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Flats, Dark Flats and Bias - Take them every session?


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I have been taking Flats and Dark Flats and Bias regularly but don't always find the time.

Can I re-use most recent files for these and save time?

Are older files better than no files at all when stacking is DSS?

Thanks in advance :) 

 

 

 

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Depends on camera that you are using.

Bias should be in principle good from single session.

You can verify this by using bias from one session and then taking another set of bias subs days or weeks later and comparing masters (subtract two master bias files and you should get just pure noise with average value of 0 ADU).

Flats and Flat darks should be taken each session unless you keep everything assembled between sessions (like in observatory or just take whole lot off the mount and store it as is). Even then - it can happen for a dust particle to move and then you need to take another set of darks.

Darks will depend on you having set point cooled camera (and since you did not mention them - I'm guessing you use DSLR?) - if you have set point cooling - then you can shoot one batch and use them for long time (until something changes like any of your settings or temperature that you can achiever - summer /  winter, or you decide to refresh your dark library).

In general - if you change parameters (gain, offset, iso, exposure length and so on) - you need to redo your calibration files.

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Bias is temperature dependant, aslong as its same ISO, give or take 5 degrees I believe is the rule. Flats are optical train dependant, you can reuse them aslong as you dont change focus, or the telescope/lens. Dark flats I just follow the same rule as Flats, if im taking flats, I retake dark flats also :)

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5 minutes ago, Grant93 said:

Bias is temperature dependant, aslong as its same ISO, give or take 5 degrees I believe is the rule. Flats are optical train dependant, you can reuse them aslong as you dont change focus, or the telescope/lens. Dark flats I just follow the same rule as Flats, if im taking flats, I retake dark flats also :)

Are you sure about bias being temperature dependent?

Bias is just read out signal and nothing more. There is no time for any sort of dark current to form. They can be noisier on higher temperatures but their signal should in no way depend on temperature.

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I have an old 1000D at the moment and I do not change focus or remove the camera.

I just bring the whole setup inside and re-polar align each session.

I used to take darks but read they were not needed so I have removed them from my process.

Thanks for the help - much appreciated as always.

Save me some time and disk space! :) 

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5 hours ago, vlaiv said:

Are you sure about bias being temperature dependent?

Bias is just read out signal and nothing more. There is no time for any sort of dark current to form. They can be noisier on higher temperatures but their signal should in no way depend on temperature.

Just thought I've read it somewhere - if not, then I've been doing it wrong this whole time hahaha. I've been making librarys from each night, feel silly now.

I trust your opinion more than my own, you're far more experienced than me.

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38 minutes ago, Grant93 said:

I trust your opinion more than my own, you're far more experienced than me.

Well, we can easily test that - we simply take set of bias subs at one temperature and on another and compare the two.

It is better to verify to be on a safe side.

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