Re: [ATM] squaring truss tubes and collimation

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Jay Kirkland asked:

>    But can you properly collimate a telescope if your spider cage is
> not centered on your primary mirror?
>  ..wouldn't
> an off-axis light path end up potentially interfering with the spider
> end of the scope, creating a kind of  tube bottleneck and all the
> diffraction effects that go with such a thing?

I can't share your pessimism. Optically, you need to line up the focuser,
secondary and primary to get the optical axes together as one, with the
secondary centered in the light path (i.e. "offset"). With one common
optical axis, you can use it as a reference.

You want the spider cage centered on the optical axis, and you do that by
offsetting the spider the proper amount away from the focuser - this done,
the optical axis (you can see the laser spot) is centered in the cage, but
this isn't much of a practical problem - look into the empty focuser, and
move your position - can you see the cage reflected in the primary from
anywhere within the focuser? Even so, the vignetting will be very mild.

Digital setting circles may require that the optical axis and the altitude
axis are perpendicular to each other or reasomnably close.
If the mirror box is tilted a little or decentered with respect to the
optical axis, so what? - as long as you can tilt the primary to make up for
it - and similarly, even if the spider cage is a little askew, the optical
parts are still lined up properly after collimation and will work as
intended.

However, with a rotatable top, you have a much more complex problem, as it
is necessary to make the axis of rotation coincide with the optical axis.

Nils Olof



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