Jon Woodhead:
Perhaps ice crystals on your sensor - they typically congregate in the middle of the field in my experience and can be very persistent, even with anti-dew heater turned on. Were you operating in particularly humid conditions with a rapid camera cool-down?
Willem Jan Drijfhout:Jon Woodhead:
Perhaps ice crystals on your sensor - they typically congregate in the middle of the field in my experience and can be very persistent, even with anti-dew heater turned on. Were you operating in particularly humid conditions with a rapid camera cool-down?
That would also be my thinking. I had this recently myself and found it on my flats first, but later also discovered it on some of my light frames. Easy to get rid of, increase camera temperature by 15-20 °C, until they're gone and then very, very slowly cool down again.
Kyle Goodwin:Willem Jan Drijfhout:Jon Woodhead:
Perhaps ice crystals on your sensor - they typically congregate in the middle of the field in my experience and can be very persistent, even with anti-dew heater turned on. Were you operating in particularly humid conditions with a rapid camera cool-down?
That would also be my thinking. I had this recently myself and found it on my flats first, but later also discovered it on some of my light frames. Easy to get rid of, increase camera temperature by 15-20 °C, until they're gone and then very, very slowly cool down again.
Unfortunately they usually (in my experience) leave a residue behind which will continue to foul up images until it is cleaned. Usually it corrects out, but you can definitely see a mess in the flats when it happens.