Hubble Space Telescope Primer for Cycle 12

4.1 Overview

Table 4.1 - 4.5 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 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 26
35 x 31

~0.05
~0.027
~0.032

3700-11,000
2000-11,000
1150-1700

29.1
28.8
25.5

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

STIS/CCD STIS/NUV STIS/FUV
52 x 52
25 x 25
25 x 25

0.05
0.024
0.024

2500-11,000
1650-3100
1150-1700

28.0
24.6
24.0

WFPC2⁴
150 x 150
35 x 35

0.10
0.0455

1200-11,000
1200-11,000

27.5
27.8

Table 4.2: HST Instrument Capabilities: Slit Spectroscopy

SI
Projected Aperture Size

Resolving
Power⁵

Wavelength
Range [Å]

Magnitude
Limit²

STIS⁶
52" x (0.05-2)" [optical]
(25-28)" x (0.05-2)"
[UV first order]

(0.1-0.2)" x (0.025-0.2)"
[UV echelle]

25" x (0.05-2)"
[NUV prism]

First order:
~8000
~700
Echelles:
~100,000
~30,000
Prism:
~150

1150-10,300
1150-10,300

1150-3100
1150-3100

1150-3100

14.9, 15.4, 20.1
16.6, 19.2, 21.9

10.9, 12.4
11.6, 14.3

19.8

Table 4.3: HST Instrument Capabilities: Slitless Spectroscopy

SI

Field of View [arcsec]

Projected Pixel Spacing on Sky [arcsec]

Resolving Power

Wavelength Range [Å]

Magnitude Limit²

ACS/WFC grism G800L
202 x 202

~0.05

~100

5500-11000

24.9

ACS/HRC grism G800L
29 x 26

~0.029

~140

5500-11000

24.1

ACS/HRC prism PR200L
29 x 26

~0.027

~100

2000-4000

23.8

ACS/SBC prism PR130L
35 x 31

~0.035

~100

1250-1800

20.7

ACS/SBC prism PR110L
35 x 31

~0.035

~100

1150-1800

19.7

NICMOS⁷
51 x 51

0.2

200

8000-25,000

21.6,21.1,18.0

STIS⁸
52 x 52
25 x 25

0.05
0.024

~700-8000
~150-8000

2000-10,300
1150-3100

See slit spectroscopy above

WFPC2⁸
10 x 10

0.1

~100

3700-9800

25

Table 4.4: HST Instrument Capabilities: Positional Astrometry

SI

Field of View

Precision

(per observation)

Wavelength Range (Å)

Magnitude

FGS1R 69 square arcmin 1 mas 2 mas 4700-7100 <14.5 <17.0

Table 4.5: 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 - 4.5
¹WFPC2, ACS, and NICMOS have polarimetric imaging capabilities. STIS, ACS, and NICMOS have coronographic capabilities.
²Limiting V magnitude for an unreddened A0 V star in order to achieve a signal-to-noise ratio of 5 in a CR-SPLIT (when appropriate) exposure time of 1 hour assuming low-background conditions (LOW; see Section 5.5.1). The limiting magnitude for imaging in the visual is strongly affected by the sky background; under normal observing conditions, the limiting magnitude can be about 0.5 brighter than listed here. 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. STIS direct imaging entries assume use of a clear filter for the CCD and the quartz filter for the UV (for sky suppression). For STIS spectroscopy to achieve the specified signal-to-noise ratio per wavelength pixel with a 0.5" slit (0.2" for the echelles), multiple values are given corresponding to 1300, 2800 and 6000 Å, respectively (if in range). The ACS/WFC, ACS/HRC and WFPC2 entries in Table 4.1 assume filter F606W. ACS spectroscopy assumes wavelengths of 1300Å (PR130L), 3000Å (PR200L), and 6800Å (G800L). The 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 ACS/SBC entry in Table 4.1 assumes filter F125LP. For NICMOS imaging, we assume filter F160W with a detector temperature of 77.1 K; the limiting H magnitude, in the Vega system, is given for a point source to reach S/N=5 in the brightest pixel and 1 hr exposure. Please see the NICMOS Instrument Handbook, Chapter 9, for details.
³With ramp filters, the FOV is smaller for the ACS/WFC. Please see the ACS Instrument Handbook for details.
⁴The 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.
⁵The resolving power is lambda/resolution; for STIS it is /2 where is the dispersion scale in Angstroms/pixel.
⁶The 25" or 28" first order slits are for the MAMA detectors, the 52" slit is for the CCD. The R ~150 entry for the prism on the NUV-MAMA is given for 2300 Å. More accurate and up to date values for spectroscopic limiting magnitudes can be found in the STIS Instrument Handbook.
⁷ NICMOS has three grisms (G096,G141,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 S/N=5 in 1 hr exposure.
⁸All STIS modes can be operated in a slitless manner by replacing the slit by a clear aperture or filter. WFPC2 has a capability of obtaining low-resolution spectra by placing a target successively at various locations in the WFPC2 linear ramp filter. STIS also has a prism for use in the UV.

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, STIS and WFPC2; WFC3 will replace WFPC2 in 2004. Spectroscopic observations are currently limited to STIS but will be expanded to include COS after the next servicing mission. 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. ACS/WFC is not designed for near-UV imaging, a capability which will be offered by WFC3/UVIS. Observers wishing to make near-UV imaging observations should trade the high throughput and angular resolution against the wider field offered by WFPC2. ACS is also a new instrument and is not yet exhibiting significant charge transfer efficiency degradation, due to radiation damage.
The ACS/HRC provides critical sampling of the PSF in the visible and high throughput in the blue. For broad-band UV imaging it can be competitive with or better than the STIS NUV-MAMA. The WFC3/UVIS channel will also be competitive with the STIS NUV-MAMA and will offer a wider field of view. WFC3/UVIS will have a short wavelength limit of ~2000Å.
ACS offers a fully apodized Visible/NUV coronagraphic imaging mode with 1.8" and 3.0" occulting spots. It also offers occulted (un-apodized) imaging. STIS offers occulted imaging with partial apodization, but smaller width occulting wedges.
The ACS/SBC channel provides FUV ( < 2000Å) imaging capability with greater sensitivity and an extended set of filters compared to the STIS FUV-MAMA. MAMA snapshots will only be available with the STIS MAMA.
NICMOS is the camera of choice of observations at wavelengths longer than about 1 micron. There is some overlap with ACS at shorter wavelengths. In future cycles the WFC3/IR channel will higher throughput and a larger FOV than NICMOS, but will have a long wavelength cutoff of 1.7 microns. NICMOS sensitivity extends to 2.5 microns.
NICMOS has a few other features that will not be not superseded by WFC3/IR. NICMOS provides higher spatial resolution (particularly in camera 1), and has facilities for coronagraphic and polarimetric measurements.
From 3200Å to 10,000Å, STIS is the only instrument allowing R>500 spectroscopy. ACS does offer high throughput grism (R~100) imaging in the WFC and prism (R~100) imaging in the ACS/HRC.
In the FUV (< 1800Å), COS should be significantly more sensitive than STIS. From 1800Å to ~2500Å, the choice between COS and STIS will depend on details of the observing program. Beyond ~2500Å, STIS CCD spectroscopy should be more sensitive than COS. Throughout the UV, STIS will remain the only choice for high spectral resolution (R~100,000) or spectroscopy of resolved structure on scales smaller than 1 arcsec. ACS offers high throughput prism (R~100) imaging in the ACS/HRC and ACS/SBC channels.

The following tables provide some basic recommendations that may be of use in deciding which HST instrument in Cycle 12. Table 4.6 summarizes typical decisions that are frequently made when choosing an HST instrument for spectroscopic observations. Table 4.7 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.
Table 4.6: Spectroscopy Decisions

Type of Observation

Recommended Instrument

Comment

Spectroscopy at R>100 STIS

slitless spectroscopy ACS (R~100) or STIS (R > 100)

Table 4.7: Imaging Decisions

Type of Observation

Recommended Instrument

Comment

Ultraviolet Observations

< 2000Å ACS/SBC Larger FOV, better sensitivity than STIS

> 2000Å WFPC2 or ACS/HRC or STIS/NUV WFPC2 has the largest FOV ACS/HRC has the highest throughput STIS/NUV has no readout noise

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

Coronography ACS/HRC

Infrared Observations

Wavelength > 1 microns NICMOS Only instrument available

4.1.2 Future Instruments

Two new science instruments, the Cosmic Origins Spectrograph (COS) and the Wide Field Camera 3 (WFC3), will be installed in HST during SM4. Instrument capabilities are outlined in the COS and WFC3 mini-handbooks released with the Cycle 12 Call for Proposals. Highlights of these instruments are described below

The Cosmic Origins Spectrograph (COS) is designed to perform high sensitivity, moderate-resolution (R ~ 16,000-24,000) and low-resolution (R ~2000-3000) spectroscopy of astronomical objects in the 1150-3200Å spectral region. COS will significantly enhance the spectroscopic capabilities of HST at ultraviolet wavelengths, and will provide observers with unparalleled opportunities for observing faint sources of ultraviolet light. Wavelength coverage at far-UV wavelengths (1150-2050Å) is expected to be ~300 to 820Å per exposure, depending upon the spectroscopic mode chosen. Far-UV light is recorded by a crossed delay-line microchannel plate (MCP) detector. Wavelength coverage at near-UV wavelengths (1700-3200Å) is expected to be ~100 to 800Å per exposure, depending upon the spectroscopic mode chosen. Near-UV light is recorded by a multi-anode microchannel array (MAMA) detector similar to the MAMA detectors used on STIS. COS has two circular science apertures that are 2.5 arcseconds in diameter. Limited COS imaging capabilities are available only at near-UV wavelengths. COS is not meant to be a replacement for STIS, which will remain in HST after the servicing mission and will be available to the community. Rather, COS will complement and extend existing HST spectroscopic capabilities. The high sensitivity ultraviolet spectroscopy enabled by COS will allow users to investigate fundamental astrophysical topics such as the ionization and baryonic content of the intergalactic medium, the origin of large scale structure in the Universe, the chemical and dynamical evolution of galaxies, and the properties of stars and planetary systems.

The Wide Field Camera 3 (WFC3) is a panchromatic, wide-field, high-throughput imaging camera that will replace WFPC2 in its radial bay during SM4. It features two independent channels - the UVIS channel, sensitive at ultraviolet and optical wavelengths (2000-10,000Å) with a FOV of 2.7 x 2.7 arcmin and a scale of 0.04 arcsec/pixel, and the IR channel, sensitive at near infrared wavelengths (8500Å - 1.7 microns) with a FOV of 2.2 x 2.2 arcmin and a scale of 0.13 arcsec/pixel. The instrument's extended wavelength range, angular resolution, and large field-of-view, along with high sensitivity and a wide selection of spectral elements (62 filters and 1 grism in the UVIS channel; 14 filters and 2 grisms in the IR channel), provides users with a vast array of options for imaging and low-dispersion, slitless spectroscopy, making WFC3 one of the most versatile instruments onboard HST.

WFC3 complements and extends the capabilities of ACS and NICMOS, and it also provides a high degree of redundancy with these instruments to help secure HST's unique imaging capabilities until the end of its mission. WFC3 is a facility instrument that will allow users to carry out a variety of key scientific investigations, including searches for galaxies at redshift up to z~10, the study of the physics of star formation in distant and nearby galaxies, accurate measures of the baryonic mass by detecting stars at the limit of the hydrogen-burning sequence and extra-solar Jupiter-like planets, and study of planetary objects in the Solar System.