ATM Lens sources.

[Anthony Stillman <atmer@flash.net>]

I new I was going to take some heat for my pointed remarks regarding lens
image quality.  It was with that in mind that I qualified them.

First, let me say, unqualified, that some of the best images I've seen were
through retractors.  I know that Bob Kestner would disagree with me, (he
already  has), but in my opinion no amount of obstructed aperture can beat
a really good APO for pure viewing joy.

Second, Bratislav, (who keeps me honest), I am aware of a 7" f/12 refractor
made with two unusual glasses which is both an APO and is color corrected
to bring five colors in the visual and near IR to focus at the same focal
plane.  Not only can't I afford this glass, if I could I don't believe I
would be couragious enough to work it.  Though, there are a few persons on
this list with whom I would intrust it.

Also, Don, Brarislav, the oil spaced lens at Riverside, to which I refered,
employed a rather large (a few mm) oil filled space as a refracting
element, not
simply to wash away the effects of two inner unpolished or poorly polished
surfaces.  Kevin Medlock who had described to me liquid lens work at LLNL,
warned me that they and some few amateurs who had also tryed this, had
difficulty sealing oil/oil-other stuff componds.  Technically he expalined,
the oil guck eats through the sealing goo.  Of course this was in the mid
and late seventies and I know that 3M has come a long way since then.

Third, Larry, I'll grant you that one can get use to the halo.  Further,
simple filters can remove most of this objectionable quality and film with
specific spectral sensitivity all but ignores it.  That said, people who
have only used reflecting instruments are oftened dismayed and dissapointed
by the image afforded from a fast achromat at high power.

I agree, large achromats are hard to find at star parties or amoung friends
willing to let others use them (maybe its just me.)  I would guess that
there are 25 to 50 12" dobs for every 6" or larger refractor at the typical
star party.

As for specific historic instruments.  I was recently out bid for a 6" f/10
Jeagers at the estate sale of George Lambert M.D.  I could have bid higher,
but it became clear to me that the man who purchased it was not going to
relent.  I did become angry when I realised this and likely added a hundard
dollars to his final price, in spite, for this I'm sorry.  He is a fair
businessman and works hard for what he earns.  I won't be suprised though
when I see it on his web site for double or half again more.   If I win the
lottery, I hope he isn't supprised when I try to buy it from him.

Yes, I would greatly like to use the 36" Lick, secondary color and all.
I've only seen this telescope from the observing area and that was when I
was a kid.  I had to pay for that trip by listning to my Mother complain
for two hours about how we were all going to crash down the mountain to our
deaths as my Father navigated the road up to and down from the observatory.
Incidently, most Clark lens use a large airspace to control coma.

As a rule of thumb ("Telescope Optics Evaluation and Design"  Rutten and
van Venrooij) for ordinary air-spaced achromats to be used visually the
focal ratio should be no less than 0.122 * D. Where D is the clear
aperature given in mm.  Hence a 4.3" achromat should be made to f/13 for
the eye not to notice (mostly) the red and blue halos.  This explains way
at f/10.5 the purple halo is only minor.  Further, I would add that cutting
nearly a foot off the tube length is worth it.

Let me add, I still harbor the belief that refractors are true telescopes
and reflectors, while certainly extending our knowledge of the universe,
are somehow less worthy.  In a book I read years ago,  Mr. Brown, while
standing beside his 200" s of finely figured pyrex on show to the public
which helped pay for it, was chided by two older women.  "You should be
ashamed of yourself", one said, "You can't even see through it."



Anthony


PS.  Please, before anyone else trashes me, with respect or otherwise,
re-read my original message.