Re: Re : ATM Diag. size

[astillma@nit.AirTouch.COM (Anthony Stillman)]

Bratislav,

There is no way that I can settle the endless argument of un-obstructed verus
obstructed but big.  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 suppose that the smallest obstruction would be a piece of dust.  Does dust 
effect image contrast?  I don't clean my optics that often, but when I do, I
notice a difference.  OK, maybe thats not completely fair, but there is a limit
to how small you can make obstructions in a functional instrument.  0.02% of
aperature, well maybe I could figure out a way to do it, but...

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.  Further, professionals are generally more concern
 with light gathering, at least on earth.  And of course there are the issues 
of gravity induced deformation, historical precident, and money.

Yes, I have seen phenomenal detail in obstructed instruments.  The most
impressive was a Takahashi Dall-Kirkham.  There was great seeing that night,
and I actually was able to pick out the ripples in the cloud structure of 
Jupiter.

If I had 10 grand to buy a telescope, would I buy a 7" APO or a really big
Newt.  OK, you got me there.  I'd buy the Newt.

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.  Ok, if you want to make
a 10 inch scope with a 1/2 inch secondary, just cause you can, great, how can
help.  Can I look too?  But the image isn't going to be much better, the 
un-vignetted field is at most 1/2 inch and you'll need to use some form of
relay.


As a member of on and on anon, I'll shut up now.

Anthony