[APML] Tube flexure - star trails Help?

["Rick Kellogg" <rkellog1@twcny.rr.com>]

All,

I have switched mounts and I am having trouble with star trails and I am
trying to isolate what the trouble is.

My setup has a Televue 101 (540 mm FL) attached to a GEM via Losmandy DUP
plate and Parallax rings, on top of the rings is another DUP, forming a
rigid structure.  Connected to the top DUP plate (perpendicular to the
scope) is another DUP plate to which I attach wide field cameras and a guide
scope.  I have a Nikon F3 connected to the TV101.   For guiding, I use the
SBIG STV connected to a Nikkor 80-200 mm F2.8 lens, set at 200 mm.  For a
wide field camera I have a meduim format cammera with a 150 mm FL F4 lens.
The mount is rugged and can hold 50-70 lbs, and should have no trouble at
this load.

I took several 45 min shots with this setup, and found star trails on the
35mm  (540 mm FL) shot, but none on the medium format (150 mm FL) taken
simultaneously.  The STV showed less than perfect tracking, but not bad.
(always < 2 arc sec average, typically < 1).

Using a 15x Peak loupe with scale, I measured the trails as about 25 micron
long on the 35 film (All stars were trailled by the same amount and
direction). While I realize it the TV101 has 3 times the FL as the medium
format shot,  I believe I could have detected 8 micron long trails.  I shot
the medium format at F5.6 and could see diffraction spikes from the
diaphragm on the bright stars.

Both cameras were aligned with the RA axis of the mount parallel to the
frame.  The star trails were at about a 30 degree angle to the frame.  The
mount was looking at DEC angles from +30 to +40 degrees, and RA axis such
that the counterweight was around 20-35 degrees below horizontal.

The pictures were taken on 2 different nights yet both nights showed similar
trails.

At this point, I believe there is some movement of the TV-101 relative to
the perpendicular DUP plate, to which both the guide scope and Hasselblad
are solidly secured to.  But I can't figure out why the trails are about 30
degrees relative to the RA axis.

I would like comments and ideas on what you think the problem is, and how to
test for it.


Rick Kellogg




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