AndreHackelbusch I shot a light frame with no cooling and it was the same pattern there as if with cooling.
Whoa... that is not likely to be a moisture problem then (I had only read Walter's posting, not yours).
OK, try this next time too. Unscrew your camera just a little (like a quarter turn or less to make sure it does not fall off :-). Check the image. If the pattern rotates relative to the frame, the problem is at the telescope end. If not, the camera end.
This will "binary chop" the debugging process (i.e., eliminate half of the possible problems; sometimes debugging is a process of elimination, rather than a process of discovery).
If the smudge does not rotate when you'd rotated the camera, the problem is somewhere between the camera's flange and the sensor.
I am guessing that the camera you bought has a built in filter wheel, in which case, you can also change the filter selection to eliminate yet another potential problem.
Your problem is actually likely to be e dust speck either on your filter or a glass element in your telescope.
Rotating your camera should tell you which.
Since your smudge is large with a somewhat radial symmetry, it is probably not dirt on the sensor itself (good news), but it could be dust on the filter (I have no idea how far your filter is away from the sensor -- I don't use cameras with integral filter wheels) or dust on any reducer that you use. Rotating the camera and choosing different filters might help pinpoint the problem further (or more precisely, what is not the problem).
By the way, a large smudge on your filter will not cause the problem that you are seeing -- that's not how optics work. If it is something on your filter (or one of the elements of the Petzval telescope), it would be a small and perhaps barely visible dust speck. Just use your air blower to see if it will clean out the shape on the sensor after checking if the problem is on the telescope side or the camera side. If you don't already own a small hand bulb air blower, you need to buy one right away, since you will need it over and over to blow out the dust every time you open any part of the optical chain up to open air.
Never, ever blow air at expensive optics with your mouth. Use one of the cheap air bulbs that are sold to photographers. There are many that filters air on the way in and emit very little dust themselves.
You can also use the "black light" lamps or flashlights to see refection from the small dust specs better.
Chen
