Re: [APML] Cassegrain notes

[Gene Horr <genehorr@texas.net>]

Tonyhallas@aol.com wrote:

>      Sorry to hog the wires but so many questions! The 14.5" f/8 Cassegrain has the coma of an equivalent Newtonian but slightly more curvature of field and a very slight increase of astigmatism in the corners.When it is collimated properly it can give "almost" perfect 35mm negatives with nothing else. The RC design has less coma in the corners but a lot more curvature of field and from what I hear, is difficult to collimate.

Tony:

The reason the RCT is used so often for research instruments is that it produces
the most even star sizes across the field.  It does not necessarily produce
the best resolution (in fact from my experience almost all other Cassegrain
variants _of_the_same_quality_ have better resolution and visual contrast).

Regarding the curvature of field, this is generally not the case.  I have
found that most RCTs on the market have a far flatter field than most 
Cassegrain variants.  But most of the is likely due to the RCTs being
optimized for imaging (with a large, lower "power" secondary) as opposed
to most Cassegrains which are optimized for high power planetary work
(small, high "power" secondaries.)  Assuming the primary and secondary
both have the same FLs the field curvature should be the same.*

* But this is a guess on my part.  I have not confirmed the last sentence.

The RCT is a pain to collimate.  But no more so than a Newtonian that is
optimized for photography.  Those can be a much bigger PITA.  

Gene Horr
genehorr@texas.net

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