[ATM] Re: Nasmyth secondary mirror

[Vladimir Sacek <vladis.2@juno.com>]

Don Anderson wrote:

>I have been figuring obstruction percent on area not radius. By area it
is
>13.22%, by radius or diameter it is 36.36%.  Which is correct?

They are both correct, and each good for different things. Linear form
directly indicate physical size of the obstruction. Areal form is more
directly related to the energy loss from the Airy disc caused by
c.obstruction: for obstructions smaller than 40% linear, percent of
energy loss is closely approximated by double the percent of areal
obstruction; for 40% obstruction, it is by 1.9 times the areal
obstruction, and for 50% c.o. 1.7 times the areal obstruction. For
instance, 36% linear obstruction has 13% areal obstruction and
1.8x13%=26% energy loss from the Airy disc due to the obstruction. This
approximates the Strehl as ~0.74 for the left side of the MTF graph,
where the main factor is out-of-Airy-disc noise created by the energy
lost to it. This is also the part of the MTF graph indicating
low-contrast performance.

For this aperture size (and more so for the 1m) criterion for acceptable
c.obstruction size is somewhat more relaxed than for smaller apertures
that can operate at lower totoal error levels. Still, as a source of
error, the smaller c.obstruction, the better.

You might consider tilting the secondary, in order to avoid double
spider. If secondary remains orthogonal to the axis of primary, it
doesn't have to be perfectly flat. A mild curve won't induce significant
spherical aberration. For instance, in this configuration, 0.1mm deep
secondary would still induce less than 1/4 wave s.a. and with 0.01mm
sagitta s.a. induced would be entirely negligible (please double check).
Striving for a perfect flat would be wast of time (or money).

Vlad
_______________________________________________
ATM mailing list http://www.atmlist.net/