Mystery donuts

Wesley Mitchell

Hi Adam,

I'm having a problem with donuts, some fairly severe. Here's what I tried so far:

Changing the camera

Changing the filter

The donuts show up in the same number, at the same positions for each experiment.

The scope is a Sharstar SCA260 f/5. It is a Cassegrain astrograph, it has a primary mirror, a secondary mirror, and a set of lenses in front of the focuser tube.

The only other thing in the imaging train is an OAG, which I will try to eliminate next.

I have tried racking the focuser all in, and cleaning the back of the rear lens.

The question: can dust or debris on a mirror cause these artifacts? When I used the calculator at astronomy.tools, it informs me that the dust particles are between 1 and 1.5 mm from the sensor. But this doesn't make sense since it is the same with different cameras (same sensor size).

The flats don't calibrate these out, and I don't understand why. Instead, it leaves a subtle but ugly mottling on the stacked images. The donuts are barely visible in the flats themselves. I'm taking exposures at the same gain. I've tried both flat panel and sky flats.

What do you think my options are at this point?

Thanks

Wes

BlockHead

Those donuts are SHARP..they must be close to the sensor/camera. If you changed the camera and filters... when all that remains is the glass in front of them both. No, dust on the mirror WILL NOT produce images showing the obstruction. You can prove this to yourself by gently placing a piece of paper towel/tissue on primary mirror partially covering it. Then take some pictures... you will NOT see the obstruction.. you will just see less light. 

Flats should calibrate this out. That might be a different issue. But first.. you have to find these particles. 

One thing I do not understand is how thin they appear. It is as if there really are little rings on that surface. 

The distance to the particles would be the same regardless of camera because you come to focus in the same place (assuming you focused on stars and things). At least I think this is right.

I would look at the front surface of the lens. I assume this is a corrector lens and cannot be removed.

-the Blockhead