Spitzer Documentation & Tools
IRAC Instrument Handbook
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Table of Contents

1     Introduction.

1.1       Document Purpose and Scope.

1.2       Basic Definitions.

1.3       IRAC Essentials.

1.4       Standard Acknowledgments for IRAC Publications.

1.5       How to Contact Us.

2     Instrument Description.

2.1       Overview

2.2       Description of Optics.

2.2.1    Field of View (FOV).

2.2.2    IRAC Image Quality.

2.2.3    Spectral Response.

2.2.4    Distortion.

2.3       Detectors.

2.3.1    Design.

2.3.2    Linearity.

2.4       Electronics.

2.4.1    Hardware.

2.4.2    Fowler Sampling.

2.4.3    Exposure Time and Frame Time.

2.4.4    Calibration Lamps.

2.4.5    Firmware.

2.5       Sensitivity and Saturation

2.5.1    Sensitivity Model

2.5.2    Saturation.

3     Operating Modes.

3.1       Readout Modes and Frame Times.

3.2       PCRS Peak-Up Observations.

3.3       Map Grid

3.4       Dithering Patterns.

4     Calibration.

4.1       Darks.

4.1.1    Shutterless Skydarks.

4.1.2    Closed Shutter Campaign.

4.2       Flat-Fields.

4.3       Photometric Calibration

4.4       Color Correction

4.5       Array Location-Dependent Photometric Corrections for Compact Sources with Stellar Spectral Slopes 

4.6       Pixel Phase-Dependent Photometric Correction for Point Sources.

4.7       IRAC Aperture Photometry Corrections.

4.8       Photometric Variations in Data Taken with Full Array, Subarray, Dithering and Staring Modes 

4.9       IRAC Point Spread and Point Response Functions.

4.9.1    Core PRFs.

4.9.2    Extended PSFs.

4.10      Calculation of IRAC zmags.

4.11      Astrometry and Pixel Scales.

4.11.1      Optical Distortion.

4.11.2      Pixel Solid Angles.

4.12      Pointing Performance.

4.12.1      Pointing Accuracy.

4.12.2      Target Acquisition Accuracy.

4.12.3      Pointing Glitches.

4.12.4      Jitter, Drift, and Wobble.

4.12.4.1     Jitter

4.12.4.2     Drift

4.12.4.3     Wobble.

4.13      Instrument Performance Trending.

4.13.1      Photometric Stability.

4.13.2      Skydark Bias Offsets.

4.13.3      Radiation Hits.

5     Pipeline Processing.

5.1       Level 1 (BCD) Pipeline.

5.1.1    sanity datatype (parameter checking).

5.1.2    sanity check (image contents checking).

5.1.3    tranhead (header processing).

5.1.4    insbposdom (InSb array sign flipping).

5.1.5    cvti2r4 (byte type changing).

5.1.6    Wraparound Correction: iracwrapdet and iracwrapcorr.

5.1.7    iracnorm (Fowler sampling renormalization).

5.1.8    snestimator (initial estimate of uncertainty).

5.1.9    iracebwc (limited cable bandwidth correction).

5.1.10      Dark Subtraction I: ffcorr (“first frame effect” correction) and labdarksub (labdark subtraction) 

5.1.11      muxbleedcorr (electronic ghosting correction).

5.1.12      darkdrift (“readout channel” bias offset correction).

5.1.13      fowlinearize (detector linearization).

5.1.14      bgmodel (zodiacal background estimation).

5.1.15      Dark Subtraction II: skydarksub (sky “delta-dark” subtraction).

5.1.16      flatap (flat-fielding).

5.1.17      imfliprot.

5.1.18      detect-radhit (cosmic ray detection).

5.1.19      dntoflux (flux calibration).

5.1.20      Pointing Transfer (calculation of pointing information).

5.1.21      predictsat (HDR saturation processing).

5.1.22      latimflag (residual image flagging).

5.2       The Artifact-Corrected BCD Pipeline.

5.2.1    Stray Light

5.2.2    Saturation.

5.2.3    Sky Background Estimation.

5.2.4    Column Pull-down.

5.2.5    Banding Correction (Channels 3 and 4).

5.2.6    Muxstripe Correction (Channels 1 and 2).

5.3       Level 2 (Post-BCD) Pipeline.

5.3.1    Pointing Refinement

5.3.2    Superboresight Pointing Refinement

5.3.3    Mosaicking and Coaddition.

6     Data Products.

6.1       File Characteristics.

6.2       File-Naming Conventions.

6.3       IRAC Specific Header Keywords.

7     Data Features and Artifacts.

7.1       Darks, Flats, Missing Data, and Bad Pixels.

7.1.1    Bad Pixels.

7.1.2    Flat-field.

7.1.3    Missing Data.

7.2       Electronic Artifacts.

7.2.1    Saturation and Nonlinearity.

7.2.2    Muxbleed (InSb).

7.2.3    Bandwidth Effect (Si:As).

7.2.4    Column  Pull-Down/Pull-Up.

7.2.5    Row Pull-Up.

7.2.6    Full-Array Pull-Up.

7.2.7    Inter-Channel Crosstalk.

7.2.8    Persistent Images.

7.2.8.1       Cryogenic Mission Persistent Images.

7.2.8.2       Warm Mission Persistent Images.

7.3       Optical Artifacts.

7.3.1    Stray Light from Array Covers.

7.3.2    Optical Banding and Internal Scattering.

7.3.3    Optical Ghosts.

7.3.4    Large Stray Light Ring and Splotches.

7.4       Cosmic Rays and Solar Protons.

8     Introduction to Data Analysis.

8.1       Point Source Photometry

8.2       Extended Source Photometry

8.2.1    Best Practices for Extended Sources.

8.2.2    Extended Source Aperture Correction.

8.2.3    Low Surface Brightness Measurements and the Maximum Scaling Factors.

8.2.4    Caveats & Cautionary Notes.

8.2.5    Faint Surface Brightness Behavior.

8.2.5.1       Binning.

8.2.5.2       Small Scales.

8.2.5.3       Medium Scales.

8.2.5.4       Large Scales.

8.2.5.5       Increasing exposure time.

8.3       IRAC High Precision Time Series Photometry

8.3.1    Time-Correlated Noise.

8.3.2    Spacecraft Motion.

8.3.3    Repeatable Pointing.

8.3.3.1       “Sweet Spot”.

8.3.3.2       Why This Sweet Spot?.

8.3.4    Mapping Intrapixel Gain.

8.3.4.1       Centroiding.

8.3.4.2       Noise Pixels.

8.3.4.3       Optimal Aperture Size.

8.3.5    Residual Nonlinearity.

8.3.6    Time-varying Bias in Point Source Data.

8.3.6.1       Staring Mode Darks.

8.3.6.2       Superdarks.

8.3.7    IRAC’s Photometric Stability.

8.3.8    Persistent Images.

8.3.9    Removing Correlated Noise – Repeatability and Accuracy.

8.3.10      Measuring Noise Reduction.

8.4       Post-BCD Data Processing.

8.4.1    Pointing Refinement

8.4.2    Overlap Correction.

8.4.3    Mosaicking of IRAC Data.

8.4.3.1       Creating a Common Fiducial Frame.

8.4.3.2       Outlier Rejection.

8.4.3.3       Mosaicker Output Files.

8.4.3.4       To Drizzle or Not to Drizzle?.

8.4.3.5       Mosaicking Moving Targets.

8.4.4    Source Extraction.

8.4.4.1       Noise Estimation.

8.4.4.2       PRF Estimation.

8.4.4.3       Background Estimation.

8.4.4.4       Source Extraction.

8.4.4.5       Outlier Rejection.

8.5       Things to Check and Important Notes for Data Analysis.

Appendix A.     Pipeline History Log.

S19.2 (Warm Mission Only; processing started on 2015 June 15)

S19.1 (Warm Mission Only; 2015 August 29)

S18.25 (Cryogenic Mission Final Processing; 2012 March 15)

S18.18 (Warm Mission Only; 2010 May 27)

S18.18 (2010 May 27)

S18.14 (2010 December 10)

S18.5 (2008 December 1)

S18.0 (2008 June 12)

S17.0 (2007 December 5)

S16.0 (2007 April 9)

S15.0 (2006 November 28)

S14.0 (2006 May 3)

S13.0 (2005 October 27)

S12.0 (2005 May 9)

S11.0 (2004 November 30)

S10.5 (2004 June 2)

S10.0 (2004 May 7)

S9.5 (2004 March 1)

S9.1 (2004 January 22)

S9.0 (2004 January 14)

S8.9 (2003 December 8)

Appendix B.     Performing Photometry on IRAC Images.

A. Point Source Photometry on a Mosaic.

B. Point Source.

Appendix C.     Point Source Fitting IRAC Images with a PRF.

C.1   Use of the Five Times Oversampled PRFs Outside of APEX

C.2   Modifications to the IRAC PRFs for Use with APEX

C.3   Results of Tests with PRF fitting.

C.3.1        Test on Calibration Stars.

C.3.2        Subpixel Response in Channels 1 and 2.

C.3.3        The Serpens Test Field.

C.3.4        The GLIMPSE Test Field.

C.3.5        Photometry of Moderately-Resolved Sources.

C.3.6        Positional Accuracy.

C.3.7        A How-To-Guide for IRAC Point Source Photometry with APEX

C.3.8        Pixel Phase.

Appendix D.     Time Series Photometry Supplement

D.1 Data Features.

D.1.1 Header Keywords.

D.1.2 Skydark Change in the Middle of an Observation.

D.1.3 Background modulation.

D.1.4 Noise Pixel Spikes.

D.1.5 Timing.

D.1.6 58th Frame in Subarray Datasets.

D.1.7 Corrupted Header.

D.1.8 Position Jumps Due to Reaction Wheel Speed Change.

D.1.9 Pointing History File Change.

D.1.10 Effects of Cosmic Rays on Centroiding Algorithm

D.1.11 Pattern noise (horizontal striping).

D.2 Time Series Photometry Tools.

Appendix E.      IRAC BCD File Headers.

E.1 Cryogenic Mission Channel 1.

E.2 Cryogenic Mission Channel 2.

E.3 Cryogenic Mission Channel 3.

E.4 Cryogenic Mission Channel 4.

E.5 Warm Mission Channel 1.

E.6 Warm Mission Channel 2.

Appendix F.      Acronyms.

Appendix G.         Acknowledgments.

Appendix H.         List of Figures.

Appendix I.       List of Tables.

Appendix J.      Version Log.

Bibliography.

 

 

 


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