Date: Fri, 22 Sep 2006 17:02:09 -0400 (EDT)
From: jeg@astro.princeton.edu
Subject: Re: 2.5m obslog 20060921 (MJD 54000)
I take it that there was no problem with *b1* ? My first impression
was that something had taken the collimator mirror way out, but if
that were the case the problems would have shown up on b1 as well,
in almost exactly the same way.
So *if* there was no problem with b1, it was not the collimator.
The images are very comatic and out-of-focus. If the CCD had moved, there
would
 	a) have been a strong focus gradient across the chip
and
 	b) there would be essentially no coma.
So it was not the CCD. Much the same can be said for the dewar, though
the field flatteners move with the dewar, and it is conceivable that
the dewar is loose (the focus ring is not tight) and that it slipped with
a little tilt which introduced the coma. A long shot, but THE FOCUS
RING SHOULD BE CHECKED.
No movement of the central optics which is believable would have led
to this, and in any case, the impact would have been much greater in the
blue.
As far as I know, that exhausts the possibilities UNLESS the r1 triplet
was not reassembled correctly. Since that was the only thing optical which
was touched recently, it is not an unlikely candidate. There are springs
which are supposed to hold that assembly in place; as far as I know, the
assembly is done with all screws bottoming, but maybe they did not
get tightened??
I strongly suggest that a series of arcs be taken with the telescope
at low elevation (10 degrees or so) with the rotator at a series of angles
separated by about 45 degrees, so as to put gravity al all possible
orientations with respect to the axis of the camera to see if we can
reproduce the trouble.
--jim
On Fri, 22 Sep 2006, Craig Loomis wrote:
>
> In case anyone was wondering, there were no motion or unexpected commands
> sent to the controller between the nice looking 53999/40713 and the nasty
> 54000/40715. And the controller reports the same position for all the
> collimator actuators throughout.
>
> - craig
>
> On Sep 22, 2006, at 7:30 AM, observer@apo.nmsu.edu wrote:
>
>>
>> During checkout we had a problem with spectrograoh again. Post-calib gave
>> us
>> bad flat f=40715 and arc f=40716. SOS gave us alarm with low ngoodfiber=201
>> for r1 camera for flat, and arc reduction had been rejected at all.
>> Visually
>> the flat and arc of r1 camera were or out of focus or with fog or dust on
>> the
>> surfaces. We repeated arc observations with the same result.
>> French and Steph checked the r1 triplet and CCD by removing r1, but did not
>> find any
>> problem with optics. Our next observations was bad as we lost the
>> connections,
>> and sos could not reduce the data, but visually we saw very good spectra on
>> the displays. We repeted postcal with flat=40720, and arc=40721, and the
>> results were very good. After that I checked and adjusted collimation for
>> 8.5C.
>>
>> Apparently removing and replacing r1 somehow fixed the problem, but we have
>> no
>> idea how. We can only speculate that either a lens or the CCD shifted, and
>> the
>> illumination pattern looks like half an out-of-focus donut. Is this a tilt?
>> If
>> so, it must be a lens tilt, because the half-donuts are the same size over
>> all
>> the CCD.  A before and after arc and flat have been saved to
>> ~snedden on sdsshost2. They are called before_arc.fit, after_arc.fit,
>> before_flat.fit and after_flat.fit. We've never seen anything like it.
>
>
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