1800 second Lights do not calibrate

[pmarshall]

Images acquired by MaximDL and processed in PixInsight 1.8.8.2
North American Nebula, small region in interior.
Narrow band Ha=600sec, O3=900sec, S2=1800sec.
Flats for each filter and Darks for each filter.
Ha and O3 calibrated fine.
It appears that S2 would calibrate for flat (vignetting eliminated), but the hot pixels were not calibrated out.

Double checked quality of Bias-Master, 1800sec-Master-Dark and S2-Master-Flat. They looked normal and compared similar to the 600sec and 900sec darks and flats for the other two filters.
Most of my imaging is done in the exposure range of 300 to 1200 seconds. This is the first time I imaged at 1800 sec (30minutes).

Is there something I am overlooking that needs to be considered for long exposures?

Thanks; Phillip

[wvanreeven]

You probably checked this already but are the binning and gain+offset or ISO (depending on which camera you used) the same for the SII darks and lights? You mention hot pixels only so that's why I ask about darks only and not about flats.


Clear skies, Wouter

[pmarshall]

The binning for all filters is 2x2.
How do I determine if the gain+offset are the same or different for S2 vs. Ha or O3? Same CCD camera, QSI-583wsg-8 is 9 years old.
I compared the RAW S2 image versus the poorly calibrated S2 and the hot pixels were in the same place and had the same value. But the vignetting was calibrated out.
This is the first 1800 second project I have done. All other calibrations worked as expected for 300, 600, 1200 second exposures.

Thanks

[pfile]

were your calibration masters made in pixinsight?

you don't have to worry about the gain of a CCD camera, it is fixed. if the binning was wrong then calibration would fail outright as a bin2 image is half the size (in each dimension) of a bin1 image.

so the darks are 900s duration? i guess there's nothing technically wrong with that but most people take darks of equal or longer length than the lights and let the software scale the darks down. i guess scaling up should work but not many people do that to my knowledge.

can you get some 1800s darks?

rob

[mschuster]

My experience with QSI-683, same sensor as yours.

2400s lights 3nm Ha, 64x 2400s darks taken within one month of lights, all binned 2x2, all at -20ºC.

Some "warm" pixels in the lights get warmer than they were in the darks, and so don't calibrate out.

Some "warm" pixels in the lights get cooler than they were in the darks, so the reverse problem.

Some "warm" pixels in the lights were normal in the darks. And so on.

Solution: 1) Don't scale dark master, that only makes the problems worse. 2) Don't use a bias master, just dark-subtract, 3) Dither, good dithering plus good rejection solves the problem completely for me.

[pfile]

i took the title of the thread too seriously... i somehow understood that the calibration was not working at all.

however as mike has correctly pointed out, scaling a dark will tend to leave residual hot pixels around.

so what you are seeing is normal.