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M20 Trifid Nebula in LRGBHa

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Posts Made By: John Biretta

April 1, 2010 04:30 PM Forum: Equipment Talk

adequate for a new coating?

Posted By John Biretta
This looks fine. This is what a good coating should look like.
The effect of the pinholes is highly exagerated by the bright
back lighting. If the entire mirror was covered by hundreds
or thousands of these pinholes you might have an issue,
but 3 or 4 microscopic ones is nothing to worry about.
No matter how good your clean room is, you can't get 100% of the
dust -- theres always a tiny bit that gets in the coating chamber
and leaves pinholes. The bigger question is how the other
99.999% of the mirror surface looks in normal lighting -- if
it looks bright and uniform then you are in good shape.
If there are big blotches or finger prints in the coating, then
it should be done again.

April 2, 2010 06:26 PM Forum: CCD Imaging and Processing/Solar System

Saturn April 2nd a.m. U.T., my best

Posted By John Biretta
Very nice!

April 2, 2010 09:38 PM Forum: CCD Imaging and Processing/Deep Sky

My MI-250 mount is on its way!

Posted By John Biretta
Be interesting to hear comparisons to G-11....

April 5, 2010 02:45 AM Forum: CCD Imaging and Processing/Solar System

Tethys' Shadow

Posted By John Biretta
Outstanding catch!

April 5, 2010 04:31 PM Forum: Equipment Talk

anti-slip "stop rings" on 1968 Newt hardware

Posted By John Biretta
I'm not very familiar with OC. What contacts the telescope
tube at the main rings? Are they mounted with felt? Rollers?
Or is there an inner metal ring screwed to the tube like
the Cave rotating rings? Cave had inner and outer rings with
Teflon plugs between them running in a machined track.
It worked pretty well, once the tension on the plugs was
set properly.

I expect if things weren't in the same plane, the stop rings
could be squeezing the main rings as it rotates. Probably
the 1/4 inch "play" is a complete non-issue.

April 8, 2010 09:32 AM Forum: Equipment Talk

Question on telescope value

Posted By John Biretta
If its a 3" it will say "D 75mm" on the focuser
and lense cell. The 3" with Alt-Az mount was model
140. With the Equatorial Mount is was model 142.
(Not sure about your model 155.)

A very complete 1955 model 142 (EQ mount,
wood tripod, complete set of wood boxes,
Unihex, 5 eyepieces, solar screens) in very
nice condition sold on eB** last November for $1650.
You can probably use that data point to
estimate an upper price limit on yours.

In 2008 a similar one which additionally had an
original Polarex electric drive motor went for
$2300. Not sure the motor adds $650 -- maybe
prices have dropped recently with the recession.

April 9, 2010 09:45 AM Forum: Reflectors

dew on primary...help

Posted By John Biretta
I added a home-made heater to the rear of the
primary in my 11" Dob. I made a couple chains
of resistors in the form of concentric circles,
and then fastened these to the rear of the
primary using a bit of blue painters tape over
each resistor. The resistors were placed to avoid
interfering with the floatation mirror cell.
Worked pretty well, though there
is a very fine line between keeping dew off, and
making tube currents. It does not need much
heat -- like 1 or 2 watts -- you'll want a
variable controller so as to use the absoulte
minimum power that keeps deew off. I had been
taking this particular scope to Winter Star Party
located 50 feet from the ocean, and sometimes
dew could get really bad (though seeing at
these times was often very steady).

A small fan mounted above and to the side of the
primary and aimed at it, will also help minimize dew.
It won't completely avoid dew, but it will
restrict the formation to a light haze, rather than dripping wet.

April 15, 2010 09:08 AM Forum: Telescope Making

mirror removal for transport

Posted By John Biretta
Some of the UO cells I've seen used whimpy, easily bent 0.05" brass
mirror clips. If thats what you've got, then yes, remove the mirror for
transport.

April 15, 2010 11:28 AM Forum: Digital SLR AstroPhotography

M81 and M82

Posted By John Biretta
Great stuff! Nice dust lanes in 81.

April 24, 2010 05:52 AM Forum: ASTRONOMY

Newby with a ton of questions

Posted By John Biretta
http://www.weasner.com/etx
http://meade.com/manuals/

From my vague recollection:

The M = RA model needs to be pointed manually, but
does have a tracking motor. It cannot be upgraded to AutoStar. Yes, it would need to be polar aligned
by you to track properly.

The EC model can be upgraded to AutoStar. No
software is required to run the scope or find objects.

The AT, Premier, and LS models include AutoStar.

Unless small size / light weight / portability are
important factors, you might look over the scopes at
Orion Telescopes
http://www.telescope.com
While I like my ETX a lot, it is still a 90mm scope.
Something like a basic 6" or 8" aperture Dob might show
many more objects for around the same price (though
will be much larger / heavier to carry around).

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