Is this artefact the primary mirror or the sensor?

[BrendanC]

So the other day I processed an image and noticed an 'anomaly'. I checked the flats and sure enough, there it is:

DSS had done a good job of applying the flat but I could just about see that there was something not quite right in the image. Et voila, there it is, in the flat (and all the other flats incidentally, and the master flat).

On looking at the primary mirror, I could see a fairly large mark where it looks like an insect may have met its maker (this is the best photo I could get):

So, my first thought was 'Obviously, it's the mirror - which probably needs cleaning anyway'.

But then I decided to do an experiment. I took a few shots with the camera in the focuser, at a low ISO and exposure, almost like taking sky flats but in the room with the light on, rotating the camera between shots. This is what I got (stretched to show details):

#

If the problem was the mirror, I would have expected 'the blob' to have moved around the image, as I moved the camera around.

But it hasn't. It's stayed put, except that its shape changes a bit. This 'shape shift' seems to correlate with the direction of the light - you can see that the dark side of the blob is always towards the light, almost like a crater on the moon.

This implies it's the camera. Given that I've checked the coma corrector, the only remaining thing is the sensor, even though I have auto sensor cleaning enabled for every time I switch it off.

So, before I either take my scope apart to clean the primary (scary) or take my camera apart to clean the sensor (very scary), what do you think? I'm kind of hoping it's the mirror.

Thanks, Brendan

Edited January 31, 2021 by BrendanC

[BrendanC]

One more thing: I just took the camera out and took a few quick exposures. Here's one, stretched:

This is without anything attached - no coma corrector, no lens, no mirrors. 

It looks clean to me.

Is this a good test? Does it now imply it's the scope? And if so, why didn't 'the blob' move around when I moved the camera?

Confused!

[andrew s]

It looks like a dust mote on or near  the sensor. It is revealed in the converging light of the telescope but with uniform illumination it won't show up as much if at all.

Regards Andrew 

[BrendanC]

Darn. I thought that was the case. So, what's the best way to go about fixing this? Sensor cleaning seems yet another circle of hell I need to pass through... 

[andrew s]

A flat field should remove it. Regards Andrew 

[BrendanC]

I guess. That's what I used before and it pretty much solved the problem, I just kind of noticed it in the stretched image.

This guy seems to have a good, safe way of cleaning the sensor: https://www.youtube.com/watch?v=b-7ciRxh6s8 

I'll persist with flats in the meantime until/unless the dust bunnies really start to bug me...

Thanks for your help.

[BrendanC]

Just figured out how to get the sensor exposed in the manual clean mode on the camera and yes, there's definitely a dust mote pretty much where that mark would correspond to.

I'm going to get a blower to see if that can shift it.

In the meantime, flats are the workaround.

On the plus side, I don't need to clean the mirror it would seem! 

Edited January 31, 2021 by BrendanC