A mirror is supported by an n-point mirrorcell (18 points in
my case). But the mirror only seldom rests with its total weight on all
18 points: only when pointed at the zenith. When the mirror is glued to
the cell with RTV it hangs most of the time on the RTV blobs with part
of its weight. Using a sling, it rests most of the time with a part of
its weight in the sling. At first I had my mirror glued the cell with
silicone RTV. The result was not what I wanted: in the startest I saw
astigmatism (don't know if it was caused by the RTV). To make sure if
the astigmatism is in the glass or not, it is necessary to be able to
rotate the mirror. If the astigmatism follows the position of the
mirror: the cause is in the glass. Rotating can't be done when the
mirror is RTV-ed to the cell. So I cut it loose with a 0.24 mm
thickness guitar wire (which was very easy). Next solution I tried was
a "traditional" 2 inch wide car safety belt sling. This had at least
four drawbacks:
- It stretched and caused me to check the
collimation often, especially when the atmosphere was moisty.
- The mirror could move sideways (without going
back to its middle position even when the mirror was pointed to about
25 degrees above the horizon), leading to a collimation problem as
well.
- Using an equatorial mount the mirror will shift
sideways. I don't have an equatorial mount by now for this scope, but I
read about this problem on one of the mailinglists I have subscribed
to.
- Potato chipping (according to kriege and Berry),
leading to astimatism.
Trying to overcome this problems I made a double sling, following an
idea of the Swedisch ATM-er Nils Olof Carlin (link on my links page). I
made it from very thin material: a 1.6 mm (0.062inch) thickness bicycle
break cable. Ithought this would cute my problems, but alas: I still
saw astigmatism sometimes. So I decided to try the "piano wire" solution (links to
Frederic Gea's page, where I read about this type of lateral support,
on my linkspage). |
