Re: [ATM] PlOp PlOp Fizz Fizz oh what a relief it isn't? Mirror cellfor an 8" mirror

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Rick,

this is just an amazing volley of thin air against PLOP. But let's discuss 
your points:


>>>I suspect PlOp is inadequate and produces false results that are
inversely proportional to the number of supports.

I don't know of that proportionality, but yes, all finite element programs 
will be in error by some percentage - like it or not. But Plop was never 
intended to give *exact* results - what it so far seems to do is optimize a 
design, calculate the figure deviation from ideal and give you a reasonably 
realistic idea of the deviations to expect. I know of a few attempts of 
comparing PLOP to professional (and expensive - far beyond the means of us 
amateurs) software, finding reasonable agreement.
http://www.cruxis.com/scope/scope1100_mirrormesh3d.htm
is an attempt to compare PLOP to another software - again, good agreement, 
but there's absolutely no need for you to be convinced.

>>. Let?s be accurate, secondaries do not affect the figure of
the primary, it may sometimes mask its errors but that mainly applies
to on axis testing and not the real world.  >>>

The art of modeling the real world is sometimes known as "physics".


>>>The idea should be to provide  support so the mirror does not sag, not so 
>>>it sags into an acceptable figure. Otherwise , why not just figure to a 
>>>sphere, ring support and
let the objective sag to correction?

You ought to know better than that... If a fixed focus is a premium 
consideration for you, as it well may be even though it may not be for other 
users, PLOP is more than happy to oblige. Just uncheck the "Refocus error 
calculation" box. If checked, PLOP tells you how much gravity - whenever 
applied - will change focus compared to weightless conditions.
>>>I haven?t dissected the code but I did not see where the software  dealt 
>>>with boundaries. >>

You have to supply the diameter. This tells PLOP where the mirror ends.

>>I did not see trans lateral vectors, there is a  lot that seems missing at 
>>this point. In dealing with the eight inch  mirror that is 15% thicker on 
>>the edge than the center. That means the  mesh elements need to represent 
>>15% more weight on the edge than the
center.  I can produce a model that seems to agree with a 40% zone for  3 
point support but then I can produce one that is more complete that  suggest 
80%.  >>>

How did you validate those models and their completeness? Anyway, trying 
mirrors of different focal lengths but otherwise similar will quickly 
convince you that PLOP does in fact consider the thinning of the mirror at 
center.

>>>For D the 50% zone would be  ((D^2)/8)^½) or  D/2*2^½ or approximately 
>>>71% of the diameter. Thus the idea would be that an annular ring at the 
>>>71% zone would support the optic. >>>

It would of course, if you are not concerned about the optical figure. PLOP 
does attempt to estimate it - perhaps rather successfully, but who knows?

>> My 6x¾? mirror has been in its cell for  38 years sitting on 3 points at 
>> 71% and still
has an excellent figure.

Shouldn't surprise me if it could stay there a couple of years more. But I 
can't think PLOP has anything to say about the matter.

>>One person said something about nine points  not be good enough, really???

Good enough for what mirror? What has been said a few times is that for a 
certain mirror, PLOP shows less deformation for a 6 point cell than for a 
conventional 9 point cell (a 9 point cell with all 9 points on a circle is 
shown to be better, though, even if perhaps less practical).

>>>Has anyone ever seen a nine point cell  fail? I have seen a few glued 
>>>down mirrors warp but the main problem I  have seen is from inadequate 
>>>lateral edge support.

PLOP has done a wonderful job to ensure adequate axial support, but does not 
attempt to optimize lateral support. If you're handy with software, why not 
write some good freeware to calculate the effects of various lateral 
supports?

> I can guarantee  you that if you designed a roof support using PlOp it 
> would be turned  down at the planning dept.

Shouldn't surprise me - anyone using PLOP to design a roof might appear a 
little less than serious.  Anyway, PLOP might (given reasonable parameters) 
estimate the deformation from gravity, but it would not estimate the 
structural strength - the planning dept. just might be more concerned with 
the latter question.

In conclusion - constructive criticism is great, and PLOP would no doubt 
benefit from it. Why not try that variety next time?

Nils Olof 

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