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Posts Made By: John Biretta

April 7, 2002 08:03 PM Forum: Telescope Making

8" Novak Mirror Cell

Posted By John Biretta

Bill, Got one I'm not using. Still new in box.
Drop a line if you want it.
grzincic (at) erols (dot) com - John B.

November 22, 2002 05:17 AM Forum: Film Astrophotography - Imaging and Processing

Tamron SP 300/2.8 LD IF?

Posted By John Biretta

Wondering if anyone out there has experience with
the Tamron SP 300/2.8 LD IF (manual focus) lens for
astrophotos. MTF numbers on seem
respectable enough, especially at the edges and
corners. I saw some nice images posted by Andrew
in July. Any suggestions for stop-down (wide open?
F4? F5.6??) or UV-blocking filters for this lens?
Thanks, - John B.

December 22, 2002 09:18 PM Forum: Telescope Making

"Unbending" a mirror?

Posted By John Biretta

Here's one for the glass pundits: I recently bought a
30 year old 6" OTA. Initially it showed a 3-symmetry
pattern in star tests. OK, so I'm thinking the three
retaining clips are too tight. I checked and indeed
the clips are fairly tight. Removing them eliminates
some of the pattern, but not all of it. I am guessing
that 30 years with tight clips has bent the mirror in
some semi-permanant fashion.

I should add that this particular style of mirror
cell had the three rear support points rotated 60 degrees
to the mirror clips -- so tight clips bends the entire
mirror, rather than just pinching it under the clips.
It is 1" thick, looks like a poured Pyrex blank.

Is it possible to "unbend" the mirror? Ideas:

(1) rotate mirror 60 degrees, re-tighten clips
and wait 30 years? Too slow....

(2) suspend the mirror on three points, and heat
to say 150F (something a little higher than any
temperature it is likely to have encountered in the
last 30 years) for a couple hours?

Other ideas (short of re-figuring)?

Thanks, - John B.

April 6, 2003 08:41 AM Forum: Equipment Talk

Re: Anyone know what this 1.25 eyepiece is?

Posted By John Biretta

Gary Russell is also my guess, especially if the
1.25" barrel is some sort of plastic.

May 1, 2003 12:28 AM Forum: Equipment Talk

Re: Head for my N.E.T.

Posted By John Biretta

Bogen 3126 is probably what you want. I had a
6 lb scope on it, and was great except for
looking straight up -- then it would tend to
tilt backwards unless you really tightened-up
the elevation drag. But 3 lb scope should be
perfect on it. Try Don't
think Anacortes carries it, but have a look.

September 27, 2003 07:03 PM Forum: Equipment Talk

Re: Need testers for new Antifog!

Posted By John Biretta

Might be worth trying on inexpensive finder devices like
Telrads and Rigels. Don't think I'd try it on a $5K lens
unless the lens maker had certified it or signed on.
I've got protective windows on some of my smaller scopes;
might try it on the window but not the lens.

October 3, 2003 02:39 AM Forum: Equipment Talk

Re: Newtonian Astigmatism

Posted By John Biretta

Check if one axis of the astigmatism pattern
lines up closely with the long axis of the tube
(long axis as you look in the eyepiece).
I have often seen this situation when there is
too much wadding in the secondary holder. The
pressure causes the secondary to bow outward,
which leads to astigmatism.

November 24, 2003 11:24 PM Forum: Equipment Talk

CG-5 mount / drive jitter issue

Posted By John Biretta

Past weekend I was comparing with two 4" refractors
side-by-side at high power, one on a GP mount and the
other on a CG-5 mount. After a while I noticed the
image from the CG-5 mounted scope was slightly
unstable -- it seemed to wander or jitter just
slightly -- maybe <1 arcsecond amplitude and
and several oscillations per second. Normally
I would have attributed this to "seeing" but
the jitter was most definitly NOT there in the
GP mounted scope ten feet away. (Seeing was
superb BTW.)

Anyone else seen this sort of thing in a CG-5?
One thought I had, was whether the stepper drive was
exciting some resonance in the mount. Does anyone
know the drive rate on these (steps per second)
or whether it is different from the GP mount?
When the CG-5 RA drive is going I can hear a 1-2 Hz
ticking sound; is that the step rate (seems too
coarse?) or something else? I suppose it could
also be an issue with rigidity of the CG-5, but
the capacity, etc. seems similar to the GP.

Thanks in advance for any help....

February 5, 2004 11:34 PM Forum: Equipment Talk

Re: I may have been ripped off, what I do now?

Posted By John Biretta

Maybe I'm all wrong on this, but it seems a bit early
to be getting this nervous. The "ship" date on stuff
seems typically anywhere from same day to 2 weeks
depending on how busy people are, at least in my
humble experience on Astromart. Maybe if you paid
for overnight shipping, and he promised you'd have
it the next day, well then maybe I'd be a little
worried. While its relatively easy to cash a check
(and probably wise to do it before shipping -- note
recent spate of fraudulent cahsiers checks in the
media), it can sometimes take days to do a
bomb-proof packing job on a big scope. People have
day jobs, stuff happens, etc. Even if he took
another week or two (or how about 6 months!),
and didn't reply to any communications, he'd still
be doing better than I've ocassionally seen from
well-known scope manufacturers/dealers out there.
Its not exactly "great service" but its a little
early to be talking police.

April 25, 2004 04:34 AM Forum: Equipment Talk