Subject: Re: IRTF WFS 2000 Sep 1
From: Chris Shelton <[email-masked]>
To: Doug Toomey <[email-masked]>
Cc: Vern Stahlberger <[email-masked]>, [email-masked], [email-masked], [email-masked]
Date: Tue, 16 Jan 2001 13:26:52 -1000

Doug,

Yes, there is a mistake in the email describing the optics for the IRTF
WFS.  The zemax and igs files are fine.  Part 2, the WFS Collimating Mirror,
does not really have the same radius of curvature as the Camera Mirror;
the correct radius is -1035 mm (concave).  As near as I can tell, the rest
of the text is ok.

Here are some new numbers, for baffle sizes.  The first three are my
readings on the baffles inside the dewar between the window and the
dichroic -- this is just a cross check.
  Surface 17  53.6 mm square
          18  69.8 mm diameter
          19  69.6 mm diameter

These baffles are in the WFS:

  Baffle 1, on the steering mirror face:  14 mm diameter.  This is the
pupil stop for the WFS.  If it isn't there, the WFS will see past the
secondary to the sky.

  Baffle 2, 300 mm downstream from the steering mirror: 18 mm diameter.
This baffle catches the rest of the field outside the region that you
have steered to.  If you do "visible science", this baffle should be 
resized to accommodate the size of your visible science field.

If you put the splitter for the visible science field just before the
membrane mirror, as we talked about today, the visible science field
will always be centered on the guide object, and not steer with the
IR field.

Besides the baffles just mentioned, there is also a field stop assembly 
just before the membrane mirror that provides the adjustable field stop 
for the AO.

You mentioned on the phone that you were looking for the zemax
file for the AO relay.  I've attached relay2000sep1a.zip, which
contains relay2000sep1a.zmx.

Regards,
Chris S

Chris Shelton wrote:
> 
> 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%, 
[This is a typo, the correct radius is -1034.56 mm]
>   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
> 
>   ------------------------------------------------------------------------------
>                           Name: wfs2000sep1c.zmx.gz
>    wfs2000sep1c.zmx.gz    Type: application/x-gzip
>                       Encoding: base64
> 
>                          Name: wfs2000sep1.igs.gz
>    wfs2000sep1.igs.gz    Type: application/x-gzip
>                      Encoding: base64