Modifying the primary cell

[Dave_D]

While i'm on a roll with modding stuff, and rooting around in sheds, i found 3 triangular metal plates about 2inches across. The primary cell on my 10" GX250 is a 3 point, which uses cheap nylon bolts to support the primary. When i added the reinforcing strip to the tube, i found that not only were the nylon bolts fairly thin (about M6) but also, the primary doesn't sit squarely on the bolts.

Would there be any benefit for a 10" mirror in switching to a 9 point cell by adding contact points to the 3 plates and replacing the nylon bolts?

[callump]

Dave, optically, a three point support for the 10" mirror should be quite adequate. 

Some like a 6 point support for 10". A 9 point is probably overkill.

You might like to download the cell design program Plop - it will tell you the best positions for the support points, etc.

http://www.davidlewistoronto.com/plop/

I'm not sure nylon bolt supports are that bad - if they don't slop then should not be a problem.

Not sure what you mean when you say the mirror does not sit squarely on the bolts - but if it's not sitting on all three bolts at the same time, then something else must be supporting or interfering - so i'd look to see what that is first...

Callum

[Dave_D]

Dave, optically, a three point support for the 10" mirror should be quite adequate. 

Not sure what you mean when you say the mirror does not sit squarely on the bolts - but if it's not sitting on all three bolts at the same time, then something else must be supporting or interfering - so i'd look to see what that is first...

Callum

the whole scope suffers from sloppy engineering. the mirror is on all 3 bolts, but if you imagine the bolt coming up through the cell but not being perpendicular, the bolt end is contacting the mirror at a slight angle so the point of contact is a lot smaller than the area of the bolt.

[ronin]

Odd but I think a 6 point mount is optimum.

The link I had no longer works, try a search for "plop optimised cells"

[coatesg]

I'm actually just starting to work out a similar optimisation for my primary cell which is another old style 3 point Orion Optics cell. Only difference is that the primary I have is 14" f4.6, with PLOP returning theoretical deformations of 1/12 wave RMS and P-V 1/3wave... hmm...

My idea is to convert this to a 6 point by adding three bar supports at ~0.59 radius which will reduce the deformations to (theoretical) 1/233 RMS and 1/45 P-V respectively. I just need to figure out the best way to do it (and need to remeasure more cell dimensions to work what is possible). Either:

a) I mount a bar on top (easiest) assuming it doesn't raise the mirror so high that the side nylon bolts miss the mirror, or

I mount the bars underneath as kind of U-shaped parts that hold the mirror at about the same height. (harder, and unsure if the central part of the support gets in the way).

Either way, they need a bit of pivot so that the mirror finds itself level across the two points on each bar. If anyone has done anything similar, and has photos or ideas, then love to hear. 

Interestingly, the 6-point cell gives a better result than an automatically generated 9-point from PLOP (with two rings of supports) and a 9-point all at same radius is about 25% better, but harder to make (and starting to get negligibly better versus the accuracy of the mirror itself which is about 1/10 wave).

[Dave_D]

I'm actually just starting to work out a similar optimisation for my primary cell which is another old style 3 point Orion Optics cell. Only difference is that the primary I have is 14" f4.6, with PLOP returning theoretical deformations of 1/12 wave RMS and P-V 1/3wave... hmm...

My idea is to convert this to a 6 point by adding three bar supports at ~0.59 radius which will reduce the deformations to (theoretical) 1/233 RMS and 1/45 P-V respectively. I just need to figure out the best way to do it (and need to remeasure more cell dimensions to work what is possible). Either:

a) I mount a bar on top (easiest) assuming it doesn't raise the mirror so high that the side nylon bolts miss the mirror, or

I mount the bars underneath as kind of U-shaped parts that hold the mirror at about the same height. (harder, and unsure if the central part of the support gets in the way).

Either way, they need a bit of pivot so that the mirror finds itself level across the two points on each bar. If anyone has done anything similar, and has photos or ideas, then love to hear. 

Interestingly, the 6-point cell gives a better result than an automatically generated 9-point from PLOP (with two rings of supports) and a 9-point all at same radius is about 25% better, but harder to make (and starting to get negligibly better versus the accuracy of the mirror itself which is about 1/10 wave).

I can't see a reason why it wouldn't work, but this is what i was thinking of doing for the pivot system. as the bolt and 3 point support plate will both be steel, the magnet, whether neodymium or steel will hold the two in place and the height can be adjusted with the bolt. Here's a (very) rough-and-ready-totally-not-to-scale drawing of what i have in mind but if anyone can think of an flaws, please say...

[coatesg]

I wouldn't rely on the magnet to hold the plate in place - if the triangular plate had a hole for registering the "bearing", then it shouldn't slip around. I guess the biggest thing is that it's only the mirror holding the cell together - as long as you were careful not to turn it upside down with the primary removed, then it might not make much difference. Does the extra height from the plates take the mirror too high in the cell at all? (which is the issue I'm suspecting I'd have - I need to wait for the rain to stop so I can get in the dome and remove the cell again...!)

You might want to consider running the plan through PLOP - you may well find the 9-point with triangles isn't quite as good as a 6-point (which is what I found). 

[Peter Drew]

Is the primary mirror showing signs of distortion on a star image with the original support, if not then I would leave it as is. There is a virtue in simplicity if it works. 

[coatesg]

I'd be tended to agree for the 10" - would be interesting to run through PLOP and see what the theory gives for that as that will probably show not much deformation - at least to the point where it might not be affecting the image. (and it'll make a difference as to what you propose to do with the scope of course).

There is likely to be a degradation for my case - even if it doesn't display as something like pinched optics, the quoted P-V error from my mirror's zygo report is a fair bit less than my calculated cell error, so this may well be dominant. (Ignoring all atmospherics, thermal issues, miscollimation, etc!!). My scope is being rebuilt now anyhow with a bigger secondary, primary needs recoating, etc so it's a good time for me to do this.