. Using a light panel of some kind is the simplest method, but since you have 50mm of aperture you can use a decent sized phone screen, tablet or laptop screen as a light source for the flats. and so probably dont need an extra tool for this. Very quick to take and so not much in the way of excuses to not do this each time you're out.

As for the multi session thing, you need to calibrate your session with matching calibration frames. So if you can take a library of darks, bias (and dark flats, if bothered) at matching temperatures where they can be reused indefinitely, the only calibration frame that is session sensitive would be the flats. Once you have calibrated the subs well it doesn't matter from which session it was anymore and you can stack any number of them together without issue and disregarding the session it was from. If you dont want to go through the trouble of storing calibrated frames you can use DeepSkyStacker with the "groups" function to drop each session to their own group. This feature calibrates each set with their own calibration frames and then stacks everything together in the end.

Some good information here and pretty much along the lines as I had been thinking.

First my DIY cooler you spotted, it takes temperature down by roughly 15 degree C to the ambient. I purposely made the cooler to not be to efficient so as to avoid condensation on the sensor. (This has been known with other ones which overcool the sensor. Apparently dedicated cooled ZWO cameras have a mini heater tape near too the sensor)

Anyway I could in theory make a library of darks using 2 degree increments for example. 

I keep my camera screwed to the scope, however like you alluded to I took a white screen "screen shot" on my phone and used this to cover the aperture last time for my flats so its really not a biggy, it is the darks above which are the biggest pain as far as I am concerned so a library I shall collate.

Thanks very much for the help.

Steve

Edited by bomberbaz
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