Re : ATM Diag. size

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

Anthony,

> I can only point out my observations.  I have done some
> simple tests with masks and a good APO a friend of mine owns, I can't afford
> one myself.  I was amazed by the how much sensitivity the eye has to spacial
> details.  Of course we knew what we were looking for ahead of time, so it was
> hardly a double blind study.

I haven't even mentioned personal experiences here. I'm talking about
some of the greatest planetary observers of all time (and PROFESSIONAL
observers too!). If they come to a concensus that obstruction of 0.2 is
about the limit where one can pick loss of detail on planets (the most
demanding objects that is), my natural reaction is to agree with that
(especially when my own experiences confirm this). In fact, some
experienced observers (name escapes me now) thought that 0.3 is closer
to that limit !
Point is, it is very difficult to be objective in a short observing
session.  Variations in the seeing, psychological condition of the
observer, knowing ahead of time what we're going to observe with, all
will contribute to (often subconscientious) biases. One easy way to
check how easy or difficult is to pick obstruction effects is to
photocopy computer simulated examples of Jupiter from an excellent S&T
article by Mark Russel, cut them into individual pieces, mix and try to
recognize unobstructed one. You'll be surprised how difficult it is.
And we don't have quirky atmosphere, shakey mounts and imperfect 
diagonals and eyepieces in the way to detract us.

> I suppose that the smallest obstruction would be a piece of dust.  Does dust 
> effect image contrast? 

I guess it does, but not much. I've seen an OTF of simulated 'dirty'
system and it hardly departs from ideal. It causes some scatter
though.

> As for the professionals, well large aperatures (>1/3 m) have another problem
> to contend with, air cells.  I sure you know that this limits the ablities of 
> a passive imaging system.

There are enough exeprienced 'planetary' amateurs that use larger
scopes. Parker, Miyazaki (0.4m scopes), Viscardy, Joyce (0.5m). Guys at
Pic replaced 0.6m refractor with 1m reflector. One of the most detailed
maps of the Moon so far has been done with 1.5m instrument.

> Further, professionals are generally more concern
> with light gathering, at least on earth. 

I'm talking specifically high-resolution-hungry planetary observation 
posts here. Not much variable star work goes at Pic du, for example.

> ... Takahashi Dall-Kirkham
> was able to pick out the ripples in the cloud structure of Jupiter.

I thought that any 0.2m and larger scope should easily do that.
Certainly my CCD images with 0.185m scope (with 0.3 obstruction to
boot) do.

> I guess what I'm saying is, as long as there is an obstruction, don't go out
> of your way to make it tiny.  Sure less than a third of apature is worth it,
> but going well below 20% starts getting hard to do, and it has questionable 
> value, especially if the intended use is not visual.

Couldn't agree more. Going for minimal diagonal size is dangerous 
if you can't verify the quality of your diagonal (especially the edges).

Bratislav