----- Original Message -----
Sent: Wednesday, March 10, 2004 6:35 AM
Subject: Re: [M]: Re: GPS mirror lock


> That's very interesting Roger. Are you saying that the
> central baffle tube itself is moving inside the OTA as the
> scope slews across the sky? The mirror lock could not do
> anything about that, since it secures the position of the
> mirror mount onto the baffle tube.
Yes. There is a measurable bend in the baffle tube. The 'key' problem to
remember, is that any component before the secondary, has it's effect on the
light path amplified by a factor of about 5*. I calculated that for an
LX200/12, the tube itself would bend by about 0.3 arc seconds through 180
degrees, with the weight of the mirror, when at the normal focal point (I
measured where my mirror was, for the scope focussed about 4.5" behind the
rear of the focusser). This then gives an 'unavoidable' error from this
source alone, of about 1.5 arc seconds. The rear plate (I was guessing on
the aluminium alloy used, but assumed 6061 - similarly I was guessing on the
plastic used for the baffle tube), gave a slightly lower distortion, but
brought the total up to just over 2 arc seconds. There was also a _tiny_
bend in the mirror itself. In a sense, there is a 'minimum' movement, that
is as far as I can see, unavoidable for this design, no matter how well
everything else is locked. I'd put this at perhaps 2 to 3 arc seconds total
for the worst case (180 degree) movement of the scope.

> Of course, if the mirror lock is disengaged to allow use of
> the coarse focus knob, then the mirror is free to flop around
> as much as ever. Resetting the lock will tend to move the mirror
> back to roughly the same angle has it had in the previous locked
> state, but this position can vary somewhat with each use of the
> lock. For instance, if you want to test GOTO accuracy, then you
> need to set the lock before the first alignment star and not touch
> it again until after the last test star is recorded.
>
> Using the coarse focus knob while the mirror lock is engaged will
> result in severe image shift, and could damage the coarse focus
> mechanism. So changing the focus with the mirror lock engaged
> requires use of a secondary focuser, such as the MicroFocuser included
> with the GPS scopes.
Yes. Moving the coarse focusser with the lock engaged, first 'rocks' the
assembly hard to one side or the other, and then tries to 'scrape' the
collet along the baffle tube, putting much more force onto the focusser
itself, and all the other components, than they are really designed to take.
With the 'lock' done up, and moving the scope round the sky, the total
movement, in the axis parallel to the line through the coarse focus knob,
and the centre of the tube, was down close to the 'bending' limits. However
at 90 degrees to this, there was still a significant shift. It makes sense
if you think of the system as being locked at the collet, and at the
focusser shaft, it can still rock at the front at 90 degrees to this line,
till the end of the outer tube touches the inner tube. The clamping force of
the collet, reduces this motion, but does not completely remove it...
I must admit I did find myself wondering about careful placement and design
of the guidescope mounts, to at least partially compensate for the motion,
and wondered if this was how some systems appear to have very good
responses. When dealing with 'pure bending' (as opposed to nasty
shifts/rocking motion), it would be possible to site the guidescope, so that
it's bending is in the same order as that of the main scope, and get a final
result that is better than one scope on it's own.
As has been mentioned by another poster, Al Testani, was doing a lot of work
in this area, trying to measure and model the errors between guidescope and
main scope across the sky. It might well be worth talking directly to him,
to see how he was getting on.

Best Wishes

> --- Bill Dougherty
>
>
> > Can anyone direct to some information on the GPS model mirror lock
> > mechanism? Did they just install the equivalent of a shipping bolt that
> can
> > be latched and unlatched or is it something more like the bisque mirror
> lock
> > collar?
> >
> > Thanks and clearest skies,
> > Walt Cooney
> > http://members.cox.net/waltc/BLACKBERRY_OBSERVATORY.htm
> It's a bit like a collet chuck. A ring is driven round by a gear, and
> tightens the collet. It works reasonably well, but _does not_ completely
> remove mirror shift. It brings it down to perhaps 1/8 to 1/4 the level on
> the older scopes, but there is still a small amount of rocking as the
> front of the baffle tube moves.
> With a guide scope, it allows exposures many times longer
> than could be achieved with the older scopes, and the shift is slow, and
> relatively 'smooth' across the sky, instead of having relatively fast
> movements in some areas. On it's own, it is perhaps the best 'extra' on
the
> latter scopes.
>
> Best Wishes

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