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Date: Wed, 16 Jul 97 14:17:23 CDT
From: (Tom Renbarger)
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Subject: Re: optics
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Status: R

Giles, Jessie:

Spoke with Ron Kappler after lunch.  Should be able to make the change with
no problems, but it will make the total price go up a little.

Here is the updated version of the optics table for the webpage.

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A revised table for the SPARO lenses.

Viper Telescope 

	F/3.5 (but see discussion below) 
	Plate Scale = 2.35 arcmin/mm
	S0 = 43.7" (distance between image of primary and focal plane)

Physical Lengths

	S1 = 5.25" (distance from focal plane to pupil, "a" in Jessie's notes)
	S2 = 5.25" (distance from pupil to array, "b" in Jessie's notes)


	Field Lens (quartz, n = 2.1075)
		f = 4.69"
		R = 5.19" (plano-convex)
physical diameter = 1.00"
    open diameter = approx. 0.75"
central thickness = 0.12"
   edge thickness = 0.10"

We have three lenses of f = 4.65", which could be used as backups in a 

	Pupil Lens (quartz, n = 2.1075)
		f = 2.625"
		R = 5.82" (convex-convex) 
physical diameter = 1.75"
    open diameter = 1.65"
central thickness = 0.23"
   edge thickness = 0.10"

We have two f = 2.53" lenses, not really good enough even as backups.

	Array Lens (quartz, n = 2.1075)
		f = 5.25"
		R = 5.81" (plano-convex)
physical diameter = 0.75"
    open diameter = 0.50"
central thickness = 0.11"
   edge thickness = 0.10"

We have two f = 5.25" lenses, so we're OK although lacking any backups.

Beam Size (assumes we have the proper plate scale)

	Beam Size = (Plate Scale)*(Winston cone aperture)*(magnification)
		  = (2.35'/mm)*(3.05 mm)*(1)
		  = 7 arcmin.


The image of the primary on Viper is at the chopper.


X-vertex and Z-vertex in Jessie's notes correspond to the point where the
central ray hits a given optical element on Viper.

	-Also true

The array will have a diagonal length of the Winston cone aperture (3.05 
times a geometrical factor of 2*sqrt(2) + 1, assuming a 3 by 3 array with
adjacent cones touching.  This length turns out to be about 0.47".  The 
lens was therefore specified to have a 0.75" diameter for relative ease of
mounting at the lowest possible price.

	-Taken into account with the 0.5" open diameter of the array lens

The spacing between array and pupil was determined for a 1.5" pupil 
at F/3.5.  A pupil lens diameter of 1.75" was chosen to give a slightly 
sized pupil even after mounting to optimize throughput.

	-Unless you've had a change of heart, this still stands, locking into
 	 place the values of S1 and S2, which in turn determines f for the
	 pupil lens.

The trouble with the illumination of the primary has evolved into the issue
of the possible discrepancy between Viper's plate scale and its effective 
length.  The focal length listed for Viper is 1.0m, while the inverse of 
plate scale is 1.5m.  This affects us in terms of the amount of the 
primary we
ultimately illuminate.  For our F/3.5 dewar, the effective diameter of the
primary is either 30cm or 45cm.  Furthermore, the effective F/# of Viper is
not affected by the change in the effective focal length.  Given the 
diameter of 2.0m, Viper is either an F/0.5 or F/0.75 telescope; faster 
than the
dewar in either case.  Since we are now calculating the effective diameter 
the primary by D = f(effective)/3.5, Viper defaults to acting as an F/3.5 
when SPARO looks through it.

	-I think this is right, but maybe I'm playing a little loose with
	 what the F/ratio is.  At any rate, still no update about this
	 discrepancy, although it does not affect the lenses we should order.

The price of two field lenses plus two pupil lenses will be around $2100, 
with the 4-5 week lead time.
	-Price will be a little more than $2100, 5 week lead time

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