Re: [ATM] Re:How much of cell induced deformation is too much?

["Mel Bartels" <mbartels@bbastrodesigns.com>]

some comments in reading snippets from the thread...

> >In regard to cell induced surface error, it is important, but shouldn't
> >be taken to the extreme. Taking into account error contributions from the
> >atmosphere, local turbulence, thermal imbalances, miscollimation,
> >c.obstruction, TE, roughness, etc. 0.80 Strehl seems to be out of the
> >reach. For apertures ~16" and larger, seeing alone will take care of it
> >nearly 100% of the time. Considering this, an attempt to preserve
> >non-existent near-perfect optical quality by nearly elliminating cell
> >induced error seems pointless.

Well, there is a definite difference in performance between my personal 20
inch at highest powers compared to other 20 inches on average.  I worked my
20 inch until I could detect no error.  I've used my 20 inch at 1500-2000x
for serious observing, and took it up to 6000x for curiosity one night.
Sometime later another amateur mentioned that they got their scope up to 10k
to 12k magnification, so at least I'm not the worst highest power nut!
<smile>

> Which I (no doubt wrongly) interpreted to mean: Any amount of quality way
over 80% Strehl ratio is not worth going for in a
> large scope, because it is unobservable. It would be a waste of time and
effort to improve the scope much past this point, since

The difference in performance between mirror most likely judged 0.8 Strehl
and one judged 0.95 Strehl is obvious even at a glance, at medium and higher
powers, even in the larger aperture sizes (16 to 40 inches in size).

> >"non-existent near-perfect optical quality"

There are some very fine mirrors that star test at highest powers extremely
well.  It takes many minutes to hours to detect any error at all using the
star test.

> You also said:
> >In order to meaningfully star-test this size telescopes, you need to use
> >800x-1000x magnification.

I use minimum of 750x to star test larger aperture scopes.

Mel Bartels

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