4.1 Overview

Tables 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 HST Instrument Handbooks. In such cases the Handbook numbers take precedence.

Notes to Tables 4.1 - 4.5

⁵ The resolving power is lambda/resolution; for STIS it is /2 where is the dispersion scale in Angstroms/pixel.

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. Spectroscopic observations are currently limited to STIS, except for the slitless capabilities of other instruments listed in Table 4.3:. 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 in Cycle 14.
• 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. 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.
• 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 10420Å, 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 and 16 times larger than 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. In future cycles the WFC3/IR channel will have 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 in the 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 to use in Cycle 13. 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 4.7: 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
Coronagraphy	ACS/HRC, STIS
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), will be installed in HST during SM4. Instrument capabilities are outlined in the COS and WFC3 mini-handbooks released with the Cycle 13 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 ~300-820Å 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 ~100-800Å 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), will provide 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 will complement and extend the capabilities of ACS and NICMOS, and will also provide 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 the study of planetary objects in the Solar System.