--- In roddier@yahoogroups.com, "zeissnut" <grzincic@...> wrote:
>
>
> Hi Mick,
>
> I'm trying to follow along the numbers in these recent
> posts. Couple random questions....
>
> So you are doing interferometry at the radius of curvature,
> which gives a large spherical error, and then manually
> correcting or "nulling" out the spherical due to the
> set up. OK, got it. What are the units on the
> interferometry "Pri Spheric -0.625" in message #673?
> Is it waves PTV, RMS, on the wavefront, on the surface, etc.?
>

Dr. Biretta - sorry for any confusion about some of these details.

All the Zernike coefficients displayed by OpenFringe are normalized,i.e., an
error magnitude in nanometers divided by the program's reference wavelength of
550 nm. The Pri Sph then needs the manipulations discussed in post #677 to
obtain a value used in calculating rms and Strehl. Each coeff should be rms
error at the surface. Overall rms is obtained by squaring each coeff, dividing
by a squared scaling factor, summing the values, taking the square root, and
then correcting for actual laser WL to ref WL (550 nm).

> I am always a little nervous when the corrections are
> 10 or 20 times bigger than the thing we are trying
> to measure. But I guess there is no way around it.
> Unless you can come up with a big flat mirror to test
> against, or something like that.
>

One of the advantages of Bath interferometry is that it doesn't need a reference
flat. It's also a "common path" system: both stimulus and response light beams
travel essentially the same spatial path, greatly reducing air turbulence
effects.

> If I just put a ruler on one of your sample igrams,
> the error is about 2 fringes or 2 waves PTV on the
> wavefront (at center of curvature). Does that sound
> about right? Or is there a factor of two missing somewhere?
>

The Bath test is a "1-pass" (or single refection) test, at least as I performed
it, so the fringe spacing is 1 wavelength bright-bright or dark-dark. Or am I
answering the wrong question?

> Back to Roddier. What camera / pixel scale / filter
> did you use again? This is the 10 inch F/6 mirror?
> What is the exact focal length?
>
> - John B.
>

The camera used was/is a Philips ToUCam Pro 840K; the pixels are 5.6 umeter
square; the camera has a built-in IR filter which is all I use with the
Newt/mirror. The igrams were obtained with lasers of 635 nm and 650 nm
wavelength; 650 nm tests were slightly more optimistic.

The mirror is 10 inches, virtually exactly 254 mm in diameter, nominal f/6. The
exact FL is 60.125 inches, 1527 mm.

Thanks for the continued interest in this.

Mick Hollimon
13 June 09
9:43 AM PDT USA