Hello! I’m getting strange results after applying my master dark frame to images. It seems inconsistent as well. For the last several nights, I’ve spent several hours collecting images of the short duration variable RR CET. B, V, and I filtered images take about 5 minutes to get, and the loop continues till the variable is under 30 degrees.
I use an older SBIG ST402 camera at 1x1 binning and -10C cooling.
I’ve attached two raw adjacent images and the processed images (dark at 120 sec and -10C and the flat frame. The strange result does not occur if I only use the flat, ad does occur if I only use the dark frame.
I’ve redone the dark without effect. I’ve used darks and images from NINA and SGPro, I get the same result when calibrating using MPO Canopus and AIP4WIN.
It looks like some setting in FITS was changed? Possible binning change? But I don’t know what to do next. Thank you four your help.
Mike
RR CET_B_120.00_-9.79_2025-12-22_0085.fits (767.8 KB)
RR CET_B_120.00_-9.79_2025-12-22_0085_P.fits (767.8 KB)
RR CET_B_120.00_-9.79_2025-12-22_0086.fits (767.8 KB)
RR CET_B_120.00_-9.79_2025-12-22_0086_P.fits (767.8 KB)
Only the image with serial number 0086_P looks really strange to me. The hotpixels have value 65535 but should be gone if calibrated with a master dark. The calibrated files are still 16 bit integer (BITPIX=16) and not floating point values (BITPIX=-32). These integers are better called a word and allow only values 0..65535. This is okay of you apply only a master dark (not with a flat). I assume the dark pulled the values below 0 which resulted with 16 bit word values probably in 65535.
Check the background pixel values in the master dark and the images. If the master dark pixel values are higher, check the gain setting or pedestal setting in your software. Reduce the pedestal setting and create new darks with a lower background value and try again.
Han
Thanks! The SBIG ST402 adds 100 ADU, but it cannot be changed. The dark appears to have higher pixels than the light frame. I’ll try retaking the darks with good light exclusion to see if that helps. Thanks
Thanks! I think I had a low level light leak when I made the darks. I’ve been wrapping my brain around possible software issues, but nothing has panned out. When I had the camera in my position, I covered the camera well to make darks even tough the ST402 is supposed to make darks without additional work. At Starfront observatories, the camera was on the scope uncovered without the front cap. So, I’m redoing my darks paying articular attention to eliminating light leaks as much as possible. The first experimental raw dark still showed a few pixel values above the lowest pixel values in the image I took last week, though much better. I’ll finish the new master darks and do a new run so that I can check completely. Thanks again for your help
Mike
Hello! I’ redone my master darks. Hopefully, my next run will be uneventful.
I am wondering if I can still process my current raw images. The dark is 200-500 ADU above the lowest pixel of the raw image. By doing pixel math and subtracting 500 ADU from the dark, I can apply it to all raw images that I've saved.
I had taken a number of images of M67 for transforms, so if the pixel math does not affect the final photometry, I would like to do that in order to save the images I've obtained for M67, as well as RR Cet and a few other variables.
Thank you for your guidance.
Mike