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CAL/FOS­132­1
FOS Target Acquisition Strategies
(SHORT VERSION for Cycle 5 Phase II Reference )
Charles D.(Tony) Keyes, Anuradha P. Koratkar, Ian N. Evans
Space Telescope Science Institute
Instrument Science Report CAL/FOS­132
January, 1995
ABSTRACT
We present tables providing commonly recommended efficient target acquisition strategies for
most FOS apertures as a function of acquisition accuracy. Many of these packaged acquisition
sequences are restricted to objects which are pointlike and which are not highly variable.
Overview
FOS target acquisitions can be divided into three broad categories: 1) those that utilize ACQ/BIN,
at least for the first stage of acquisition; 2) those that require ACQ/PEAK; and 3) those that
require consultation. The applicability of one or more of these categories to a particular acquisi­
tion is determined by many characteristics of the target (e.g., brightness, variability, brightness
distribution, geometry, target environment) and by the pointing accuracy required for the science.
1. Objects that can be acquired by using ACQ/BIN (at least for the first stage of acquisition)
include all single, not very bright, not highly variable (± 1 magnitude) objects (e.g. stars, some
QSOs, extragalactic planetary nebulae which are not extended, and AGN with dominant point­
like nuclei in the 2000­5000 å range).
2. ACQ/PEAK is used for any needed later stages of acquisitions commencing with ACQ/BIN,
for acquisitions of objects too bright for the FOS MIRROR (see FOS Instrument Handbook Table
1.3.1 for FOS brightness limits), for variable objects, for AGN which do not have dominant point
source nuclei in the 2000­5000 å range, or for any observations requiring precise centering.
3. These categories of objects require the help of an Instrument Scientist or Program Coordinator:
(a) extended (>0.2''),
(b) crowded fields (even 2 objects separated by <0.3''),
(c) SLIT, BAR, or 0.1 aperture observations,
(d) objects fainter than V=19 mag,
(e) objects with emission lines, but little continuum,
(f) peculiar geometry as in gravitational lenses,
(g) objects with E(B­V) > 0.1, or
(h) many acquisitions using offset stars.
Table 1 presents a summary of each individual pattern (or step) that is commonly used in the
acquisition sequences recommended in this document. Table 2 presents commonly recommended
sequences as a function of science aperture and desired pointing accuracy. Note that, for strate­
gies commencing with ACQ/BIN, different sequences are employed for FOS/RED and FOS/
BLUE detectors, but sequences commencing with ACQ/PEAK are independent of detector. Fur­
ther, ACQ/BIN pointing accuracies are one­sigma values, whereas ACQ/PEAK values derive
directly from pattern geometry and are upper limits to the pointing uncertainty.

CAL/FOS­132­2
Table 1: FOS Target Acquisition Pattern Pointing Accuracies and Overheads
Aperture Pattern
Name
Search­
size­X
Search­
size­Y
Step­size­X
(arcsec)
Step­size­Y
(arcsec)
Pointing
Accuracy
(arcsec)
Overhead
(minutes)
4.3 A 1 3 ­ 1.23 ­ 7
1.0 B1 6 2 0.61 0.61 0.43 13
0.5 C1 3 3 0.29 0.29 0.21 11
0.3 D1 5 5 0.17 0.17 0.12 22
D2 5 5 0.11 0.11 0.08 22
D3 5 5 0.052 0.052 0.04 22
E1 4 4 0.17 0.17 0.12 16
E2 4 4 0.11 0.11 0.08 16
F 3 3 0.17 0.17 0.12 11
1.0­PAIR B2 6 2 0.61 0.61 0.43 13
0.5­PAIR C2 3 3 0.29 0.29 0.21 11
0.25­PAIR P1 5 5 0.17 0.17 0.12 22
P2 5 5 0.11 0.11 0.08 22
P3 5 5 0.052 0.052 0.04 22
P4 4 4 0.11 0.11 0.08 16
2.0­BAR BD1 1 11 ­ 0.052 0.03 12
0.7­BAR BD2 1 11 ­ 0.052 0.03 12
SLIT S 9 1 0.057 ­ 0.03 11
ACQ/BIN
RED
Z ­ ­ ­ ­ 0.12
(1­sigma)
9
ACQ/BIN
BLUE
Z ­ ­ ­ ­ 0.08
(1­sigma)
9

CAL/FOS­132­3
Table 2: Commonly Recommended Target Acquisition Sequences
Science
Observation
Aperture
Required
Pointing
Accuracy
(arcsec)
Sequences Starting with ACQ/BIN Sequences
Starting with
ACQ/PEAK
(both detectors)
RED
Detector
Over
­head
BLUE
Detector
Over
­head
Over
­head
4.3 >0.1 Z 9 Z 9 A+B1+C1 31
0.12 Z+E1 25 Z+F 20 A+B1+D1 42
0.08 Z+D2 31 Z+E2 25 A+B1+C1+E2 47
0.04 Z+D1+D3 53 Z+D1+D3 53 A+B1+D1+D3 64
1.0 >0.1 Z 9 Z 9 A+B1+C1 31
0.12 Z+E1 25 Z+F 20 A+B1+D1 42
0.08 Z+D2 31 Z+E2 25 A+B1+C1+E2 47
0.04 Z+D1+D3 53 Z+D1+D3 53 A+B1+D1+D3 64
0.5 0.12 Z+E1 25 Z+F 20 A+B1+D1 42
0.08 Z+D2 31 Z+E2 25 A+B1+C1+E2 47
0.04 Z+D1+D3 53 Z+D1+D3 53 A+B1+D1+D3 64
0.3 0.08 Z+C1+E2 36 Z+C1+E2 36 A+B1+C1+E2 47
0.04 Z+D1+D3 53 Z+D1+D3 53 A+B1+D1+D3 64
1.0­PAIR >0.1 Z 9 Z 9 A+B2+C2 31
0.12 Z+E1 25 Z+F 20 A+B2+P1 42
0.08 Z+P2 31 Z+P4 25 A+B2+C2+P4 47
0.04 Z+P1+P3 53 Z+P1+P3 53 A+B2+P1+P3 64
0.5­PAIR 0.12 Z+E1 25 Z+F 20 A+B2+P1 42
0.08 Z+P2 31 Z+P4 25 A+B2+C2+P4 47
0.04 Z+P1+P3 53 Z+P1+P3 53 A+B2+P1+P3 64
0.25­PAIR 0.08 Z+C2+P4 42 Z+C2+P4 42 A+B2+C2+P4 53
0.04 Z+P1+P3 53 Z+P1+P3 53 A+B2+P1+P3 64
0.1­PAIR,
2.0­BAR,
0.7­BAR, SLIT
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