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Date: Fri, 1 Aug 97 22:58:06 CDT
From: (Giles Novak)
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Subject: meeting with Jesse, etc.
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Here is an update on a few issues that have become decided
as a result of Jesse and my meeting, and some other work I
did recently.  I decided to just Email the information to
you.  I don't think it is necessary to put it on the web
in the form of "minutes to a meeting" as I had originally
thought I would do.

(A) Final positions of the optical rays are now decided:

They are as given in your optical design, with the following refinements:

(1) Left-right inversion:  Viewed from the optics snout, the first grid 
the incoming light reaches diverts the light towards the left, and the 
second grid diverts the light towards the right.

(2) Angle of beam after reflection by mirror:  At point of reflection by 
mirror, reflected beam makes an angle of 22 degrees with respect to radial 
direction.  (With respect to the radial direction, the beam is deflected 
towards the right.)

(3) The order of the pupil-stop optics will be hwp (in snout), aperture 
filter, pupil lens.

(4) All of the optical elements will be displaced by 0.15" towards 
the He-4 cold surface, as compared to what is given in your design.

(3 and 4 were necessitated due to an interference problem between a
grid and the filter-holder)

(B) Array supports:

We are in luck.  The He-3 pot is parallel to the He-4 cold surface to 
better than half-a-degree.  (Measured carefully by Jesse and I).  
Also, I think that the angle 
between the two sets of hole patterns is probably recoverable to
within a degree or two.  Finally, you told me that the He-3
pot is centered, and I did a quick check of this and verified that
it is centered to a mm - prob better.

So we will mount the arrays on a precision,
copper U-shaped piece that is mounted to the He-3 pot - the sides
of the U support the arrays.  This can be made out of a single piece of 
copper, or a few pieces.  All optical alignments will be made by adjusting
grids and the mirror.

I will give Jesse the positions of the arrays relative to the center
of the He-3 pot.  Simultaneously, he will sketch up a drawing that I will 
so that it puts the arrays in almost exactly the right place.  We agreed
to make the array supports out of a single copper block but since
then I am thinking that this is unecessary - we can make the
U-shaped piece out of three pieces - a shallow U and two sides,
that can still be assembled into a single U with great precision
but will be much easier to machine.

(C)  Space considerations that became apparent once we put the
transparency of the optics on the dewar:

(1) For ease of wiring, minor mods will need to be made to the
copper link that runs from the 3 K stage copper ring to the
heat switch.  Jesse will tell D. Pernic

(2) No plumbing or other interference should extend by more than
3.75 inches above the He-4 surface in the area just below the
grids (see photo).  Jesse will tell D. Pernic.  

(D) Finally, we went over the plans on the web page, and Wirth is
confident that he can deliver on his items.

Just today, another issue has come up that I need to discuss
with you.  Jim and I took apart the STOKES
hwp and have crudely sketched up the SPARO hwp and identifed
the bearings, gears, etc that we need to order.  But we are
running into some real troubles fitting the 1.75 inch hwp.
One possibility is we can order another one that is 1.625"
instead.  We'll still get the 1.5 inch. clear that we need,
so this is doable.  But this is an expensive item.  The other 
possibility is that
you and Jeff could open up the bore on the snout just a little.  If
you could go from 2.75 to 2.875 on the i.d. of the snout then
we can use the 1.75 inch hwp.

Let me know what you think.

(Note that we only need 1.5 inch clear so were not talking
about losing throughput if you can't open up the snout, just
ordering a new hwp.)

Well, I hope you had, will have, or are having a productive
trip to CMU.  See you Monday the 11th.