Mirror clip diffraction patterns?

[JonathanA]

Hi,

After a lot of work I am finally getting astigmatism free images from my ES 152 MN (Comethunter).

But now that the images are astigmatism free I can now see very clear 'shadows' of the primary mirror clips in images with bright stars. I am guessing that they are formed by shadows in the diffraction from the mirror edge??? I attach a highly zoomed image of the problem. The stars concerned are about Mag 6-7.

I would be interested to see what anyone thinks. Would a mask around the edge of the mirror clean up the images? If so how wide would the mask need to be?

Thanks,

Jon

 

[wimvb]

To get rid of them, you probably need to make a baffle ring to put on the primary. The ring should then cover the clips, about 5 - 10 mm smaller diameter than the primary. The ring should be matt black and have very clean edges, probably laser or water cut from thin sheet metal. You could also try to replace the clips by smaller size ones.

Anything in the light path causes diffraction. After the clips, you'll probably see diffraction from the focuser drawtube.

Diffraction is something that comes with a newt. Only refractors will give you pin point stars. Personally, if the diffraction isn't too bad, I wouldn't bother.

[andrew s]

3 hours ago, wimvb said:

Only refractors will give you pin point stars.

No they won't they give you a circular diffraction pattern called an Airy Disk. However, you normally get a  roughly Gaussian point spread function due to various aberrations, seeing or a low image scale.

[Stub Mandrel]

I was impressed by the cheap 50mm achromat lens from Astromedia in my guide scope. Every so often when the seeing was good the graph in PHD2 showed a tiny peak either side of the main one, a perfect slice through an airy disc!

[wimvb]

17 hours ago, andrew s said:

No they won't they give you a circular diffraction pattern called an Airy Disk. However, you normally get a  roughly Gaussian point spread function due to various aberrations, seeing or a low image scale.

The Airy disk is the best we can ever hope to achieve, since it is the diffraction pattern caused by the limited aperture of any optical system. Maybe I should have written "as close as we can ever get to pin point stars."

[JonathanA]

19 hours ago, wimvb said:

Personally, if the diffraction isn't too bad, I wouldn't bother

Yes, I think you are probably right. It's small enough to live with - losing aperture doesn't seem the way to go. Ironically, the pattern only became as apparent as this once I had worked through all the other factors that were reducing image quality - over tightened corrector (it's a Mak Newt), floppy draw tube etc.

 

[andrew s]

7 hours ago, wimvb said:

The Airy disk is the best we can ever hope to achieve, since it is the diffraction pattern caused by the limited aperture of any optical system. Maybe I should have written "as close as we can ever get to pin point stars."

Sorry I did not mean to be picky. I read your post as implying Newts suffer from diffraction while refactors do not. Clearly you did not mean to imply that.

Regards Andrew

[wimvb]

No worry, Andrew.