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Posts Made By: John Biretta

March 4, 2010 11:30 AM Forum: Equipment Talk

Re: 2 inch ap diagonal vs tv diagonal

Posted By John Biretta

I think its very difficult to tell from the
advertising precisely what the standard is
-- is it Wavefront? Surface? Peak-to-Valley?
RMS? What area -- 1/4 inch? Entire mirror?
Normal incidence? 45 degrees? Tested before
or after they coated it? Do they test each one,
or only 1 in 10? 1 in 100? How is the mirror
mounted? What is the substrate -- Coke bottles?
BK7? Quartz?

I think the only way to know what you've got, is
to put it in a scope and try it out. Do some
star tests at high power with and without it.
If you don't like it, send it back.

If you want a stronger test, you can put an
extension tube between the diagonal and
eyepiece -- effectively using a larger beam at
the mirror surface. That will make the errors
easier to see (though you probably won't
actually observe in that mode).

I've tested about 8 of these from AP, TV, etc.
The ones from AP and TV were pretty good -- there
are only minor issues visible in high power
star testing by looking at the images inside and
outside focus. Most had slight astigmatism or
rarely coma, but probably better than 1/10 wave
on the wavefront. Most folks would probably never
notice these small errors in real use.

The best one was something I cobbled together myself
from a TV NEAF "blem" mirror element (with no blem)
and a beat-up old 2" diagonal shell I got off AM.
Ugly housing, but the wavefront is perfect.
If you want a perfect one, you'll to need to
test them yourself.

The ones from Vernonscope are probably worth a look
-- quartz, protected silver, and interferogram
included -- if you are trying to find the best
wavefront. Not tried them myself yet.
http://www.vernonscope.com/frame_astronomical.html
The advertising hype will tell you that silver
mirrors rapidly degrade, are unstable, etc., but
I've got 30+ year old ones that are good as new.
The coating is thin (vs thick dielectric) and
will not warp the substrate.

March 20, 2010 02:51 AM Forum: Telescope Making

Oiling a doublet with failing cement?

Posted By John Biretta

Here's one from left field.... I recently picked up an old
camera lens. One of the cemented elements has de-bonded
around the outer 10% radius or so -- I can see colorful Newton's
rings all around the edge, and the wavefront goes to pot in this
de-bonded area. I was wondering if there might be some way
to flow a liquid or oil into debonded area -- some way to
re-couple the glasses -- sort of a cheap repair. Anyone try
this with any success? I'd really like to avoid splitting
them apart and re-cementing, if possible.

April 16, 2010 12:58 AM Forum: Reflectors

Re: Mirror help

Posted By John Biretta

I don't have any experience getting grape jelly off mirrors,
but here's my advice:

First figure out what coating you've got. Bare aluminum
coatings needed to age befor cleaning -- the purpose was to
allow the surface to oxidize and form a layer of aluminum
oxide, which is much harder than aluminum. But it is very rare
to have bare aluminum coatings these days. Modern coatings will
usually have some hard coating applied on top of the aluminum,
so there is much less to worry about.

I would probably try to get the stuff off of there sooner
rather than later. What I would do is first make some tests on
a small spot, and figure out what will safely remove the stuff.
Get some qtips and distilled water, and gently work one
of the typical spots and see if that removes it. Work the spot
and then rinse the entire mirror with distilled water, and
inspect it. If that doesn't do anything, try a mixture of
1 part distilled water, 1 part rubbing alcohol, and a tiny drop
of dish soap, and again work the spot with that, and then rinse the
entire mirror with distilled water. See if the spot was removed
or improved, and check for any damage or sleeks in the area you
were working. Shine a light through the back of the mirror and
see if there is anything odd about the area you were just cleaning
-- like extra light coming through the coating. If still there is
no improvement, try straight rubbing alcohol (no dilution).
Once you figure out how to safely remove one spot, go on to do the
entire affected area. Maybe use cotton balls (larger) instead of q-tips.

It is also possible the junk you see on the mirror is coming from
the cotton balls you are using. Try to get some different ones,
or preferably the sterile kind. I've sometimes run into this where
there is some oily junk on the cotton balls (or qtips), and
continued cleaning makes a big mess. The factory may have sprayed oil
or lotion on them to make them feel softer, etc.

Sometimes I've used "spring water" to clean mirrors in an emergency
when distilled was not available, and saw no problems. I don't recommend
it, but I've never seen anything like what you have there.

I'm not sure what to make of the gold color of the spots. It seems
like an important clue.

May 4, 2010 02:22 AM Forum: CCD Imaging and Processing/Deep Sky

Re: New equipment

Posted By John Biretta

What the heck is a TeleVue AT130?

October 26, 2010 07:36 AM Forum: Audio

Re: general question on dual cassette decks

Posted By John Biretta

My recollection from ~1977 is that Technics was the better brand.
I shopped low- and mid-priced units from both and got a Technics.
It is certainly possible they have comparable high-end models.

November 12, 2010 11:49 AM Forum: CCD Imaging and Processing/Deep Sky

Re: Horsehead

Posted By John Biretta

Very nice! The reflection nebula in the bottom left takes on some interesting details as well.

March 28, 2011 10:11 PM Forum: Eyepieces

Re: Combining 2X Powermate with Televue Ethos

Posted By John Biretta

Haven't tried it myself. One minor issue I can think
of -- there will be considerable torque from
the weight of the eyepiece + lever arm of the
Powermate. Stuff might happen -- rotating tubes
will want to rotate. On refractors and Cass scopes
the diagonal will want to rotate in the focuser -- or
the focuser will unscrew from the scope --
ending up with the eyepiece pointing at the ground
-- probably the safety under-cut on the 2" barrel will
catch the setscrew and save the eyepiece.
On Dobs any flexture in the upper tube will be
made worse by the large weight far off-axis.

OTOH if you've got a beefy focuser strongly
attached to a rigid tube, and very strong set screws,
you should be fine.

May 6, 2011 11:35 PM Forum: Refractors

Re: Clearance in an objective cell

Posted By John Biretta

Robert Apruzese said:
My question is: how much clearance
should there be between the cell and the lens to allow for expansion and contraction. Assume I will be using the lens in temperatures from +10F to +90F.

Why not measure it directly? Use a digital caliper
to measure the lens elements and cell at room temperature.
Then put them (but not the caliper) in the freezer
at 10 F and measure again. Easy to calculate how
much everything contracts at 10 F, then contraction
difference between cell and glass, and finally required
room temperature cell ID to avoid pinching at 10 F.

Problems generally arise since metal contracts
faster than glass, and hence there is pinching at
cold temps.

August 29, 2012 11:39 PM Forum: AstroMart FAQ

Delay in seeing new ads under new annual fee system

Posted By John Biretta

OK, so I've paid my $15 under the new annual subscription system. What if any delay should I experience in seeing newly posted ads? I have supported at various times in the past and believe I was seeing a 1 hour delay before implementation of the new system, per the policy in ad:
http://www.astromart.com/classifieds/details.asp?classified_id=720347

Would it be possible to state somewhere on the account menu or left menu what delay you are experiencing? For example, where it says "20 sponsor ads remaining" could it also say something like "you will see new ads with a XX hour delay." It might clarify things a bit, and might even motivate some to support?

Thanks, - John B.

July 16, 2013 04:01 PM Forum: Equipment Talk

Re: Astrophotography New comer needs your opinion

Posted By John Biretta

If I understand correctly, the scope is pretty much repaired and OK now? Do you see any remaining residue from the sticker when you look in the front of the scope? (This sticker is usually on the secondary mirror mount, and must have somehow migrated from there to the primary during shipping.) If the scope needs further work, you might get an estimate from Vixen for the work, and then ask the seller to cover half the cost (including shipping to and from Vixen). It is also possible the scope might still be covered under warrantee from Vixen, and that they might repair it for free (especially since you have a photo of the sticker on the primary mirror to document the original problem). But I would only ask the seller to pay if in fact it needs further work, and you do send it to Vixen. Since you've already done work on the scope yourself, rather than returning it, I feel it would not be appropraite to ask the seller to cover full costs of further work (as odd as that may seem).

Since you've already worked on the scope yourself, I feel it would not be appropriate to ask the seller to take it back. A better course of action might have been to return the scope to the seller in the original condition as you received it. But it is too late for that now. Ultimately you have to decide whether to fix the scope yourself or return it, and then live with the decision. With a new item there might be a sticker that says "removing this screw will void the warrantee" and things are hence more clearly defined. Things are not so clearly defined here.

Probably in this case it was faster to fix it yourself, as compared to sending it to Vixen (and risking further shipping damage, etc.), or returning it to the seller and waiting to see another one on Astromart. So it may have been the correct decision. If the scope is now working well and appears to be completely repaired, I think you should just enjoy the scope, and have some pride that you were able to fix it yourself. Someday when the optics need cleaning, you'll know how to do it.

Unfortunately, it is not rare to receive both new and used optical goods with a loose screw or something bent or broken or not right. I don't think any manufacturer is likely to compensate you for time and effort dealing with a scope received in poor condition. They may offer to fix it for free, and possibly pay return shipping, but that is all. (It sounds like the seller did make some such offer.) Manufacturers are not likely to pay for your time and effort. A few Astromart sellers are probably nice enough to do that, but it I don't think it is "required."

If you had some material cost to repair the scope, you could ask the seller to cover it. But if its just cotton balls and rubbing alcohol to clean the primary, then I'd say its not worth it. (If the primary mirror were actually broken, you could ask for funds to replace it, but that is not the case here.)

Ultimately you can leave a rating for the seller. Some accurate but brief description of the events might be in order.

I am impreseed that Vixen spent that much time with you on the phone! That is the good news in all of this. Customer service is alive and well, and living at Vixen it seems. Kudos to Vixen!