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  1. #1
    Uncle Bob's Avatar
    Uncle Bob Guest

    Default 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.



  2. #2
    Brian Tung's Avatar
    Brian Tung Guest

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

    Uncle Bob wrote:

    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 <brian@isi.edu>
    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

  3. #3
    Uncle Bob's Avatar
    Uncle Bob Guest

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

    On Mon, 24 Jul 2006 09:49:53 -0700, Brian Tung wrote:



    I know, it's strange. Imagine the central obstruction shadow with a single
    ring of light around it. No concentric rings other than that single skinny
    (about the thickness of the usual edge ring in a normal de-focused star
    image.
    The other thing is the CO shadow appeared bigger on one side than the
    other side of focus, even though we were pretty close to the same distance
    from focus.



    Well, the background isn't really black. There are effects from the SFO
    light dome. But the shadow of the CO is black. The resemblance to M57
    was a metaphor offered by another observer. The actual appearance of the
    disk/ring was noted by a number of experienced observers and they were as
    puzzled as I was. I have star tested several telescopes under varying
    conditions and never have I seen this effect, nor had anyone I was able to
    coax to the eyepiece. It was pretty weird. It made me do an equipment
    check, then a sanity check, then have others verify it (and they did).

    Corrector plate was clear. Dewpoint was very low and we never got near
    that.
    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.
    Regards,
    Uncle Bob


  4. #4
    Brian Tung's Avatar
    Brian Tung Guest

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

    Uncle Bob wrote:

    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?


    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 <brian@isi.edu>
    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

  5. #5
    Uncle Bob's Avatar
    Uncle Bob Guest

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

    On Mon, 24 Jul 2006 11:34:48 -0700, Brian Tung wrote:


    OK. I'm buying that!

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



    The effect was radially symmetric. It didn't occur to me to rotate the
    EP, but id did occur to try without barlow, with barlow, 8mm and 14mm
    radians, and to try the same EPs/Barlow on another scope (C8) which showed
    none of the effect.


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

    Regards,
    Uncle Bob



  6. #6
    Brian Tung's Avatar
    Brian Tung Guest

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

    Uncle Bob wrote:

    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.)


    No star diagonal?

    --
    Brian Tung <brian@isi.edu>
    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

  7. #7
    Regina Roper's Avatar
    Regina Roper Guest

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

    On Mon, 24 Jul 2006 08:16:43 -0700, Uncle Bob <here@home.org> wrote:


    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.



  8. #8
    Uncle Bob's Avatar
    Uncle Bob Guest

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

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


    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

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



    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


  9. #9
    Uncle Bob's Avatar
    Uncle Bob Guest

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

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


    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


  10. #10
    Gil's Avatar
    Gil Guest

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

    Maybe instead of Polaris you WERE looking at M57.

    It could happen.


 

 
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