# HETG Location in Flight

Revised 7/17/99 dd
( go to HETG location at XRCF )

### Introduction

The axial (along X axis) location of the gratings determines their dispersion scale through the Rowland spacing; the Rowland spacing of the grating is the on-axis Rowland circle intercept to focal plane distance. This spacing must be inferred from measurements between the HETG and HRMA as well as assumed/measured location of the HRMA focus w.r.t. the HRMA fiducial location, e.g., CAP center line.

### Grating and HRMA Axial Reference Locations

In order to put measured values into context it is important to have various axial reference points of the HRMA and gratings specified relative to each other:

Positive direction is away from the focal plane.
Grating
Axial Locations
HETG Xg ValueLETG Xg Value
"Xg = 0"0.00.0
HESS/GESS surface
(metrology reference)
0.800"18.648 mm
[0.734"]
ARM reflective
surface
1.500"1.700"
OTG Datum -D-
1.670"1.670"
Rowland intercept2.500"2.500"
 Flight HRMAAxial Locations Value w.r.t. CAP CL(July 4, '96) Value w.r.t. CAP CL(July '99) CAP CL(midplane) 0.0 0.0 HRMA node 0.371" 49.911/2. - 18.15 mm = 6.805 mm= 0.268" CAP Datum -A-"= P-side of CAP"CAP width is2x0.9825 0.9825" 49.911/2 mm = 0.9825" HRMA focus -395.909" 10079.77 - 49.911/2.0 = 10054.814 mm = -395.859" HRMA focallength(node-to-focus) 25.4*(395.909 + 0.371) = 10065.51 mm 10079.77 - 18.15 = 10061.62 mm

### Design and Mechanical Measurements

The following equation gives the Rowland spacing in terms of other measurable values:

``` RC spacing  =  Focus to CAP CL  -  Design CAP CL to OTGorigin
+  OTGorigin to RC  + Install Offset
```
Values for these quantities are tabulated below for three flight cases:
• i) Flight design drawing (301331, July 4, 1996)
• ii) July '99 values: Includes July '99 HRMA focus value and OTG-installation corrections. Scott Texter reports that EK measures the as-installed HETG to be 0.009 inches closer to the HRMA than designed; the LETG is 0.0043" further from HRMA than designed.

 Case) Grating RC spacing (inches) Focus to CAP CL (inches) CAP CL to OTGorigin (inches) OTGorigin to RC (inches) Installoffset(inches) RC spacing (mm) i) Both 8633.69 339.909 395.909 56.83 + 1.67 = 58.50 2.5 0.0 ii) HETG 8632.65 339.868 395.859 58.50 2.5 0.009 ii) LETG 8632.31 339.855 395.859 58.50 2.5 -0.0043
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