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

[Dominic-Luc Webb <dlwebb@canit.se>]

On Thu, 8 Feb 2007, Richard wrote:

> 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."


Well, fair enough! We looked at this a bit differently. I ignored
their comparison with the Ritchey Chrétien. It seems like bad science
to compare two systems with a curved focus in which the one for sale
has a field flattener while the comparison design does not. Since
the planewave scope has some form of corrector plate in front of
the system, I thought of it as catadioptric as well. I do not
normally regard the Ritchey Chrétien as catadioptric. The spot
diagram you provided a link to...

http://www.atmlist.net/contrib/cnc-at-cncservo-dot-co-dot-uk/rc20corr.jpg

only really suffers from bad spelling (I believe it is Ritchey-Chrétien).
I am curious what form of correcting is meant... field flattener near
focus or is there a corrector plate in front like a Schmidt or LH, etc
would have? Your RC design also looks to have somewhat more uniform
spot size across the field (I'm still not sure what this means
for image quality). Anyway, I guess your F/8.5 spot diagrams are
smaller than the planewave F/6.8 design. Do you propose your system is
more appropriate for comparison, and did you manage to make a system
that approached that spot diagram?

Dominic-Luc Webb


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