Re: [ATM] Ghosts in the Coulter

["Stephen Koehler" <s.c.koehler@gmail.com>]

All,

Here's a data point in the discussion on the ghost images.  I made a
quick simulation of planetary ghost images, then reverse engineered it
to get a mirror surface map that would cause that distortion.  Here's
the result:

http://www.atmlist.net/contrib/s-dot-c-dot-koehler-at-gmail-dot-com/misc/davis.png

Warning: I could have a number of things wrong with this simulation,
so take the results with a grain of salt.  Here's what I did.

A planetary image can be considered to be formed by a convolution of a
perfect image and the "optical transfer function", which is the
Fourier transform of the "point spread function".  The point spread
function is what you see in the focal plane, i.e., a star test image.
Pane one shows the PSF that leads to the ghost image.  As you might
expect, there are multiple stars.  I made two of them somewhat dimmer
than the main one.

The second image shows the simulation of the planet--Jupiter, in this
case.  This uses the same algorithm that Aberrator uses.

The third pane shows what I get if I reverse engineer the star test to
see what wavefront phase gives rise to the image.  It consists of an
odd checkerboard pattern.

The last pane shows an "unwrapping" of this phase to get a
wavefront/surface map.  It consists of a tilted grid of small peaks
and valleys overlaid on a relatively flat surface.

Assuming this is all valid, I conclude that primary mirror distortion
is highly unlikely to cause the ghost images.  This argument does not
say anything about what the secondary mirror could do, since that case
is a bit different.
-- 
Steve Koehler
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