Sponsored By:

Subject: Re: PICTOR: 201XT Guiding Anomaly?
From: Howard C. Anderson
Reply To: mapug@shore.net
Date: Wed Jul 17 07:11:11 1996

>I continue to be very happy with the Pictor 201XT, though I've recently found
>that it does strange things under certain guiding conditions. Please could
>anyone with a 201XT / off-axis guider combination verify this and/or suggest
>a cure?
>
>The anomaly occurs when the OA guider has to be rotated more than a certain
>amount from the "centre position" in order to find a guide star. "Centre
>position" is when the OA guider tube is in vertical alignment with the top
>centre of the scope tube (where the piggy-back bracket fits). It seems to be
>OK up to about 45 deg either side of centre position, but much more than that
>(say 90 deg) and things start to go wrong.

I encountered something similar during one of my expeditions on 28 April 96
and reported what I had observed here on the list. The guide correction
slowly became (4,8) and stayed that way. The position of the off-axis
guider was in the vicinity of 90 degrees as you reported. The problem
was repeatable. I finally chose another star at another angle.

>
>An excellent example of this problem is when trying to guide on M51. With M51
>centred in the camera viewfinder, there are no potential guide stars anywhere
>near "centre position". I find I have to rotate the OA guider approx 90 deg
>anticlockwise to find a useable guide star. This star is very faint (mag 11,
>I suspect) so the Pictor needs an exposure time of 15 to get a brightness
>reading above 10 - however, it is fully useable.

On 18 May 96, I shot M51 using SAO 44642 (Magnitude 6.9) as the guide star.
Two hour shot guided perfectly. I do not recall the position of the off-axis
guider but it looks from MegaStar as if it should have been in the vicinity
of 90 degrees.

>
>Here's what happens: the Pictor calibrates itself based on this star
>correctly and without apparent problem. When guiding starts, everything seems
>fine also. But after a short while the tracking correction numbers start to
>increase. They start at 1 or 2 (normal) then gradually increase over about 30
>minutes until they reach 9. Shortly after that it bombs out with "GE" (Guide
>Error).

Mine only went to (4,8) then stayed there indefinitly. I also heard no DEC
corrections being made. That was what made me suspicious and led to my
aborting the shot after 10 minutes or so. I tried it again and had to abort
the second shot also. I was trying to shoot the dumbbell nebula at that
time. After choosing another guide star, I got a perfect 75 minute
Pictor 201XT autoguided shot.

>
>Here's the strange thing: during normal guiding you can hear guiding
>corrections being implemented by means of a subtle "clunk" noise from the Dec
>drive. While in this error condition, you hear no such noise - it seems that
>there are no Dec corrections being done (although maybe RA). At the end of
>the "mis-guiding" process, the guide star is still very close to its original
>position (this is with very accurate polar alignment).

This is what I observed also *except* that the guide star had drifted in the
field and the photos I got had star trails.

>
>This condition is totally repeatable - I've had it happen three times with
>M51 (different nights) and a few times with other targets (can't remember
>which). The error sequence is always identical. For M51, I had to end up
>doing a 40 minute hand-guided exposure - there was no other solution. On the
>other hand I always get perfect guiding providing I can find a star within 45
>deg or so of OA guider centre position.

Somehow it is possible to guide on M51 because I have the photos to prove it.
Wonder if it is date dependent?

>
>The summary seems to be that the 201XT cannot handle guiding when the OA
>guider is rotated by 90 deg (or thereabouts) from its centre position, even
>though calibration in this position is performed flawlessly (which is very
>strange).

My original posting to this list also speculated similarly. I wondered if
there were a mathematical discontinuity in the vicinity of 90 degrees.
Have not encountered the problem since April but I have been somewhat
conscious of the possibility of such a problem so I may be choosing guide
stars that do not involve 90 degree orientation. Why I was able to handle
M51 though is then a puzzle.

>
>What am I missing - can anyone shed any light or suggest a solution?

The only thing that might be a factor, and I just thought of it, is that
subsequent to the problems I observed, I went to a great deal of trouble
to try to eliminate DEC retrograde and backlash movements. I was rather
successful. I am wondering if Pictor 201XT calibration, done with DEC
motion anomolies, might result in the problems we observed...

>
>Dark skies, beautiful images
>
>Philip Perkins -- philip.perkins@pop3.hiway.co.uk
>51 27'N 1 36'W -- dark skies, dark clouds (mostly)
>
>

Thanks,

Howard