Primary mirror behaviour solved?

[dsoscope (Dan)]

I need some coroborating advice from those with more expertise with primary mirrors.
I bought an Orion Optics UK mirror 250mm F4.8 research grade 10 years ago and as I progressed in the hobby, I slowly realised that my imaging was on the soft side. Getting to the bottom of this, I embarked on a process of elimination to seek a potential cause.

Was it the coma corrector? I have two CC's (MPCCmk3 and an ES) and the mirror behaviour was identical. Was it potential mirror Pinching? My mirror is completely free floating now and still questional mirror behaviour. Was it focuser miss alignment? I shimmed the focusser so that the laser dot remains exactly in place on the primary fully racked in and out. I recently did a star test closely watching the intra and extra focus behaviour of a star. What I found was horrendous!

On slight extra focus I see a badly misshapened donut ( twice as high as it is long ) and intra focus was a perpendicular spike pattern , and on best focus, I have to settle on a triangle shaped star. When I look at the entire image frame (4/3 size) triangle stars right accross the frame. well, slightly spikier on one side and slightly donutty on the other......not good!

So.... astigmatism? Really? On a certified 0.985 strehl mirror? I did read some scathing opinions on what orion optics test certificates are worth, but I thought they were far outliers.
Well of course there's the secondary. I dont have an elimination process for it other than full replacement. Given the astigmatic behaviour through the focus point, it is the primary right?
So anyway, its fun learning new things and look forward to further enlightenment on the this subject.......Cheers
Dan

[Stefan Buda]

Just rotate the primary 45 degrees, re-collimate and see if the orientation of the startest astigmatism has changed accordingly. If it remains the same then you need to investigate the secondary. And you should do the testing without a CC in the optical train.
If the orientation of astigmatism changes 45 degrees then there is not much you can do with that mirror. If you received o test certificate showing a very good figure, it means that they turned off the astigmatic component in software - that is cheating.

[By.Jove (Jove)]

Originally Posted by dsoscope

On slight extra focus I see a badly misshapened donut ( twice as high as it is long ) and intra focus was a perpendicular spike pattern , and on best focus, I have to settle on a triangle shaped star

I'm guessing this is a thin mirror, right ? That is astigmatism, and it was baked in when they ground and polished the mirror. Perhaps try a Televue Dioptrix but I doubt it will help much.

Minimizing flexure during grinding and polishing - as well as in the scope - was the reason why long ago the old-school types used full-thickness mirrors, a 10" would have been close to 45-50mm thick.

[dsoscope (Dan)]

Many thanks everyone for the feedback. That was quick!

Yeah Jove. Spot on. It looks to be 25mm. So with the substrate they call Zenador I think, a fused quartz like substrate, I would have thought it make a difference limiting deformation.

Also, that's a great suggestion Stefan, I'll try the mirror rotation tonight.
So..... these testing results could be tailored for a set outcome.......Why doesnt that surprise me!

Anyway I can plan to move forward from here after a mirror rotation check.

So contemplating an F5 GSO mirror but Ill have to extend the scope. It looks like the quality range is OK to excellent and all diffraction limited as a minimum. With GSO, they have had years of fine tuning behind them by now, so I imagine they're a good bet. Their mirrors seem thick and weigh a kilo more. Its got to be better than what I have now I imagine!
Also, I tracked down a German company, AstroReflect and they work with TS doing premium mirrors. I can get an F4.8 but at 2.5x the cost of a GSO.

Any suggestions appreciated.

Thanks again..
Dan

[Startrek (Martin)]

I’ve used GSO mirrors ( 6” to 12” ) in both metal and carbon fibre OTA’s for nearly 10 years now ( both visual and imaging ) and don’t have an issue with them at focal ratios between f6 down to f4.9.
At f4 and faster I always recommend folk investing in premium higher end scopes. The budget Skywatcher , Saxon , GSO etc…. just don’t cut it with fast optics. They use the same design engineering and hardware in f6 as they do for f4 which is ridiculous. Every year we have folk struggling with various issues on this forum with f4 budget scopes both visual and imaging. Some have reasonable success resolving their issues , some put up with the problems and some give up.

My 2 cents ……,

Clear Skies

Martin

[dsoscope (Dan)]

Your 2 cents Martin is great info. It gives me more confidence in the F ratios I'm using for considering a GSO mirror.

Now having come back from further star testing. I seems that my primary has settled down a little. I have elongated donuts inside and outside focus now but still in perpendicular directions.
I moved the mirror twice, 40deg and further to 60deg and the "pie section triangle" stars I found, pretty much stayed in the same orientation on all the frames. It was a bit hard to determine because my focus point is vague. A Batinov mask on a small star that gets best focus makes large stars deform out of focus, and vice versa. Also simply rotating the batinov mask on any star also causes big changes in the focus point. Too much going on at once......Very interesting to interpret the data.

I'm thinking, surely all this astigmatic spot and out of focus donut behaviour cant just be the secondary....Can it? Has anyone seen this sort of thing before with a secondary?
Or is it most likely that both mirrors contribute to the mayhem?

cheers
Dan

[OzEclipse (Joe Cali)]

Dan,

A thin 10" primary mirror needs to be mounted in a mirror cell with at least 9 points of floatation. If you are on a three point, that might be causing the triangles.

Triangles can be formed by 3 points on the back of the mirror and too much tension on the corresponding mirror clips.

Same with a secondary.

Joe

[dsoscope (Dan)]

Hi Joe.

Ah yes, good pickup. My mirror was on a 9 point floatation setup...still is, but my secondary was attached to the 30mm diameter block fully spread with silicone and cured to the mirror.
So you're saying that could be problematic? I was thinking at the time that silicone has enough flex in it for the mirror. Perhaps a thicker layer for more give?
What's the alternative to this sort of attachment?
An alternative would be something I would try if mounting the secondary again.

Thanks for the tip

Dan.

[Stefan Buda]

Originally Posted by dsoscope

Now having come back from further star testing. I seems that my primary has settled down a little. I have elongated donuts inside and outside focus now but still in perpendicular directions.
I moved the mirror twice, 40deg and further to 60deg and the "pie section triangle" stars I found, pretty much stayed in the same orientation on all the frames. It was a bit hard to determine because my focus point is vague. A Batinov mask on a small star that gets best focus makes large stars deform out of focus, and vice versa. Also simply rotating the batinov mask on any star also causes big changes in the focus point. Too much going on at once......Very interesting to interpret the data.

I'm thinking, surely all this astigmatic spot and out of focus donut behaviour cant just be the secondary....Can it? Has anyone seen this sort of thing before with a secondary?
Or is it most likely that both mirrors contribute to the mayhem?

cheers
Dan

I was hoping that you would assess the orientation of the astigmatic figure on a slightly defocused star, always on the same side of focus, as it is much easier to see. A perfectly symmetrical astigmatic figure is hard to see in focus, as it tends to produce a small cross or squarish star. With a triangular star image it is even harder to assess the orientation, that is why a slight defocus makes it much easier.
The secondary should be attached to the, presumably aluminium, holder with 3 blobs of silicon that are not thinner than 1mm to allow for differential thermal expansion/contraction.
Bahtinov mask focusing is very sensitive to the presence of astigmatism.

[doug mc]

Usually secondaries flex across the minor axis. This makes a focused star X shaped. If you move a focused star away from the centre of the eyepiece field along the major axis of your secondary it should show this aberration reducing to a better shape if the secondary is the issue. Best seen at higher powers.

[iborg (Philip)]

Originally Posted by dsoscope

but my secondary was attached to the 30mm diameter block fully spread with silicone and cured to the mirror.
So you're saying that could be problematic? I was thinking at the time that silicone has enough flex in it for the mirror. Perhaps a thicker layer for more give?

Hi Dan, I don't know if the silicon is an issue or not, but, I am reasonably confidant that a thinner layer will provide more give, not a thicker one, for this type of situation.


Philip

[OzEclipse (Joe Cali)]

Originally Posted by dsoscope

Hi Joe.

Ah yes, good pickup. My mirror was on a 9 point floatation setup...still is, but my secondary was attached to the 30mm diameter block fully spread with silicone and cured to the mirror.
So you're saying that could be problematic? I was thinking at the time that silicone has enough flex in it for the mirror. Perhaps a thicker layer for more give?
What's the alternative to this sort of attachment?
An alternative would be something I would try if mounting the secondary again.

Thanks for the tip

Dan.

More of a problem with overtightened edge clip type secondary holders not silicon.

Have you loosened the clips on your primary? Enough that the mirror isn't under tension even if it is 9 points. Not enough that it could fall out.

I have a 6"f7 with a near perfect figure on the mirror. I ground and polished the mirror back in the 1970's. When I first assembled the OTA, I put too much tension on the mirror. The result was triangular stars.

Those Orion UK mirrors are supposed to be pretty good. Not related to the American Orion brands. I think Mike Sidonio uses a 12" f3.8. So I wouldn't give up on that mirror just yet.

Joe

[dsoscope (Dan)]

Yes Stefan, by batinov focusing the large stars, It turned out that the small stars had an extended triangle shape and that's how I established what was happening for the 3 primary mirror positions.

My secondary holder was actually 50mm wide and hardwood. Full coverage of silicone at 1mm or less thick. Sounds like you may have pointed to a potential issue here.
Anyway, the secondary assembly is fully apart and I have developed an alloy mirror holder instead. The mirror fits with an ever so small free play and temporarily firmed up with side tape.

Thanks Phillip, Perhaps too much coverage area also makes silicone too rigid. I think a thin x 3 pea size points seems to be the consensus, which I didnt have.

Hey Joe, as yet, I am still hopeful that it's a mirror mounting issue, So with an absolute pinch free optical train coming together, we'll know for sure.

I didnt know of a secondary mirror test Doug. I'm curious on how that goes next time.

Thanks everyone for the help and direction,
Look forward to getting under the stars soon (after my silicone cures)

Dan.