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RCompassi

Help with weird diffraction pattern on SW 200p

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Hello!

I have a Skywatcher 200p (200/ 1000mm), and I'm having some weird issues with (I guess) stray reflections.

The problem appears as a fan (I don't know the exact word), that always points to the center of the frame.

I'm using a Canon T1i (modded), GSO coma corrector and Astronomik CLS CCD clip filter.

I already tested without one of each, and both, and the problem continues exactly the same, ruling them out.

The focuser doesn't get in the light path (it's flush with the tube), tried covering the internal bolts (from the focuser and the finderscope holder), no change in the pattern.

The stars looks like this (ignore the orange ring)

com leque.jpg

 

I tried SEVERAL ways of blocking internal reflection, trying to pin down what is causing this. Included: covered the spider vanes, covered the back of the secundary (maybe the bolts there?), blackened the edges of the secundary to no avail.

Then I created a frontal baffle, like this:

baffle.jpg

 

And the ligh fan is gone!

sem leque.jpg

 

Here is a comparison of the pleiades:

No baffle:

(you can see the light fans pointing to the center of the frame)

leque.png

 

With the baffle:

(no more light fans)

sem leque.png

 

Any thoughs on what should I do?

Thanks you very much for any response.

 

Rafael Compassi

 

Edited by RCompassi

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Hi. I think that's the secondary not covering part of the light cone; it needs twisting -clockwise or anticlockwise as you look down the tube-  so that it covers the whole of the primary. The only way I can do it is by centring the secondary over the primary with a laser and then using a Cheshire to centre the primary. And/or get a bigger secondary. I think it needs to be 60mm. HTH.

Collim_steps_L.jpg

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I tried several times to collimate again, but it is spot on. 

 

I will try a BIG secondary, a 70mm one.

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Are the secondary vanes twisted? 

Alscollimationapp is very useful tool to check secondary is centred over primary.

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No, they are very straight. I even masked thtem to see if the pattern changes, but no.

I used a collimation cap too, but the mirror is centered over the primary.

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How wide is/was the baffle? Is it big enough to block light that would normally be reflected by the edge of the primary mirror? Could such an effect be caused by a mirror edge that deviates from a true parabola? 

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alscollimationapp is not a cap, it is a small program you can use to overlay an image either live from your focuser or a still you had taken to check for concentric circles between primary and secondary mirrors.

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15 minutes ago, Ricochet said:

How wide is/was the baffle? Is it big enough to block light that would normally be reflected by the edge of the primary mirror? Could such an effect be caused by a mirror edge that deviates from a true parabola? 

About 2cm wide.

 

11 minutes ago, happy-kat said:

alscollimationapp is not a cap, it is a small program you can use to overlay an image either live from your focuser or a still you had taken to check for concentric circles between primary and secondary mirrors.

Ah, sorry, I misreaded that. I used mire de colimation, it does the same overlay with bullseye

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