Hubble Space Telescope observers are presented with many choices of instruments. In many situations, the observer will have to make a selection between complementary cameras, for example, ACS/WFC and WFC3/UVIS, or complementary spectrographs, COS and STIS. The oldest instruments STIS and ACS suffer more severely from CTE losses due to the high radiation environment. The damaging effects of continuous radiation exposure on the CCDs are unavoidable, regardless of whether the camera is operating or not. For STIS, a “Summary of changes after SM4” is to be found at the end of
Section 4.5. The CTE of ACS/WFC has degraded to the level expected for 9+ years aboard
HST. Much effort, both internal and external to the ACS Team, has gone into mitigating the science impact of ACS/WFC charge transfer inefficiency. Recently, the ACS Team has focussed on employing the pixel-based CTE correction technique of Anderson & Bedin (2010, PASP, 122, 1035) to correct for CTE losses. This correction technique has recently been incorporated in the CALACS pipeline. Please consult the
ACS Web page for the latest details. CTE can also be mitigated by positioning a target close to one of the readout amplifiers.
In this section, we make some general comparisons between instruments and their modes to help provide some basic criteria for specific instrument choice. If the choice is not immediately clear, the observer should read through the relevant sections of the appropriate Instrument Handbooks and carry out modeling of the astronomical fields using the provided tools. For this cycle, Python Exposure Time Calculators (ETCs) for all instruments are available and can be found at:
They use a new computing tool called PySynphot, in place of the previously used STSDAS synthetic photometry (
synphot) software package, and access reference files updated with the most recent on-orbit data. Please consult the
Cycle 20 Announcement Web page for up to date information on the status of HST instrumentation.
The cameras we consider in this section are ACS, STIS, and WFC3. In order to accomplish the proposed science, decisions will be made based largely on wavelength and areal coverage, spatial resolution, sensitivity, and the availability of specific spectral elements or observing modes. In Figures
4.1 to
4.4 we have provided a set of plots which compare throughput and discovery efficiency for the main cameras, based on SMOV data for WFC3 (for the IR throughputs, please refer to
WFC3 ISR 2009-30 and for the UVIS,
WFC3 ISR 2009-31). These figures will help to illustrate the recommendations below.
In the far-ultraviolet (
λ< 2000 Å) the ACS/SBC is more sensitive and has a larger number of available filters than the STIS/FUV-MAMA and therefore, it is the recommended choice. For the near-ultraviolet (~2000
<λ< 3500 Å) the generally recommended camera would be WFC3/UVIS because of its larger field of view and superior sensitivity when compared to STIS/NUV-MAMA.
Table 4.1 contains a detailed comparison between WFC3/UVIS and STIS/NUV.
WFC3/UVIS has a higher throughput than any HST instrument over the wavelength range extending from its blue cutoff (at 200 nm) to ~400 nm. Beyond this wavelength, the choice of which
HST instrument is best suited for users depends on the specific requirements of the science program. Although the absolute throughput of ACS is higher at optical wavelengths, the excellent WFC3 efficiency in the range 400-700 nm, coupled with its 20% smaller pixels, 50% lower readnoise, relatively small CTE corrections, and much lower dark current can make it the preferred instrument for some investigations. Specifically, some of the primary science observations with
HST require using several orbits to coadd signal from many exposures, in which case WFC3 may be the preferred instrument for even broadband F606W and F814W observations of faint sources. The choice between the two instruments will require careful predictions from the respective ETCs, factoring in detailed observational setup. Of course, WFC3 contains many more filters over its complete wavelength range than ACS/WFC, yet ACS offers a 50% larger field of view, both considerations potentially important for users.
Table 4.2 presents a detailed comparison between WFC3/UVIS and ACS/WFC.
Table 4.2: Imaging at Optical Wavelengths (350 - 1000 nm).
In the near-infrared (8000 <λ< 25,000 Å) the WFC3/IR has superior throughput and a much larger field of view than NICMOS. The data obtained with WFC3/IR should be easier to reduce and calibrate due to accurate bias subtraction made possible by the presence of reference pixels. WFC3/IR is currently the only option to perform near-infrared observations with
HST because NICMOS will not be available for observations in Cycle 20.
Table 4.3 provides a comparison between WFC3/IR and three NICMOS channels.
Table 4.3: Imaging at Near-Infrared Wavelengths (800 - 2500 nm).
In the following four figures, we present a graphical comparison of the HST imaging detectors with respect to several useful parameters: system throughput, discovery efficiency, limiting magnitude, and extended source survey time.
System throughputs as a function of wavelength are shown in Figure 4.1. The plotted quantities are end-to-end throughputs, including filter transmissions calculated at the pivot wavelength of each broad-band filter.
The discovery efficiencies of the cameras, defined as the system throughput multiplied by the area of the field-of-view, are shown in
Figure 4.2.
The point-source limiting magnitude achieved with a signal to noise of 5 in a 10 hour long exposure with optimal extraction is shown in
Figure 4.3.
Figure 4.4 shows the extended-source survey time to attain ABMAG=26 mag in an area of 100 arcmin
2.
COS is more sensitive than STIS by factors of 10 to 30 in the far-ultraviolet (~1150 <
λ< 2050 Å) and by factors of 2 to 3 in the near-ultraviolet (~1700 <
λ< 3200 Å.) COS also has a unique, but limited, capability to observe wavelengths between 900 and 1150 Å. Please refer to the
COS Instrument Handbook for important factors concerning COS observations in this wavelength range. COS is optimized for observations of faint point source targets, so it will be the instrument of choice for such objects. COS has an aperture of 2.5" in diameter. The COS FUV spatial resolution is limited to 1", while the COS NUV spatial resolution is approximately 0.05". However, for sources larger than approximately 1" perpendicular to the dispersion direction, portions of the spectrum may overlap on the NUV detector. Therefore, when spatial resolution is required, STIS will usually be the preferred instrument. STIS first order NUV and FUV MAMA modes have a spatial resolution of about 0.05" over a 25" long slit, while STIS CCD spectral modes (1900-10,200 Å) have a spatial resolution of about 0.1" with a 52" long slit. COS does not operate in the optical.
STIS high dispersion echelle modes also have significantly higher spectral resolution than COS (R~100,000 vs. R~20,000), which will be essential for some science programs. In addition, the STIS NUV medium resolution echelle mode E230M has a wider wavelength coverage in a single exposure (~800 Å) than does a single COS NUV medium resolution exposure (100 to 120 Å in three discontinuous pieces). Despite the sensitivity advantage of COS in the NUV, when complete coverage of a broad wavelength range is needed, STIS may be a more efficient choice. STIS also has a wider variety of apertures than does COS, including a number of neutral density apertures, and so STIS may be preferred for many UV bright objects. Given the low usage of most COS NUV modes, it is likely that calibrations for the comparable STIS NUV modes will be more robust.
The throughputs of COS and STIS in the ultraviolet are shown in Figure 4.5. The effect of the
HST OTA is included. STIS throughputs do not include slit losses.The effective area of COS modes below 1150 Å is shown in
Figure 4.6.
