Re : ATM Diag. size

[Bratislav Curcic <epabcc@epa.ericsson.se>]

Anthony Stillman wrote 

> my experience has been that un-obstructed light paths
> are far better than even the smallest obstruction.

This is quite a bold statement. "even the smallest" ? I know that even
die-hards like Kutter admitted that they couldn't see the difference
when obstruction got to about 0.1. The more realistic (as in "not
optical designers of unobstructed systems"), and more experienced
observers (e.g. Antoniadi) have reported that 0.15 to 0.20 is a much more
likely limit where eye can pick the injurous effects of obstruction.
What is the smallest obstruction instrument you have observed with ?
0.05 ? 0.02 ?  What object ? Seeing the central star in M57 is
incidently NOT very taxing object for testing OTF of an obstructed
instrument. A well baffled, well made Newtonian with clean optics will
do there just the same as megabuck APO, even inch for inch. It may fall
behind on ultrafine details on Moon or Mars though, but add few inches of
aperture and it will pick up again. The problem may simply be that
quality of obstructed instruments you've experienced simply wasn't on a
par with (mostly excellent) APOs. I will again quote Jean Dragesco here
in saying that his 10" Newtonian is the best instrument he owned (among
31 'other' scopes were 7.1" Starfire, 10" Takahashi, some TCTs and
C14). And he _knows_ what a good instrument is, being a serious
planetary nut for quiiiiite a while (in fact longer than I live).

> If you are using a CCD or grain film, you should consider wheather
> or not an obstruction will significantly effect the image.

I can quicky answer that. It will NOT significantly affect the image.
In fact it will not affect it, period. Carefuly check the images in
Dragesco's book. Not a single APO image there (OK it may be one or two
Nemec's images, mostly for historical reasons). But check all the other
ones (and I challenge all APO owners out there to REPEAT resolutions
achieved with obstructed scopes used there, let alone surpass - and
embarassingly enough, there is one C8 with 0.34 obstruction among
these). Shouldn't be that difficult, should it ? All APO owners keep 
going on and on how 4" Travellers and likes beat the pants of 8" SCTs on
Moon/planets. Well, 'Hic Rhodos, hic salta !' Let's see some results. I
also have some planetary images done with (incidently almost EXACTLY
7.1") instrument with 0.3 obstruction. Any proud 7.1" Starfire owner
out there who care to exchange some CCD images ?
 
Notice obstructions among the professional scopes. Not many APOs
and TCTs there ? Even Pic du Midi changed refractor for an "inferior"
Cassegrain. And this is primarily PLANETARY site. Also check ALPO
archives. Again inferior, obstructed and obsolete Newtonians lead 
the way. Hmmm... 

There is no qestion on how WE perceive images delivered by
unobstructed, low scatter telescopes (= APOs). I like them too. In
fact, it they were cheap, I'd gladly own 10 or 12 inch APO, thank you.
But if you use OBJECTIVE device (like film or CCD), you'll quickly find
that there's only one variable that really counts (for a given optical
quality) - aperture. You can have world's most perfect 6" APO, a
good 10 incher will simply blow you away as far as recording details
on planets (and mostly everything else too).

Bratislav