Re: [ATM] Cassegrain vs. Gregorian (secondary testing)

["Richard F.L.R. Snashall" <rflrs@rcn.com>]

Bob May wrote:
> The comment was made that the cass secondary with a e of -4+ would be easier
> to build than a gregorian of -.5 would be and in this, I beg to differ.

This is not quite the comparison.  The Cassegrain with the same
characteristics will have RoCs for the curves that are quite a
bit longer than those of the Gregorian.  The actual amount of
material removed will actually be less, even though the e value
is larger (in magnitude).

> The methods that I know of for testing a cass secondary are the Hindle test
> (large perforated sphere needed), through the back (need to grind and polish
> the back to a near ROC of the front surface to minimize the errors from
> refraction or run a raytrace to see what happens to the light as it goes
> through the back surface and glass) or using a match plate (making a concave
> surface to match the curve of the secondary).

And now the test I described.  The difference is that the lens
setup must be zonally calibrated (against the perfect sphere)
and that calibration must be subtracted from the final zonal
differences.

> Bob May
> http://nav.to/bobmay
> bobmay@nethere.com
> NEW! http://bobmay.astronomy.net


-- 

Rick S.

http://users.rcn.com/rflrs

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