Re: [ATM] A Null Test

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

Ric Rokosz wrote:
>
> --- "Richard F.L.R. Snashall" <rflrs@rcn.com> wrote:
>
>>  From what I've read, the Ross Null Test is good at
>> f/4 or slower.  
>
>
> >From my experience that isn't true.Have done a 24
> incher f/3.3 and a 18 inch f/2.8 with a 6 inch custom
> made lens I did and a 13 inch lens where I redid the
> convex side.
>
With three BK7/N-BK7 lenses (deriving from Richard's Offner test):

        a) CVI PLCX-38.1-51.5-C    38.1 mm diameter 100 mm f.l. planoconvex
        b) Optosigma  015-0475     30.0 mm diameter -170 mm f.l. 
planoconcave
        c) 25-25.4 mm diameter variable f.l. (c. 100 to 200 mm) biconvex

I selected the diameter of the variable f.l. lens, as that seems to be a
popular size.  Not all of that diameter is used.  I also selected the output
speed of f/16, i.e. in all the tests, the mirror subtends the same angle as
would a f/8 mirror in a standard test setup.

Varying the spacing, I was able to generate null tests for conic constants
-0.8 to -1.2, f-numbers 2.5 to 5.5, and diameters 200 to 600 mm.

Pushing it to at least f/7.0 seems to be okay.  Larger than 600 mm diameter,
the limitation appears to be the size of the first lens -- this one should
still be okay up to 800 mm and maybe up to 1000 mm (I didn't check.).  
Smaller
than 200 mm, there is a minimal air-space limitation that was hit; and even
a bit at 200 mm, but it looks like not enough to hurt the test a lot.

By allowing a slower output speed or changing the f.l. of the first lens
(possibly needing more aperture on all the lenses) may allow some testing
below 200 mm.

I think also, that increasing the f.l. of the first lens might allow for
testing at conic constants larger than -0.8 (ellipsoids).

I have not investigated changing the f.l. of the middle planoconcave lens --
the available selection of planoconcave lenses seems to be smaller.
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