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| Hubble Space Telescope Primer for Cycle 17
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4.8 Instrument Comparisons
If Servicing Mission 4 is successful in installing WFC3 and COS, and returning STIS and ACS to full functionality, then HST observers will be presented with a rich choice 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. For this Cycle, we do not have on-orbit performance data to help inform many of the choices that must be made. Both STIS and ACS will have continued to age in the high-radiation environment since they were last used, and we can not predict how they will perform in Cycle 17. One obvious question, for example, is whether the charge transfer efficiency for ACS/WFC has degraded.
In this section, we make some general comparisons between instruments and their modes to help provide some basic criteria for specific instrument choice. Where the choice is not immediately clear, the observer should read through the relevant sections of the Instrument Handbooks and carry out modeling of the astronomical fields using tools provided.
4.8.1 HST Imaging
The cameras we consider in this section are WFC3, ACS, NICMOS and STIS. Decisions will be made based largely on wavelength and areal coverage, spatial resolution, sensitivity, and the availability of specific spectral elements or observing modes, to accomplish the proposed science. 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 earlier on-orbit data or laboratory data using for WFC3. Current WFC3 information is based on detectors which are subject to change before SM4. These figures will help to illustrate the recommendations below.
In the far-ultraviolet (FUV < 200 nm) the ACS/SBC is more sensitive and has more filters than the STIS FUV-MAMA and is the recommended choice. In the near-ultraviolet (NUV ~ 200 - 350 nm) the WFC3/UVIS has a superior field-of-view and sensitivity than ACS/HRC and STIS/NUV-MAMA and is the recommended choice, except for those specific occasions when the highest resolution is required, as ACS/HRC provides better sampling (44% smaller pixels) than WFC3/UVIS. Table 4.1 contains a detailed comparison.
Table 4.1: Imaging at Near-UV Wavelengths (200 - 350 nm)
1 These polarizers are optimized for the UV and the ACS/HRC field-of-view but can in principle be used with the ACS/WFC in the optical.
In the optical (350 - 1000 nm) the choice between ACS/WFC and WFC3/UVIS may be a difficult one. The ACS/WFC has a wider field of view than WFC3/UVIS and the ACS/HRC has smaller pixels than WFC3/UVIS. The WFC3/UVIS does have the better point-source sensitivity below 450 nm, between 450 and 600 nm the two cameras are comparable, while above 600 nm ACS/WFC has a slightly better point-source sensitivity. For extended-source sensitivity and survey efficiency, WFC3/UVIS is better below 400 nm and ACS/WFC is better above 400 nm. Table 4.2 contains a detailed comparison.
In the near-infrared (800 - 2500 nm), the WFC3/IR has superior throughput and a much larger field-of-view than NICMOS, and the data should be easier to reduce and calibrate, due to accurate bias subtraction made possible by the presence of reference pixels. WFC3/IR is the clear recommendation for most near-infrared observing. However, NICMOS/NIC1 and NIC2 do have smaller pixels than WFC3/IR and the wavelength range of NICMOS does extend to 2500 nm, while WFC3/IR coverage ends at 1700 nm. Table 4.3 contains a detailed comparison.
Table 4.2: Imaging at Optical Wavelengths (350 - 1000 nm).
1Average throughput at 10 nm bandpass at the pivot wavelength (V=F606W; I=F814W; z=F850LP).
2Some of these filters cover only the ACS/HRC field-of-view but can in principle be used in the ACS/WFC.
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.
Figure 4.1: HST Total System Throughputs
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.
Figure 4.2: HST Survey Discovery Efficiencies
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 4.3: Point-Source Limiting Magnitude
The next figure, Figure 4.4, shows the extended-source survey time to attain ABMAG=26 in an area of 100 arcmin2.
Figure 4.4: Extended-Source Survey Time
4.8.2 Spectrographs
In the far-ultraviolet (FUV ~ 115 to 205 nm), COS is more sensitive than STIS by factors of 10 to 30, while in the near-ultraviolet (NUV ~ 170-320 nm) COS sensitivity is two to three times that of STIS. COS is optimized for observations of point source targets, so it will be the instrument of choice for faint point source targets. Its aperture is 2.5 arcsec in diameter. Its 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 1st order NUV and FUV MAMA modes have spatial resolution of about 0.05" over a 25" long slit, while STIS CCD spectral modes (190-1020 nm) have 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 (100,000 vs. 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 (~80 nm) than does a single COS NUV medium resolution exposure (10 to 12 nm in three discontinuous pieces), and so, 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.
A useful comparison of COS and STIS at ultraviolet wavelengths is given in Table 4.4.
Table 4.4: Spectroscopy at Ultraviolet Wavelengths
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.
Figure 4.5: Throughputs for COS and STIS in the FUV and NUV.
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Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |
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