Internal reflections ?

[Packen104]

Bright stars cause an out of focus donut shaped image to appear in my images.
I am using a Celestron CPC925 with f6.3 field flatener and ZWO ASI 1600mm camera.
The artifact occurs even if the bright star is not directly in the camera frame.
It appears to be out of focus which to me indicates a different optical path.
I would appreciate any advice on the cause how to eliminate this problem
Ken

[Joshua Bunn]

Hi Ken,
I Think you will find this Is a dust doughnut, the dust will be on a Filter or the sensor cover glass. A flat frame should fix it, or cleaning the optical train.


Josh

[bojan]

If it moves as bright star image moves across the field, than it is internal reflection.
Not much you can do about it... try different flattener (with better anti-reflection coating).

[gregbradley]

What Josh said.
A dust donut. Have a look at the sensor and filters with a bright torch held at an angle to show up anything on the surface of them.

Clean them up with a blower or a microfibre cloth.

Its best practice to keep these clean and to clean them occassionally rather than rely on flats (and their vagaries) to remove them.

Greg.

[The_bluester]

Actually I am going to suggest an internal reflection as well. If it was a dust donut it should be darker than the area around it. not lighter.

To add to what Bojan wrote, try reframing slightly and see if it moves around the frame, if it stays in the same place on the frame when you reposition the bright star then it is dust, if it moves, it is a reflection.

I had similar issues with my C925 (And other scopes) when using my ASI294MC camera, which had an antireflection coated sensor chamber window, not a UV/IR cut one. Fitting an IR cut filter almost completely eliminated the problem. The reflections from a star like Alnitak were spectacular without it.

I believe there is or at least used to be an IR cut window available for the 1600 cams which was not the case for the 294, so I had to use an external one.

[multiweb]

That's the out of focus reflection of that bright star bouncing back to your sensor likely from your flattener. You could try to increase the distance between the flattener and your camera slightly by moving the primary forward. That would make the reflection bigger and fainter.

[RB]

Yep, I agree, that's what I'd say it is too.

:thumbsup:

[gregbradley]

Reflections from flatteners and reducers became a real issue about 5 years ago when these large format sensors like the 16803 became popular.

Scope designs changed and most now have a corrector of some sort at the back of the scope.

This was usually an issue with filters not being antireflection coated well enough and led to mark 2 versions of Baader and Astrodons.

As Marc said it was a reflection between the flattener and the filters back.

If that proves to be the cause you may need to upgrade the filters.

I am curious to see how this plays out.

Greg

[Packen104]

Yes the donut moves away from the bright star as you scan across.
Thanks you are confirming it is an internal reflection. I will look into the ir cut filter solution, that seems a cheap fix (if it works).
I will also try a small change on the spacer between the camera and reducer.
It might be a while until we get some reasonable weather the wet seems to have started and cloud is king.

[gregbradley]

You also may try making a narrow baffle. That is a common solution.
Greg.

[Packen104]

I removed the flattener. problem solved.
Greg's comments about the reflection between flattener and filter must be the cause.


Thanks all for you comments.