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Re: [APML]: Mirror Flop
> From: Rob Roy <royrg@mcmail.cis.mcmaster.ca>
> I have some 300mm shots with trails that I blamed on the front end
> heaviness of the lens itself. Now I'm beginning to wonder if might be this
> mirror shift problem since I used the main scope for guiding.
In my case (300mm lens, C-11 OTA) I originally thought it was mirror
flop and then found it was the camera body flexing (my 300mm lens
does NOT have a 1/4-20 tripod socket near the center of gravity, so
the rig was a large cantilever - just begging for flexure problems!),
so I built a plywood holder for the end of the 300mm lens. Now I
support the piggyback rig at both ends and the flexure problem is not
a factor.
I'm still glad I drilled/tapped three holes into the rear casting for
the C-11. Helps for more accurate drift alignment analysis.
> The 8" LX200 doesn't come with a shipping screw hole. Has anyone drilled
> and threaded a 1/4-20 hole in the back of one? If so, exactly where is the
> best location to do so? I've had the mirror out, but don't remember what's
> in behind it for support.
Much of the outer portion of the mirror is not supported, so if you
put the holes farther out than about 33% of the mirror's radius the
bolts will touch the mirror. In my case I used nylon threaded rod so
that even though they touch the mirror's back they won't scratch.
However, too much pressure puts astigmatism in the mirror, which
shows up at the eyepiece. Back off on the pressure and the image is
good once again.
NOTE: My approach did NOT remove all mirror flop. I'm not certain,
but my hunch is that there's enough slop (even a few thousandths of
an inch) between the baffle tube and mirror cell that the mirror can
still move side-to-side. My three bolts in the back of the mirror
just keep it from tilting/rocking, but don't constrain in any way the
possibility of side-to-side motion.
Tom Krajci
Capt Tom Krajci
B-52 Intelligence Officer
"There are only two possible outcomes in any war.
Operational success or intelligence failure!"
http://spur.barksdale.af.mil