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Re: Re[2]: [ATM] A two way null test for convex hyperboloids
Richard,
On Apr 6, 2005 12:22 AM, Richard <cnc@cncservo.co.uk> wrote:
> Hi Mauritz,
>
> Yes, that's more or less what I'm ray tracing now. It seems that
> the null for those specific raidii and K is quite dependant on
> the glass type. The 10 wave abberation I saw is with BK7. If I
> change the glass to fused silica I can re-optimise focal
> distance to get down to about 1/2 wave, but with generic quartz
> I can't get any better than 15 waves. What exactly is the
> "QUARTZ C" that oslo is using?
>
Yes, it does depend on the index of refraction.
The choice of a quartz subtrate was arbitrary, but perhaps a bit stupid.
Here is the prescription for a BK7 substrate instead,
*LENS DATA
Wavelength 632.8 nm
SRF RADIUS THICKNESS APERTURE RADIUS GLASS SPE NOTE
OBJ -- 674.432508 V 1.000000 AIR
1 1.4140e+03 20.000000 78.097034 S BK7 C cc -6.35
2 -1.3746e+03 -20.000000 P 79.230497 AS REFL_HATCH cc -5.907
3 1.4140e+03 P -674.432508 P 78.096979 S AIR * cc -6.35
(The other surface should now be 1414 mm RoC instead and converges to a
conic konstant of -6.35. Still the 1374 mm surface converges to the same conic
as before -5.907 as we want.)
The sensitivity to testing wavelength is not too big, this example would give an
ok null also for Na light 589 nm with a source/knifeddge distance of 673.4 mm.
> Personally, although quite an original idea, I don't see the
> benefit over using a plano back to the mirror and getting the
> null from a ross or offner null setup. At least that keeps the
> number of aspheric surfaces down to one, and you can be sure of
> surface that null does represent. With the bi-convex idea, I
> would be very worried by the interdependance of the two radii
> and conic constants - maybe if the refractive index of the glass
> is slightly different, the two surfaces may converge to
> something altogether different.
This interdependence is what I'm using :-) The point is that if you
can measure the RoC for the two surfaces with a spherometer, one can
calculate to which aspheric surfaces the testing and polishing
procedure converges to.
If it is simpler to polish and verify a flat and use an aux lens for
nulling instead, I'm sure you can jugde better than I. I'm just
tossing out an idea for discussion and feedback.
Thanks,
Mauritz
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