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Diffraction Pattern Question

Reflectors Advanced Collimation Coma Tilt Troubleshooting
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Josh Jones avatar
Josh Jones avatar

So I’ve acquired a small Sharpstar 15028hnt F/2.8 newt and been playing with it to see if it’s truly capable of delivering or working across a full frame sensor. It’s been a couple weeks of precise bench collimation, star testing, tilting, backfocus adjusting, etc. I do run my own ASG electronic tilt adjuster and am able to adjust backfocus and tilt down to about 1 micron. So far it’s working ok, but I get a diffraction pattern off-axis that is bugging me.

At this point, I feel collimation is good, central star collimation is good, even repeatedly pulling camera off and testing over several days it’s holding well. I’ve ran a lot of focus curves and adjusted things like tilt and backfocus and it’s very repeatable, consistent, and feel I got this down pretty good as well.

I use multiple tools like hocus focus curves, ASTAP, and CCD inspector to get it really as good as this thing will get (in my opinion) across a full frame camera. But what I’m getting is a good central diffraction pattern, but a large pull in off-axis diffraction coming toward the center.

smaller stars in these off-axis areas seem good and round and don’t present a lot of coma like structures, so I’m thinking it’s something shiny causing this perhaps? The primary does have a blackened ring around it but there are a variety of primary adjuster bolts back there that are shiny and off-axis, would this cause the diffraction patterns like this?

I’m now starting to think the central star shouldn’t have so much diffraction other than the spider vanes? Curious if this is obvious to others before I start pulling the primary out and covering things.

📷 sharpstar 15028hnt primary.jpgsharpstar 15028hnt primary.jpg

📷 Clamshell FWHM Analysis.pngClamshell FWHM Analysis.png📷 central star consistent.pngcentral star consistent.png📷 Bottom Left corner.pngBottom Left corner.png📷 Top Right corner.pngTop Right corner.png

andrea tasselli avatar
andrea tasselli avatar
Clipping of the marginal rays seems the most probable reason. I see the primary is masked so you won't get much pattern out of it unless the mask's edge is rough. Really rough.
CraigT82 avatar
CraigT82 avatar

How long is the dew shield? Have you tried without it?

Well Written
Josh Jones avatar
Josh Jones avatar

andrea tasselli · Aug 27, 2025, 04:36 PM

Clipping of the marginal rays seems the most probable reason. I see the primary is masked so you won't get much pattern out of it unless the mask's edge is rough. Really rough.

makes sense… I’ll try bit without dew shield and see if results are better.

Josh Jones avatar
Josh Jones avatar

Well, the removal of a dew shield didn’t work well.

Another question here for thought… I see these scope manufacturers often throttling down with aperture stops at the corrector level. Why do they do this?

As an example, my RASA 11 optical window is some 70mm and yet they only ship a M42 or M48 camera mount right above this. I build a M58 and use an M68 and it’s great with a full frame, but why would they do this?

I see epsilons do this but also this sharpstar 15028hnt has a 44mm imaging circle, but at the corrector they put an M48 mount above the corrector? Why not M54 in order to get tot he 44mm imaging circle that is 55mm away at f/2.8?

Could this aperture stop be the source of my marginal ray clipping.

andrea tasselli avatar
andrea tasselli avatar
Josh Jones:
Could this aperture stop be the source of my marginal ray clipping.


Quite likely. Either there or at the secondary, although this seems a bit far-fetched.
Josh Jones avatar
Josh Jones avatar

So just to close out and follow up on this sharpstar 150 diffraction issue, I had time to pull the primary mirror and make a better mask for it by 3d printing a ring that covers all the forward facing shiny bits and bolts. I also noticed the primary was pinched a bit tight in flocking so added some PTFE tape around it to make it slide and seems to have done the trick here.

Off-axis stars are much better, diffraction patterns are not pulling anymore. I have some more testing to do, but if anyone else runs one of these scopes, happy to give you the STL file for a primary mask ring that is a bit better than the stock setup. I also made some bolt covers for the front end to help cover things like focuser bolts and secondary bolts.📷 20250911_161011.jpg20250911_161011.jpg📷 20250911_155544.jpg20250911_155544.jpg📷 20250911_160114.jpg20250911_160114.jpg📷 20250911_161000.jpg20250911_161000.jpg

on axis and off axis stars look much better now… So end conclusion is it was either pinched primary with stress on it or it was resolved with a better primary mirror mask ring.

📷 Screenshot 2025-09-15 115440.pngScreenshot 2025-09-15 115440.png📷 Screenshot 2025-09-15 115351.pngScreenshot 2025-09-15 115351.png

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andrea tasselli avatar
andrea tasselli avatar
I'd opt for the latter. If it were pinched mirror you'll have seen tri-foil deformation of the PSF and I couldn't see any. You just moved (with the mirror mask) the exit pupil where it belongs; the primary.