ASI ZWO 533 MC - dark pixels with colorful halo - faulty camera?

Jan75

Hello,

I purchased the camera in March and it worked well for 6 months. Since about 3 days I have observed dark pixels appearing in the middle of the frame, surrounded with color pixels. These dark pixels appear on Light and Flat frames but not on Dark frames. They appear on exposure times ranging from 0.5s to 60s (the picture attached is a 60s frame with STF applied in Pixinsight) at -10°C. Weirdly, they seem to appear randomly, meaning they are visible on most frames but not always. Has anyone had a similar issue? Is is a HW fault and should I return the camera?
Thank you!

kvastronomer

This is dew formed due to rapid cooling. Wait 20 minutes and it will be gone or cool gradually.
I noticed it too on my 533.

Jan75

kvastronomer You might be right after all. I just tested again and there were no visible dark pixels when I shot a flat frame at room temperature. Then I started cooling down to -10°C and the dark pixels appeared! I then waited for about 15 minutes (all while still cooling at -10°C all along) and... the dark pixels were gone again! I didn't change anything else in the setup so this indeed looks like a fast cooling could be the cause. For some undetermined reason the dew would always "reveal" the same dark pixels which was what initially misled me to think that it should be a different cause, maybe tied with the sensor. I am at loss for other ideas so yours is currently my #1 candidate for a good explanation! Thank you!

Jan75

Thank you @kvastronomer - that's an interesting explanation, however unfortunately in my case the cause is most probably different. I have these two pictures taken on two different days (Sept. 25 indoors flat, Sept. 26 outdoors light), with two different exposure times (0.5s, 60s), yet the dark pixels are exactly at the same place. If this was dew wouldn't it form at random spots? See the capture below - in Pixinsight those two pictures, when superposed, show the exact same pixels are impacted on both pictures.

Jan75

@tech@zwo what is your opinion on this? To me this looks like a faulty camera, doesn't it?

Jan75

Also I should add that these dark pixels did not appear in the dark-flat frames taken at the same time as the flats on the previous picture, same temperature, same exposure time. Therefore they cannot be removed through frame calibration...

wvreeven

Jan75 Darks only help to identify hot pixels. Flats are used to identify dark pixels. If you take a raw dark and a raw flat and determine the (x, y) coordinates of a given dark pixel in the flat, then the corresponding pixel in the dark should have more or less the same value. Do you mean to say that that is not the case?

Jan75

Indeed @wvreeven the raw dark and raw flat frames taken within the same 5 minute period don't seem to exhibit the same values as one can see on the screenshot (this time the flat is not debayered but the dark pixels still show). The red half-circle shows where the white pixel from the flat frame should be on the dark frame yet the value is completely different.

kvastronomer

More specifically, it is a frost forming on surface imperfections like dust particles, so it could be in the same spots.
I usually cool my 533 to 0C, wait for 10-15 minutes and then down to -10C.

Jan75

@kvastronomer thank you very much, your answer really helped. I was almost decided to send the camera to customer service - now I'm practically sure there is nothing wrong with the camera. The sales person from the store also mentioned he wouldn't even recommend "deep" cooling with the camera and that 0°C is usually enough to lower the noise to unimportant levels. Again, thank you!