Re: ATM Optical standards

[epabcc@epa.ericsson.se (Bratislav Curcic)]

Bob Goff wrote :

> different grades of telescopes?

Yes. We can define high resolution scopes (planetary etc), and 
photographic (CCDgraphic). The requirement for a telescope that
is supposed to do Nova patrol or comet/asteroid orbit evaluation,
or photometry is far, far more relaxed than for a telescope intended
to monitor Mars year around (with its diameter rarely going above
5-6 arc sec most of the time).

> What does "diffraction limited" mean

As far as I am concerned, we'd be better WITHOUT this unfortunate
attribute. It is so strechable, and it has been so many times beaten to
death by commercial vendors that frankly it has lost any credibility as
means to describe quality of an astronomical telescope.
What we need is some sort of Pickering scale applied to telescopes;
zero being totaly unusable, 10 meaning absolute perfection (say no
discernible defects on extra focal images). Telescopes above say 6 or 7
would be capable of forming clean Airy discs, but with some other kind
of optical defects, detectable by extrafocal image observing, or observing
of low contrast extended objects.

> Or, does diffraction limited mean with all
> geometric rays of light passing through the Airy disc, and are different
> qualities specified as to how small the bundle of rays are going through the
> Airy disc? 

I would leave geometric ray analysis only to photographic (CCDgrahic)
scopes, and even that only for theoretical evaluation. Once we get
close to REALLY good optics, I don't think we can rely on geometric
optics. Of course, there is industry standard based on it (Strehl
ratio), but I feel it is of very little use for amateur optics
evaluation. How would an average Joe Blow find what Strehl ratio of his
just finished 6" f/8 or whatever is ? I feel that for amateur optics we
cannot have reliable QUANTITATIVE description (say 1/24 wave RMS, or
1/9 wave p-p). Foucault is not precise enough for this, Ronchi is not
quantitative at all, as it is star test (to great extent, although
Suiter did great job in defining 1/4 wave by looks of extrafocal images
in an obstructed instrument), Gaviola (caustic) is a bit more precise,
but not that widespread, even less true for Hartmann, and most null
test setups are next to impossible to give any idea about quality in
real terms (Ross with its reduction of final f/ratio makes Foucault 
tetsing especially insensitive ).

> Please e-mail or fax me, (520) 882-4924, by Friday, Sept. 20 AT LATEST (I
> leave early Sept.21 for Germany).

Should have communicated this earlier ...

> I am the USA representative to the working committee on
> optical standards for amateur telescopes

Oops, take it all back ! 
BTW, who is Australian representative, if any ?

Bratislav