Secondary mirror woes

[markse68]

I’ve had an issue with astigmatism with my scope since I got it and tried a few things but haven’t cured it yet. It shows as stars under high magnification blurring out linearly then perpendicular to that when racking focus back and forth. It’s making splitting doubles very hard and I feel the scope is capable of better.I think my collimation is ok and the primary is meant to be a Jim Hysom  who apparently didn’t make bad mirrors.

 My suspicion is it’s caused by the secondary and more specifically the mount. I liked the mount at first- it separates tilt from rotate and height so nice and straight forward to dial in. But it’s design is I think flawed, inevitably imparting a bending moment on the mirror- especially the way the mirror is mounted to it with double sided foam tape. Differential thermal expansion could also be adding to it. I tried to remedy this by sticking it just at 2 places rather than over whole surface but it made no difference- possibly worsened it. The problem is to adjust the angle and lock it in place you must apply some torque to the 2 adjusting screws, pulling the mount and mirror against the fulcrum bar- not good.

i like the look of the AstroSystems mounts- I think the way they support the mirror around it’s edge is probably ideal with no forces on the mirror other than gravity. So I may order one but am also toying with making my own mount- can’t be that hard can it? 🤔

Ive ordered a Russian optical flat to check the secondary flatness but it hasn’t arrived yet and I can’t wait...

I might look to get a smaller secondary as I think this one’s a bit big (44mm) for an 8.5” f7.5 scope? Certainly the focal plane is quite a long 120mm out from the tube wall which seems a bit unnecessary- a smaller mirror further up the tube might be better. Need to play with the numbers in Newt.

in the mean time I’m going to try another mirror mounting  bodge with this mount to see if that helps and my theory is right...

Edited August 17, 2019 by markse68

[markse68]

Oh in case the primary does have a rolled edge (not sure that would cause such astigmatism though?) I made up a mask that shaves 3mm off the outside edge, still leaving 8.5” clear aperture as the mirror is closer to 8.75” actually. 

[paul mc c]

Have you checked that its not the primary....

[Peter Drew]

I think it highly unlikely that a Jim Hysom mirror would have a rolled edge or native astigmatism. It is quite easy to introduce astigmatismt mechanically. Make sure that the three retaining clips are barely touching the mirror. The design of the secondary mirror adjustment looks as though it could stress the aluminium backplate which might print through to the mirror and introduce astigmatism.   😀

[markse68]

Hi Paul, no I don’t know that it’s not the primary but I’ll cry if it is. The secondary mount just seems to be the most likely culprit

@Peter Drew I was told it was a Hysom primary but have no way of being certain- hope it is though. It’s not the best primary cell design by modern standard but I have freed up the mirror- removing the felt pads that jammed it in around its periphery and the 3 cork pads that supported its base. I replaced them with ptfe tape and the mirror can now slide around fairly freely in all dimensions.

This is today’s bodge to hopefully see if it is the secondary mounting causing the issue. I removed the larger pads of UHB tape I’d previously used and instead used 4 small pads of pe foamed tape whose height equals its width. This makes them quite wobbly and should hopefully decouple the mirror from any bending of the support plate and it’s thermal movement. Not trusting the small area of double sided bonding I’ve reinforced it with thin strips of self adhesive aluminium tape doubled up. This should still allow the mirror some movement but hopefully stop it dropping on the primary! Fingers crossed .

I think I will invest in a new mirror as this one doesn’t look great under a microscope. It has scratches and small chips in the glass and the backside actually seems better polished than the silvered side, though that might just be the coating.

[paul mc c]

If you can turn the primary in its cell,this can tell you if it might be the primary,if the de focused star pattern turns as the mirror is turned then it will be the primary,if not then it will be the secondary.

[markse68]

I will try that next for sure but I think it will throw collimation too as the rim of the mirror isn’t perfectly parallel to the bottom surface. I actually taped it to prevent it rotating in case that happens 

[markse68]

Well it seems improved but still there 😞 Artificial star is a light on a distant crane (a bit windy this evening!)