ImageCalibration treatment of Flats for when calibration box is checked

[astrospotter]

I am sure this is 'somewhere' but in finding this answer I have timed out in a search.

In ImageCalibration if I use the method for controlled temp and light box flats discussed in the 'tips' forum it says after creating a simple integrated master flat (in my case pedestal not used) then for light frame calibration to check 'Calibrate'.

The pop-up help for 'Calibrate' for master flat section discusses the overscan and master bias subtraction (so far so good)  but then it say the master flat will be dark-subtracted.   THIS worries me as I see no place to enter some sort of master dark for the exposure used for the flat frames which in my case is about 40 seconds for these Ha lights I will be calibrating.

 My master Dark is for 20 minute Ha exposures and that just cannot be right to subtract away from such short flat frame exposures.   I would be happy to see or know of some checkbox to say 'don't even dark subtract for the master flat'.    I realize the master dark is a far lower signal than the flat but what is the point of subtracting away a grosely long exposure?   

So I hope that tool pop-up tip is incorrect and the 'Calibrate' for master flats does NO dark subtraction as there is not even a place to put in the dark master for the flats (which I don't typically take anyway).

Also maybe the dark subtraction mentioned there has nothing to do with clicking Calibrate and it is really discussing part of the image cal that is to be done anyway to the lights (dark subtraction to light frames part)

Thanks.

I ask this as I want to squeeze everything I can from this and use of the very big image cal script has led me to believe flat correction in that is not quite right so I will try that another time.  Today I want to use raw pixinsight low-level calibrate tool.

[Cleon_Wells]

This may not answer all your questions, but read this link on Calibrating/bias subtracting your Dark and Flat subs, then your integrated Master Dark and Master Flat files have the pedestal removed, leaving the Cal boxes unchecked during light calibration.
http://pixinsight.com/forum/index.php?topic=2570.msg19019#msg19019
Cleon

[mmirot]

Might want to check the amount of dark current accumilated in 40 Seconds with your camera. Often times the noise error in the dark frame subtration is greater or as great the just subtracting a bias.

Max

[mmirot]

Here is an example.
My KAF 16803 has dark current of 0.037 e/sec at -15C.
 At  60 seconds this is only 2.2e.
I currently run at between -25 and -35 C so my dark current is between to 0.016 -0.05 e/sec.
This put me in the less than one electron range. I think a bias should still be ok.

Max

[astrospotter]

I'll try to state the problem in a shorter way.  I have done the full cal and kept the 'Calibrate' checked and things look very good.

The issue is the pop-up for the master flat for 'Calibrate' says it is going to do something that I believe is not what is desired.  It says it will correct the master flat by subtracting out the dark frame.  Normally the dark frame for such a subtraction must be a dark=flat as it is called or a dark taken with same exposure as the flat frame.

There is no way I want my short 30 second flat to have subtracted from it the very long 20 minute darks that show up as master dark.   

That is what is misleading or just downright wrong.

I do understand the process of calibration.  I simply feel the tool tip is not saying what PixInsight is really going to do but is misleading.  I am positive PixInsight is NOT going to use the incorrect master flat for the 20 minute exposure as a 'dark-flat'  it is just that it is implying it will do as such.

Mark

[Cleon_Wells]

Mark, doesn't the MasterDark Optimization (if checked) scale the MasterDark to fit the UnCalibrated MasterFlat, what is your MasterDark to the UnCal MasterFlat scaling factor?
Cleon