A correct way to use light integration with master bias and calibrated master dark

Leonardo Andrei Arismendi Díaz

At the beginning of my time using PixInsight (4 years ago), I use to calibrate my master data using the following steps:

1) Integrate bias in a master bias

2) Calibrate and optimize each dark with master bias (following a recipe from a web site)

3) Integrate each calibrated dark in a master dark

4) Delete original bias and dark files and store master bias and master dark for further use in reprocessing light data.

After see fundamentals videos about master files and calibration, I have a doubt. Which will be the correct use of the previous master bias and master dark described to apply on lights in order to reprocess images and make a correct calibration of them?

Because in my case, the order of producing a master darks wasn´t the recommended way, this master dark is usable? is it valid calibrate lights with master bias and this particular master dark?

Thanks,

Leonardo Arismendi

Wade Hilmo

I think that step 2, above, is a potential problem.  The issue is that for cameras with low read noise and dark current, calibrating each dark with the master bias is likely to black clip a bunch of pixels.  Even using the master bias to calibrate a master dark could black clip pixels.

Most of the time with my current cameras, I use an uncalibrated master dark and a bias calibrated master flat to calibrate the light frames.

Alternately, if you want to use both a master bias and master dark (perhaps to optimize darks with a CCD camera), I would suggest using an uncalibrated master dark and letting the ImageCalibration process use the master bias to calibrate the master dark internally.  Since it all happens internally, ImageCalibration can use signed, floating point numbers to handle negative numbers instead of black clipping.

Leonardo Andrei Arismendi Díaz

Thanks for your recommendations.

My concern is more about how I can use the ones that already exist.

Or is there definitely no way to use them, but only master_bias, completely losing the use of those master_darks?

Wade Hilmo

If I understand your situation, you have old data that you want to process, but the master bias and dark are questionable.

I am trying to figure out the best way to answer the question as to whether the existing masters are usable.

On the one hand, dark calibration is about the simplest thing possible.  The value of each pixel in the master dark is just subtracted from the value of the same pixel in each light frame.  The software isn't going to fail because you use a non-optimal master.  You could use literally any monochrome image with matching geometry as the master dark.

So are your existing masters usable?  In the above sense, yes, they are.  Are they going to give you be best result?  Maybe.  It depends on whether clipping occurred when the bias master was subtracted from each dark sub.

One thing to try would be to examine your existing master dark and look for black clipped pixels.  They will have a value of zero.  If there are just a handful of them, I wouldn't worry about it.  If there are many hundreds or thousands, then I would consider the master dark to be suspect.

If the lights are less than about 6 months old, I would just re-take the bias and dark frames and make masters using the best technique.

If the lights are older than that, you could try to calibrate them twice, once with the old masters, and once with new ones.  You'd then keep the best result.

Leonardo Andrei Arismendi Díaz

Thanks a lot x1000!
I will execute your recommendations.