Re: [APML] Cassegrain Question

[Aplanatic@aol.com]

Hi Bill,

> Gentlement,

Yes, I usually mean to be gentle but the dark side takes over from time to time.  ;)
 
> Assuming that the secondary support mechanism is centered within the OTA
> (I realise this is a big if :^) ) and the rear cell is in place, no
> optics installed. If a laser is placed into the focuser shouldn't the
> laser exit through the central hole in the secondary ? It seems to miss
> by a large margin and only by wedging the rear cell a bit can I make
> this happen. Furthermore the focuser cannot be shimmed due to the design
> (this bothers me a bit). With the rear cell wedged, the support bolts
> cannot be fastened due to the missalignment.

Yuck, sounds like the back plate is not perpendicular to the tube.  The impact of this would be (assuming otherwise perfect optical collimation) that the focal plane of the camera would not coincide with the focal surface of the Cassegrain if you left the secondary mirror in the center of the OTA.  Not good, especially for film imaging.

A couple of procedural issues:  1) Make sure that the laser collimator is itself correctly aligned.  Do so by rotating the laser and making sure that the spot does not move w.r.t. the secondary holder.  2) By direct measurement find the center of the OTA and verify that the secondary holder is close to being centered.

Depending on the degree of misalignment, you may be able to fix this problem by centering the secondary holder on the laser spot.  The only impact in moving the secondary off-center is potentially a slight increase in vignetting due to the tube.  It all depends on how far you must move the secondary to align it with the laser spot, and thereby get it on the optical axis as defined by the back plate and focuser.
 
> PS: Star Instruments 10 inch f/15 Cassegrain

Do they have a web page?

-Dave Rowe-

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