Mars drawings - Lick 36" Clark refractor

On Wed, 22 Oct 2003 19:19:16 +0900, Trane Francks wrote:


From page 6 of the writings: "In order to eliminate chromatic
aberration, we stopped the aperture to 20 inches."

Even 20" must have provides positively stunning views.


Thanks Trane... Didn't see that.

Mark

On 21 Oct 2003 23:30:30 -0700, (ValeryD)
wrote:

I am sure, that observers saw MORE, than were documented on these
drawings. One should be truly artist to document all details he saw
during best moments through such large telescope.

I have been a professional artist most of my life, and I do not
believe ANYONE could draw EVERY detail seen on Mars with its correct
relative contrast to the other markings this apparition in such a
large aperture instrument with good seeing. There simply isn't enough
time within the rotational constraints--unless one is endowed with a
truely photographic memory (truely rare!!).

Very nice drawings. I see, that Barnard's drawings were far not
as detailed as performed at this apparition.

The thing that disappoints me about Barnard's drawings is the
oversimplified sweeps he gives to the forms. It has an unfortunate
tendency to make a stylized image, to the point of looking not quite
right, if not far too generalized.

Because atmosphere is the main limiting factor, even much smaller
instruments
with excellent optics sometime allow to see quite similar pictures.

Interestingly enough, Sol Robbins drew nearly the same amount
of detail this appartion with a 150mm instrument . The seeing this
Summer was so bad around my location, I rarely approached capturing
anything close to what Sol did.

Dan Chaffee

(Dan Chaffee) wrote in message ...


Because atmosphere is the main limiting factor, even much smaller
instruments
with excellent optics sometime allow to see quite similar pictures.

Interestingly enough, Sol Robbins drew nearly the same amount
of detail this appartion with a 150mm instrument . The seeing this
Summer was so bad around my location, I rarely approached capturing
anything close to what Sol did.

Dan Chaffee

Yes, Sol's drawings are amazingly detailed. He told me, that time to
time his seeing is almost perfect. But I am sure he did a "patroling"
of such nights/hours.
Also his instrument has almost perfect color correction and variable
spherical aberration correction with Chromacor-II movement along optical
axis. This allow him to maximize his scope performance at any given moment.
And he is artist in profession, which is greatly help to document most of
details.
I saw a lot of details on Mars this summer, but I am not any kind of
artist!

V.D.

(Mark Wagner) wrote in message

Objective Diameter 36''
Focal ratio f/19, Focal length 57' 10''
Tube length 52'
Crown lens thickness 1.96'' at center, 0.60'' at edge
Flint lens thickness 0.93'' at center, 1.65'' at edge
The two lenses are about 6.5'' apart

Okay, I have a question:

Given that a 36" mirror 2" thick would need a pretty hefty multi-point
mirror cell to prevent warping, how does a lens this thin remain
usable with only edge support?

-Jeff

Given that a 36" mirror 2" thick would need a pretty hefty multi-point
mirror cell to prevent warping, how does a lens this thin remain
usable with only edge support?


1 - Lenses will have 4 times less wavefront aberration than a mirror with the
same amount of warping.

2 - both front and rear will warp by approximately the same amount so that the
overall wavefront is hardly affected.

3 - large lenses are sometimes designed in such a way as to minimize the sag
with gravity, much like bridge structures. One of the best forms for very low
sag is the Flint leading achromat. This design is curved toward the sky and the
resultant arch resists sagging.

Roland Christen