Hello! As I’ve posted previously, I’m getting a number of images that have the darkest pixels about 300 ADU below the darkeest pixels of the master dark in my SBIG ST402 camera with BVIC filters - 65000 full well. The images have darkest pixels about 800 and the master darks have darkest pixels about 1100 ADU.
In trying to solve this, I’ve exhausted most possibilities, and I am exploring camera specific electronics aging issues.
I am wondering how subtracting 400 ADU from my master darks would affect the quality of photometry. With simple subtraction, the darkest pixel in the master dark would be about 700 ADU - below the darkest pixel of the image.
If the photometry is still good, then there may not be a need to pursue expensive repair of an 18-year-old legacy camera (or to replace the entire imaging train!) at this time, and I would be able to pursue such if electronic drift/noise gets worse.
Thank you for your guidance and best regards.
Mike
Can you tell us more about these images? What are the exposure times of the darks and of the target images? How far apart in time did you take them? Did the temperature of the air or the camera change during the interval between darks and target images?
Thank you for your note. I’ve been making runs of RR CET SPV. B filter at 120 seconds. I rotate through I and V filters and then back to B. In my run last night, all were affected, though in previous runs, some I and V images were OK. There was no evidence of rapid temperature change that I could see, and the ST402 compensates fairly quickly. I made the new master darks about a week ago, making sure there were no light leaks - telescope capped and make during the night. The ST402 has an internal shutter as well to make dark frames
I use a master dark calibrated with a median combine of 25 dark frames at the same temperature and time. I use twilight flats at 1-second integrations, using a median normalized combine since ADUs vary between 5000 and 60000 - the linear range of my ST402 from previous testing.
I found this from Arne Hendon in the AAVSO forum from 08-January-2023. (There were a couple of threads dealing with negative pixels over the last few years.)
“I think the reason you are getting negative pixels is because you don’t have much sky background. Subtracting a dark frame with its noise then gives you positive and negative pixels, with a mean around zero. This is perfectly normal. Do your dark subtraction and flatfielding, and then apply an offset, in that order.”
When I try this in MPO PhotoRed Utilities, it does not appear to allow negative pixels before the offset is applied. It appears to show a wrap situation. I’ve included an image of RR CET processed through MPO Canopus before application of the offset. (The image ends in P). There are no negative pixels, and the histogram appears to show a wrap situation. It would look fairly Gaussian if the brightest pixels were brought to the lower part of the histogram.
I tried the same with AIP4WIN. It kept negative pixels, but even after the addition of an offset of 400 aftercalibration, the histogram does not appear Gaussian, so I am not sure if it is handling things well. I have attached that file.
I’ve asked Mr. Warner at MPO Canopus how the program handles negative pixels during calibration. The nice thing about MPO Canopus is that it handles batch calibration well, so I can calibrate dozens of images from a single SPV during a night’s run easily.
Are there any other programs that handle negative pixels and offset addition after calibration well? Perhaps PixInsight?
I am still curious how the photometry might be affected if the offset is subtracted from the master dark and then applied to the image, rather than applying the offset after dark and flat application.
Thank you for having a look at these images and best regards.
Mike
RR CET_LIGHT_B_120.00_-9.79_2026-01-09_0045_P.fits (767.8 KB)
RR CET Test AIP4WIN_0001_S2F.FIT (765 KB)
Mike