PLate OPtimizer

Lore & Background

PLOP was developed by telescope maker David Lewis and first described in 1999. It was based on Toshimi Taki's 1993 program PLATE, but with a simplified user interface, which contributed to its wide acceptance among makers of large Dobsonian style amateur telescopes. The program is used to simplify calculations needed in the design of mirror support cells, particularly for mirrors as thin as two inches for a diameter of thirty inches.

Many amateur telescope makers use cells designed via equal area rule calculation, using programs such as David Chandler's public domain program, Cell. However, such calculation does not account for mechanical stresses introduced in one part of a telescope mirror by another part, whereas finite element analysis can be used to reduce such stress. Although general finite element analysis programs such as Nastran will work for mirror cells, an advantage of PLOP is that it can be set to ignore deformation that merely results in refocus of a mirror's parabola. PLOP can be used to calculate floating support points for a mirror's axial (rear) support; however, additional tools are needed to calculate potential error from a mirror's lateral (edge) support.

Mirror cell calculations, whether using PLOP or another program, do not overcome errors introduced by gluing the mirror to its cell, excessive tightening of edge supports, nor impingement of the cell structure onto the mirror as a result of differential cooling shrinkage. The significantly more complex calculations arising from the support needs of large honeycomb mirrors and those using active optics systems are outside the design parameters of such programs.

Reader's Guide

PLOP's significance lies in its role as a specialized finite element analysis tool for amateur telescope makers, particularly those building large Dobsonian style telescopes with thin mirrors. The article notes that while basic mirror cells can be built with minimal calculation, amateurs seeking larger and thinner mirrors found such designs inadequate. PLOP addresses this by simplifying calculations for mirror support cells, based on Toshimi Taki's earlier program but with a more accessible interface. Its key advantage over general finite element analysis programs like Nastran is its ability to ignore deformation that merely results in refocus of a mirror's parabola, streamlining the design process. However, the article also emphasizes limitations: PLOP only calculates axial support points, not lateral support; it cannot overcome errors from gluing, excessive edge support tightening, or differential cooling shrinkage; and it is not designed for large honeycomb mirrors or active optics systems. The program's legacy is thus as a practical, accessible tool for a specific niche in amateur telescope making, with clear boundaries on its applicability.