Re: [APML] Cassegrain Question

[William Mattil <wrmattil@ix.netcom.com>]

Aplanatic@aol.com wrote:

> Yuck, sounds like the back plate is not perpendicular to the tube.

Errr, well maybe. The focuser itself is swaged into the back plate and that could account for this too. It is not collimatable (is that even a word ? :^) ) If it were then problem solved.

>  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.

Its not possible that the laser could be that far out of wack <laughs> But When rotating it, it makes no difference. And I checked with another laser to be certain.

>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.

Close enough. At least at this point. The laser spot completely misses the secondary support mechanism so it is at least 1/2 inch off. In about 30 inches.

>Depending on the degree of misalignment, you may be able to fix this problem by centering the secondary holder on the laser spot.

Someone else suggested that as well. But I doubt that moving the secondary 1/2 to 3/4 inch off center would be wise. Would you agree ?

> 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?
>

No. Its an older instrument.

Best Regards

Bill




_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo