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Date: 24 Jul 2006 08:16:43
From: Uncle Bob
Subject: Unexplained transient abberation in C 9-1/2 Celestron SCT
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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.
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Date: 24 Jul 2006 17:41:18
From: Wade A. Hilmo
Subject: Re: Observing Barnard's Galaxy (NGC6822)
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I saw Barnard's Galaxy at OSP last year.
The observer in the camp site next to mine was trying to confirm that he had
found it and wanted me to take a look through his 18" dob. To say that the
surface brightness was low is a bit of an understatement. OSP's skies are
pretty dark, and this object was visible only as a large, subtle area where
the background wasn't quite as dark. It was completely featureless, except
for its general shape. We spent a few minutes discussing where we thought
it was in relation to the star field before we were convinced that we were
both seeing the same object (or even that were seeing an object at all.)
I also looked at it through my 14" scope and it was similarly hard to
detect. I've read that it can be seen in small instruments or even
binoculars, but I don't think that my eyes could do it. I'm going to OSP
again next month and I'll give it a try.
-Wade
<tony_flanders@yahoo.com > wrote in message
news:1153751683.974253.274530@i3g2000cwc.googlegroups.com...
> Dennis Woos wrote:
>
> > Anybody have any reports/tips on observing Barnard's Galaxy? ...
> > I now read that this is a famous object, and that it can be challenging
> > to observe because of it's low surface brightness.
>
> What you said -- it can be challenging to observe because of its low
> surface brightness. The fact that it's pretty low in the sky as seen
> from
> most of the U.S. doesn't help either. Don't even *think* about
> observing
> it from the suburbs. It's possible under semirural skies (mag 21.0 per
> square arcsec on the Sky Quality Meter) but *much* easier under truly
> dark skies, where it stands out surprisingly well in an 8-inch scope.
> Some people have reported seeing it in binoculars, but I've never
> succeeded with anything smaller than 4" aperture.
>
> As for the object itself, it's large (about 15'), amorphous, and
> utterly
> lacking any bright concentration. Stands out best at around 100X,
> in my experience.
>
> - Tony Flanders
>
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