Looking for flexure

At the IRTF, we tested EXES for flexure by going to a telescope position that mimicked mounting on SOFIA.  We then did 6" nods along the slit direction, although we were using the decker position with a pinhole arc.

ND difference showing night sky through offset pinholes.  Emission lines are shifted from spectrum to spectrum because of their azimuthal offsets at the slit plane.  

A cut along column 400 of the above image showing the values through the 5 pinholes (not the pinhole near row 600 wasn't looking at sky so the the ND difference was very small.  This is to show the data number level on the sky.

By looking at the difference between the two sides of the nod, can look for any change in the location of the pinholes on the detector that might indicate flexure within EXES.  By looking closely at one of the pinhole spectra and shifting the column cut by +/- 1 pixel, we find that the nod difference shows no evidence for flexure.

A random nod difference along column 400 near one pinhole spectrum showing the difference (white), the difference with +/- 1 pixel shifts to one side of the nod (green/blue), and the column DN before difference (red).

Made similar observations at +/- 1 hour to simulate SOFIA telescope rotations of elevation = 25 and elevation = 55 (on SOFIA).  Looking at the difference in pinhole positions for the different hour angles, find change of <3 pixels between "elevation = 25" and "elevation = 55".  At the focal plane, this is <75 micron.  It is a smooth change at the positions examined with half coming between HA = +1 and HA = 0, and half coming between HA = 0 and HA = -1.

Looking at the position of the night sky lines, did not notice any shift with hour angle.  This suggests the echelle grating is the most likely source of the 3 pixel shift.  If this is true, the angular change in the grating to give a 3 pixel shift would be the linear shift / focal length or 75 microns / 1 m = 15 arcseconds or 0.004 degrees.

Mapping beam translation caused by ellipsoid translation

Mark and I mapped out how the beam translates at particular parts of the instrument in response to fore-optics ellipsoid translations. Note that we did these measurements before and sent out an email report, but there was a mistake in the recording of those values (actually, a pair of axes were swapped). The following measurements were redone on 9/19/2011.

Below is a chart of our measurements and a photo to define the axes. We set out to measure beam translations at these locations: slit (front wall of the slit/filter/decker cassette), echelle grating, and echelle paraboloid.

Here are our measurements:

ellipsoid motion beam measurement location beam motion
---------------------------------------------------------------------------------------------
-3mm, EE front of slit cassette -5mm, EE
-6mm, EE -10mm, EE
-3mm, FE -5mm, Z
-5.4mm, FE -8mm, Z

-3mm, FE echelle paraboloid -13mm, FE, +2mm EE
-3mm, EE -12mm, EE, -2mm, FE

-3mm, FE echelle grating +7mm, FE
-5.4mm, FE +13mm, FE
-3mm, EE -7mm, EE

How much will the hold time of the EXES Helium dewar change if we install a copper grounding wire?

The current hold time is 25 hrs, for 9.5 L of He.
relevant constants:
L He density: 124.96 kg/m^3
He heat of vaporization: 20.3 kJ/kg

Heat load on the dewar now:

9.5 L * (124.96kg /m^3 )(m^3/1000 L)*(20.3 kJ)= 24.1 kJ per 25 hr --> 0.27 W

The copper wire that will be installed is a 2m long 36 AWG (0.005 inches in diameter).
Lakeshore* has values for the thermal conductivity of Cu at cryogenic temperatures:
4K 300 W/m/K
10K 700 W/m/K
20K 1100 W/m/K
80K 800 W/m/K

The wire connects the 4K He dewar to the 77 N dewar.
As a worst case, I adopt the 1100 W/m/K.

The heat load from the wire is:

1100 W/mK *(77-4 K) A / L, where A is the cross-sectional area of the wire and L is the length, 2 m.
A= (0.005/2 inches)^2 *pi * (2.54 cm/inch * 1 m / 100 cm)^2 = 1.27e-8 m^2

1100 W/mK * 73 K * 1.27e-8 m^2 /2 m = 0.00051 W, compared to 0.27 W currently in the dewar.

This will reduce the hold time by: 0.05 hrs or 3 min.

* http://www.lakeshore.com/temp/acc/am_wirets.html

Entrance window thicknesses

I used the ISP website on Pressure window design to recheck the thickness for our entrance windows.  The unsupported diameter is 1.85 inches for the center of the o-ring.  With the safety factor = 4, I got thicknesses of 6.6 mm for CsI and 3.0mm for KRS-5.  I had told Dana slightly different numbers, but I bet we're getting edge thickness that are close to correct.  They will at least have edge thickness sufficient to give safety factor >3.