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Dust in optical train somewhere (advice)

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Hi, I've come to start imaging this year again and have noticed what looks like dust bunnies somewhere in my optical train. I've cleaned all the bits I can think of - ccd sensor, reducer lens, filter wheel etc.

If you look at the images below you can see the position of the dust... I've really overstretched one to make it obvious.

Whats even more frustrating is that I've rotated the camera several times and the dust doesn't move (see the 4 images, before and after rotation).

What does the fact the dust doesn't move mean then? Can anyone tell me where the dust is likely to be? I always thought out of focus dust was close to the sensor, but i've done my level best at cleaning it... 

I've not done anything with the objective lens, as you can see in the image, there are some marks, but they don't look like they'd be the cause... can I/should I clean these? How?

My setup is kept in my DIY observatory year round.

Any advice appreciated...




Screen Shot 2023-05-04 at 17.58.48.png

Screen Shot 2023-02-14 at 18.41.10.png

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Looking at the images, given  the size, not moving as you rotate the camera and (lack of) sharpness, I would suspect that the dust spots are on the surface of the camera window.  They are certainly not  sharp or small enough to be on the sensor itself . I would guess that they are on the outer surface of the  window.  The number you have is actually not that bad.  I know it is always nice to have a pristine optical train, but that is sometimes just to hard and cleaning  only results in a different set of dust spots.  From thta image, I would expect that they will calibrate  out very easily with a good set of flats and flat darks.


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Posted (edited)

Hi, I had a similar problem some time ago. A calculator that very accurately determines the location of a speck of dust from the camera sensor turned out to be very helpful. In the calculator, you enter the specifications of the camera, the telescope and Dust Doughnut or Reflection Shadow Size counted in px (use big magnification in any graphic software to count its size in pixels)


Edited by RafalT73
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Posted (edited)
5 hours ago, joecoyle said:

I've not done anything with the objective lens, as you can see in the image, there are some marks, but they don't look like they'd be the cause... can I/should I clean these? How?

Hi Joe.

Others have already replied to the part of your post regarding the dust shadows and judging from experience those in your sample image are most likely caused by dust on the filters.

Your photo of the lens cell does not have sufficient resolution to be certain but it does appear to show signs of lens fungus, which needs to be treated otherwise it will spread across the surface of the lens and etch the glass.

If you examine the lens closely and can see what looks like strands of fine cotton-wool, grouped in clumps or “fingers”, then you probably have some fungus growth and that will already have begun etching the glass.

Fungus on the outermost sides of a lens element can be killed by exposure to strong UV-C light and fungus residue removed using regular lens cleaner but any etching damage to the lens is permanent.
There are no engineered radiation sources available to the general public that are strong enough to penetrate more than a few mm of glass and fungus growing on the rear side of lens elements in a cell can only be killed by disassembly of the cell so that the inner surfaces can be accessed.

Glass fungus thrives in dark, warm and humid conditions so if you are in the habit of capping the lens when the observatory is closed for long periods, or if the observatory itself is warm and humid, then you have ideal conditions for the airborne fungus spores to grow on the lens.

Regular exposure to sunlight and fresh air is said to help kill the fungus.

Hopefully, the suspected appearance of lens fungus in your lens-cell photo is just an artefact, if not, then this reply might help you avoid further lens damage now, or in the future.


Edited by Oddsocks
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If the dust bunny doesn't change positron when you rotates the camera, it's on the sensor window. It can be really hard to see and also hard to shift, even with a cleaning fluid. 

I've had similar issues. I had to get myself some jewellers loupes or watch glasses and hold the camera so I could see the light from a window reflected back to me.

I needed to look really really closely, turning the camera so I could see into the edges and corners.

I eventuality spotted the microscopic spec and I had to use Baader WonderFluid on a cloth and a sensor cleaning wand to push the cloth over the window surface to finally shift it.

Now I'm obsessive about avoiding dust but I also take flats. You never know when another dust bunny is going to creep in. 

Good luck. 




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