Re: [ATM] 6-points cell voor 20 inch mirror?

[Mark Holm <mdholm@telerama.com>]

For a 508mm diameter, 50.8mm thickness, f/5 Pyrex mirror, with an 80mm
diameter secondary on a 6 point cell, Plop gives 3.6nm RMS residual
deformation.

I have been advising an upper cut off of 2.5 nm, so I would not usually
say this cell is good enough for this mirror.  I know 2.5nm sounds
awfully low.  There is a reason.  Jeff Anderson Lee made a study with
Plop, investigating the effect of construction errors on the resulting
deformations.  He found that errors of a magnitude that could reasonably
be expected in home workshop construction, 1 to 2mm, can worsen residual
deformation by a factor of 2 to 4.  If you multiply 2.5 nm by a factor
of 2 to 4, you get into the maximum range you would want to really have
present.  Multiplying 3.6 times 2 to 4 starts getting into the range of
too much.

This argument is based on the assumption that you want a telescope
capable of good high power views.  If you are only aiming for low power
viewing, or deep sky imaging, you can probably relax the criterion by
another factor of 2.

My graphical summary of cell limits is at
http://www.atmsite.org/contrib/Holm/Plop_optimized_cells/design_space.html

Mark Holm
mdholm@telerama.com

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