Re: [ATM] Are these kits good?

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

momsoldguns@qwest.net wrote:

>Richard, 
>
>I had a couple of questions, and there were a couple of sticky points in
>fabrication for me ... I'm sure that yours are probably more optically
>oriented than mine ... I thought he went a little over the board on
>measuring the radius of r-3 though, and I thought he should have spent more
>time on test plates ... what did you see? 
>
>Coyoté
>
I took his discussion to mean that this design was better since
it was the canonical CF correction.  I don't consider that as
an absolute.  (The design is not quite the canonical CF
correction -- that design is visually even worse.)

He used a comparison at 0.5 degrees off axis.  By increasing the
air-space, lateral color is introduced.  The result is visually
worse than the original Bray design.  Actually, in spite of the
discussion, the design is visually worse than the Bray design at
the center as well.  As I mentioned off-line to Brian (other thread),
there are better solutions [optically], e.g.:

   Surf      Radius   Thickness   Glass   Diameter
    OBJ    Infinity    Infinity                  0
    STO    Infinity  -0.0871689                  6
      2     51.6675        0.63   N-BK7       6.12
      3     -29.798    0.296797               6.12
      4     -29.798        0.44      F5       6.12
      5    -154.965    88.72998               6.12
    IMA    -32.0381                       1.578593

exceeds the Bray design [but it is not OTS, of course] in the center;
it still loses from the lateral color at the edge, though.

I got the impression that he had to consider wedge so much since
he was not using the original prescription and was, in a sense,
"hogging" new curves (in spite of the way he described it).

I really didn't see a ghosting problem with the Bray prescription.
The inner radii appear different enough to push the ghost away from
the focal surface.

All that being said -- the solution is valid (I've mentioned this
before); since the two inner radii are designed to be equal and
he verifies that interferometrically, the tolerance on the actual
inner radius value is quite a bit looser.  It's when the radii differ
in opposite directions from the design that there is a problem; I'm
surprised that didn't come out in the analysis.

End of venting session.....


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