Re: (IAAC) Markarian 205 - 8inch F/10

At 09:31 AM 6/5/98 +0200, you wrote:

>it performed like the first
>generation of the Nagler series giving what we call in France (I don't
>remember the proper term in english) "flying shadows" problems, and a
>bright background sky.

The American vernacular for this is "Kidney Bean" effect. I can't recall
the older, more traditional term.

>I would like to know if netastro observers have experienced big instruments
>(specially professional ones) and would like to share their thoughts.

I worked a bit in observatory automation at Ohio State University, which is
a major player in American ground-based astronomy. I did get to look
through several gigantic telescopes a few times when I was there. (And I am
talking about scopes well out of the realm of amateur experience - 60" and
larger, up to somewhere short of 3.5 meters IIRC.)

I have also used the 31" f/7 Newtonian at Warren Rupp Observatory a fair
amount, as well as some large (28-36") dobs here in Arizona.

I think you may be on to something when identifying the cause of the
'disappointment factor'. Much higher magnifications must be used to coax
the eye-brain system into maximizing the contrast between target and sky.
Showpiece objects - stuff such as M13 and the like - are so bright that the
effects of seeing are immediately noticeable when viewing them.

As you say, construction of large instruments happens along different lines
than amateurs pursue, and differences in baffling and the like could
account for some of the difference, I suppose. OTOH, here in the US the
great majority of amateur telescopes have little in the way of baffling or
shielding to attenuate stray light. Reflective optics scatter a great deal
more light from their surface coatings than refractors do from their
air-glass transitions, and in very large apertures such scattering in even
a well-baffled system becomes far more noticeable. This might also play a

Seeing is often not the greatest in professional facilities, especially
when no "real work" is being done and people are looking through the
telescope. During such times it is fairly common to turn off environmental
controls that would attenuate bad seeing if they were left on. As I
mentioned before, seeing will be far more noticeable in a very large

On the whole, I think there is a point of diminishing returns that is met
when using very large telescopes visually. Anyone who gets the chance to
use such a scope visually should do so, of course - it is still an
experience not to be missed - but many people who do are indeed
disappointed with the experience. And many of the few who have done so many
times seem to think that a large amateur instrument is just fine.

Jeff Medkeff          | Acting Assistant Coordinator
Rockland Observatory  | Association of Lunar and Planetary
Sierra Vista, Arizona | Observers, Solar Section

On the web at http://shutter.vet.ohio-state.edu/