Unexplained transient abberation in C 9-1/2 Celestron SCT

Greets, brothers and sisters!

At Saturday's star party (elev. 2900, temp:95f rh:33%), around midnight,
I tried doing a star test on a recently acquired 9.25" Celestron. Seeing
wasn't wonderful, but my intent was to try a collimation check, in the
roughest sense.
I placed an 8mm radian directly into the visual back, focused on Polaris,
which was boiling just a bit (I could see the companion star, though).
Next, I turned the focuser clockwise about 1/3 turn. Classic donut
shaped image appeared, with secondary shadow centered. So far OK. Then
back to focus, and continuing through, to about 1/3 turn counter
clockwise, and I couldn't believe my eyes...the central obstruction
shadow, which was tiny, continued to grow with the star image as I turned
the knob. At 1/2-1/3 turn from focus, the image was bizarre. It looked
like a black disk, about 1/4 the diameter of the field of view,
surrounded by a single undulating ring of light. Reminded me of an
artist's conception of a black hole and accretion disk. No donut, just
the hole and a ring of light dancing around it. Hardly a symmetrical star
test. I freaked. Heh heh.
I called a few other observers over to check, and they remarked they'd
never seen anything like it.
I repeated the procedure with a 14mm radian, and a barlowed (2x) 14mm
radian. Same results. By now, I'm thinking there is something terribly
wrong with the scope. My fellow observers, one armed with a C-8 of
excellent quality, took the same EP and tried the same test on his scope
and had the expected results--no black hole and ring. Some remarked that
my out of focus image resembled the ring nebula (M 57).One last thing, as
I continued de-focusing past the "black hole" the familiar donut re-appeared,
but not at the same
distance from focus--it took another 1/2-3/4 turn of the focuser to get
it back. Some other details--the scope had been out in the open air for
3-4 hours, since about 8pm, so it was at ambient. There is nothing
loose (mirror, secondary, corrector plate, etc) in the OTA. There is no
apparent pinching of the tube, although I wouldn't be able to detect any
differential expansion stress between the OTA and the Losmandy dovetail
plate that was attached to it, but perhaps, owing to the unusually high
temperatures, differential expansion may have been an issue.

So all day Sunday, I'm thinking my scope has hosed optics, but on Sunday
night, I tried the test again--same mount, same star roughly the same
temperature--maybe 5F cooler, same EP and barlow. And the black hole
was gone. In its place, a normal donut. WTF!! I tried it on Vega, just
to be sure, and the star test was normal. Seeing was a little better
than the previous night, too, but I can't feature seeing causing the
strange results of the previous night. Further, the star test indicated
normal correction (perhaps a tiny bit of an edge, but negligible).

So what do you think caused this strange star test? I am at a total loss
to account for it. I wasn't the only one to see it, either.
Thanks for reading this too-long post, but I'd really like to get some
idea from the SCT experts in the group as to possible causes of the
transient abberation. Looked to be grossly over/under corrected.

Uncle Bob wrote:
At Saturday's star party (elev. 2900, temp:95f rh:33%), around midnight,
I tried doing a star test on a recently acquired 9.25" Celestron. Seeing
wasn't wonderful, but my intent was to try a collimation check, in the
roughest sense.
I placed an 8mm radian directly into the visual back, focused on Polaris,
which was boiling just a bit (I could see the companion star, though).
Next, I turned the focuser clockwise about 1/3 turn. Classic donut
shaped image appeared, with secondary shadow centered. So far OK. Then
back to focus, and continuing through, to about 1/3 turn counter
clockwise, and I couldn't believe my eyes...the central obstruction
shadow, which was tiny, continued to grow with the star image as I turned
the knob. At 1/2-1/3 turn from focus, the image was bizarre. It looked
like a black disk, about 1/4 the diameter of the field of view,
surrounded by a single undulating ring of light. Reminded me of an
artist's conception of a black hole and accretion disk. No donut, just
the hole and a ring of light dancing around it. Hardly a symmetrical star
test. I freaked. Heh heh.

I'm having trouble visualizing this. Clearly, the undulating is due to
the seeing, but you're not concerned about that, I gather. So it must
be the rest of it.

You mention a black disc, but obviously, it's no blacker than the rest
of the background, right? It's just an absence of light. So am I right
in guessing that what you saw on one side of focus was a single ring of
light? On the other hand, that doesn't resemble M57 in the slightest;
M57 looks an oval with a dimming in the middle--but it certainly isn't
black there. So please clarify.

If the image is filling even 10 percent the width of the field at 300x
(that's an actual angular size of a bit over one arcminute), you've gone
too far for star testing anyway. I'm wondering whether you had humidity
issues. A 95-degree night with 33 percent relative humidity is pretty
muggy. Did you check the condition of your corrector plate? How humid
was it the following night--Sunday? Mind you, I'm not sure how this
would cause what you saw, exactly, but there's only so many things that
could be there one night and gone the next.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

Uncle Bob wrote:
Well, the background isn't really black. There are effects from the SFO
light dome. But the shadow of the CO is black.

The shadow can't actually be blacker than the background. There may be
contrast effects that make it appear as though it's blacker, but it
can't actually be blacker, because defocused light from the background
is mixing in with the shadow.

Do you remember whether the ring was inside focus or outside it? Or,
equivalently, which way on the focus ring is toward infinity? Clockwise
or counter-clockwise?

The second night it was a bit less muggy, though I left my psychrometer at
home. I was relieved to find the abberation had disappeared. I'm
wondering if differential expansion of the dovetail plate could have
stressed the OTA.

It could have, though I don't see how that would have created that
effect, either. Whatever aberration it caused, and the effects that it
resulted in, should not have been radially symmetric, and then we could
see whether or not the effect rotated with the eyepiece/observer's eye.

Did you have to refocus substantially at any time between the two tests?

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

Uncle Bob wrote:
Do you remember whether the ring was inside focus or outside it? Or,
equivalently, which way on the focus ring is toward infinity? Clockwise
or counter-clockwise?

CCW is toward infinity. The weird effect was, as I recall, CCW from focus.

OK, so it's like a kind of super SA. Could there have been any stress
on the corrector plate? Bowed outward perhaps? (It might not take
much.)

Did you have to refocus substantially at any time between the two tests?

No, I used the same configuration (8mm/14mm directly into the visual back).

No star diagonal?

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

On Mon, 24 Jul 2006 08:16:43 -0700, Uncle Bob wrote:

At Saturday's star party (elev. 2900, temp:95f rh:33%), around midnight,
I tried doing a star test on a recently acquired 9.25" Celestron. Seeing
wasn't wonderful, but my intent was to try a collimation check, in the
roughest sense.
I placed an 8mm radian directly into the visual back, focused on Polaris,
which was boiling just a bit (I could see the companion star, though).
Next, I turned the focuser clockwise about 1/3 turn. Classic donut
shaped image appeared, with secondary shadow centered. So far OK. Then
back to focus, and continuing through, to about 1/3 turn counter
clockwise, and I couldn't believe my eyes...the central obstruction
shadow, which was tiny, continued to grow with the star image as I turned
the knob. At 1/2-1/3 turn from focus, the image was bizarre. It looked
like a black disk, about 1/4 the diameter of the field of view,
surrounded by a single undulating ring of light.

I dunno, but this sounds like "normal" when you are defocusing way
out, with a scope that has a lot of magnification, using a high power
eyepiece.

Look at this article on our website:
http://home.earthlink.net/~steve_wal...tal/webcam.htm

Halfway down there is a picture that I took of my husband getting set
up to do an image with his C-11 (on the right) and of the result shown
in the computer screen (on the left) when he was racking thru the
extremes of travel while focusing the scope on Mars. He had to go
way out of focus for the dim image of the "donut" to show up in the
photograph.

To find the picture, if it is not immediately apparent, search the
article text for

As explained above, my "control point" is about 15 feet away

which is the start of the next paragraph right below the picture.

If you are looking at a really bright star and defocus a lot, you will
get a similar "ring". You will also see this using fairly low power.
When you use very high power, and defocus just a LITTLE bit, you will
see the concentric rings with sharp central peak that are useful for
star testing and alignment.

If this is not what you are describing -- then, apologies for wasting
everybody's time!

Regina R.

On Mon, 24 Jul 2006 12:00:59 -0700, Brian Tung wrote:

Uncle Bob wrote:
Do you remember whether the ring was inside focus or outside it? Or,
equivalently, which way on the focus ring is toward infinity? Clockwise
or counter-clockwise?

CCW is toward infinity. The weird effect was, as I recall, CCW from focus.

OK, so it's like a kind of super SA.

Yes indeed. I think it resembled a super-dooper SA. Which is why I was
considering sending it back to Celestron. But when it isn't reproducable,
that's a problem.

Could there have been any stress
on the corrector plate? Bowed outward perhaps? (It might not take
much.)

I can't suggest any strain mechanism that wouldn't have been there on both
nights, except, perhaps, a slightly higher temperature (~10-15F).


Did you have to refocus substantially at any time between the two tests?

No, I used the same configuration (8mm/14mm directly into the visual back).

No star diagonal?

No, I'll be using the scope for imaging, so collimation at the visual back
is probably going to be closer to the mark that through a diagonal, at
least, that is my belief.

Regards
Ubcle Bob

On Mon, 24 Jul 2006 19:43:56 +0000, Regina Roper wrote:

On Mon, 24 Jul 2006 08:16:43 -0700, Uncle Bob wrote:

At Saturday's star party (elev. 2900, temp:95f rh:33%), around midnight,
I tried doing a star test on a recently acquired 9.25" Celestron. Seeing
wasn't wonderful, but my intent was to try a collimation check, in the
roughest sense.
I placed an 8mm radian directly into the visual back, focused on Polaris,
which was boiling just a bit (I could see the companion star, though).
Next, I turned the focuser clockwise about 1/3 turn. Classic donut
shaped image appeared, with secondary shadow centered. So far OK. Then
back to focus, and continuing through, to about 1/3 turn counter
clockwise, and I couldn't believe my eyes...the central obstruction
shadow, which was tiny, continued to grow with the star image as I turned
the knob. At 1/2-1/3 turn from focus, the image was bizarre. It looked
like a black disk, about 1/4 the diameter of the field of view,
surrounded by a single undulating ring of light.

I dunno, but this sounds like "normal" when you are defocusing way
out, with a scope that has a lot of magnification, using a high power
eyepiece.

Look at this article on our website:
http://home.earthlink.net/~steve_wal...tal/webcam.htm
Halfway down there is a picture that I took of my husband getting set
up to do an image with his C-11 (on the right) and of the result shown
in the computer screen (on the left) when he was racking thru the
extremes of travel while focusing the scope on Mars. He had to go
way out of focus for the dim image of the "donut" to show up in the
photograph.

To find the picture, if it is not immediately apparent, search the
article text for

As explained above, my "control point" is about 15 feet away

which is the start of the next paragraph right below the picture.

If you are looking at a really bright star and defocus a lot, you will
get a similar "ring". You will also see this using fairly low power.
When you use very high power, and defocus just a LITTLE bit, you will
see the concentric rings with sharp central peak that are useful for
star testing and alignment.


Yes, thanks. What we were seeing was different from the normal donut
image a defocused star presents. Imagine the donut image, with the hole
intact, but with the donut (dim starlight) replaced by a single
slender ring. Very strange indeed.
Uncle Bob

Maybe instead of Polaris you WERE looking at M57.

It could happen.

On Tue, 25 Jul 2006 10:12:42 -0700, Gil wrote:

Maybe instead of Polaris you WERE looking at M57.

It could happen.

That's true. I wasn't using a GOTO mount. ;-)

Uncle Bob

What we were seeing was different from the normal donut
image a defocused star presents. Imagine the donut image, with the hole
intact, but with the donut (dim starlight) replaced by a single
slender ring. Very strange indeed.

Srw "on duty" now, at RR's laptop. She missed your point, I guess;
but it seems to me that since virtually any EP has certain internal
reflections, you had a sort of magical confluence of those when the
star image was in a position in the field where a reflection became
visible, and the interference pattern, with the image defocused,
created the ring. If I use fairly lowish power with my C-11 (which
has a significant central obstruction, as your C-9 does) and just
start to defocus a star field, each star suddenly turns into a "sharp
undulating ring" and then, the sharp rings grow into donuts. With
Vega maybe all the overpowering light just wiped out your discernment,
or perhaps you had not lined things up exactly the same way, and the
reflection(s) either did not occur, the star not positioned in the
field *exactly* comparably to your situation with Polaris, or the sign
of the interference pattern was different, causing less of the effect.
I am not making myself entirely clear as I'm not an optics designer
and I am only "vamping", but I am sure that others here, or on a Yahoo
optical experts group, could weigh in if my suggestion is pure
nonsense...

Srw