Re: [ATM] Focuser, Derotator, Off Axis Guider

[Andy Saulietis <iss@pvtnetworks.net>]

Martin C writes:


>I plan to take photographs with my 14" dobson telescope.
>Because of the field rotation the best guiding solution
>seems to be an off axis guider.
>Using a secong telescope for guiding is more complicated
>because this scope has to be rotated too and the rotating
>assembly must be very precisely parallel to the main scope's
>optical axis.
>
>So I need
>1. field derotator around the focuser
>2. off axis guider
>3. low profile focuser with extension for viual use
>. . . very expensive.
>
>I think it should be possible to build a focuser with integrated
>off axis guider. The the guider doesn't need additional backfocus.
>If a derotator is already there I could seek for a guide star
>by rotating the whole focuser instead of rotating the OAG alone.
>
>The focuser housing around the tube must be open to one side to
>allow the second tube of the OAG to be mounted.

Chuck Shaw and I built several variants of field de-rotated guidescopes,
they work but are mechanically complex.

One version has the guidescope mounted inside a rotationg tube
the axis of which is parallel to the main scope field rotator axis.
The guide scope can be offset inside the rotating tube to find guide stars.

The version I'm working on has a fixed guide scope with field rotator
alined to the main scope, with an x-y carriage inside the rotator that
is used to move the guider camera to acquire the guide star. In my
case, the guide scope is a 6" f20 refractor, with the lens and field rotator
separately mounted to the main OTA to save weight and improve rigidity.
A lightweight baffle connect the 2 units to exclude stray light. The
jury is still out on this approach. Anothe feature of this system will
be to simplify guide star acquisition by showing the current location of the
guider field on a display showing the locations of GSC stars within the
3" x-y field of view. One would just move the x-y carriage to place the guider
fov on the desired star. This would be useful for a remotely operated
telescope.

These system have a problem in that the guider axes rotate relative to the
mount axes, so if the mount is doing the guiding the corrections applied
will be progressively more in error. Mel Bartel's controller software
can compensate for this problem.

A 'strapdown' guider with no moving parts might be the best solution for both
alt-az and equatorial mounts..in this approach the guider camera 
field has to be
large enough to guarantee a useable guide star is in the field without having
to move tod scope. In an alt-az design, the field rotation is mathematically
compensated for to obtain the residual guiding errors. The problem with this
is that a guider with a *large* fov is needed..very expensive today.

Finally, a 14" scope IMHO is on the borderline for using an equatorial mount
or plaform
Andy Saulietis
ISS Enterprises
10 Milky Way
Mayhill NM 88339


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