Re: [ATM] Flat Field Cass with Hougthon subaperture corrector. WAS:corrected dall-kirkham telescope

[Richard <cnc@cncservo.co.uk>]

Hi Dominic-Luc,

Thursday, February 8, 2007, 1:20:09 PM, you wrote:

DLW> Checking the link to the (corporate ?) website, the theoretical
DLW> spot diagrams are truly impressive,

Well, not *that* impressive. Impressive for spherical surface, yes.

They make the statement :

"However the Ritchey does nothing to eliminate the damaging effects
of off-axis astigmatism and field curvature."

"The CDK design tackles the off-axis coma problem by integrating
a pair of correcting lenses into a two mirror design."

But then go on to use the corrector on a two mirror design that bears
no resemblance to a Ritchie Cretien. I am currently building a 20" RC
with a two element corrector. The corrector elements are both BK7, one
PCX, one PCV , about 6" from the focal point, and close to each other.
Light enters the plano side of the PCX from the secondary and leaves
the plano side of the PCV and goes on to the focal plane. The pair
makes very little difference to the overall focal length of the
instrument, but has a drastic effect on field curvature and the final
spot diagram. At f/8.5, over the visible spectrum from 480nm to 644nm,
the entire spot is still well inside the Airy disk, from field
diameter of 0mm to 50mm and beyond. Worst case RMS spot radius at 25mm
field height is 0.9 microns compared to planewaves' 6 microns. Yes,
these are theoretical, not measured, results, but so are those on PW's
website.

Here's the spot diagram :
http://www.atmlist.net/contrib/cnc-at-cncservo-dot-co-dot-uk/rc20corr.jpg

There is nothing new about correcting Cassegrain type constructs with
lens elements. Papers have been written on the subject going back many
decades.

-- 

Best regards,
  Richard in the UK

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