Sensor concern with ZWO 294mc pro

[JohnJONeill]

Hi 

I bought this camera preowned from Astro buy and sell. I wonder if experienced imagers can give their opinion on the attached non processed image.  it has been stacked in Siril with Lights only. Each sub was about 120 sec long and there was about 30 images. I know there is amp glow with this camera. However, I’m wondering about the dark areas. One looks like a circle. I’m concerned that I been sold a camera with a dirty sensor. However, this camera has sealed sensor so dust shouldn’t enter the sensor. Opinions greatly appreciated 
 

[Vroobel]

How you know that the dark areas aren't caused by dust in your optical train? 😏

That's funny, two days ago I perfectly cleaned my refractor, reducer, filters and camera, but one stubborn spot was still in the flat frames. Nothing on the glass and nothing on the camera as well. 

Do one proper flat frame at home then rotate the camera by 90 degrees and do another flat frame. If the spots are still in the same place then they are on the camera indeed. If they move then they are elsewhere. 

My spot moved, so I gave up. Flat frames are just for that to remove the dust moths. 

BTW, any dust located directly on the sensor surface creates small and sharp shadow. 

Edited April 14 by Vroobel

[JohnJONeill]

Many thanks for the sound advice. I have another cooled camera so I will see if the same happens with this. I will then know  if it’s elsewhere in the optical train

cheers

John

[Elp]

I'd say it's the lack of applied flats, you can't really do respectable AP without them.

More of a concern with that sensor is whether it'll suffer from the rainbow pattern you get with them, it happens with mine. If so you'll struggle to really stretch your images as you'll have random red/green patterns across the image which don't calibrate out.

[JohnJONeill]

Thanks Elp

Ive not not managed to do calibration frames yet as part of my workflow. Thanks for the the reply and perceptions. 
John

[Vroobel]

The flat frames (hopefully) will eliminate effects, but not the reason. It's worth to know where is the problem. 🙂

[Elp]

It's actually quite difficult keeping everything pristine clean hence the need for flats. The only time I've found you can likely get away with the dust motes (note not the vignetting, that needs flats) is when imaging at F2 (little to no dust motes to be seen).