Mirror cell PLOP/FEA scepticism.

[marcopolo]

Hi all

I have a bit of a techie question for any prepared to dive in and get dirty!!

Years back I changed my cell design (8.75in 20mm thick primary) from an 'orthodox' 9 point as per the books to a PLOP optimized layout. I remember having a brief debate with Richard Schwartz (the designer of PLOP) at the time about FEA and it's usefullness (or not!!) in the real world as we use it a lot in the Aerospace industry (my occupation). The model is only as good as it's inputs.

The bottom line was I though the FEA was itself correct but missing some parameters that would untimately place the support points unintuitively too close to the centre of the mirror in real life . In my book if it looks wrong it probably is and in engineering design gut instrinct is a good thing (sometimes).

To this date I've still not heard of a single analysis either by star testing, lab testing or otherwise that proved PLOP optimised cells are better than the orthodox way. it certainly didn't seem to make a difference in my scope even though the models showed there would be a significant improvement in the order of 1/4-1/6wv p-v

Even this summary of it does not adress the key issue: is it better?

Or has it proven to be better?:

Mirror Cell FAQ

Any thoughts appreciated.

Marco.

[Glasspusher]

Marco,

Put simply, a lot of telescopes have been made with mirror cells based on the equal area principle, and lot based upon a PLOP analysis. We don't see the forums choked with posts advocating one over the other. It seems that both are capable of producing a cell that does the job. I have just finished making a PLOP designed cell for my 18 inch, if the cell does not perform as expected you will be hearing from me!!!

John

[Peter Drew]

During my career as a telescope maker the conventional wisdom was for supports at the 70% radius, this was later revised to 40%, Jean Texereau in his respected book advocated support at the outer edge, so quite a spread of possibility. Not having access in those days to sophisticated computer analysis for cell design the only option was experimentation. I made cells with standard 3 point mirror support as well as multi point versions, quite a large number of 6"- 10" mirrors were actually bonded to flat plate cells with a layer of felt between the bonding glue (Evostick Timebond), larger mirrors up to 30" actually had a bed of a heavy duty type of bubble pack for support, hundreds of supports and good airflow under the mirror. All these methods, which would give a current designer a nervous breakdown, worked well. So where do you stand?

[Glasspusher]

Peter, I once started to polish a 16 inch mirror with bubble pack under it. As the polishing mixture made its way off the mirror and onto the bubble pack it became obvious that the bubble pack was not providing uniform support as areas of poor contact became evident. The pockets of air were not uniform. Retrospectively, it was probably wrong to assume that a cheap low tech packing material would provide the uniform contact that I was looking for.

John

[Peter Drew]

Agreed John.

"Bubble pack" was the nearest description to the material I used. This was a heavy duty material used primarily for covering swimming pools, the "bubbles" were about 12mm diameter and could not be deflated by strong fingure pressure (as is the delight of children). Standard packaging material always deflated over time.