Dust hunting

[discardedastro]

I've got a consistent dust problem in my otherwise (largely) spotless setup, and it's annoying me! Very aggressively crushed image below to shot them, top left of the image:

These rings have persisted in the same place for absolutely ages - over a year - and having dismantled my entire optical train I can't spot a single speck of dust. There's actually two rings, a 130px wide one and a 700px wide one. http://www.wilmslowastro.com/software/formulae.htm suggests distances of 8.4mm and 1.58mm from the sensor plane respectively which would be within the sealed camera (I think). It does persist on other filters (above is L). The optical train is:

• 200PDS telescope
• Baader MPCC MkIII
• Baader Varilock adjustable T2 extender set at 26mm from the front of the camera
• ZWO EFW Mini
• Two thin delrin shims (spacer to accommodate the ZWO anti-dew heater)
• ASI183MM-PRO

I had a look in the front of the camera but couldn't see anything on the AR window or the sensor itself.

Any hints for where to go from here? What else could this be?

As an aside, is the vignetting shown in the image above significant? My understanding was that the 200PDS had a larger secondary than the 200P so that for imaging the field would be essentially vignetting-free - it's only slight.

[michael8554]

It's funny how the brain sees patterns in random noise, to my mind there are 5 or 6 bunnies in your image!

Show us one of your flats, stretched, that would be revealing. 

Even better, if you're sure it's the camera, make a flat with nothing attached. 

Michael 

[Davey-T]

To quote Mick Dundee " That's not dust bunnies, this is dust bunnies " or more correctly donuts 

Dave

[discardedastro]

I've not got a reliable mechanism for making flats at present - a Lacerta LED flat box is next on the shopping list once payday rolls around! That's part of what makes this so annoying. Having said that, I'm glad I've not got as many doughnuts as Davey-T 😧

The two-ring pattern is consistent across multiple images/targets - I've not noticed any others, but then I haven't had much light lately!

I did take some flats with the t-shirt at dusk approach about 9 months ago but they were free of any dust, so not a lot of use. I'll pop the 'scope off the mount tomorrow - I need to rotate the tube rings around to get the focuser pointing at the dovetail for radial balancing, anyway - and do some flats while I'm at it. Oddly hadn't thought about doing a flat without the rest of the optical train as a means of debugging!

[michael8554]

Nothing scientific required for the test camera flat, just point it at a piece of brown/grey/white card during the day in dim light conditions. 

Michael 

[alan potts]

I have spots on mine and as far as I am concerned the scope and everything else is spotless, I have given up, good luck.

Alan

[discardedastro]

Well, here's a flat, which has picked up that vignetting and another dust spot lower right. I've jury-rigged a flat generator with a spare LCD monitor I had lying around, which seems to work OK.

In lieu of finding any dust, this at least works for calibrating out. The vignetting is actually concerning me more at the moment - makes me wonder if my secondary isn't quite positioned right.