Date: Sat, 27 Aug 2005 22:02:06 -0400 (EDT)
From: jegunn
Subject: Re: 2.5m obslog 20050826 (MJD 53609)
Sorry, Dan, I fired off my reply before getting to yours. Do you
remember the readhead moving before? You and I argued for a long time
about a -0.01 degree offset on the rotator, but if things had ever
changed by .14 degrees in the past we would have certainly screamed.
????
thanks
--jim
On Sat, 27 Aug 2005, Dan Long wrote:
> Hi Steph,
>
>     I recall when the altitude read head was modified, getting the
> axis to index properly was a bit tricky.  I think there was some
> extra step involved, like resetting the canonical fiducial.  Since
> the rotator is a complete tape, I don't know if the rotator even has
> a canonical fiducial.
>
>     In any case, I don't believe you should have to take another
> pointing model if all you do is change the rotator fiducial table.
> I'm pretty sure the model doesn't even consider rotation.  That would
> be handled by the instrument block.  The only effect of a rotator
> error would be to introduce an error in the correction proportional
> to the angular error (and .1 degrees for the small corrections
> applied in the model would I think be insignificant).
>
>     If you choose not to take more rotator fiducial data, you will
> need to adjust the instrument blocks of the imager and spectrographs
> to compensate.  rot_inst_ang would need change by .14 degrees (not
> sure about sign).  Probably the simplest way to get the exact amount
> would be to mount the imager with cleared offsets, let skippy or
> ltmatch give you the rotator error, then use inst_edit to
> interactively change rot_inst_ang while tracking.  If you get the
> sign right, the error should go away.  If wrong, the error should
> double.
>
>
>    Once you get rot_inst_ang where you want it, make sure you edit the
> instrument blocks to make the change permanent (it should be the same
> change for both imager and spectrographs).
>
>     Note that you would still have to do instrument changes at -.14
> degrees mount to get things to line up, until the fiducial table is
> corrected.
>
> 				Cheers,
>
> 				  Dan
>
>
> >Thanks for the fixes. A few questions (hopefuly not too dumb).
> >
> >  What would cause this offset after Wendell took rotator fiducial
> >data? If we took new data, why wouldn't we get the same offset? It
> >sounds like the read head has moved, and there's no point in getting
> >new fiducial crossings in that case.
> >
> >Doesn't this mean the instruments will be off by this amount in the
> >rotator when we go to a field? Won't we have to correct this each
> >time?
> >
> >
> >On Aug 27, 2005, at 1:54 PM, Jon Brinkmann wrote:
> >>
> >>Steve and I investigated the problems mounting the imager.  We found
> >>and corrected two items:
> >>
> >>1)    The physical home position of the rotator is at about -0.14
> >>     degrees as shown on the MCP.  This was determined by checking
> >>     the pointer on the east side of the PSS against the mark on the
> >>     rotator.  We're not sure how this occured, but there are two
> >>     solutions:
> >>
> >>     1)    Move the rotator to -0.14 degrees (instead of 0.0)
> >>         for instrument change.
> >>
> >>     2)    Take new rotator fiducual data, generate a new table,
> >>         and obtain a new pointing model.
> >>
> >>     Of the two, I prefer the first, at least until next bright time
> >>     when we will take new fiducial data on all the axes.  The
> >>     second solution would consume precious time on the sky.
> >>
> >>2)    The east saddle locating pin was mis-positioned.  We loosened
> >>     the pin, mounted the imager, and centered the pin.
> >>
> >>After correcting these items, we mounted the imager three times with no
> >>problems.
> >>
> >>Steve & Jon
>
>
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