8.4 Sensitivities and Exposure-Time Estimation

Table 8.2 lists current estimates of the detection limits for slitless spectroscopy with the UVIS and IR modes. The numbers in the table are the V magnitudes of point sources for which a 1-hour exposure yields a spectrum with a S/N of 5 per resolution element, with extraction boxes of 1 × 5 and 1 × 3 pixels for the UVIS and IR, respectively. V magnitudes are given for three different assumed source spectral types.

Table 8.2: V-band detection limits for the WFC3 grism modes for 1-hour exposure and 5σ detection.

Mode	Spectral Type			Wavelength (nm)
Mode	O3 V	A0 V (Vega)	G2 V	Wavelength (nm)
UVIS2/G280	25.0	21.9	18.2	250
IR/G102	21.3	22.0	22.9	1050
IR/G141	20.5	21.3	22.8	1550

For more accurate estimations of exposure times, observers should use the WFC3 Exposure Time Calculator (ETC). The ETC supports all three spectroscopic modes.

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WFC3 Instrument Handbook
- • Acknowledgments
- Chapter 1: Introduction to WFC3
  - • 1.1 New for Cycle 32
  - • 1.2. Overview
  - • 1.3 Key Features of WFC3
  - • 1.4 WFC3 Quick Reference Guide
  - • 1.5 Special Considerations for Cycle 32
  - • 1.6 Sources of Further Information
  - • 1.7 The WFC3 Instrument Team at STScI
- Chapter 2: WFC3 Instrument Description
- Chapter 3: Choosing the Optimum HST Instrument
- Chapter 4: Designing a Phase I WFC3 Proposal
  - • 4.1 Phase I and Phase II Proposals
  - • 4.2 Preparing a Phase I Proposal
- Chapter 5: WFC3 Detector Characteristics and Performance
- Chapter 6: UVIS Imaging with WFC3
  - • 6.1 WFC3 UVIS Imaging
  - • 6.2 Specifying a UVIS Observation
  - • 6.3 UVIS Channel Characteristics
  - • 6.4 UVIS Field Geometry
  - • 6.5 UVIS Spectral Elements
  - • 6.6 UVIS Optical Performance
  - • 6.7 UVIS Exposure and Readout
  - • 6.8 UVIS Sensitivity
  - • 6.9 Charge Transfer Efficiency
  - • 6.10 Other Considerations for UVIS Imaging
  - • 6.11 UVIS Observing Strategies
- Chapter 7: IR Imaging with WFC3
  - • 7.1 WFC3 IR Imaging
  - • 7.2 Specifying an IR Observation
  - • 7.3 IR Channel Characteristics
  - • 7.4 IR Field Geometry
  - • 7.5 IR Spectral Elements
  - • 7.6 IR Optical Performance
  - • 7.7 IR Exposure and Readout
  - • 7.8 IR Sensitivity
  - • 7.9 Other Considerations for IR Imaging
  - • 7.10 IR Observing Strategies
- Chapter 8: Slitless Spectroscopy with WFC3
- Chapter 9: WFC3 Exposure-Time Calculation
  - • 9.1 Overview
  - • 9.2 The WFC3 Exposure Time Calculator - ETC
  - • 9.3 Calculating Sensitivities from Tabulated Data
  - • 9.4 Count Rates: Imaging
  - • 9.5 Count Rates: Slitless Spectroscopy
  - • 9.6 Estimating Exposure Times
  - • 9.7 Sky Background
  - • 9.8 Interstellar Extinction
  - • 9.9 Exposure-Time Calculation Examples
- Chapter 10: Overheads and Orbit Time Determinations
  - • 10.1 Overview
  - • 10.2 Observatory Overheads
  - • 10.3 Instrument Overheads
  - • 10.4 Orbit Use Examples
- Appendix A: WFC3 Filter Throughputs
  - • A.1 Introduction
  - A.2 Throughputs and Signal-to-Noise Ratio Data
    - • UVIS F200LP
    - • UVIS F218W
    - • UVIS F225W
    - • UVIS F275W
    - • UVIS F280N
    - • UVIS F300X
    - • UVIS F336W
    - • UVIS F343N
    - • UVIS F350LP
    - • UVIS F373N
    - • UVIS F390M
    - • UVIS F390W
    - • UVIS F395N
    - • UVIS F410M
    - • UVIS F438W
    - • UVIS F467M
    - • UVIS F469N
    - • UVIS F475W
    - • UVIS F475X
    - • UVIS F487N
    - • UVIS F502N
    - • UVIS F547M
    - • UVIS F555W
    - • UVIS F600LP
    - • UVIS F606W
    - • UVIS F621M
    - • UVIS F625W
    - • UVIS F631N
    - • UVIS F645N
    - • UVIS F656N
    - • UVIS F657N
    - • UVIS F658N
    - • UVIS F665N
    - • UVIS F673N
    - • UVIS F680N
    - • UVIS F689M
    - • UVIS F763M
    - • UVIS F775W
    - • UVIS F814W
    - • UVIS F845M
    - • UVIS F850LP
    - • UVIS F953N
    - • UVIS FQ232N
    - • UVIS FQ243N
    - • UVIS FQ378N
    - • UVIS FQ387N
    - • UVIS FQ422M
    - • UVIS FQ436N
    - • UVIS FQ437N
    - • UVIS FQ492N
    - • UVIS FQ508N
    - • UVIS FQ575N
    - • UVIS FQ619N
    - • UVIS FQ634N
    - • UVIS FQ672N
    - • UVIS FQ674N
    - • UVIS FQ727N
    - • UVIS FQ750N
    - • UVIS FQ889N
    - • UVIS FQ906N
    - • UVIS FQ924N
    - • UVIS FQ937N
    - • IR F098M
    - • IR F105W
    - • IR F110W
    - • IR F125W
    - • IR F126N
    - • IR F127M
    - • IR F128N
    - • IR F130N
    - • IR F132N
    - • IR F139M
    - • IR F140W
    - • IR F153M
    - • IR F160W
    - • IR F164N
    - • IR F167N
- Appendix B: Geometric Distortion
  - • B.1 Overview
  - • B.2 UVIS Channel
  - • B.3 IR Channel
- Appendix C: Dithering and Mosaicking
  - • C.1 Why Mosaicking and Dithering are Needed
  - • C.2 WFC3 Patterns
- Appendix D: Bright-Object Constraints and Image Persistence
  - • D.1 UVIS Channel
  - • D.2 IR Channel
- Appendix E: Reduction and Calibration of WFC3 Data
  - • E.1 Overview
  - • E.2 The STScI Reduction and Calibration Pipeline
  - • E.3 The SMOV Calibration Plan
  - • E.4 The Cycle 17 Calibration Plan
  - • E.5 The Cycle 18 Calibration Plan
  - • E.6 The Cycle 19 Calibration Plan
  - • E.7 The Cycle 20 Calibration Plan
  - • E.8 The Cycle 21 Calibration Plan
  - • E.9 The Cycle 22 Calibration Plan
  - • E.10 The Cycle 23 Calibration Plan
  - • E.11 The Cycle 24 Calibration Plan
  - • E.12 The Cycle 25 Calibration Plan
  - • E.13 The Cycle 26 Calibration Plan
  - • E.14 The Cycle 27 Calibration Plan
  - • E.15 The Cycle 28 Calibration Plan
  - • E.16 The Cycle 29 Calibration Plan
  - • E.17 The Cycle 30 Calibration Plan
  - • E.18 The Cycle 31 Calibration Plan
- • Glossary

8.4 Sensitivities and Exposure-Time Estimation

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