You should first determine where the differential movement is (listed in
order of likelyhood):

8" primary mirror
ETX primary mirror
Flexure in ETX mounting bracket
Flexure in ST4-to-ETX connection
Flexure in Camera-to-LX200 connection

Do you get significant mirror slop when focussing in-and-out on a star?
- if so, a re-lube of the mirror slider tube should reduce it to
acceptable levels.
Is there significant backlash in the focussing mechanism? - If there is
then the mirror will be free to move forwards/backwards to the extent of
the backlash. Take the focusser mechanism apart and correct it (see
Doc's page).
ETX primary - I don't have one of these to play with, but the likely
problem and solution are as with the LX200. There may perhaps be a way
of locking the mirror down after focussing.
I don't know what method you use for mounting the ETX, but you need to
ensure it doesn't move *at all* relative to the LX200 main tube.
The other connections should ideally be all-screwed connectors, 1-1/4"
push-fit with a single locking screw is OK for eyepieces but just
doesn't cut it for mounting cameras.
Field rotation due to a 1-1/2 degree offset of the guidescope is not a
problem (though you don't say whether your LX200 is wedge mounted or
ALT-AZ?).

Chris


>----------
>Sent: 09 June 1998 03:22
>Subject: [M]: Mirror Shift?? Tracking errors? HELP!!
>
>What do you do if you suspect you have mirror shift in an 8 inch LX200
>that is causing tracking errors in photographs? I use an ETX spotting
>scope as a guide scope and track with the ST-4. The ST-4 is telling me
>my tracking is excellent, the focal length of my guide scope and the
>LX200 are the same (I use an f6.3 focal reducer), I am using a 1 second
>exposure with the ST-4, and I am still getting streaked stars. Thus, one
>strong possibility is that I am getting mirror shift in either my LX200
>or my guide scope! Is there a way to tighten the mirror myself (easily)
>or do I have to send it back to Meade? Also, what is the likelihood
>that the mirror shift is occurring with the EXT instead of the LX200?
>Does anyone have any information, advice or opinions on this problem?
>Does anyone know if the LX200 with the ETX as a guide scope can cause
>what SBIG calls "differential deflection"? The only other possibility
>is that I am getting field rotation. For my last shot (which resulted
>in noticably streaked stars), I guided on a star about 1.5 degrees
>off-axis (and, no, that star was not within the field of view of my
>final photograph) and my polar alignment caused drift from the center of
>the box on a barlowed 12mm reticle (magnification was 210X) to the
>northern border of the box in a 5 minute period. Thus, my alignment was
>not perfect but neither was it totally terrible. Does anyone know if
>such an alignment error and extent of off-axis guiding can caused
>noticable star streaks in a 60 minute guided photograph? Help me
>please-- I'm losing my mind trying to figure this one out.
>
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