|Space Telescope Science Institute|
|Call for Proposals and HST Primer|
Hubble Space Telescope observers are presented with several choices of instruments. In many situations, an 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 available at the end of Section 4.5.CTE for ACS/WFC has degraded to the level expected for more than eleven 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. The ACS Team has focused on developing a pixel-based CTE correction technique described by 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 webpage for the latest details. CTE can also be mitigated by positioning a target close to one of the readout amplifiers.For this cycle, Python Exposure Time Calculators (ETCs) for all instruments are available and can be found at http://etc.stsci.edu/. ETCs use a new computing tool called PySynphot (that has replaced the STSDAS synphot software package), and has access to the most up-to-date reference files.As always, please consult the Cycle 23 Announcement webpage for the latest information on the status of HST instrumentation.4.7.1 HST ImagingACS, STIS, and WFC3 cameras are available for Cycle 23. Decisions on which camera to use will be made based on wavelength, field of view, spatial resolution, sensitivity, and the availability of specific spectral elements or observing modes. Figures 4.2 to 4.5 provide a set of plots (based on WFC3 SMOV data1 but still valid for current WFC3 operations) comparing throughput and discovery efficiency for the cameras.In the far-ultraviolet (λ < 2000 Å) the ACS/SBC is the recommended camera over STIS/FUV-MAMA because it is more sensitive and has a larger number of available filters. For the near-ultraviolet (~2000 Å < λ < 3500 Å), WFC3/UVIS is generally recommended over STIS/NUV-MAMA because of its larger field of view and superior sensitivity. Table 4.1 contains a detailed comparison between WFC3/UVIS and STIS/NUV-MAMA.
7 (e-/pix/hr) WFC3/UVIS has the highest throughput among HST imaging instruments in the wavelength range extending from its blue cut-off (at 200 nm) to ~400 nm. Beyond that wavelength range, the choice of which HST instrument is best suited for observations depends on the specific requirements of the science program.Although the absolute throughput of ACS is higher at optical wavelengths, the excellent WFC3/UVIS efficiency in the range of 400 nm to 700 nm, coupled with its 20% smaller pixels, 25% lower read noise, relatively small CTE corrections, and much lower dark current can make it the preferred instrument in some cases of optical imaging. Alternately, the 50% larger ACS/WFC field of view or the availability of imaging polarizers can be considerations potentially important for users. The optimal choice between the two instruments may require careful predictions from their respective ETCs, factoring in detailed observational setup. Table 4.2 presents a detailed comparison between WFC3/UVIS and ACS/WFC.
Broad-band throughput1 @ B,V, I, z Dark current (e-/pix/hr) Average throughput at 10 nm bandpass at the pivot wavelength (B = F438W, V = F606W, I = F814W, z = F850LP).
In the near-infrared (8000 Å < λ < 25,000 Å) the WFC3/IR has superior throughput and a much larger field of view than the now-defunct 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 since NICMOS is no longer available. For reference purposes, Table 4.3 provides a comparison between WFC3/IR and three NICMOS channels.
The following four figures 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 4.2. The plotted quantities are end-to-end throughputs, including filter transmissions calculated at the pivot wavelength of each broad-band filter.Figure 4.2: HST Total System ThroughputsFull lines represent instruments that will be offered during Cycle 23. Dashed lines represent instruments that are not available for Cycle 23 (HRC, NICMOS) and a previously flown instrument (WFPC2).Discovery efficiencies of the cameras, defined as the system throughput multiplied by the field of view area, are shown in Figure 4.3.Figure 4.3: HST Survey Discovery EfficienciesFull lines represent instruments that will be offered during Cycle 23. Dashed lines represent instruments that are not available for Cycle 23 (HRC, NICMOS) and a previously flown instrument (WFPC2).The point source limiting magnitude achieved with a signal-to-noise of 5 in a 10 hour exposure time with optimal extraction is shown in Figure 4.4.Figure 4.4: Point-Source Limiting MagnitudeFull lines represent instruments that will be offered during Cycle 23. Dashed lines represent instruments that are not available for Cycle 23 (HRC, NICMOS) and a previously flown instrument (WFPC2).Figure 4.5 shows the extended source survey time to attain ABMAG = 26 mag in an area of 100 arcmin2.Figure 4.5: Extended-Source Survey TimeFull lines represent instruments that will be offered in Cycle 23. Dashed lines represent instruments that are not available for Cycle 23 (HRC, NICMOS, and WFPC2).4.7.2 SpectrographsFor observations requiring resolutions of R > 15,000, COS is more sensitive than STIS by factors of 10 to 30 in the far-ultraviolet (~1150 Å < λ < 2050 Å) and by factors of two to three in the near-ultraviolet (~1700 Å < λ < 3200 Å). COS also has a unique but limited capability to observe wavelengths between 900 Å and 1150 Å. However, it does not operate in the optical. Please refer to the COS Instrument Handbook for important factors concerning COS observations in this wavelength range.COS is optimized for observing faint point source targets, making it the instrument of choice for such objects. COS has an aperture of 2.5" in diameter. COS/FUV spatial resolution is limited to 1", while COS/NUV spatial resolution is approximately 0.06". 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 is usually 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 Å - 10,200 Å) have a spatial resolution of about 0.1" with a 52" long slit.STIS high dispersion echelle modes have significantly higher spectral resolution than COS (R ~100,000 vs. R ~20,000), which is 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 Å 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 is the more efficient choice. STIS also has a wider variety of apertures than COS; this includes a number of neutral density apertures that makes STIS the preferred instrument for many UV bright objects. Given the low usage of most COS/NUV modes, it is likely that calibrations for comparable STIS/NUV modes will be more robust.Table 4.4: Spectroscopy at Ultraviolet Wavelengths
R = λ/dλ M (λ < 1150) Background 2 (counts/sec/resel) 2.40 x 10-4 (FUVA)2.38 x 10-4 (FUVB) 1.5 x 10-3 9.0x10-3 (ergs/cm2/sec/Å) 7.6 x 10-17 4.8 x 10-15 9.0 x 10-15 8.0 x10-15The first number is number of pixels along the dispersion per segment; the second number is for both segmentsOne resel is considered 6x10 pixels for COS/FUV, 3x3 pixels for COS/NUV, 2x5 pixels for STIS/FUV, and 2x3 pixels for STIS/NUV.
The throughputs of COS and STIS in the ultraviolet, including the effect of the HST OTA, are shown in Figure 4.6. STIS throughputs do not include slit losses. The effective area of COS modes below 1150 Å is shown in Figure 4.7.Figure 4.6: Throughputs for COS and STIS in the FUV and NUV.Figure 4.7: Effective Area for COS Below 1150 ÅA comparison of the effective area of the G130M modes (red) with that of G140L (blue). The G130M effective area below 920 Å is not well characterized, so the value at 920 Å is used to extrapolate to shorter wavelengths, shown in yellow. The new mode (G130M/1222 Å) is also shown.For IR throughputs, please refer to WFC3 ISR 2009-30 and WFC3 ISR 2011-08. For UVIS throughputs, WFC3 ISR 2009-31 and WFC3 ISR 2010-14.