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GHRS Instrument Handbook 6.0 81
Chap t er 7 Target Acquisition
Reference Information
7.1 Predicting Target Acquisition Count Rates for Stars 82
7.1.1 Alternate Method for Predicting Target Acquisition Count Rates 83
7.1.2 Two Examples 83
7.2 Constraints on Acquisitions and the STEP­TIME Parameter 86
7.2.1 Very Bright and Blue Stars 86
7.2.2 STEP­TIME Constraints 86
7.2.3 ``Wrapping'' of the Counters 86
7.3 Acquisition Count Rates for Extended Objects 90
7.4 Other Acquisition Information 92
7.4.1 Effective Areas of the Acquisition Mirrors 92
7.4.2 Geocoronal Lyman­alpha Background 94
7.5 Obsolescent Acquisition Parameters 94
7.5.1 Interactive Acquisitions 94
7.5.2 Early Acquisitions 94
7.5.3 MAPs 95

Target Acquisition Reference Information
82 GHRS Instrument Handbook 6.0
7.1 Predicting Target Acquisition Count Rates for Stars
We have calculated GHRS target acquisition count rates for the spectra of a subset of
the 175 stars contained in the Bruzual­Persson­Gunn­Stryker (BPGS) Library of Stellar
Spectra by using calcphot, a task in the synphot package in stsdas. GHRS target
acquisition count rates for objects similar to those in the BPGS catalog can be predicted
by following the procedures described here. Constraints on the value ofSTEP­TIME
are discussed. Please note that the values tabulated are the total count rate for a star, and
that the count rate for a particular diode will depend on that portion of the Point Spread
Function that strikes it. That can influence the degree, for example, to which the paired­
pulse correction applies. However, the acquisition procedure sums the counts over the
eight science diodes upon which the LSA is imaged, so for most objects these values
may be used with confidence.
Do not forget to reduce these values by a factor of 0.3 if the focus diodes are being used
for an IMAGE; this is because of the reduced area of the focus diodes compared to using
eight normal diodes for an acquisition. This factor applies to when a focus diode is cen­
tered on a point source.
The flux distributions in the BPGS catalog include ultraviolet wavelengths and can be
used for planning GHRS target acquisitions. Each spectrum in the catalog was dered­
dened and scaled toV 0 = 0.0. Thecalcphot task in thesynphot package of stsdas was
used to convolve the catalog flux distributions with the effective areas of the acquisition
mirrors. Table 7­1 on page88 contains columns giving the BPGS catalog object name,
spectral type, (B--V) 0 , count rate for the acquisition mirror with no reddening, and scale
factors (per unit magnitude) indicating the relative count rate observed at given amounts
of reddening compared to the count rate with no reddening.
To use Table 7­1 on page88 to predict target acquisition count rates:
. Determine the intrinsic color, (B -- V) 0 , and intrinsic magnitude,V 0 , of your object as
well as its color excess,E(B -- V).
. Find an entry in Table 7­1 that has similar spectral characteristics to your object [by
spectral type or (B -- V) 0 and note that luminosity class is important for the coolest
stars]. The table is sorted by increasing(B -- V) 0 . Make sure you pick from the col­
umn corresponding to the acquisition mirror that you plan to use.
. Scale the predicted count rate found in the previous step by the ratio of apparent
brightness of your object to an object of magnitude zero, i.e., multiply by .
. To obtain the scale factor by which the unreddened count rate will be reduced for an
amount of reddening appropriate to your object (the reddening reduction factor),
multiply the count rate from the previous step by this factor:
. The GHRS detectors are nonlinear at high count rates: this phenomenon is referred
to as the ``dead­time'' or ``paired­pulse'' effect. Consequently, the predicted count
rate from the previous step must be reduced to yield the actual count rate that GHRS
will measure. Multiply the count rate you just determined by the ``fraction detected''
value determined from Figure 7­2. on page85 to obtain the final predicted count
rate.
10 0.4V 0
--
10 scale factor E B V
--
( )
â

GHRS Instrument Handbook 6.0 83
Target Acquisition Reference Information
. This final value should be reliable to within a factor of two, which is adequate for
acquisition purposes in almost all instances.
7.1.1 Alternate Method for Predicting Target Acquisition Count Rates
Figure 7­1. on page84 shows mean predicted count rates as a function of(B -- V) 0 color
for the four acquisition mirrors. Also shown are the fits to the predicted count rates for
different amounts of reddening. The label for each curve represents the color excess,
E(B -- V), applied to the spectra.
You can estimate target acquisition count rates using the plots instead of Table 7­1 on
page 88:
. Determine the intrinsic color and magnitude of your object as well as its color
excess.
. Read from the Figures (depending on the mirror used) the predicted count rate.
. Scale the predicted count rate by the ratio of apparent brightness of your object to an
object with zero magnitude.
. Use Figure 7­2. on page85 to correct for the ``paired­pulse'' effect.
7.1.2 Two Examples
First, suppose you want to observeµ Col, which has the following properties:
Using the table, you would see that HR 8023, an O6 star with is the
closest match, giving a predicted count rate for the A2 mirror of 1.3x 10 7 counts s --1 for
a V 0 = 0 star. Multiplying this count rate by 10 ­0.4â5.13 gives 1.2 â 10 5 counts s --1 . Red­
dening will decrease the counts slightly; calculation of the scale factor indicates that you
should multiply by 0.93, giving a new count rate of 1.1â 10 5 counts s --1 . The dead­time
correction factor estimates that only 83% of those counts will be detected, so one would
expect approximately 94000 counts s --1 with this star. In fact, whenµ Col was observed
early in Cycle 4, 19,600 counts were obtained in 0.2seconds with the A2 mirror, which
works out to 98,000counts s ­1 , which is within 5% of the calculated value.
Second, consider a very red star such as Aldebaran:
Using the table, you would see that BD--1 o 3113 has a similar spectral type (K5III) and
color (1.61). The calculated count rate for mirror A2 is then counts per sec­
ond. The adjustment for apparent magnitude is , which yields a count rate of
530 per second, or 210 in 0.2 seconds. Early in Cycle 4a Tau was acquired with mirror
A2 and the count rate seen was 246 in 0.2 seconds, within 15% of the predicted value.
Name Sp. Type. V B -- V E(B -- V)
µ Columbae O9V 5.16 --0.29 0.01
Name Sp. Type. V B -- V E(B -- V)
a Tauri K5III 0.84 1.54 0.00
B V
--
( ) 0.313
--
=
1.6 10 3
â
10 0.4 0.84
â
--

Target Acquisition Reference Information
84 GHRS Instrument Handbook 6.0
Figure
7­1.
Mean
target
acquisition
count
rates
for
stars
with
the
four
mirrors
of
the
GHRS.
The
numbers
at
the
right
indicate
the
appropriate
E(B
--
V)
for
each
curve.

GHRS Instrument Handbook 6.0 85
Target Acquisition Reference Information
Figure 7­2. Fraction of counts detected as a function of the true count rate,
i.e., before the paired pulse correction.

Target Acquisition Reference Information
86 GHRS Instrument Handbook 6.0
7.2 Constraints on Acquisitions and theSTEP­TIME Parameter
7.2.1 Very Bright and Blue Stars
There are limits to the brightest and bluest stars that can be acquired with mirrors N1
and N2 because of the very high fluxes of these objects. Figure 7­3. on page87 shows
the limits for these two mirrors as set in the Constraints and Restrictions Document
(CARD). Objects that are too blue and too bright relative to these limits should be
acquired with mirror A1 or A2.
7.2.2 STEP­TIME Constraints
Once you have predicted an acquisition count rate, you then determine the integration
time per dwell point during the target acquisition: STEP­TIME. The goal is to have
GHRS see 10 3 to 10 4 counts at the peak dwell point of the spiral search for either an
acquisition or a peakup. A minimum of about 100 counts is acceptable for fainter
objects. The STEP­TIME then is just the number of counts desired (10 3 to 10 5 , but at
least 100) divided by the predicted count rate. Remember that valid values ofSTEP­
TIME range from 0.2 seconds to 12.75 seconds. If you have questions about acquiring
very bright and faint objects you may wish to consult a GHRS Instrument Scientist.
7.2.3 ``Wrapping'' of the Counters
To avoid the possibility of a failedACQ/PEAKUP, you should ensure that no single
diode of the eight used for target acquisition exceeds 65,000 total counts (the number
accommodated in a 16 bit register) in a givenSTEP­TIME. While this is usually a
problem only for very bright targets, it can always be avoided by using a STEP­TIME
of 0.6 seconds or less. This problem is avoided if BRIGHT=RETURN is specified
because a 32­bit on­board register is used. If instead aBRIGHT limit is explicitly given,
the register that holds it is limited to 16 bits.
Figure 7­3 is provided to allow for a visual check of potential problems arising from the
choice of STEP­TIME. This Figure shows the constraints placed on using a particular
mirror and STEP­TIME.

GHRS Instrument Handbook 6.0 87
Target Acquisition Reference Information
Figure 7­3. Target acquisition constraints.
N2R: Constraints and Restrictions Document (CARD) upper limit for use of Mirror­N2. Observing
objects that are bluer and brighter than indicated by this line would result in degraded perfor­
mance and possible damage to the instrument. Brighter and bluer objects must be acquired with
mirrors A2 or A1.
N1R Constraints and Restrictions Document (CARD) upper limit for use of Mirror­N1. Observing
objects that are bluer and brighter than indicated by this line would result in degraded perfor­
mance and possible damage to the instrument. Brighter and bluer objects must be acquired with
mirrors A2 or A1.

Target Acquisition Reference Information
88 GHRS Instrument Handbook 6.0
Table
7­1
Predicted
target
acquisition
count
rates
for
stars,
reduced
toV
0
=
0.
Star
Name
Spectral Type
(B--V)
0
count
rate
for
mirror
reddening
reduction
factor
a
N2
A2
N1
A1
N2
A2
N1
A1
9
Sgr
O5
­0.337
8.595E9
2.098E7
4.598E8
2.657E5
­3.034
­3.108
­3.428
­3.130
HR
8023
O6
­0.313
5.684E9
1.348E7
2.545E8
1.690E5
­2.955
­3.021
­3.382
­3.114
60
Cyg
B1V
­0.269
5.367E9
1.293E7
2.794E8
1.767E5
­3.002
­3.070
­3.393
­3.123
69
Cyg
B0IB
­0.234
4.192E9
9.901E6
1.869E8
1.209E5
­2.935
­3.003
­3.384
­3.110
i
Her
B3V
­0.203
2.764E9
6.529E6
1.232E8
7.970E4
­2.935
­3.003
­3.384
­3.110
HR
7467
B3III
­0.182
2.436E9
5.755E6
1.087E8
7.028E4
­2.936
­3.004
­3.384
­3.110
20
Aql
B3IV
­0.156
2.268E9
5.357E6
1.012E8
6.542E4
­2.936
­3.004
­3.384
­3.110
38
Oph
A1V
­0.139
1.776E9
4.108E6
6.421E7
4.582E4
­2.891
­2.953
­3.347
­3.101
HR
7346
B7III
­0.108
1.368E9
3.165E6
4.948E7
3.531E4
­2.892
­2.953
­3.347
­3.101
HD
189689
B9V
­0.081
1.008E9
2.303E6
2.763E7
2.306E4
­2.864
­2.921
­3.317
­3.097
59
Her
A3III
­0.045
5.948E8
1.363E6
1.700E7
1.407E4
­2.873
­2.932
­3.320
­3.099
11
Sge
B9IV
­0.027
5.745E8
1.293E6
1.168E7
1.182E4
­2.819
­2.873
­3.281
­3.088
69
Her
A2V
0.000
4.918E8
1.107E6
9.997E6
1.011E4
­2.819
­2.873
­3.281
­3.088
HR
8020
B8IA
0.027
1.063E9
2.386E6
1.940E7
2.154E4
­2.820
­2.872
­3.255
­3.089
78
Her
B9V
0.036
4.532E8
1.018E6
8.272E6
9.187E3
­2.820
­2.872
­3.255
­3.089
58
Aql
A0V
0.057
4.291E8
9.634E5
7.830E6
8.696E3
­2.820
­2.872
­3.255
­3.089
60
Her
A3IV
0.085
3.197E8
7.086E5
3.280E6
5.497E3
­2.758
­2.810
­3.217
­3.077
HR
6570
A7V
0.107
2.999E8
6.647E5
3.079E6
5.161E3
­2.759
­2.811
­3.217
­3.077
HD192285
A4IV
0.124
2.628E8
5.771E5
1.444E6
3.569E3
­2.719
­2.771
­3.184
­3.071
q
1
Ser
A5V
0.143
2.669E8
5.862E5
1.466E6
3.623E3
­2.718
­2.770
­3.184
­3.071
KW
114
0.175
2.395E8
5.240E5
9.954E5
2.843E3
­2.703
­2.755
­3.168
­3.069
KW
154
0.219
2.050E8
4.439E5
4.497E5
1.636E3
­2.666
­2.715
­3.166
­3.063
c
Ser
F0IV
0.240
1.829E8
3.958E5
4.004E5
1.457E3
­2.665
­2.714
­3.166
­3.062
HD5132
F0IV
0.287
2.011E8
4.354E5
4.412E5
1.605E3
­2.666
­2.715
­3.166
­3.063
HD508
A9IV
0.305
1.718E8
3.720E5
3.771E5
1.372E3
­2.666
­2.716
­3.166
­3.063
r
Cap
F2IV
0.339
1.125E8
2.322E5
2.619E4
1.014E2
­2.484
­2.517
­3.181
­3.023
KW
332
0.389
1.177E8
2.431E5
2.751E4
1.065E2
­2.486
­2.519
­3.181
­3.023
HD7331
F7IV
0.427
9.807E7
2.012E5
2.256E4
8.772E1
­2.451
­2.482
­3.179
­3.015
BD+63
o
13
F5IV
0.444
1.139E8
2.362E5
6.070E4
2.395E2
­2.488
­2.523
­3.171
­3.024
HD35296
F8V
0.489
1.089E8
2.259E5
5.804E4
2.290E2
­2.488
­2.523
­3.171
­3.024
vB
1
0.530
6.757E7
1.374E5
2.147E4
8.448E1
­2.407
­2.437
­3.175
­3.006
HD154760
G2V
0.586
6.393E7
1.285E5
3.115E3
1.151E1
­2.365
­2.386
­3.206
­2.998

Target Acquisition Reference Information
GHRS Instrument Handbook 6.0 89
a.
use
factor
f
to
reduce
count
rate
by
.
HD139777A
K0V
0.631
6.040E7
1.215E5
2.954E3
1.091E1
­2.367
­2.387
­3.206
­2.998
HR
6516
G6IV
0.651
5.167E7
1.039E5
2.518E3
9.301E0
­2.365
­2.386
­3.206
­2.998
HD136274
G8V
0.672
4.901E7
9.855E4
2.392E3
8.837E0
­2.366
­2.387
­3.206
­2.998
HD150205
G5V
0.700
4.650E7
9.352E4
2.275E3
8.404E0
­2.367
­2.388
­3.206
­2.998
31
Aql
G8IV
0.732
4.042E7
8.123E4
1.960E3
7.242E0
­2.364
­2.384
­3.206
­2.997
vB
21
0.766
3.890E7
7.818E4
1.887E3
6.972E0
­2.364
­2.385
­3.206
­2.997
BD--2
o
4018
G5IV
0.791
3.516E7
7.068E4
1.713E3
6.328E0
­2.365
­2.386
­3.206
­2.998
HD190571
G8V
0.816
1.587E7
3.086E4
1.165E1
4.76E­4
­2.233
­2.244
­3.719
­3.247
HD11004
G5IV
0.825
1.587E7
3.086E4
1.169E1
4.78E­4
­2.234
­2.244
­3.719
­3.247
HD56176
G7IV
0.857
1.510E7
2.937E4
1.109E1
4.54E­4
­2.233
­2.244
­3.719
­3.247
HD190470
K3V
0.895
1.237E7
2.403E4
8.968E0
3.67E­4
­2.230
­2.241
­3.719
­3.246
q
1
Tau
G8III
0.904
1.950E7
3.862E4
2.639E1
2.67E­4
­2.306
­2.320
­3.750
­3.262
HD170527
G5IV
0.918
2.157E7
4.277E4
2.944E1
2.98E­4
­2.309
­2.323
­3.750
­3.263
HD191615
G8IV
0.947
1.596E7
3.159E4
2.136E1
2.16E­4
­2.303
­2.317
­3.750
­3.261
HD4744
G8IV
0.989
1.087E7
2.119E4
8.241E0
3.37E­4
­2.240
­2.250
­3.719
­3.248
91
Aqr
K0III
1.010
8.535E6
1.662E4
6.381E0
2.61E­4
­2.237
­2.247
­3.719
­3.247
HD95272
K0III
1.041
8.184E6
1.593E4
6.086E0
2.49E­4
­2.236
­2.246
­3.719
­3.247
y
UMa
K1III
1.081
6.894E6
1.342E4
5.143E0
2.10E­4
­2.237
­2.247
­3.719
­3.247
BD+1
o
3131
K0III
1.143
4.403E6
8.511E3
7.928E0
2.54E­4
­2.210
­2.221
­3.718
­3.242
vB
173
1.202
6.753E6
1.337E4
2.396E1
7.80E­4
­2.296
­2.311
­3.721
­3.260
HD166780
K5III
1.323
2.059E6
3.986E3
3.791E0
1.21E­4
­2.214
­2.225
­3.718
­3.243
RZ
Her
M6III
1.380
5.352E6
1.042E4
1.057E1
3.39E­4
­2.228
­2.239
­3.718
­3.246
HD116870
M0III
1.413
1.414E6
2.734E3
2.228E0
7.87E­5
­2.207
­2.218
­3.717
­3.241
M67
IV­202
1.463
1.170E6
2.263E3
1.857E0
6.56E­5
­2.208
­2.219
­3.717
­3.242
HD104216
M2III
1.523
1.423E6
2.795E3
4.603E0
1.50E­4
­2.271
­2.287
­3.721
­3.255
BD--1
o
3113
K5III
1.609
8.070E5
1.588E3
2.681E0
8.72E­5
­2.277
­2.293
­3.721
­3.256
HD142804
M1III
1.694
7.419E5
1.463E3
2.517E0
8.19E­5
­2.283
­2.299
­3.721
­3.258
Gl
15B
M6V
1.707
2.391E6
4.742E3
8.704E0
2.83E­4
­2.303
­2.318
­3.721
­3.262
Gl
65
M5V
1.768
1.075E7
2.122E4
3.642E1
1.18E­3
­2.284
­2.300
­3.721
­3.258
HD
151658
M2III
1.770
7.242E5
1.423E3
2.359E0
7.67E­5
­2.272
­2.288
­3.721
­3.255
R
Leo
1.918
4.550E6
8.879E3
1.308E1
4.26E­4
­2.245
­2.260
­3.721
­3.250
WZ
Cas
N
2.636
1.987E5
3.867E2
5.89E­1
1.92E­5
­2.246
­2.263
­3.721
­3.250
AW
Cyg
N
3.790
1.951E5
3.796E2
5.70E­1
1.85E­5
­2.243
­2.260
­3.721
­3.249
Table
7­1
Predicted
target
acquisition
count
rates
for
stars,
reduced
toV
0
=
0.
(Continued)
Star
Name
Spectral Type
(B--V)
0
count
rate
for
mirror
reddening
reduction
factor
a
N2
A2
N1
A1
N2
A2
N1
A1

Target Acquisition Reference Information
90 GHRS Instrument Handbook 6.0
7.3 Acquisition Count Rates for Extended Objects
Chapter 4 mentions acquisition methodologies for extended objects, and, in particular,
the use of the EXTENDED Optional Parameter. In this section we provide some guid­
ance on predicting the count rates to be expected during an acquisition of an extended
object, especially one beyond our own Galaxy. The renewed availability of Side 1 of the
GHRS and its G140L grating make it possible to get high­quality spectra of faint objects
efficiently.
Whenever possible, we recommend that faint objects be acquired by offsetting from a
nearby and brighter point source. If accurate coordinates are used, this method should
be reliable. However, such objects are not always present next to targets of astrophysical
interest. Also, obtaining a good astrometric position of an extended source can be pre­
vented by its large saturated area on the photographic plates upon which the Guide Star
Catalog is based. In these cases a direct target acquisition will need to be attempted, and
it should succeed if the object provides enough ultraviolet photons. The procedure
closely follows that for point sources just described:
. Find a star in Table 7­1 on page88 with a spectral energy distribution like that of
your object, or consult the IUE Atlas of Star­Forming Galaxies, by Kinney et al.
(1993, ApJS, 86, 5). This publication provides representative spectra of many
classes of galaxies and compares their shapes to ones of stellar spectra (whence the
``spectral types'' listed).
. Estimate the flux that will fall within the LSA from either Table 7­1 on page88 or
Table 7­2 on page91, as appropriate.
. Calculate STEP­TIME in the same manner as for stars. If necessary, you may use
mirror N2 on Side 2 for your acquisition, even if Side 1 is being used to observe, but
doing so will add about 40 minutes of time in order to switch from one Side to the
other.
. If your object does not fall within these categories, please consult us.

GHRS Instrument Handbook 6.0 91
Target Acquisition Reference Information
a. use factor f to reduce count rate by .
Table 7­2 Predicted count rates for non­stellar objects
``Spectral
Type''
count rate for mirror reddening reduction factor a
N2 A2 N1 A1 N2 A2 N1 A1
o3_6v 8.843E9 2.116E7 3.990E8 2.726E5 ­3.059 ­3.112 ­3.362 ­3.134
o4_9i 8.047E9 1.934E7 3.731E8 2.427E5 ­3.054 ­3.109 ­3.380 ­3.133
o5_6iii 8.935E9 2.140E7 3.961E8 2.687E5 ­3.068 ­3.121 ­3.364 ­3.136
o7_b0v 7.407E9 1.771E7 3.282E8 2.065E5 ­3.028 ­3.086 ­3.388 ­3.129
o9_b0iv 6.413E9 1.533E7 2.931E8 1.784E5 ­3.012 ­3.072 ­3.396 ­3.125
b0_2i 4.314E9 1.005E7 1.372E8 9.530E4 ­2.949 ­3.005 ­3.378 ­3.113
b0_2iii 5.144E9 1.214E7 1.908E8 1.231E5 ­2.994 ­3.051 ­3.388 ­3.122
b2_4v 2.889E9 6.765E6 1.037E8 7.131E4 ­2.964 ­3.021 ­3.364 ­3.116
b2_5iv 2.754E9 6.444E6 9.673E7 6.712E4 ­2.960 ­3.017 ­3.367 ­3.115
b3_5i 1.694E9 3.836E6 3.581E7 3.042E4 ­2.859 ­2.910 ­3.328 ­3.095
b3_6iii 2.344E9 5.448E6 7.404E7 5.400E4 ­2.943 ­2.998 ­3.353 ­3.112
b5_8v 1.713E9 3.952E6 4.849E7 3.716E4 ­2.922 ­2.976 ­3.337 ­3.108
b6_9i 9.689E8 2.187E6 2.021E7 1.765E4 ­2.831 ­2.882 ­3.313 ­3.089
b7_9iii 8.700E8 1.975E6 1.845E7 1.663E4 ­2.867 ­2.918 ­3.305 ­3.098
b8_9iv 9.701E8 2.216E6 2.337E7 1.984E4 ­2.889 ­2.941 ­3.316 ­3.102
f2_7iv 1.007E8 2.108E5 2.968E4 1.206E2 ­2.547 ­2.580 ­3.201 ­3.043
f2_8i 4.536E7 9.480E4 1.403E4 5.634E1 ­2.535 ­2.569 ­3.201 ­3.041
f5_7v 1.079E8 2.254E5 1.782E4 ­2.538 ­2.571 ­3.238
f6iii 9.009E7 1.862E5 8.586E2 ­2.499 ­2.526 ­3.830
f8_9v 6.664E7 1.367E5 3.996E3 ­2.460 ­2.484 ­3.245
g0_2iv 4.451E7 9.041E4 3.614E2 ­2.415 ­2.434 ­3.604
g0_3i 1.971E7 4.015E4 ­2.429 ­2.444
g0_5iii 3.349E7 6.822E4 ­2.431 ­2.450
g0_5v 5.372E7 1.095E5 ­2.428 ­2.449
g5_8i 4.802E6 9.721E3 ­2.399 ­2.414
g5_8iv 2.123E7 4.242E4 ­2.344 ­2.354
g5_k0iii 6.969E6 1.384E4 ­2.320 ­2.330
g6_9v 2.588E7 5.189E4 ­2.360 ­2.372
g8_k1iv 8.008E6 1.583E4 ­2.297 ­2.304
k0_1v 1.307E7 2.606E4 ­2.337 ­2.348
k0_2iii 5.918E6 1.165E4 ­2.285 ­2.290
k1_3i 2.497E6 5.057E3 ­2.397 ­2.411
k2_3v 9.035E6 1.800E4 ­2.327 ­2.336
k2iii 2.092E6 4.107E3 ­2.268 ­2.273
k3iii 1.157E6 2.266E3 ­2.259 ­2.264
k4_5iii 5.490E5 1.085E3 ­2.290 ­2.296
k5_m0v 2.607E6 5.167E3 ­2.304 ­2.311
k5_m5i 9.714E5 1.976E3 ­2.413 ­2.426
k7_m3iii 5.420E5 1.081E3 ­2.323 ­2.330
10 f E B V
--
( )
â

Target Acquisition Reference Information
92 GHRS Instrument Handbook 6.0
7.4 Other Acquisition Information
7.4.1 Effective Areas of the Acquisition Mirrors
Table 7­3 on page93 lists the effective areas of the four acquisition mirrors (in cm 2 ) as a
function of wavelength, to use to predict acquisition count rates. These values are those
from the Science Verification Report for the GHRS, adjusted by the observed ratio of
post­ to pre­COSTAR sensitivities. Note that A1 and N1 may only be used with detector
D1, and A2 and N2 with detector D2.
Also note that the proper use of this table requires compensation for the different ener­
gies of photons of different wavelengths, hence the last column, which is in picoergs per
photon.
Figure 7­4. Relative sensitivities of the GHRS acquisition mirrors. The effec­
tive areas shown are from Table 7­3 on page 93.

GHRS Instrument Handbook 6.0 93
Target Acquisition Reference Information
Table 7­3 Effective areas of the four GHRS acquisition mirrors
Wavelength (å)
Effective Area (cm 2 )
perg
photon ­1
A1 N1 A2 N2
1100 0.0025 0.00085 0.175 18.1
1150 4.89 0.12 23.36 17.3
1200 16.79 0.29 54.18 16.6
1250 28.54 0.34 76.50 15.9
1300 34.10 0.37 96.22 15.3
1350 36.64 0.34 97.03 14.7
1400 36.05 0.28 96.88 14.2
1450 35.35 0.28 105.46 13.7
1500 35.49 0.27 113.43 13.2
1550 0.000172 34.43 0.29 133.84 12.8
1600 0.00298 30.18 0.31 155.76 12.4
1650 0.0317 29.37 0.37 165.92 12.0
1700 0.0843 25.91 0.44 176.10 11.7
1750 0.0923 18.11 0.56 216.91 11.4
1800 0.0680 11.82 0.68 259.65 11.0
1850 0.0261 7.38 0.74 281.80 10.7
1900 0.0094 3.04 0.79 302.40 10.5
1950 0.0021 1.04 1.04 362.78 10.2
2000 0.55 1.04 363.32 9.93
2050 0.30 1.08 406.51 9.69
2100 0.16 1.13 449.71 9.46
2200 1.43 594.61 9.03
2300 1.55 663.74 8.64
2400 1.75 776.52 8.28
2500 1.87 847.76 7.95
2600 1.85 854.42 7.64
2700 1.83 866.65 7.36
2800 1.68 813.29 7.09
2900 1.50 754.81 6.85
3000 0.76 398.89 6.62
3100 0.58 309.31 6.41
3200 0.38 194.15 6.21
3300 0.21 116.67 6.02
3400 0.14 80. 5.84

Target Acquisition Reference Information
94 GHRS Instrument Handbook 6.0
7.4.2 Geocoronal Lyman­alpha Background
The acquisition mirror sensitivity curves illustrated above show that the mirrors still
reflect well at the Lyman­a line at 1216 å. Note that Ly­a is suppressed by mirror A1.
Tests have shown that the count rate from geocoronal Lyman­a can be as high as 12
counts per second per diode. If you wish to acquire a faint target with the least Ly­a
contamination, we suggest that you specifySHADOW as a Special Requirement in Phase
II. Doing so, however, will limit the schedulability of your program.
7.5 Obsolescent Acquisition Parameters
We mention here for completeness several aspects of acquisitions that are now unused
but which may be helpful in some circumstances. We are not aware of any usage of
these in Cycles 4 or 5. There is nothing wrong with these features, but few or no GOs
have found them necessary.
7.5.1 Interactive Acquisitions
We first discuss interactive acquisitions (INT ACQ) in the hope of dissuading you from
using them. AnINT ACQ requires real­time contact between the ground andHST. Real­
time contact is a limited and expensive resource that should only be used as a last resort.
In almost all cases where an onboard acquisition will not work (because the object is in
a crowded field, or is variable, or is a moving target), it is sufficient to use an early
acquisition to get a WFPC2 or FOC image a few weeks in advance of the GHRS obser­
vation. The image can then be analyzed to pinpoint the source to be observed without
requiring real­time contact. INT ACQ may be needed in a few instances where the
object changes in its ultraviolet brightness unpredictably. We suggest that you consult
with us before requestingINT ACQ.
If an interactive acquisition with the GHRS has been specified, a spiral search will be
run after HST makes its initial pointing. A map of the LSA is made at each dwell point
and each map is then downlinked to STScI in real time and may be viewed almost
immediately in OPUS. After the spiral is complete, the telescope remains at its final
dwell point, awaiting instructions. The final 9 (or 25) maps are assembled into a mosaic
and displayed for the observer to identify the target, either from a cursor position or
from calculation of a centroid. The motion needed to center the specified position in the
LSA is computed and uplinked toHST. The recentering of the target usually takes place
about an orbit after the spiral search, and must be scheduled for a specific time. If you
have so requested in your proposal, an image of the LSA will be made after the recen­
tering so that you may confirm the position of the target (but additional interaction at
that time is not normally possible).
7.5.2 Early Acquisitions
In an early acquisition an image of the field of interest is obtained several weeks (8 or
more) in advance of the spectroscopic observation that the GHRS is to make. The image
may be obtained with WFPC2, with the FOC (especially if an ultraviolet image is
desired), or with the imaging capability of the GHRS itself. The GHRS is relatively
slow at getting images, so if you wish to map an area much larger than about 2â 2 arc­

GHRS Instrument Handbook 6.0 95
Target Acquisition Reference Information
sec we recommend that you consider WFPC2 or FOC. However, the GHRS has the
capability of obtaining a monochromatic map (see below and Section4.2 on page 45) in
IMAGE mode, which can be useful in some situations. For an early acquisition, be sure
to note the relationship of the image to the spectroscopic observations as a Special
Requirement (use ON HOLD). Also, you should plan ahead so that the early acquisition
image can be analyzed quickly and the positions measured sent back to STScI for incor­
poration into the telescope observing schedule.
7.5.2.1 Explicitly Specifying BRIGHT and FAINT limits
Explicit BRIGHT and FAINT limits may be specified if you desire, although there is an
increased risk of a failed acquisition unless you are confident of those fluxes. Also, a
few very bright stars cannot be acquired with Side 2 if an explicitBRIGHT value is
given but they can be acquired automatically withBRIGHT=RETURN (this is because
different bit levels of counters apply in the two cases).
Details on computing BRIGHT and FAINT limits are given in Section7.1 on page 82.
Please note that although we discourage the use of explicitBRIGHT and FAINT values
unless they are unavoidable, you still need to estimate the target acquisition count rate in
order to ensure that you choose the acquisition mirror correctly and that theSTEP­
TIME is determined properly.
Please note that using BRIGHT=RETURN and explicitly specifying BRIGHT and
FAINT limits result in fundamentally different acquisition procedures. If BRIGHT and
FAINT are specified, the acquisition stops as soon as those conditions are met and the
point at which that happened is moved to the center of the LSA. With
BRIGHT=RETURN, the entire spiral search region is sampled and the brightest object in
it determined before any movement is made to center on the target. Both procedures
require the same amount of telescope time because the schedule must allow for the
entire region to be sampled.
7.5.3 MAPs
The GHRS has the ability to make aMAP of the LSA by raster scanning one or both of
its small focus diodes over the aperture. You may, for example, want a map to confirm
the pointing at the time your spectrum was taken. The default forONBOARD ACQuisi­
tions is to make no map. If you ask forMAP=END­POINT, you will get a map after the
spiral search has found your target, but before it has been centered (withLOCATE) in
the LSA. If you want a mapafter the final centering, you can add a single line using
IMAGE mode. An IMAGE may also be obtained of the SSA, which can be a usefula
posteriori means of determining what was observed in a crowded field. TheMAP=ALL­
POINTS option may not be used with anONBOARD ACQuisition.
The time per exposure is calculated from
where (i.e., 1, 9, or 25), and is the number of
dwell points mapped (=1 ifMAP=END­POINT is chosen and = if MAP=ALL­
POINTS. MAP=ALL­POINTS can only be used withINT ACQ or an early acquisi­
t exp 128 N MAP
â N SEARCH
+
( ) STEP­TIME
â
=
N SEARCH SEARCH­SIZE
( ) 2
= N MAP
N SEARCH

Target Acquisition Reference Information
96 GHRS Instrument Handbook 6.0
tion.). Please note the value ofSTEP­TIME you want as aCOMMENT on the expo­
sure line.