Bad C11 optics? HELP!

THE PROBLEM:

I've got an old used black-tube C11 with "Starbright" coatings that appears to me to have REALLY bad optics.

Maybe I'm missing something, tho, so I'm asking for some EXPERT advice.

Please note that the questions I ask below are really only directed at you IF you actually KNOW the answers, or have some firm basis for offering your opinion.

I'm not too interested in wild guesses etc. What I'd really appreciate are reliable thoughtful answers from knowledgeable individuals who actually really know what they are talking about wrt non-obvious causes of poor optical performance, and/or who know a LOT about C11s.

(sorry if that sounded a bit rude. Didn't it mean to. I was just trying to be very clear about what I'm looking for here.)


THE SYMPTOMS:

The views this C11 produces are absolutely WORTHLESS. Simply atrocious. Not even in the ballpark.

Regardless of power, all the way down to 77x (24mm PAN w/0.66 reducer), EVERYTHING has a fuzzy/spikey colorless glowing edge. Stars, Jupiter, The Moon, everything. In-focus stars are little, round, fuzzy/spikey blobs. Uggh.

At higher powers this fuzzy/spikey edge grows until at a mere 280x (10mm Radian), Jupiter looks like a fuzzy white cotton ball. No detail at all. Just a fuzzy blob. Horrendous.

Also, there's LOTS OF COLOR in the INTRAfocal out-of-focus diffraction pattern (lots of orange with some blues & reds near the inner and outer rings). NO color at all on the EXTRAfocal side. This is the case regardless which eyepiece, reducer, or barlow is used.

INTRAfocal diffraction rings are a brightly colored fuzzy/spikey very LOW CONTRAST mess with 5-6 very fuzzy rings showing, but on the EXTRAfocal side, the rings are tight, contrasty & colorless.

There is no astigmatism apparent.

I'm not yet an expert at interpreting star test rings, so I can't tell you anything reliable about the degree of correction, zones, smoothness or other aberrations based on what I see in the eyepiece. All I can tell you is what I see described above.


CONDITIONS:

I'm using Radians & Panoptics with a Denkmeier power switch diagonal.

The OTA is perfectly collimated (per star test, concentric rings and all that).

All the optical components (primary, secondary & corrector) are all rotationally aligned with their alignment marks at "3 o'clock" as I understand they should be.

I even tweaked the primary alignment so that the primary focal point is dead on the optical axis of the primary baffle tube and is hitting the secondary dead center. Everything in this scope is aligned as perfectly as I can make it, and I'm quite capable of lining up optical components with high accuracy.

No tube currents were in evidence. The exterior of the OTA cylinder is insulated with silvered plastic bubble wrap insulation to reduce cooling issues (works great! Buy the bubble wrap at any Lowe's, it's in the insulation dept. Looks good on the OTA, too!)

The seeing was not perfect, but my Intes M703 was tack sharp under the same atmospheric conditions, using the same diagonal & eyepieces... and bubble wrap. ;]


QUESTIONS:

Is it possible for a REVERSED CORRECTOR to cause these problems? By "reversed" here, I mean "with the wrong side of the corrector is facing the stars." The scope was used when I bought it, so who knows what somebody might have done to it.

Is it really possible for a production C11 that passed Celestron QA 10-15 years ago or so to have optics that produce images this lousy? I find it hard to believe.

Shouldn't a typical C11 perform VERY noticably better than a high quality 7" Mak-Cass on Jupiter under the same 7-8 out of 10 seeing conditions?

What are the odds that a visit to Celestron could eliminate the "fuzzy" blur in the images and turn this scope into fine, sharp instrument (short of replacing all the optical components, that is)?

Do you have any other suggestions?


SUMMARY:

I don't know what to do. This is the only C11 I've ever looked thru. I can't believe they are all this bad. The only scope I can compare it to is my beloved Intes M703, and the C11 is optically not even in the same galaxy as the M703.

I'd love to turn this scope into a nice 11" light bucket, but the images need to be SHARP or what's the point?

I'm considering sending it to Celestron for a tune up, but after speaking to them, I did not have any confidence that they would be able to fix what's wrong with it.

On the phone, Celestron (even after getting someone from their optical shop involved) could not tell me with any certainty at all what could even be the most likely causes of all the "fuzz" or even the COLOR in the INTRAfocal diffraction rings. They just seemed really uncertain. I'm afraid that I'll spend $200 on shipping, then another $300 or so for a "tune up", and still have a really bad C11.

If you made it this far, thanks indeed for your attention!
Any help will be greatly appreciated.

Cheers!
John Coffey

THE PROBLEM:
I've got an old used black-tube C11 with "Starbright" coatings that appears to me to have REALLY bad optics.
Maybe I'm missing something, tho, so I'm asking for some EXPERT advice.
THE SYMPTOMS:
The views this C11 produces are absolutely WORTHLESS. Simply atrocious. Not even in the ballpark.
Regardless of power, all the way down to 77x (24mm PAN w/0.66 reducer), EVERYTHING has a fuzzy/spikey colorless glowing edge. Stars, Jupiter, The Moon, everything. In-focus stars are little, round, fuzzy/spikey blobs. Uggh.
At higher powers this fuzzy/spikey edge grows until at a mere 280x (10mm Radian), Jupiter looks like a fuzzy white cotton ball. No detail at all. Just a fuzzy blob. Horrendous.

If the patterns are very stable from one second to another, the problem would seem to be
in the optics. If the spikey/color patterns change from one second to
another, that might be seeing. Although thermal problems like warm air pooling
in the tube can make long-lived spike patterns. How stable was the outdoor temperature
-- was it 1 degree F change per hour or less -- thats what you need for really good seeing
and minimal thermal problems. Also if the different optics are at different temperatures,
you can get bad spherical errors.

Also, there's LOTS OF COLOR in the INTRAfocal out-of-focus diffraction pattern (lots of orange with some blues & reds near the inner and outer rings). NO color at all on the EXTRAfocal side. This is the case regardless which eyepiece, reducer, or barlow is used.

A small amount of color is normal in SCTs. Seeing effects can make it seem worse.

INTRAfocal diffraction rings are a brightly colored fuzzy/spikey very LOW CONTRAST mess with 5-6 very fuzzy rings showing, but on the EXTRAfocal side, the rings are tight, contrasty & colorless.

This sounds like a little bit of spherical error. You can get that if the
primary is still cooling down. The primary/secondary/corrector plate all cool
at different rates. If outside temperatures are falling quickly, the primary is never
going to catch up to the corrector plate and secondary.

There is no astigmatism apparent.
I'm not yet an expert at interpreting star test rings, so I can't tell you anything reliable about the degree of correction, zones, smoothness or other aberrations based on what I see in the eyepiece. All I can tell you is what I see described above.
CONDITIONS:
I'm using Radians & Panoptics with a Denkmeier power switch diagonal.

Try testing without the power switch. Test straight through if possible,
or use a simple mirror diagonal of known excellent quality. I've seen some
Denk reducer lenses that were pretty messed-up; very bad turn-edge that made
colorful spikes and halos on everything.

The OTA is perfectly collimated (per star test, concentric rings and all that).

The fact you are seeing concentric rings is good.

No tube currents were in evidence. The exterior of the OTA cylinder is insulated with silvered plastic bubble wrap insulation to reduce cooling issues (works great! Buy the bubble wrap at any Lowe's, it's in the insulation dept. Looks good on the OTA, too!)

The insulation may be a mixed blessing. I mean the optics -- big primary mirror --
needs to cool somehow. Insulation will not speed this process. It can do some
cooling through the rear casting, but most of the tube area is in the side walls
-- thats where most of the cooling would occur. You might not have air currents in
the tube, but the primary is still too warm.

The seeing was not perfect, but my Intes M703 was tack sharp under the same atmospheric conditions, using the same diagonal & eyepieces... and bubble wrap. ;]

This may not mean anything. The C11 will need much better seeing conditions
to make good images -- just because the aperture is much larger.

Is it possible for a REVERSED CORRECTOR to cause these problems? By "reversed" here, I mean "with the wrong side of the corrector is facing the stars." The scope was used when I bought it, so who knows what somebody might have done to it.

A reversed corrector will have almost no effect.

Shouldn't a typical C11 perform VERY noticably better than a high quality 7" Mak-Cass on Jupiter under the same 7-8 out of 10 seeing conditions?

Not necessarily. The C11 will need much better seeing conditions to
produce good images. Thermal properties are different, too.

On the phone, Celestron (even after getting someone from their optical shop involved) could not tell me with any certainty at all what could even be the most likely causes of all the "fuzz" or even the COLOR in the INTRAfocal diffraction rings. They just seemed really uncertain. I'm afraid that I'll spend $200 on shipping, then another $300 or so for a "tune up", and still have a really bad C11.

In fairness to Celestron, I can't tell from your descriptions exactly what the problem is either.
I've seen some SCTs that were dead perfect on an optical bench in stable lab conditions, and
just horrible outside on real stars. Thermal issues can make a real mess of good optics.
SCTs are especially bad since you've got a big (warm) mirror at the bottom and the tube is all
closed up -- can't cool very efficiently.

John Coffey said:

THE PROBLEM:

I've got an old used black-tube C11 with "Starbright" coatings that appears to me to have REALLY bad optics.

Hard to say without taking a look through the scope, but this mostly sounds like seeing/cooldown issues.

All of these SCT companies produce lemons from time to time. Maybe the owner broke the real corrector and replaced it with a piece of plate glass 8O . My experience with a 14" Meade earlier this year taught me a lot about these larger SCTs. They all have noticeable color, cool down takes a long time, and in my sample really poor (0.7 wave PTV by Roddier tests) optics made it out the door of Meade's brand new factory. To their credit they replaced the OTA and the new one has a 0.15 wave PTV optics set. Seeing affects these apertures more than smaller scopes and you just have to live with that fact.

It sounds like you did everything you could to set up the scope. Get a second opinion and/or run the Roddier test. If the optics are a dud, the fix is not going to be cheap.

I am not experienced enough to offer any suggestions but I can tell you I have an older C11 also with starbright coating......the grey model and have never seen seeing/cool down issues that created symptoms as bad as you describe. Even with no cool down time and stars twinkling to beat the band I am still able to distingush at least 2 bands on jupiter and usually more. Based on your description I am inclined to think there is something wrong but my C11 is the only SCT I have any experience on so I will leave the diagnosis to the experts.

y to tell if the optics are bad is to do a Roddier test. Easy to do, fast, simple and effective. Will tell you exactly what the P-V, RMS and Strehl is.

Rolando

One of the issues not mentioned is that you may have some kind of secondary or even primary mirror flop. You can have the star collimated in one part of the sky but when you swing to Jupiter the gravity shifts. So when you say the scope is collimated with concentric stars (a very positive sign) do a check on Jupiter's moons (throw them out of focus like you would a star and see whether you get a good donut: the moon must be in the center of the field).

Your fuzzy Jupiter sounds exactly like what I was seeing with extremely bad collimation. In that circumstance, stars appear flared.

Secondary mirror flop is usually due to some looseness in the screws or the ring that attaches the secondary housing to the corrector.

Primary mirror flop is harder to deal with.

An optical defect that is found in your generation of C11 is called "print through." Put the scope on a very bright star and throw it out of focus on either side at high power (say, a 5 to 8 mm eyepiece). If you see 8 spikes in four pairs of two (each pair is parallel) then you have print through and that is in fact a dog meat SCT. Production practices have since been changed to eliminate this defect.

good luck

Greg N

Your description of the diffraction pattern is classic severe spherical aberration, as in a scope with a fast spherical mirror = an SCT without the corrector geometry. I would seriously investigate the corrector plate for authenticity.

-drl