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 instrument 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 Limit4 |
| ACS/WFC2
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 |
| WFPC23 |
150 x 150 35 x 35 |
0.10 0.0455 |
1150-11,000 1150-11,000 |
27.5 27.8 |
- Notes to Table 4.1
1. WFPC2, ACS, and NICMOS have polarimetric imaging capabilities. ACS and NICMOS have coronagraphic capabilities.
2. With ramp filters, the FOV is smaller for the ACS/WFC. Please see the ACS Instrument Handbook for details.
3. The WFPC2 has four CCD chips that are exposed simultaneously. Three are "wide-field" chips, each covering a 75" x 75" field and arranged in an "L" shape, and the fourth is a "planetary" chip covering a 35" x 35" field. See Figure 2.2
4. The 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. Details for each of the instrument configurations follow.
The ACS/WFC, ACS/HRC, and ACS/SBC entries in the table 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.
Table 4.2: HST Instrument Capabilities: Slitless Spectroscopy
| SI |
Field of View [arcsec] |
Projected Pixel Spacing on Sky [arcsec] |
Resolving Power1 |
Wavelength Range [Å] |
Magnitude Limit4 |
| 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 |
| NICMOS2 |
51 x 51 |
0.2 |
200 |
8000-25,000 |
21.6,21.1,18.0 |
| WFPC23 |
10 x 10 |
0.1 |
~ 100 |
3700-9800 |
25 |
- Notes to Table 4.2
1. The resolving power is lambda/resolution.
2. 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.
3. WFPC2 is capable of obtaining low-resolution spectra by placing a target successively at various locations in the WFPC2 linear ramp filter.
4. See Note 4 to Table 4.1
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 |
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 ACS/WFC camera has a larger field of view than WFPC2, significantly higher throughput over a wide spectral range, lower read-out noise, better sampling of the PSF, and a factor of 15 increase in dynamic range.
- Observers wishing to make near-UV imaging observations should consider the high throughput and angular resolution of the ACS/HRC versus the wider field offered by WFPC2.
- The ACS/HRC provides critical sampling of the PSF in the visible and high throughput in the blue.
- ACS/HRC offers a fully apodized Visible/NUV coronagraphic imaging mode with 1.8" and 3.0" occulting spots. It also offers occulted (un-apodized) imaging.
- The ACS/SBC channel provides FUV (
< 2000Å) imaging capability with good sensitivity and an extended set of filters.
- The WFPC2 instrument provides a wider range of narrow-band capabilities than ACS, with a total of 13 narrow-band and 5 medium-band filters with central wavelengths ranging from 1220 to 10,420Å, as well as 4 linear ramp filters and 2 quad filters that yield 8 different central wavelengths on the 4 WFPC2 chips.
- The F850LP filter is a unique ACS capability which, because it is a narrower filter, offers better wavelength resolution in the red than WFPC2 broad filters such as F814W. When used with two adjacent filters, F850LP enables high redshift systems (z
6) to be separated out using the Lyman break technique.
- At wavelengths below 3700Å, WFPC2 provides the widest field of view covering a total of 5 square arcminutes, which is 24 times larger than the ACS/HRC field and 16 times larger than that of the ACS/SBC. (The ACS/WFC cuts off below 3700Å.) However, the ACS/HRC and ACS/SBC detectors are factors of 5 and 10 times more sensitive, respectively, than the WFPC2 detectors.
- NICMOS is the camera of choice for observations at wavelengths longer than about 1 micron. There is some overlap with ACS at shorter wavelengths. NICMOS also provides good spatial resolution (particularly in camera 1), and has facilities for coronagraphic and polarimetric measurements.
- ACS offers high throughput grism (R ~ 100) imaging in the WFC and in the HRC.
- ACS offers high throughput prism (R ~ 100) imaging in the HRC and SBC channels.
The following tables provide some basic recommendations that may be useful in deciding which HST instrument to use in Cycle 16. 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 for the proposer to decide.
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STIS suspended operations on 3 August 2004, and is not available for scheduling in Cycle 16. If SM4 is authorized by the NASA administrator, an attempt to refurbish STIS will be made on a best effort basis. If successful, STIS will become operational starting in Cycle 17.
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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 |
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4.1.2 Future Instruments
Two new science instruments, the Cosmic Origins Spectrograph (COS) and the Wide Field Camera 3 (WFC3), are to be installed during SM4. The servicing mission is expected to take place in December 2007. The new instruments will become available during Cycle 17. Instrument capabilities are outlined in the COS and WFC3 Mini Handbooks.
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