HETG Location at XRCF

Revised 7/17/99 dd
( go to HETG location in Flight )


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.
Axial Locations
HETG Xg ValueLETG Xg Value
"Xg = 0"0.00.0
HESS/GESS surface
(metrology reference)
0.800"18.648 mm
ARM reflective
OTG Datum -D-
alignment pad surface
Rowland intercept2.500"2.500"
Axial Locations
Value w.r.t. CAP CL
(Spring, '98)
Value w.r.t. CAP CL
(July '99)
HRMA node0.371" 49.911/2 - 18.15
= 6.805 mm
= 0.268"
CAP Datum -A-
"= P-side of CAP"
CAP width is
0.9825" 49.911/2 mm
= 0.9825"
XRCF ARM2.846"2.846"
HRMA focus-403.512" 10274.81 - 49.911/2.
= 10249.85 mm
= -403.538"
HRMA focal
25.4*(403.512 + 0.371)
= 10258.63 mm
10274.81 - 18.15
= 10256.66

Design and Mechanical Measurements

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

 RC spacing  =  Focus to CAP CL  -  CAP CL to OTGorigin  +  OTGorigin to RC
Values for these quantities are tabulated below for three cases: The error on the Rowland Spacing at XRCF is at least 0.5 mm due to error in determining the "Focus to CAP CL" distance. Additionally, the "CAP CL to OTGorigin" distances were measured with a laser ranger, measuring the distance between the grating ARM reflective surface and the HRMA "XRCF ARM" surface. This measurement requires compensation for traversing the HRMA XRCF ARM glass substrate - systematics here could allow a common axial shift of the gratings.

Case) Grating RC spacing
RC spacing
Focus to CAP CL
CAP CL to OTGorigin
OTGorigin to RC
i) Both8772.83345.387403.560.6132.5
ii) Both8782.79345.779403.560.2212.5
iii) LETG8788.10345.988403.51260.0242.5
iii) HETG8787.99345.984403.51260.0282.5
iv) LETG8788.76 346.014 403.53860.0242.5
iv) HETG8788.65346.01 403.53860.0282.5
iv) HETG-LETG average8788.71346.012 n/an/an/a

HETG Spacing at XRCF: X-ray Data

X-ray measurements at XRCF can be used to verify the grating periods and Rowland spacing. Fits to the XRCF data using a Rowland spacing of 8788.04 mm (the HETG-LETG mean in (iii) above) show a discrepancy between the HEG/MEG sub-assembly periods and the X-ray derived periods. Because both the HEG and MEG periods appear in error by the same fraction, an HETG Rowland spacing error is a simpler explanation than coincident period errors. The Rowland spacing which agrees with the data is close to the expected design value given in (ii) above. To minimize the number of Rowland spacings kicking around, the current working hypothesis is that the HETG was installed at XRCF with a Rowland Spacing of 8782.8 mm. MIT Accessibility