----------
From: Chris Vedeler
Sent: Saturday, March 14, 1998 9:05 PM
Subject: Re: [M]: Mirror shift


<<snip>>

> It is not supported anywhere but in the middle.
> The mirror is also not flat on the back. It is about 3 times as thick in the
> middle as it is on the edges. I would like to see a whole new mirror mount
> made which would support the mirror not just in the middle, but on at least
> three points towards the outside. I'd also like to see a way to tap three bolts
> from the outside at 120 degree angles from each other, to truly lock the
> mirror mount (and thus the mirror) down for astrophotography. I can now see
> how just using the shipping lock down screw to try and avoid mirror flop
> would not work 100% as the mirror is not supported very well in it's own
> mount and would be prone to some slight movement in spite of the fact that
> it is locked down. I've been bra! in storming all day on a way to redo the
> mount system, but I think it will require a serious machine shop.

Chris Vedeler


As you observed, the SCT primary mirror back is dished and tapers unlike
a flat back, constant thickness Newtonian mirror. The SCT primary mirror
is designed to be self-supporting from it's much thicker center. This mirror
will not support itself in a conventional mirror cell. Semi-permanent fixing
of the primary is quite easy, just wedge the slider to the baffle tube with
shim stock. You will need to bore holes in the rear casting or remove the
metal tube to wedge the baffle tube behind the mirror, of course.

Temporary fixing should be done around the back of the primary mirror
slider so as to not alter the mirror's figure. Anything other than very light
pressure on the back of the primary mirror will cause it to become
astigmatic because the SCT primary mirror is as described above, not
full thickness, tapered and self-supporting from the center.

I suggest you try the Semi-permanent fixing method first. I believe you will
obtain the results I described in a previous post. Since you already have your
scope apart, why not see for yourself? Carefully wedge the baffle tube to the
slider, take several long exposures of an hour or more. Then, carefully gauge
the results you obtain with a high power loupe or better still, use a good
high-res negative scanner for comparison on your computer monitor.



--
Michael Hart
Husen Observatory

View index by [date] [author] [subject]
Previous message: Re: [M]: 8" LX50 vs. Celestar 8 Deluxe, steve sanacore
Next message: Re: [M]: LX200 Dust seal, Bill Arnett
Next message in thread: RE: [M]: Mirror shift, Heapy Chris CG
Previous message in thread: Re: [M]: Mirror shift, Chris Vedeler