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GHRS Instrument Handbook 5.0 55
Chapter 6 Design and Construction
of the GHRS
6.1 The HST Focal Plane and the GHRS Apertures 56
6.2 Gratings and Optical Elements 60
6.3 The Digicon Detectors 62

Design and Construction of the GHRS
56 GHRS Instrument Handbook 5.0
6.1 The HST Focal Plane and the GHRS Apertures
We provide here a description of the instrument in largely pictorial terms. More illustra­
tions and full technical descriptions of the GHRS may be found in the references (see
Section 9.2 on page 102).
Figure 6­1. The Hubble Space Telescope and its components, with the
locations of important operational elements shown.

GHRS Instrument Handbook 5.0 57
Design and Construction of the GHRS
Figure 6­2. The focal plane of HST and the definitions of the V2, V3 and U2,
U3 coordinate systems of the spacecraft.

Design and Construction of the GHRS
58 GHRS Instrument Handbook 5.0
Figure 6­3. Locations of GHRS apertures relative to spacecraft axes. Note
that the sense of the x and y motions are shown by the arrows, but that the
zero point for each aperture (SSA and LSA) is located at its center. COSTAR
does not, of course, change the layout of the entrance apertures, but it does
alter the way that the sky is imaged onto the focal plane. The sense is easy
to remember: the COSTAR mirrors invert the sense of the original image,
which means that the signs of motions in both coordinates, V2 and V3, (or U2
and U3) are reversed.

GHRS Instrument Handbook 5.0 59
Design and Construction of the GHRS
Figure 6­4. Optical schematic of the GHRS.

Design and Construction of the GHRS
60 GHRS Instrument Handbook 5.0
6.2 Gratings and Optical Elements
Note that the ``CD'' gratings are cross­dispersers for the echelles. CD­A has a focal
length of 1460 mm and CD­B has a focal length of 1340 mm. Note also that the ``M''
gratings are holographic and that the blaze angle quoted formally is that which correctly
predicts the center of the wavelength region the grating is optimized for. G140L is a
ruled grating. ``Ech­A'' and ``Ech­B'' refer to two modes of operation that use the same
echelle grating but different cross­dispersers and detectors.
a. Mirror N2 is actually ``D'' shaped, being a circle with a small slice off one
side. It is about 60â 80 mm.
TABLE 6­1 Properties of GHRS Gratings
Name
Grooves
per mm
Blaze
Angle
Order of
use
Angle of
Incidence
Diffraction
Angle
Deviation
Angle Detector
G140L 600 2.6 1 9.0 -- 10.3 ­5.3 -- ­4.0 14.25 D1
G140M 6000 23 1 26 -- 38 11 -- 24 14.25 D1
G160M 4960 19 1 21 -- 33 14 -- 27 6.25 D2
G200M 4320 26 1 23 -- 34 17 -- 28 6.25 D2
G270M 3600 28 1 27 -- 38 20 -- 32 6.25 D2
Ech­A 316 63.4 33 -- 53 68 -- 74 54 -- 61 13.25 D1
Ech­B 316 63.4 17 -- 33 63 -- 72 58 -- 66 5.75 D2
CD­A 194.6 0.75 1 D1
CD­B 85.7 0.54 1 D2
TABLE 6­2 Properties of Other GHRS Optical Components
Name
Clear Aperture
(mm)
Focal Length
(mm) Detector
LSA = ``2.0'' 0.559 D1, D2
SSA = ``0.25'' 0.067 D1, D2
Collimator 80 1850 D1, D2
Mirror N2 a 80 D2
Mirror A2 20 D2
Mirror N1 80 D1
Mirror A1 20 D1
Cam­A 84 1425 D1
Cam­B 86 1350 D2
D1 22 â 28
D2 22 â 28

GHRS Instrument Handbook 5.0 61
Design and Construction of the GHRS
Figure 6­5. Schematic diagram of GHRS' acquisition optics. The ``main
acquisition mirror'' is N2.

Design and Construction of the GHRS
62 GHRS Instrument Handbook 5.0
6.3 The Digicon Detectors
Figure 6­6. Cutaway view of a Digicon.

GHRS Instrument Handbook 5.0 63
Design and Construction of the GHRS
Figure 6­7. Cross­sectional view of a Digicon and views of its faceplate and
diode arrays.

Design and Construction of the GHRS
64 GHRS Instrument Handbook 5.0
Figure 6­8. A view from the cross­dispersers toward the Digicon detectors to
illustrate the senses of x and y motions and of increasing wavelength.

GHRS Instrument Handbook 5.0 65
Design and Construction of the GHRS
Figure 6­9. A detailed layout of the diodes in the Digicon detectors. Note
the 6 large ``corner diodes'' and the 6 ``focus diodes'' (numbers 4, 5, and 6, for
example).

Design and Construction of the GHRS
66 GHRS Instrument Handbook 5.0