Re: ATM Ultra-Light Dobs mounts

["Chris Westland" <westland@uxmail.ust.hk>]

With an open tube, I think it is probably possible to position the light
source and reflector precisely where you want them, shining between the
truss tubes ... and the reflecting plane (i.e. the square of plastic) will
naturally be overhead if your scope is 1/2 meter or above, and you are under
2 meters tall (my situation) ....

For the plastic, the Telrads us a cheap plastic plate maybe 1"x1.5" ... this
seems to create a good image.  Even an opaque sheet will work if you keep
both eyes openned.   I'm not sure that surface quality is that important for
Telrads of any size.

For a light source, how about a laser pointer ... powerful and directed.  I
think you could probably get a reticle small enough to just cover the
diameter of the beam.  Of course, you couldn't regulate the brightness of
the beam (or could you? ... I haven't tried putting a potentiometer on a
laser pointer).

-c


>Yes, the very off-axis light path is no problem at all (due to the
>extremely high focal ratio - like 2 m f.l. and 7 mm aperture=pupil). What I
>did have some problem with was glass plates that were not quite optically
>flat (like from slide mountings), until I used the camera skylight filter.
>I suspect you will have trouble with a plastic sheet that is not highly
>flat - it may work in transmission but not in reflection.
>
>With a closed tube you would need a cutout for light from mirror to
>glass/plastic plate, this will be a lot easier to manage with an open tube
>even with a shroud.
>
>The reticle may not be quite one focal length away, this may give some
>parallax (reticle seen as perhaps 20 m away instead of infinity would mean
>1/1000 radians error if you look 2 cm off center)), this was no practical
>problem but if you worry you can use a small lens or a mirror to increase
>optical path length.
>
>Nils Olof
>