Re: [APML]: Mirror Flop

[mark.kaye@sympatico.ca]

>I have a Meade 10"LX50.  What is mirror flop and what can be done on my 
>telescope to mitigate its effect.  I assume, from what I have read on the 
>Mapug mailing list and APML list that mirror flop will hamper my ability to 
>perform piggyback photography.

   Mirror flop is caused by the mirror moving as the scope tracks the stars. 
 The mirror of an SCT is mounted around the central baffle attached to the 
rear of the scope.  To focus, it slides up and down this baffle.  Given that 
the system must have enough play to move along the baffle, the problem of 
mirror slop comes to the fore.  As the scope changes orientation, the weight 
of the mirror causes it to change its position slightly with respect to the 
baffle.  If you are guiding on a star through the main scope, you will guide 
out the slop, but any piggy-backed device will not follow exactly the same 
path.
   There are several ways to get around mirror flop.  The effect of the tilt 
is most pronounced as you cross over the meridian.  So avoid doing this when 
taking a picture.  Always focus by moving your mirror up to the star.  Go 
through focus so the mirror is as far back in the housing as possible and 
then focus your guide star.  Do not overshoot, if you do, back the mirror off 
and try again.  You can move the scope's mirror to the all the way forward 
position and use an external focuser.  Finally, for the adventurous, you can 
tap three screw holes 120 degrees apart in the rear of your cell and put in 
three nylon bolts.  Focus your star and then carefully move the bolts up to 
just touch the mirror.
   We used to own an SCT and from the factory, its mirror slop was terrible. 
 It also used to freeze solidly in temperatures below 0C and if it got cold, 
it would become impossible to focus.  I carefully took the scope apart, 
marking the orientation of each part and then reassembled it using snowmobile 
grease, instead of the thick paste that it was greased with originally.  Once 
I adjusted the nylon bolts that centred the mirror on the baffle, carefully 
and lightly regreased the scope, the mirror slop was reduced to negligible 
and the scope would focus at any temperature.  Again, this is not for the 
faint of heart.
   All that said, mirror slop is not that much of a problem unless the focal 
length of your piggy-backed lens is long.  For anything under 200mm, you 
should not notice anything but the most horrendous of mirror slop, especially 
if you do not let your scope cross the meridian.
   Clear skies!
               MK

http://www3.sympatico.ca/mark.kaye/