A few days ago when taking flats with my CCD47-10, I experienced an unusual source of noise in my images. It looked like dark noise or cosmic rays, but there were residual signatures around each "hot" pixel. Strangely, this did not appear when taking darks, only when exposed to light. This occurred for all the filters I had, and even when I did not use a filter. I took the camera off the telescope to see if anything was obviously wrong on the CCD window, but I saw nothing. After maybe 30 minutes, the noise went away. Unfortunately I was not able to continue observations that night, so I can't say that the camera was stable. I have no idea what I would have done that would have made the noise go away, and unfortunately I did not save any of the "fixed" images with no noise. The camera was cooled to -30 C and the cooler was able to hold steady at this temperature. This was the first time I had used it since June. I did not have this problem then. I'm including one of the flats with this noise in it, and a dark without the noise. Please let me know if you'd like any more information.
I agree with Colin - that is certainly something to check. Frost can take on a wide variety of appearances. Also of great importance - this is a windowless sensor. It could be damaged by exposure to moisture. At the very least it may be impossible to clean, and it is always potentially risky to clean a windowless sensor. I strongly recommend periodic maintenance. Do not wait for frost to appear! Recharge the desiccant every two years if you are in a low-humidity region. If you are in a high-humidity conditions then recharge the desiccant annually!
I got the camera last October and have not baked it since. I am in a high-humidity region, although the camera has been stored indoors when not in use. Should I be recharging this twice a year instead?
Yes, in some situations it may be advisable to bake it twice a year. Temperature swings caused by moving between indoor and outdoor environments may actually accelerate moisture ingress. If possible, my typical recommendation is to leave the camera permanently installed, but keep the power on with the cooler turned off when not in use. That keeps the camera slightly warm, which helps prevent gradual moisture ingress through the seals.
You may wish to order a second desiccant plug. We usually have them in stock or less than a week: https://diffractionlimited.com/product/replacement-desiccant-plugs/ That way you can bake and swap, and keep contamination out.
Sounds good, I'll try recharging it. Related question - how do you get the thing out? I tried twisting it 90 degrees and pulling on it, but it's not budging. Do I need to get pliers to pull it out?
Ok, I tried the camera again after baking out the desiccant. I am still seeing the same pattern, and it looks like its the same pixels. I tried adjusting the temperature and it looks like the effect goes away if it's above -25 C. Is there anything else I can do, or should I keep it above -25 C until the humidity goes down?
Sounds like there is still enough moisture in the chamber to cause frost below a certain temperature. You should wait 24 hours after recharging (and reinstalling) the desiccant, to allow the chamber to dry out. If there is a lot of moisture you might have to bake the desiccant twice (24 hours apart). Aside from frost, I can't explain the behavior. You definitely don't want any moisture on the surface of your sensor, as it can cause corrosion. Windowless sensors should always have their desiccant recharged before moisture becomes a problem.
I did wait 24 hours before observing, but perhaps there was still too much moisture. I'll try baking it out again. Looking at another thread here (https://www.cloudynights.com/topic/866454-ice-crystals-on-ccd/) shows similar effects on their CCD, so it does seem like frost is the cause.
I re-baked the desiccant and let it sit in the camera over the weekend. There was still some frost at -30 C but not the wide spread bright dots like in the previous images. Instead, there were two star-like objects which were dark in the center. I took the temperature up to -15 C which cleared them, and was able to bring it back down to -30 C with no frost artifacts. I'll keep an eye on it but I'm not sure if there is anything else I can do.
