Re: Mount Wilson Observatory centennial noted by BBC
From: Mike Simmons (mikes_at_nospam.mtwilson.edu)
Date: 12/22/04
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Date: Tue, 21 Dec 2004 16:49:30 -0800
The solution for not having laser interferometry is lots and lots of
time. Especially when you're inventing the procedures as you go. There
were actually significant difficulties in testing the 100" mirror because
it had to be tipped up on its side and tested horizontally like smaller
mirrors. With the larger diameter, though, the vertical temperature
gradient of the air in the hall (even using fans to mix the air) made the
test results uncertain. It wasn't until the first star tests that they
could be certain of their work. There's more in the articles I mentioned.
I haven't seen either Jupiter or Saturn in the 100" but I have seen
Neptune (poor seeing, nothing special except that Triton was as bright as
Neptune in a smaller scope even at f/30) and Mars last year on the night
of its closest approach. Seeing for the latter night was good but not
great but everyone went home happy. I'm more used to observing with the
60" which is used exclusively for that purpose right now (yours for only
$900/night or $450/half-night). Jupiter is unbelievable when the seeing
is good. There's so much detail in the belts you'd need an hour to look
at it all and by then it's rotated noticeably. Saturn is appropriately
awesome as well; not as much detail on the planet but there's more than
enough to look at just in the rings when it's really steady. I haven't
tried the usual filters on Saturn but that would probably help a lot.
Part of the difference between a smaller scope and a really big one is
something unexpected -- the planets look truly 3-dimensional. There's
just an effect I can't describe that's more like looking at a globe in
space than a picture. The Moon can be equally extraordinary -- scanning
the scope across it is like being in orbit. Think of the difference
between seeing a picture and then seeing the object in a scope -- similar
idea (though not necessarily a similar effect size).
Very experienced amateur observers who have had a chance to observe with
the 60" are certainly impressed with it. For example, during Mars' close
opposition last year I apologized for poor seeing one night only to have a
serious observer with 30 years experience say it's the best he's ever seen
the planet.
Comparing the 60" and 100" on objects I've seen in both the differences
are about what would be expected. Given good conditions (and it doesn't
have to be perfect) the 100" is noticeably better. Aperture really does
matter, even in that range, and not just on the nights of best seeing.
So the bottom line of 10"/12" vs. 60" on planets is: no comparison, given
reasonably good conditions, of course.
I don't have direct knowledge of the 200" mirror figure but have heard it
wasn't made to the same specs. I've heard that there was less concern
about the figure of later mirrors because they were never going to be able
to realize their full potential anyway because of the atmosphere. The
adaptive optics came along. But I don't really know if this is true and I
put it out there as a nasty rumor to be confirmed or debunked by someone
who knows better. I do know that the 60" is considered by many to be the
best large mirror (if one can still consider 1.5 meters large) ever made.
George Ritchey made it and the auxiliary optics. Ritchey started the 100"
but was removed from the project before it was completed. Having used the
scopes on good nights with famed Mt. Wilson seeing, I don't doubt for a
moment that the test results are correct.
I did have a chance to look through the 200" once and saw both Jupiter and
Saturn. The seeing was fair and the coude room filled with people, which
causes body heat to rise into the optical path through the polar axis, but
Saturn filled the field (about 1400x, I think) and just looked
maaaarvelous. I was suitably impressed and awed by the experience.
Now that I've spilled out all this stuff about the wonders of big scopes
I'd like to make another observation -- seeing Saturn through a small
scope still fills me with awe. Jupiter and its moons in a even a spotting
scope fills me with wonder and carrying big binoculars outside to gaze at
M31, M42 or star clusters is still an unqualified joy. Different but as
spoiled as I am with big scopes they don't for a second make any other
instrument obsolete.
We're hoping to expand the 60" program in the near future and I'm hoping
we can share this experience with many more amateurs. We have a 16"
LX-200 as well and an old Brashear 6" refractor that's a pure joy to use
will be reinstalled in the near future. There are some other plans not
ready to be announced yet but I hope to meet many of you at Mt. Wilson
some time.
Mike Simmons
On Tue, 21 Dec 2004 23:11:13 -0000, Glen Baker <Lens@nwc.edu> wrote:
> Those are impressive specs for instruments made before laser
> interferometry. How does Jupiter or Saturn look compared to a good 10" or
> 12" telescope?
>
> Any idea on the accuracy of the 200" mirror?
>
> Thanks
>
> Glen
>>
>> Very. At the time of the 60" and 100" telescope's design and
>> construction they were going well beyond what was well-known from
>> previous telescope projects. The designs were probably overkill and
>> the care in making the optics -- the first of the successful large
>> reflectors -- was great. One astronomer did some tests on the
>> accuracy of the optical trains of both scopes and told me they are 1/8
>> and 1/4 wave total through three mirrors (folded Cass or coude) for
>> the 60" and 100", respectively. I know nothing about his test or
>> exactly what those figures represent, though. He is one of the key
>> people working on the UV laser guide star adaptive optics system on
>> the 100" (at coude).
>>
>> Mike Simmons
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