Darks in Demetra

Darks in Demetra

Postby David Cejudo » Sun Jan 03, 2021 12:14 am

Hello.

In Demetra, when darks are added, i use the darks that best match the images. That is, if the spectrum has 60 seconds exposition, the darks i add to Demetra will last 60 seconds or so.

My question is, what about calibration and flat images? Those are only around 10 seconds. Should i add those images already calibrated with bias and darks? Or will Demetra not apply darks to those images? Or will Demetra apply the 60 seconds darks to them?

Thanks.

David Cejudo.
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Re: Darks in Demetra

Postby Nicolas Durand » Fri Jan 15, 2021 9:30 pm

Hello David,

Demetra use offset and dark images to compute "intermediate" darks for any requested duration, provided the duration is shorter or equal to dark basket images duration.
-> You have to put dark images with an exposure duration at least equal to the maximum duration of other image baskets (flat / calibration / object).
-> You have to put images with exactly the same exposure duration for each image you put in a basket (don't put dark with different exposure duration in a same image basket.)

Regards,

Nicolas
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Re: Darks in Demetra

Postby David Cejudo » Mon Jan 18, 2021 4:38 pm

Hello, Nicolas.

I understand the process, i have very often used darks with an exposition longer than my images. But not much longer.
Let us suppose that i acquire spectra 300 seconds long. My flats are 15 seconds long. Darks used by Demetra will be 300 seconds long.
Does Demetra scale down 300 seconds darks to apply them with 15 seconds flats?

The difference is great, i do not know if that is a correct procedure. Wouldn´t be better to use shorter darks for the flats?
David Cejudo.
Observatorio El gallinero.
El berrueco, Madrid.
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Re: Darks in Demetra

Postby Robin Leadbeater » Tue Jan 26, 2021 12:28 pm

Hi David,

Demetra (and ISIS) subtract the bias from the dark to produce the thermal signal. This thermal signal is then scaled for the length of the exposure. The flat (and all other images) are then corrected by subtracting both the bias and the scaled thermal signal from them

Cheers
Robin
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