Re: [ATM] Are these kits good? & Refractor Kits

["Jerry" <wa4guu@verizon.net>]

The Fraunhofer design LA graph Richard Parker showed had obviously not been
optimized ... There is no reason the C & F couldn't have been brought to
together. It should have been better than the Clark type design with the
same radius for R3 and R4. I don't know about the Bray design, but it
sounded like a Clark to me.

Newport's web site seems to say that Max Bray designed "both systems" in a
paragraph on the cemented lens. I take that to mean the f10 and f14.1
cemented designs. 

The paragraph on the air-spaced design credits James G. Baker.

Mr. Parker mentions BK7 and F4 so I guess he got the right glass for the
air-spaced design. So where did all his talk of Max Bray relate to the
air-spaced design? Did he use the wrong glass for his LA graph of the
Fraunhofer? Or was that graph really something else entirely, like a single
element lens?

I thought the R3 measurement was more elaborate than I would do but I guess
he was thinking that R2 being the most critical that he had to have R3 very
accurate since he was testing R2's radius against R3.

I didn't think he overdid wedge. He did say how easy it was. 

I started with rough sawed blanks. So I ground one curve on each and then
rounded the edges on axis with the first curve. Then I ground the other side
of each and removed wedge by grinding only the second sides of each glass.
May be that with pre generated curves his method of removing half of the
wedge on each side would be better. 

I have no room to criticize anyone for not being precise enough. When I made
my 4" BK7 F4 achromat I just measured the concave tools for the lens convex
curves by the usual Foucault like test as I use to measure the radius of a
mirror. No figuring or test plates. Just polished and put it in a cell.
Works great. May be some day I will make the test plates, flat and so on and
tune it up right just to see if it actually will make it work better. It
works as good as any 4" achromat I've looked through. But that is a small
number... about one hand full of fingers. I will probably just wait until I
do a 6" or larger to do the more elaborate testing.


All in all Parker gave a good presentation. Talks like that are mostly for
inspiration. Then the inspired need to study on the requirements and plan
the details and try not to miss any of them.

Jerry


-----Original Message-----
From: Richard F.L.R. Snashall
 
momsoldguns@qwest.net wrote:

>Richard, 


>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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