Dark Flats Instead of Bias - Calibration?

[Rastaban]

Hi all,

I've been making use of dark flats in place of bias frames, and I'm wondering if I'm doing this right. With regards to Master Bias and Master Dark, should I treat Dark Flats as bias frames? In other words, do I just need one Dark Flat library for each filter/gain/temp configuration which have been calibrated with a Master Dark Flat?

Thanks,

[pfile]

are you using BPP?

i'm not sure i understand the question though - are you asking if you can load the master dark flat into ImageCalibration as a bias frame?

rob

[Rastaban]

Rob,

No, I'm doing the calibration and integration by hand. I'm asking if I treat dark flats the same as I would a bias in terms of calibrating darks with an integrated master dark flat in order to create a master dark. This is my current workflow, adapted from Light Vortex, and modified to use dark flats:

Stack bias dark flat frames to produce a master dark flat. Transform this into a noise-free master superbias.
Calibrate dark frames with master bias dark flat. Stack these dark frames to produce a master dark.
Calibrate flat frames with master bias dark flat and master dark. Stack these flat frames to produce a master flat.
Calibrate the light frames to stack with the master bias dark flat, master dark and master flat.

Is this correct? I hope this is clearer

PS: If my master dark frames are of the same exposure length of my flat or light, I don't need to optimize, right?

[pfile]

ok, i see.

here's what i would do:

1) Stack dark flat frames to produce a master dark flat
2) Calibrate flat frames with master dark flat. Stack these flat frames to produce a master flat.
3) Calibrate the light frames to stack with the master bias, master dark and master flat.

the dark flat duration matches the flats... that's what makes them 'dark flats' so they don't need bias subtraction because they are not going to be scaled (as you point out.) so in your flow you end up calibrating the master dark flat and then loading the bias into ImageCalibration anyway.

also the master dark flat is out of the picture once the flats have been calibrated. you use the master bias, the master dark and the (calibrated) master flat only to calibrate your lights.

rob

[Rastaban]

Ah, so I still need my bias files, if I'm reading this correctly (#3).

Thanks!

[pfile]

yes that's right. the first step is calibration and integration of the flats, which is done using the matching dark for the flat. you end up with a master flat which is bias and dark subtracted.

the 2nd step is calibration of the lights, which assuming you want to scale the master dark, you'll need either 1) a calibrated (bias-subtracted) master dark and a master bias frame, or 2) an uncalibrated master dark and master bias frame (but in this case you would tick "calibrate" and "optimize" under the master dark entry of ImageCalibration - IC then subtracts the master bias from the master dark before scaling the dark.)

in my case, my flats are short enough (~5s) and my setpoint low enough (-30C) that i simply use a master bias to calibrate the flat subs. there's not a lot of time for dark current to build up @ 5s at that temperature, at least with my sensor.

rob

[Rastaban]

Great, thanks Rob! I'll give it a try.

[stevek]

yes that's right. the first step is calibration and integration of the flats, which is done using the matching dark for the flat. you end up with a master flat which is bias and dark subtracted.

the 2nd step is calibration of the lights, which assuming you want to scale the master dark, you'll need either 1) a calibrated (bias-subtracted) master dark and a master bias frame, or 2) an uncalibrated master dark and master bias frame (but in this case you would tick "calibrate" and "optimize" under the master dark entry of ImageCalibration - IC then subtracts the master bias from the master dark before scaling the dark.)

in my case, my flats are short enough (~5s) and my setpoint low enough (-30C) that i simply use a master bias to calibrate the flat subs. there's not a lot of time for dark current to build up @ 5s at that temperature, at least with my sensor.

rob

I read that the dark should be automatically scaled to match the flats during flat calibration with the tool tick box selected.  But I notice that every time I run calibration of the flats with the master dark (to match the much shorter exposure of the flats) I get an error message in the console, words to the effect "No correlation of the dark to flat" (not at my computer at moment so cannot quote exact error).

That would imply flat-darks are needed.  But the PI documentation states that don't waste time with flat-darks due to the above scaling....

Confused.....!

[pfile]

there's lots of ways to skin a cat. dark scaling is a tried and true technique, but sometimes it does not work due to non-scalable artifacts in the darks (sensors with amp glow, or RBI turned on.)

specifically the error message you are seeing means that PI scaled the dark looking for a minimum noise in the calibrated frame and did not find a minimum. this usually means that there's very little dark signal in the frame being calibrated. i would take this as a sign that you can probably get away with calibrating your flats with a master bias only.

rob