Subject: Re: IRTF WFS 2000 Sep 1
From: Chris Shelton <[email-masked]>
Cc: Doug Toomey <[email-masked]>, Vern Stahlberger <[email-masked]>, [email-masked], [email-masked], [email-masked]
Date: Mon, 18 Sep 2000 11:04:30 -1000

Doug,

Here is the final optical design package for the wavefront sensor (WFS)
for the IRTF telescope.

The zemax and iges files have not changed since September 1, but I am
attaching them anyway to provide a complete documentation package in
one place.  They are:

  wfs2000sep1.igs.gz
  wfs2000sep1c.zmx.gz

In this final version, all mirrors are flat or concave spheres.

The design successfully matches the astigmatism of the WFS to that of the 
science instrument closely enough to get high strehl ratios with all
spherical optics.  For this to work, the astigmatism direction as well
as magnitude is matched.  Cancellation is perfect at a dichroic thickness
of 11.4 mm, giving an effective on-axis strehl of 0.999.  With the design 
thickness of CaF2 of 6.5 mm, the on-axis strehl is still 0.97, with no
software offsets.  This is at 0.85 micron.

The off-axis strehl ratio, at the four corners of the 80 arcsecond square 
field, is 0.95, 0.81, 0.78, 0.80, for 0.85 microns and a 6.5 mm dichroic.

The prescription for the clear aperture of the mirrors was chosen to give
an instantaneous field of view of the WFS of 8.1 arcsec diameter, 
steerable over a square field 80 arcseconds on an edge.  

The surface flatness spec allows a combined strehl degradation of 10% at 
633 nm from the four mirrors before the membrane mirror.  This not 
unnecessarily tight, as the bulk of the non-common-path OPD error budget 
will need to go to the dewar optics.

All the sizes given are minimum sizes to give 80% clear aperture.
A larger part can of course be used if it fits and is a catalog item.

The thicknesses are for a 6:1 thickness to width ratio.

  ***  Prescriptions for the optics *** 

All parts are mirrors, with 
  Pyrex or equivalent substrates,
  Enhanced aluminum coating,
  To be used over 0.5-1.0 micron,
  60-40 scratch-dig.  

1. WFS Fold mirror
  Flat, rectangular,
  58 x 82 mm minimum (10% margin all around clear aperture),
  14 mm thick, min.,
  51.6 x 73.4 mm clear aperture,
  1/20 wave PV surface accuracy at 633 nm over the clear aperture.
The clearance between this part and the rays returning from
the collimating mirror is tight, so attention should be given to
having a small polishing margin on one of the long sides.
The vendor may want to make this size part of silica or zerodur
in order to meet the flatness spec.

2. WFS Collimating mirror
  Concave sphere, round,
  1620 mm radius of curvature +/- 0.5%,
  4.00 inch diameter,
  17 mm center thickness, min.,
  80% clear aperture,
  1/20 wave PV surface accuracy at 633 nm over the clear aperture.
The vendor may want to make this size part of silica or zerodur
in order to meet the flatness spec.

3. WFS Steering mirror
  Flat, round,
  1.0 inch diameter, 
  5 mm thick, min.,
  55% clear aperture (only central 14 mm diam. is used),
  1/20 wave PV surface accuracy at 633 nm over the clear aperture.

4. WFS Camera mirror
  Concave sphere, round,
  1620 mm radius of curvature +/- 0.5%,
  1.0 inch diameter
  5 mm center thickness, min.,
  80% clear aperture,
  1/20 wave PV surface accuracy at 633 nm over the clear aperture.

5. WFS Lenslet mirror 1
  Concave sphere, round,
  3119 mm radius of curvature +/- 1%,
  1.0 inch diameter,
  5 mm center thickness, min.,
  80% clear aperture,
  1/4 wave PV surface accuracy at 633 nm over the clear aperture.

6. WFS Lenslet mirror 2
  Concave sphere, round,
  4990 mm radius of curvature +/- 1%,
  1.0 inch diameter,
  5 mm center thickness, min.,
  80% clear aperture,
  1/4 wave PV surface accuracy at 633 nm over the clear aperture.

This should finish Phase II of the IRTF AO optical design.  Please
let me know if there is anything else needed, or if we can declare
victory for this round and move on.

Regards,

Chris S