What (where) are these image blemishes?

[CombatCoder]

Hi,

I started to connect a ZWO 224MC to my SkyMax 127 Mak-Cass and initially just wanted to see something so used PHD2 looping to get an idea of local daytime focus.  I was surprised to see a number of circular blemishes on the unfocussed image as shown in the attached image.  These hadn't shown up when doing the same through a guide scope but were also visible when using a different ZWO 120 camera, this led me to believe it was related to the scope not the CCD.  So a good clean of the corrector plate inside and out followed, and the mirror looked clean, but no change.  More confusing is that if I rotate the camera, assuming the marks are due to the scope, I would expect the marks to rotate within the image as the camera rotates.  Not so.  As a result, I'm stuck as where these marks are coming from and how to get rid of them.    They appear to be foreground to the main image as I can see the main background rotate behind them as I rotate the camera.  It is also not the (albeit dirty) windows through which I'm testing this as the marks remain no matter what I point the scope at.  

Any help as to where these marks might be and therefore what to do to remove them would be much appreciated.

Thanks,

Adie 

 

[vlaiv]

These are shadows from dust particles - but in order to show on sensor - dust must be fairly close to sensor - otherwise "dust doughnut" will be too large and "out of focus" to show.

You can calculate approximate distance from sensor.

Try one of these:

https://astronomy.tools/calculators/dust_reflection_calculator

https://www.wilmslowastro.com/software/formulae.htm#Dust

My guess that they are on sensor cover window glass of camera unless you use some sort of filters / filter wheel - in which case, they are on filters.

[CombatCoder]

Thanks for the help.  I'll have a good look.