Dirty Mark or Reflection?

[Mikey0368]

I have a relatively new SW Esprit 100ED and I'm getting a mark on my images. I've checked the camera (it's a DSLR) and flattener and can't see anything obvious, but the lenses in the telescope do look a little dirty. 

Rather than a smudge somewhere, could it just be reflection? It's pretty much a perfect circle and I wouldn't expect a dirty mark to be so circular.

Here's a screenshot taken during processing as it sticks out like a sore thumb at this point. Once processed it's much less obvious but still there.

Anyone experienced something like this?

Thanks.

 

[CloudMagnet]

Could try looking through the scope during the day with an eyepiece and see if the mark still appears. If it does, that rules out the camera being the cause.

Were you pointing near any light sources for the pictures?

[almcl]

I have something pretty similar on my images:  

Putting the figures for mine (49 pixels diameter, F 4.9 scope) into the Dust Donut Calculator here https://www.ccdware.com/resources/dust.cfm

suggests that it is 1.08 mm from the sensor, which, I think, means it is resting on the Baader filter .

Quick way to check if it's the camera or the scope would be to take an image and then rotate the camera through 90° and take a second one.  If the mark is in the same place on the image, it's the camera.

It should be possible to calibrate out with flats.

[Mikey0368]

Thanks folks. The moon was fairly full that night so that could be the culprit I suppose.

There's a bit of trial and error involved at the moment and if the cloud ever disappears it'll help!

[Mikey0368]

I was out again last night and it's still there. There was no moon so it aint that, and everything's been cleaned.

Again, it shows up much more during processing......

[almcl]

Did you try rotating the camera through 90 degrees and shooting a few subs?

[Mikey0368]

Not yet. I'll be trying a few things on Sunday.

[kirkster501]

I doubt it’s the objective lens, any mark on that would be so far out of focus that you wouldn’t see it.  It has to be something nearer to the chip. 

[Skipper Billy]

Condensation on the chip cover glass?