To Plop or not to Plop

[AlentejoSkies]

So I have a David Hinds mirror set, the generous gift of a certain Tom Yates (he's a jolly good fellow!)

It's present home is a darkstar telescope that is not really doing justice to such fine optics. As time and budget allows, it's going to become my favourite instrument...

For now, secondary is going in the post to Orion Optics for a recoat, and the primary - coating still fine - is getting a new cell.

First discovery is that the 8 inch primary is actually 223mm diameter. That's nine inches. Great! (Is that normal?)

Second is that it's quite thin. 25mm.

And now the question: Is my planned mounting method of three blobs of silicone at around 2/3 of the diameter a good idea, or do I need to go into this a bit more deeply? It looks like there's a whole world of mirror cell optimisation out there. Do I need to open that chapter?

Thanks for any  advice,

Douglas

 

[mapstar]

Hi Douglas

Does the darkstar give fine views already? If so why fix something that isn't broken.

Maybe it would be slightly better for cooling, rigidity and optical alignment if it was on a solid floating cell. But if you dont see any faults in the current mounting then why not stick with it

 

[Peter Drew]

Along with David Hinds and Rob Miller I was the third founder of AstroSystems. We exclusively used Hinds optics, the best available commercially in their day. 8" mirrors were always 8.5" diameter in low expansion glass and between 8.5" and 8.75" diameter in 1" thick plate glass. To mount theses mirrors we routinely bonded them with impact adhesive to a flat aluminium backplate but with a sandwich of 1/8" felt between the two. This method was structually safe, imparted no stress on the mirror, had no clips to mar the surface or introduce diffraction effects and did not come out of collimation once set.   

[rl]

Yes, it's worth a go with GUI plop. The program is free and the new version lets you do distortions with a tilt on the mirror as well. At least you know what could be achieved. I guess it's important not to be too obsessed with the results it spews out if the practical results are ok but it might be possible to optimize the position of the silicone blobs. Don't forget that the output errors need to be doubled to get the results on the wavefront. To be honest an 8" at 25mm thick isn't that far off the classic 1:6 ratio which is supposed to be thick enough to minimize the effects of sag, but there again a mirror perfectly supported pointing vertically up might be good for 1/8 wavelength but degrade to 1/3 wavelength ay 70 degrees tilt. On a lot of nights you would not notice it because 8" is about the point where the atmosphere becomes the limiting factor, but given the declination of Mars and Saturn at the moment you might as well get the best out of it if it involves very little extra effort!

I can testify to Peter Drew's remarks about astrosystems. I had one of their 6" scopes and the optics were excellent even with a basically fixed collimation on the primary! It's one of those I lent to somebody and never got back...

I've just finished an 18.5"  (45mm thick) using plop. The modeling results using a simple 3-point suspension, no sling,  tilted at 60 degrees are truly frightening.....I would have got that one badly wrong had I not modelled it.

rl

[Astrobits]

Three blobs of silicone will be fine on your mirror. I have mounted a number of 8" and 10" mirrors which were only 19mm thick successfully with three blobs. Just make sure that they are about 1" -1 1/4" diameter and at least the thickness of a matchstick.

Nigel

[AlentejoSkies]

Thank you for your comments.

As far as leaving well-enough alone, the Darkstar did need some attention coming into my hands after a good few years unused: missing focuser, damaged spider, corroded collimation screws, coating damage to secondary... so it is anyway a project - and with it in pieces anyway the temptation to tinker is just too high... and then I would like to add a fan, for which the exisiting cell is not ideal...

Actually I just enjoy it. And now reading Suiter's inspirational book I'm in the mood to play around a bit. But slowly, slowly.

Looks like there is already a range of opinions on this one. Good, I can enjoy the planning!

Douglas

[AlentejoSkies]

So, blobs it will be. I think three underneath, correctly sized, and three at the sides.

And then star testing to check for asymmetry. But that will have to wait until the secondary is recoated and so on.

[cs1cjc]

For three blobs, the correct spacing, from PLOP, is 0.5 of the radius, not 0.7 as was commonly advised in the past.  A 6-point support (at 0.7 radius) would be much better, but indeed may not be necessary.

The following page: Mirror_Cell_Design_Techniques summarises the results for common cells.  Unfortunately It does not cover the 6-point cell (but see for example: DobSTUFF_mirror_cells) which is better than the equal force 9-point cell.  The best unequal force 9-point cell (as used by Orion UK) is slightly better, but more difficult to make by hand because of narrow spacing and tight tolerance on the scalene triangles.

[AlentejoSkies]

Thanks for the info Chris. Interestingly counter intuitive with 0.5 of the radius, at least to my intuition.

Douglas