Posts Made By: John Biretta

November 20, 2011 06:17 PM Forum: Refractors

Newton's Rings?

Posted By John Biretta

I'm fairly certain the pattern is due to stuff deposited on the front surface. It reminds me of the Edmund Scientific "Astroscan" scopes. These were 4.25" F/4.2 and the mirrors were (according to rumor) parabolized in the coating chamber by depositing stuff of varying thickness. Perhaps thats what you have here, sans the Aluminum top coat.

http://www.scientificsonline.com/astroscan-plus-telescope.html

December 30, 2011 05:24 AM Forum: Refractors

Aries SAFIX

Posted By John Biretta

Are you perhaps thinking of the Aries CHROMACOR instead? It is a chromatic aberration correction device optimized for the C6R. Works very well. There is typically some work to align the device with respect to the optical axis of the scope and main lens. They came in several corrections (O1, N, U1, etc.) to also optimize the spherical aberration. With a good alignment and match to the C6R, the performance at the field center was close to a true apochromat like the Astro-Physics 155 EDF. I did some side-by-side testing on Jupiter of these two setups, and visibility of small features in the cloud bands was very similar. This assumes you have a good C6R to start with -- one with only mild spherical aberration in green light, and no coma or astigmatism.

The Aries SAFIX was intended only to correct spherical aberration. It works well also. It can be used with any scope slower than F/5. But I suspect it is not what you are after. The main issue with the C6R design is the chromatic aberration, not the spherical aberration.

March 23, 2012 02:41 PM Forum: Refractors

Wide field refractor

Posted By John Biretta

Yes, a 4" F/5 or F/6 achromat should do quite well for nebula and clusters. The eye is most sensitive to green light for these faint objects, and a simple achromat should focus green light quite well. For the same reasons it will work well with OIII and UHC filters -- again these are transmitting green light.

Yes, it should compete very well on faint objects with an 80mm or 90mm APO. In fact, what you really need is aperture for faint objects, so the larger 4" will be an advantage.

Of course, the same is NOT true for planets. They are bright enough that the eye can see all the colors, and the achromat's poorer performance at the violet (and to lesser extent red) ends of the spectrum will be obvious.

April 12, 2012 03:53 PM Forum: Refractors

Under or Overcorrection via Interferogram?

Posted By John Biretta

Yes all these parameters can be easily determined. Usually the interferogram will already have been processed through some software, and the quantities will be listed in thr report. In addition, an overall quality rating called the "Strehl ratio" will be listed, and the closer this is to 1.00 the better the lens. Strehl ratios values above 0.96 generally indicate a high-quality lens.

If you have only the raw interferogram image to look at, in general you want to look for very straight, evenly spaced, lines (often called fringes) in the interferogram.

There are many examples of interferograms and their interpretation in Wolfgang Rohr's postings at this website. Here are a few quick examples. You can learn a lot by perusing his postings. Here are links to just a few on refractors:

http://www.astro-foren.de/showthread.php?12497-Sechs-reisetaugliche-Refraktoren

http://www.astro-foren.de/showthread.php?11530-Vixen-ED-APO-115-S-F-890

http://www.astro-foren.de/showthread.php?11436-Takahashi-TSA-120-900

http://www.astro-foren.de/showthread.php?10123-LOMO-Triplett-80-600

http://www.astro-foren.de/showthread.php?11698-Lzos-apo-115-805-354




May 14, 2012 06:06 AM Forum: Equipment Talk

Cave Astrola - just how good?

Posted By John Biretta

I've seen a wide range in the quality of Cave optics. I'd start by star testing the scope as best you can, or getting the mirror to someone who can evaluate it. You could have a wonderful high-performance mirror, or a pyrex doorstop, or anywhere in between.

May 19, 2012 04:26 PM Forum: Equipment Talk

T2 to T2 connection

Posted By John Biretta

For an adapter with male T on both ends:

http://www.scopestuff.com/ss_temm.htm

http://www.edmundoptics.com/mechanics/modular-mounting-components/t-mount-components/t-mount-double-thread-rings/1382

http://www.alpineastro.com/eyepiece_adapters/eyepiece_adapters.htm#Threaded Couplings
(T2-26) you may need to type link directly in since some browsers dont like the space before "Couplings"

If you need female T on both ends:

http://www.scopestuff.com/ss_taff.htm

http://www.edmundoptics.com/mechanics/modular-mounting-components/t-mount-components/t-mount-double-thread-rings/1382

http://www.alpineastro.com/product_listing.htm
and look up T2-31. I'm pretty sure the "Double T 1.25" Filter Holder" does what you want.
I've got one here somewhere. But you might get them on the phone and verify.

June 24, 2012 06:53 PM Forum: Equipment Talk

Flying with Celestron SC

Posted By John Biretta

I'd probably try to take it as a carry-on. Get a reasonably strong bag meeting all the carry-on requirements for size. Put the scope in a plastic bag, then pack lots of clothes around it. You need to avoid the phenomenon where the airline decides at the last minute that your bag needs to be checked -- because it won't be packed well enough for that -- arrange to be early in the boarding sequence to you can find space in an overhead bin.

If you need to check it, I like the Pelican 1600 series cases. Something like a 1620 might work for your C-6. They are big and heavy, but I've never had anything damaged in one.
http://www.pelicancases.com/1620-p/1620.htm
I've sometimes seen bags go off the side of the bag loader / conveyor belt as they are loading the plane -- about a 10 foot fall down to the tarmac. Checked baggage can get pretty rough treatment.

From what I've seen around the web, the corrector plates are what usually get broken. Try to make sure nothing is pressing on the center of the front lens cap. Not sure about the C-6, but in some of the other Celestron SCTs the flexible lens cap is in direct contact with the secondary mirror assembly. If something tweaks the front of the scope in transit, and secondary and corrector plate are history. The lens cap gives essentially no protection from mechanical shock or intrusion.

August 30, 2012 08:20 PM Forum: AstroMart FAQ

Delay in seeing new ads under new annual fee system

Posted By John Biretta

I tried to sign-up for the e-mail ads, but I got a message that my support had "Expired." (Even though at the left it says my "Support ends 8/29/2013" - somewhat confusing.) I take it that the "annual subscription" does not entitle you to the same stuff that supporting does? You would need to BOTH
"subscribe" for the $15 and "support" for additional money to get full access? Would be nice if this were clarified somewhere. Thanks, - John B.

September 10, 2012 05:48 AM Forum: AstroMart FAQ

Determining if a User has signed up for New Subscr

Posted By John Biretta

My understanding is that folks with last names ending in "B" and "N" who have paid the $15 annual subscription will appear as "Purple Level" subscribers: http://www.astromart.com/aboutus/supporters.asp

It is also possible that they might have chosen to "support" at a different level ("Green", etc.) before the "subscription" system was enforced.

Herb would know the details, of course.

September 14, 2012 05:25 PM Forum: Equipment Talk

Telescope Rentals

Posted By John Biretta

Its a very interesting idea. Random thoughts....

There are many pro level camera lens rental companies, and these seem to have survived. They rent lenses selling for prices comparable to the APOs you mention. Could a telescope rental company be modeled around these? Of course their market is much larger...
http://www.lensprotogo.com/
http://www.borrowlenses.com/

Somehow the thought of shipping a high-end APO several times a month makes one worry. Are they designed for that abuse? Would it stay in collimation? Would you test the scopes before shipping to make sure they were still OK optically?

Might be interesting to rent SCTs also, but they are probably more fragile and need collimation more often. Probably any scope that needs frequent collimation or that has difficult collimation would be problematic. Would you really want renters (unknown skill level) attempting to collimate stuff? (Indeed you do not specifically mention SCTs.)

Rugged / simple Dob scopes like Meade LightBridge would also be a candidate.

Eyepieces seem like an easy item to rent -- small, relatively inexpensive, easily replaced, no collimation.

In a sense, Astromart would be your competition. You can buy a scope here and re-sell, probably for less total cost than a rental. But granted, you might have to wait a while for a certain APO to appear on Astromart. And there would be a large cash out-lay while you own it.

There have been a few scope rental outfits before, but they seem to quickly disappear....