Re: [APML] SCT mirror lock

[Glenn Graham <glenng@synopsys.com>]

Bobby, that would depend on whether you have an 8"/10" or the 12" since the 12"
has a spring to resist the pull of gravity. Unfortulately, how much it will
compensate depends on how vertical the OTA is. For my 12" it actually
over-compensates throughout the range and I actually get a more stable focus
when I turn the knob to lower/pull the mirror down onto the spring.

-Glenn

Bobby Middleton wrote:

> ----- Original Message -----
> From: Steve Walters <swalters@att.net>
>
> >But if I crank the focuser to the far ends of its
> > travel in both directions a few times, the system shows virtually zero
> > mirror shift when focusing. This is at 200x. I believe Doc G on MAPUG
>
> Mapug did have some good info and Doc G had about as much as anybody. I
> always focused with the final tweek in a clockwise (I think......it's been 3
> years) motion. Whichever the correct direction, the point was to RAISE the
> mirror instead of lower it. If one lowered it, it might hang on a glob of
> grease and settle during the exposure.
>
> > I am not saying this eliminates mirror shift all together. I still use an
> > OAG for photography. But I have tried various lockdowns and convinced
> myself
> > that they don't do much for the shifting. And, if you forget it's locked,
> > you could be in for an expensive repair. It's something else to remember
> to
> > do/undo. At 3 am in 14 deg weather, my IQ drops about 70 points. So I
> > decided against using these gizmos.
> > A minority view, no doubt.......YMMV.....
>
> I was in the minority too; I didn't use lock-down. I also was concerned
> about forgetting the bolt was in there and anybody who does much
> astrophotography knows exactly what you mean by IQ dropping at 3 am.
> I locked the mirror down a few times. I thought that I pulled the system out
> of focus when that screw got tight but I never heard anybody else say this.
> Bobby Middleton
>
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