Wide Field Camera 3 Instrument Handbook for Cycle 19 - Space ...

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CHAPTER 3: Choosing the Optimum HST Instrument In this chapter . . . 3.1 Overview / 16 3.2 Choosing Between Instruments / 17 3.3 Comparison of WFC3 with Other HST Imaging Instruments / 18 3.1 Overview This chapter addresses the general questions that arise when observers choose between proposing to use WFC3 or one or more of the other imaging instruments that are available on HST. Following SM4, the observatory is at its highest level of capability: new instruments have been installed, both STIS and ACS/WFC have been repaired, and it is possible that NICMOS will be reactivated (see HST Call for Proposals). In choosing between instruments for their particular projects, observers should carefully evaluate the capabilities of WFC3 and compare them to those of the other HST instruments, in the context of their own scientific goals. There is some intentional redundancy between WFC3, ACS, STIS and NICMOS, to provide a degree of protection against degradation or failure of the instruments. However, the instruments largely complement one another, and therefore it is likely that one instrument will be preferred for any science program. Therefore, observers do need to give careful consideration to instrument capabilities in order to optimize their observations. They should refer to the HST Call for Proposals for any policy issues, and the HST Primer for additional comparison information. 16
Choosing Between Instruments 17 3.2 Choosing Between Instruments The primary factors to consider in choosing the preferred instrument are areal coverage, spatial resolution, wavelength coverage, sensitivity, and availability of specific spectral elements. Table 3.1 lists the primary characteristics of the imaging instruments currently available on HST. For some research programs, the instrument choice may be dictated by the need for a particular spectral element. In this regard, WFC3 offers considerable capability because of its broad complement of wide-, medium-, and narrow-band filters both at UV/optical and near-IR wavelengths, as well as one UV grism and two near-IR grisms for slitless spectroscopy. For studies at optical wavelengths, the trade-offs to consider when deciding between WFC3/UVIS and ACS/WFC include pixel size, field of view and, to some extent, throughput. WFC3 is generally preferable when angular resolution has higher priority than field of view, because of its finer pixel size. On the other hand, ACS/WFC has higher throughput than WFC3/UVIS at wavelengths longward of ~400 nm (see Figure 3.2), and hence may be the best choice when the highest possible sensitivity at such wavelengths is crucial. However, considerations of degraded charge-transfer efficiency (CTE) should be kept in mind, since ACS has endured the high-radiation space environment for more than seven years longer than has WFC3. At UV wavelengths, WFC3/UVIS is the only imager on HST to offer a large field of view combined with high throughput. However, its spectral coverage does not extend shortward of 200 nm, whereas ACS/SBC and STIS/FUV-MAMA both reach down to 115 nm (STIS/NUV-MAMA reaches 160 nm), and also offer finer spatial sampling (see Section 3.3.3). Thus, WFC3 will be the choice whenever both large field of view and coverage down to 200 nm are required (e.g., multi-wavelength surveys). However, if observations at extreme far-UV wavelengths are necessary, or if the highest available spatial sampling at UV wavelengths is a primary requirement, then ACS/SBC or the STIS UV channels should be considered. Because ACS/HRC could not be repaired during SM4, WFC3 offers imaging at the finest pixel scale of any HST instrument at optical wavelengths. At near-IR wavelengths WFC3/IR offers a much larger field of view and, generally, higher throughput than NICMOS. It also offers greatly improved sensitivity and ease of data reduction and calibration, due to its more uniform flat field and the accurate bias subtraction made possible by the presence of reference pixels. However, WFC3 sensitivity is limited to wavelengths shortward of ~1700 nm, and WFC3 has coarser pixel sizes than NIC1 and NIC2, and does not offer a coronagraph or polarizers. For IR observations of bright targets, NICMOS may also be preferred because observations may be obtained with the instrument intentionally defocused to avoid saturation. This is not permitted with WFC3.
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Page 3 and 4: Table of Contents Acknowledgments .
Page 5 and 6: Table of Contents v 5.5 The WFC3 IR
Page 7 and 8: Table of Contents vii 7.8 IR Sensit
Page 9 and 10: Table of Contents ix Appendix C: Di
Page 11 and 12: xi Past Science IPT Members Elizabe
Page 13 and 14: CHAPTER 1: Introduction to WFC3 In
Page 15 and 16: WFC3 Quick Reference Guide 3 • UV
Page 17 and 18: Sources of Further Information 5 1.
Page 19 and 20: CHAPTER 2: WFC3 Instrument Descript
Page 21 and 22: Figure 2.1: Schematic optical layou
Page 23 and 24: Spectral Elements 11 elongation of
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Page 27: Detector Read-Out Modes and Ditheri
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Page 33 and 34: Comparison of WFC3 with Other HST I
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Page 37 and 38: Preparing a Phase I Proposal 25 4.2
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Page 41 and 42: CHAPTER 5: WFC3 Detector Characteri
Page 43 and 44: The WFC3 UVIS Channel CCD Detectors
Page 45 and 46: WFC3 CCD Readout Formats 33 5.3 WFC
Page 47 and 48: WFC3 CCD Characteristics and Perfor
Page 61 and 62: 5.4.12 Crosstalk WFC3 CCD Character
Page 63 and 64: The WFC3 IR Channel Detector 51 Fig
Page 65 and 66: WFC3 IR Readout Formats 53 Figure 5
Page 67 and 68: WFC3 IR Readout Formats 55 Full-fra
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Page 77 and 78: Specifying a UVIS Observation 65 6.
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UVIS Field Geometry 67 Figure 6.1:
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6.4.4 Subarrays and On-Chip Binning
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UVIS Field Geometry 71 target at th
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UVIS Spectral Elements 73 6.5 UVIS
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UVIS Spectral Elements 75 Table 6.2
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Figure 6.3: Integrated system throu
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UVIS Spectral Elements 81 UV Filter
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UVIS Spectral Elements 83 Table 6.3
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UVIS Spectral Elements 85 Figure 6.
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6.5.3 Ghosts UVIS Spectral Elements
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UVIS Optical Performance 89 Table 6
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UVIS Optical Performance 91 Figure
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UVIS Optical Performance 93 Table 6
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UVIS Optical Performance 95 Figure
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UVIS Exposure and Readout 97 (1.0,
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UVIS Sensitivity 99 contains 1402×
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6.9.3 Charge-Transfer Efficiency Ot
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UVIS Observing Strategies 103 6.10
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CHAPTER 7: IR Imaging with WFC3 In
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IR Field Geometry 107 The WFC3 IR d
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IR Field Geometry 109 The POS TARG
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IR Spectral Elements 111 Table 7.1:
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IR Spectral Elements 115 Wide-band
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IR Optical Performance 117 Table 7.
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IR Optical Performance 119 Figure 7
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IR Optical Performance 121 Table 7.
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IR Exposure and Readout 123 Inter-p
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IR Exposure and Readout 125 • the
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Table 7.8: Sample times of 1024×10
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IR Sensitivity 129 Table 7.9: Suppo
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Other Considerations for IR Imaging
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IR Observing Strategies 137 sub-pix
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IR Observing Strategies 139 Table 7
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CHAPTER 8: Slitless Spectroscopy wi
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Slitless Spectroscopy with the UVIS
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Slitless Spectroscopy with the IR G
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Slitless Spectroscopy with the IR G
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Sensitivities and Exposure-Time Est
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CHAPTER 9: WFC3 Exposure-Time Calcu
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Calculating Sensitivities from Tabu
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Calculating Sensitivities from Tabu
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Count Rates (Imaging) 157 Table 9.2
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9.4.2 Diffuse Sources Count Rates (
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The S/N ratio Σ achieved in exposu
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Sky Background 163 • I λ is the
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Sky Background 165 Figure 9.1: Sky
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Sky Background 167 Table 9.4: Appro
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Exposure-Time Calculation Examples
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Exposure-Time Calculation Examples
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Observatory Overheads 173 activitie
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Instrument Overheads 175 Table 10.1
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Instrument Overheads 177 IR paralle
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Orbit Use Examples 179 In the IR ch
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Orbit Use Examples 181 Table 10.4:
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Orbit Use Examples 183 Table 10.6:
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APPENDIX A: WFC3 Filter Throughputs
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Throughputs and S/N Ratio Data 187
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% Throughput 30 25 20 15 UVIS/F200L
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% Throughput UVIS/F225W Description
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% Throughput UVIS/F280N Description
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% Throughput UVIS/F350LP Descriptio
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% Throughput UVIS/F390M Description
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% Throughput UVIS/F475X Description
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% Throughput UVIS/F763M Description
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% Throughput UVIS/F850LP Descriptio
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% Throughput UVIS/FQ232N Descriptio
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% Throughput UVIS/FQ422M Descriptio
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% Throughput IR/F098M Description B
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% Throughput IR/F110W Description W
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% Throughput IR/F126N Description [
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% Throughput IR/F128N Description P
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% Throughput IR/F132N Description P
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% Throughput IR /F167N Description
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APPENDIX B: Geometric Distortion In
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UVIS Channel 277 See http://www.sts
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IR Channel 279 Figure B.3: Linear c
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APPENDIX C: Dithering and Mosaickin
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WFC3 Patterns 283 special-purpose m
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WFC3 Patterns 285 Table C.3:Steps i
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APPENDIX D: Bright-Object Constrain
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IR Channel 289 available. To use th
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IR Channel 291 Table D.4: Count Rat
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The STScI Reduction and Calibration
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The STScI Reduction and Calibration
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The SMOV Calibration Plan 297 Propo
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The Cycle 17 Calibration Plan 299 P
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The Cycle 18 Calibration Plan 301 P
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Glossary 303 FGS: Fine Guidance Sen
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Index A ACCUM mode UVIS 96 Aperture
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Index 307 UVIS/FQ634N 242 UVIS/FQ67
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Index 309 comparing HST instruments
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