Space Telescope Science Institute  Chapter 4:  Cycle 15 Scientific Instruments  4.2 Advanced Camera for Surveys (ACS)

4.1 Overview


Tables 4.1 - 4.4 summarize the capabilities of the SIs. For some applications, more than one instrument can accomplish a given task, but not necessarily with equal quality or speed. Note that there may be small differences between the numbers quoted here and those quoted in the HST Instrument Handbooks. In such cases the Handbook numbers take precedence.


Table 4.1: HST Instrument Capabilities: Direct Imaging1
SI
Field of View [arcsec]
Projected Pixel Spacing on Sky [arcsec]
Wavelength Range [Å]
Magnitude Limit2
ACS/WFC3
ACS/HRC
ACS/SBC
202 x 202
29 x 25
34 x 31
0.05
0.027
0.032
3700-11,000
2000-11,000
1150-1700
27.8
27.3
23.2
NICMOS/NIC1
NICMOS/NIC2
NICMOS/NIC3
11 x 11
19 x 19
51 x 51
0.043
0.076
0.20
8000-19,000
8000-25,000
8000-25,000
23.2
24.7
25.6
WFPC24
150 x 150
35 x 35
0.10
0.0455
1150-11,000
1150-11,000
27.5
27.8

Table 4.2: HST Instrument Capabilities: Slitless Spectroscopy
SI
Field of View [arcsec]
Projected Pixel Spacing on Sky [arcsec]
Resolving Power5
Wavelength Range [Å]
Magnitude Limit2
ACS/WFC grism G800L
202 x 202
0.05
100
5500-11,000
24.4
ACS/HRC grism G800L
29 x 25
0.027
140
5500-11,000
23.6
ACS/HRC prism PR200L
21 x 25
0.027
100
2000-4000
22.7
ACS/SBC prism PR130L
28 x 31
0.032
100
1250-1800
21.5
ACS/SBC
prism PR110L
28 x 31
0.032
100
1150-1800
20.9
NICMOS6
51 x 51
0.2
200
8000-25,000
21.6,21.1,18.0
WFPC27
10 x 10
0.1
100
3700-9800
25

Table 4.3: HST Instrument Capabilities: Positional Astrometry
SI
Field of View
Precision
(per observation)
Wavelength Range (Å)
Magnitude
FGS1R
69 square arcmin
1 mas
4700-7100
<16.7

Table 4.4: HST Instrument Capabilities: Binary Star Resolution and Measurements
SI
Field of View
Minimum
Separation [mas]
Accuracy [mas]
Delta Magnitude (max)
Primary Star Magnitude
FGS1R
aperture center
5" x 5" IFOV
8
10
15
20
30
1
1
1
1
1
0.6
1.0
1.0
2.5
4.0
<14.5
<14.5
<16.6
<16.3
<15.0
Notes to Tables 4.1 - 4.4
1 WFPC2, ACS, and NICMOS have polarimetric imaging capabilities. ACS and NICMOS have coronagraphic capabilities.
2The limiting magnitude for imaging in the visual is strongly affected by the sky background; the limiting magnitude can be about 0.5 fainter or brighter than listed here, depending on the calculation parameters specified below for each instrument. Please note that low-sky conditions limit flexibility in scheduling and are not compatible with observing in the CVZ. Single entries refer to wavelengths near the center of the indicated wavelength range.
The ACS/WFC, ACS/HRC, and ACS/SBC entries in Table 4.1 refer to V magnitude (Johnson) of an unreddened A0 V star (Vega), for a one-hour integration. WFC and HRC use filter F606W, GAIN=2, and CR-SPLIT=2; SBC uses F125LP. Magnitudes are for a signal-to-noise ratio of 5 in a circle of radius 0.2 arcsec for the WFC and HRC, and 0.5 arcsec for the SBC. Observations assume average background conditions. Limiting magnitude values were derived without CTE corrections.
The ACS spectroscopy entries refer to the limiting V magnitude (Johnson) of an unreddened A0 V star (Vega) in order to achieve a signal-to-noise ratio of 5 in an exposure time of one hour assuming average background conditions. CCD spectroscopy magnitude limits are for GAIN=2 and CR-SPLIT=2. Assumed wavelengths are 1500Å (PR110L and PR130L), 2500Å (PR200L), and 7000Å (G800L).
The WFPC2 entries refer to the limiting V magnitude in F606W of an unreddened A0 V star in order to achieve a signal-to-noise ratio of 5 in a CR-SPLIT exposure time of one hour assuming low-background conditions. WFPC2 Charge Transfer Efficiency (CTE) losses are negligible for this filter due to the significant sky background accumulated over 3600 sec in F606W. However, note that WFPC2 images of faint point sources with little sky background can experience significant CTE losses; please see the WFPC2 Instrument Handbook for details.
The NICMOS entries refer to the limiting H magnitude in the Vega system of an unreddened A0 V star in order to achieve a signal-to-noise ratio of 5 in the brightest pixel in an exposure time of one hour assuming "average" zodiacal light. For NICMOS imaging, we assume filter F160W with a detector temperature of 77.1 K. Please see the NICMOS Instrument Handbook, Chapter 9, for details.
3 With ramp filters, the FOV is smaller for the ACS/WFC. Please see the ACS Instrument Handbook for details.
4 The WFPC2 has four CCD chips that are exposed simultaneously. Three are "wide-field" chips, each covering a 75x 75" field and arranged in an "L" shape, and the fourth is a "planetary" chip covering a 35x 35" field.
5 The resolving power is lambda/resolution.
6 NICMOS has three grisms (G096,G141, and G206) for use in NIC3. We assume a detector temperature of 77.1 K and "average" zodiacal light; the limiting Vega system H magnitude for spectroscopy is given for a point source to reach a signal-to-noise ratio of 5 in a one hour exposure.
7 WFPC2 has a capability of obtaining low-resolution spectra by placing a target successively at various locations in the WFPC2 linear ramp filter.

4.1.1 Instrument Comparison

Observers often face the choice of deciding which HST instrument is best-suited for a particular observation. In some cases, the choice is limited to one instrument, but in many situations the proposer must decide from among several possibilities. Instrument choices for imaging observations currently include ACS, NICMOS, and WFPC2. Spectroscopic observations are currently limited to the slitless capabilities of instruments listed in Table 4.2:. Some general considerations follow, and further details can be found in the individual instrument handbooks.

The following tables provide some basic recommendations that may be useful in deciding which HST instrument to use in Cycle 15. Table 4.5 summarizes HST instrument choices currently available for spectroscopic observations. Table 4.6 lists typical decisions that are often made for imaging observations. All recommendations should be considered general in nature and are meant to provide high-level guidance to observers. The ultimate choice of instrument for a particular observation may depend upon a variety of competing factors and is left as a decision to be made by the proposer.

STIS suspended operations on 3 August 2004. STIS will not be available for scheduling in Cycle 15.


Table 4.5: Spectroscopy Decisions
Type of Observation
Recommended Instrument
Slitless spectroscopy
ACS (R ~ 100) or NICMOS grism (R ~ 200)

Table 4.6: Imaging Decisions
Type of Observation
Recommended Instrument
Comment
Ultraviolet Observations
< 2000Å
ACS/SBC

> 2000Å
WFPC2 or ACS/HRC
WFPC2 has the largest FOV.
ACS/HRC has the highest throughput.
Optical Observations
Broadband > 4000Å
ACS/WFC
Wide field, high throughput.
Narrowband > 4000Å
WFPC2 or ACS
WFPC2 has more filter choices.
ACS has a few narrow filters.
Ramp filters are available for
smaller areas (both instruments).
High resolution
ACS/HRC
Best sampled PSF.
Coronagraphy
ACS/HRC

Infrared Observations
Wavelength > 1 micron
NICMOS
Only instrument available.
Coronagraphy
NICMOS

4.1.2 Future Instruments

Two new science instruments, the Cosmic Origins Spectrograph (COS) and the Wide Field Camera 3 (WFC3), were to have been installed in SM4. These instruments may be installed in HST in the future. Instrument capabilities are outlined in the COS and WFC3 mini-handbooks.


Chapter 4:  Cycle 15 Scientific Instruments  4.2 Advanced Camera for Surveys (ACS)
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