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Posts Made By: John Biretta

November 16, 2015 07:48 PM Forum: Equipment Talk

C14 broken corrector.

Posted By John Biretta

I'd be curious to hear how it got broken, so the rest of us can learn. Was it shipping damage? Was it packed in the original factory packaging?

November 27, 2015 10:06 AM Forum: Astro Binoculars

What are these?

Posted By John Biretta

It looks like one of these: http://www.bigbinoculars.com/2540m.htm

Price will depend on condition and whether the tripod is included.
I see this one sold for $690 here on Astromart:
http://www.astromart.com/classifieds/details.asp?classified_id=802462

And another for $750:
http://www.astromart.com/classifieds/details.asp?classified_id=699273

You might get a little more since they are currently unavailable new. On the other hand there were many different versions of these on the market over many years, so there are many out there, which might erode the re-sale value somewhat vs. new prices.

April 4, 2016 11:36 PM Forum: CCD Imaging and Processing/Solar System

Jupiter 3/22/2016

Posted By John Biretta

Impressive for a 5 inch scope! Very nice!

April 5, 2016 03:16 PM Forum: Equipment Talk

Collimating question about centering the star imag

Posted By John Biretta

Collimation for relatively simple scopes (Newts, SCTs, etc) is all about getting the field point with "perfect images" moved to the center of the FOV. Once the collimation is sort of close (i.e. the location of perfect images in somewhere in the eyepiece), it's just a matter of moving those perfect images to the center of the eyepiece. In fact, you can usually tell what screws to adjust which way, if you find those perfect images somewhere in the eyepiece, and think what adjustment is needed to move that image to the field center.

There are several problems with having the "perfect images" too far off-axis. For example, if you switch to a very high power eyepiece, you may find those perfect images are now outside the (narrower) field of view. Also, most eyepieces will give their best images on-axis, so if the scope itself has the best images off-axis, there may be no location where the images are as good as they could be. For example, a Newtonian will have off-axis coma, and an eyepiece may have lateral color or spectrum-like color smears or astigmatism off-axis, so if it is mis-collimated there may be no location with ideal images. (On the other hand there are rare instances where eyepiece aberrations might cancel some other aberration in the scope - like a refractor with a wedged lens might be cancelled if the eyepiece has the opposite color aberration.)

May 17, 2016 06:48 PM Forum: Antique/Classic/Vintage Optical Instruments

Value of Polarex Unitron 132F 4" telescope

Posted By John Biretta

I'm not aware of any sales of the 132F model (4" folded compact model), so can't say exactly. I would expect it to have about the same pricing as the 152 model (4" full length scope), which for "very good condition" is around $3000 to $3200.

The term "very good condition" if subject to some interpretation. I would say it means fully usable scope with no major dents or damage. Maybe a few minor scrapes in the paint would be allowed; maybe some oxidation or traces of rust that could be mostly cleaned-up. Coatings on the lens should be mostly intact with little damage.

The compact 132F model is extremely rare, but I am not sure that increases the price. Personally I prefer the full-sized model, and would not pay extra for the compact. But if you found the right collector who absolutely needed one to complete his / her collection, then I suppose the pricing could go much higher. Also if it is merely "very good condition" vs "mint" a collector might be less interested.

Here are some examples of the 152:

Mint, motor $3750 SOLD
http://www.astromart.com/classifieds/details.asp?classified_id=908648

Excellent, NO motor $3000 SOLD
http://www.astromart.com/classifieds/details.asp?classified_id=870594

Very good to excellent, motor $3100 SOLD
http://www.astromart.com/classifieds/details.asp?classified_id=878808


January 11, 2017 11:54 AM Forum: Equipment Talk

Telescope equipment rental

Posted By John Biretta

There are places that rent high-end cameras and lenses. Some of these have been around for many years and are obviously surviving. You might have a look at them as a model:
www.borrowlenses.com
www.lensrentals.com

A $13K lens goes for about $500/week and requires either proof of insurance, purchase of insurance, or a deposit. They are careful to protect themselves, and reserve the right to refuse service. Perhaps a scope is more delicate, but probably not by much.

November 15, 2017 11:59 PM Forum: Telescope Making

Another Recirculation C8

Posted By John Biretta

Looks good. Lots of air motion without filters. No fans in the light path.

How well does it work? If you've got a boiling image from a warm primary mirror, how quickly do the fans fix it?

If you look at an out-of-focus star, do you see any air currents from the fans?

November 30, 2004 12:57 PM Forum: Polls

I ......

Posted By John Biretta

I also think $12 is cheap. And I also worry how
many people will leave, thus increasing the price
for remaining members. One can envision some
downward spiral where in 2 years there will be
30 members at $1000/year each.

A mitigating factor, I suppose, is that reducing
membership will reduce pressure on the servers,
etc. So it will also be cheaper to run.

What about advertisers? I don't know how the
fees are structured, but if enough people leave,
advertisers may pull out also, further compounding
problems.

Other options? Fees for placing ads? Fees for
images in ads? Eliminating some features to reduce
costs? It would be nice to figure out why it costs
so much, and target fees in those areas. Rather than
blanket fees on all users.