Image calibration question

[andyc67]

I see in the image calibration tool for darks flats and bias there is a box that says "calibrate". I am confused on when and if I should check it. Can someone explain to me what the calibrate option actually does?
Thanks

Andy

[pfile]

calibrating a bias frame is kind of obscure - it is only necessary if your sensor has "overscan regions".

calibrating a dark means to subtract a master bias from it. if you remove the bias signal from a dark, then you can scale the dark ("optimize" it in PI's language) to make the dark signal in the dark master match the dark signal in your lights. for instance if your darks are 1200s and your lights are 600s, you probably want to do this.

if you hover your mouse over the "calibrate" tick boxes in ImageIntegration, you'll get a little tooltip with similar info.

rob

[chris.bailey]

Andy

http://www.pixinsight.com/tutorials/master-frames/index.html is well worth a read as I think it explains things well and for me is the bible of image calibration.

Chris

[andyc67]

Thanks! I read that page before but it doesn't exactly explain what the calibrate option does. I see that optimize and calibrate are two separate options. I understand dark scaling and I always use it, however are you saying that unless I check the calibrate option my darks won't be scaled?
The tooltip says that my darks will be bias subtracted. Why would I want to do this if I want my darks to actually contain my bias. They can be scaled but still contain my bias so that they can be accurately subtracted from my light.
Same goes for my flats. If I have a master flat, my master flat should have already been calibrated so why would I need to bias subtract it again?

[pfile]

dark frames can not be scaled unless the bias is subtracted first. the reason being that bias signal is fixed, but dark signal can vary depending on the length (and temperature) of the dark/light. so to scale a dark without first subtracting the bias is an error - you scaled the bias signal as well, and then over- or under- subtracted it from the light.

the calibrate checkbox is there so that if you've made a master dark from non-bias subtracted dark subs, you can subtract the bias signal from the dark when you run ImageCalibration. that gives you a little more flexibility, i suppose. if you choose not to optimize, you can calibrate your lights only with the master dark. if you want to optimize, you load a master bias and the master dark, and tick both "calibrate" and "optimize" for the master dark.

the same goes for flats, but the reason is a little different. flats need to be scaled (normalized) so that the brightest part = 1. the presence of the bias signal while trying to compute the flat scaling factor messes up the calculation.

rob

[andyc67]

Thanks rob.
If your darks have already been bias subtracted and your flats are already calibrated then do you not select the calibrate option in the flats and in the darks section?

[pfile]

that is correct, in that situation, don't select the calibrate option.

rob

[andyc67]

Ok that is logical.
Do you know what kind of master files the pre processing script expects?
So far I always assumed it needed the following:

- master dark not bias subtracted
- master flat bias and dark corrected
- master bias or superbias.

Is that correct?

Lastly, since my master darks aren't bias corrected I must always check the calibrate option for the master dark in the calibration tool, right?

[pfile]

that's a good question - i'd like to know that too. it's one reason why i never use BPP - at the time it came out i already had dark masters made the old-fashioned way. i think i ran into the script setting the FITS reader direction differently that i had set it. in the end i was not able to re-use my masters and so i just kept going with the manual method. also BPP does not seem to have the option to only calibrate flats with bias frames - it always wanted to scale my (really long) darks to match the flat. of course i could make dark flats or otherwise force a master bias to look like a dark flat, but i figured why bother.

so since the master darks include the bias, you have two possibilities - don't load a bias frame, and leave "optimize" and "calibrate" unchecked on the master dark. that assumes your lights are of similar length to the dark.

the other option is as you say - load a master bias, load the master dark, and tick both optimize and calibrate on the darks. that will properly scale your darks.

rob

[andyc67]

Since I always like to scale my darks I can see two scenarios.

1) Bias subtracted dark
- Optimize:yes
- Calibrate:no

2) Non bias subtracted dark
- Optimize: yes
- Calibrate: yes

I am uncertain how the calibration tool would operate in option 1. I think that I would still need a master bias loaded because the lights need to be dark and bias subtracted.

I also wonder who would clarify what kind of master dark and flat are needed by the BPP script? Anyone have the answer to that?

[pfile]

#1 is how i've been using ImageCalibration from day 1… it works right. and yes because the master dark is already bias-subtracted, you have to load the master bias. the math is then (light - master_bias - (dark*scaling_factor)).

on BPP unfortunately it might be "use the force" / "read the source" 

rob