Space-Based Galaxy Survey Exposure Time Calculator

This exposure time calculator was written for the Wide Field Infrared Space Telescope (WFIRST) Interim Design Reference Mission (IDRM) report, which can be found here. There have been subsequent improvements to support the 2012 WFIRST Science Definition Team activities, and to incorporate more recent data. Important feedback and input on the calculator, associated models, and documentation were provided by many people, including Peter Capak, Ed Cheng, Dave Content, Neil Gehrels, Zeljko Ivezic, Cliff Jackson, Jean-Paul Kneib, Jeff Kruk, Jason Rhodes, David Sobral, Yun Wang, Sherry Wheelock, and Julien Zoubian.

The ETC is intended to go beyond simply calculating the S/N for a particular object: in either the weak lensing or baryon oscillation modes, it also generates the population of galaxies observed (i.e. with shapes in the WL mode or with detected Hα emission lines in the BAO mode). As a consequence, it usually takes several minutes to run on a modern computer (e.g. my MacBook with a 2.1 GHz processor).

It is anticipated that the program may be useful outside of WFIRST, but no warranties are made regarding its suitability for general purposes.

The reference for the ETC is: Christopher M Hirata, Neil Gehrels, Jean-Paul Kneib, Jeffrey Kruk, Jason Rhodes, Yun Wang, Julien Zoubian, "The WFIRST Galaxy Survey Exposure Time Calculator" 2012 (arXiv:1204.5151). Please also indicate the version you used.

The latest version (v10) of the source code can be obtained here:

• Manual (Please read at least the relevant sections of this first to see what it is you are actually computing.)
• Code can be downloaded here.

Version	Date	Download	Comments
v10	2012-03-28	Code * Manual
v9	2012-03-07	Code * Manual
v8	2011-10-01	Code * Manual	Used for the fall 2011 performance update.
v7	2011-05-04	Code * Manual	Used for the IDRM report.

(Note: v6 and earlier were not used for any published work and are considered obsolete.)

The changes to the ETC itself since the IDRM report are rather modest (e.g. inclusion of minor noise terms or additional user options). The largest change has been to the galaxy population model for the BAO program: the new Sobral et al (2012) Hα luminosity function is significantly lower than the previous model.

The WFIRST configurations are described below, including the 2011 IDRM as well as the 2012 DRM1+DRM1A.

2011 IDRM: The IDRM configuration files can be downloaded for the imaging channel and the the spectroscopy channels. The IDRM1 is described in detail here; it has 3 channels, 1 for imaging and 2 for spectroscopy.

• The imaging channel is a 7x4 layout of H2RG detectors at 0.18"/p.
• Each spectroscopy channel is a 2x2 layout of H2RG detectors at 0.45"/p.
• WFIRST IDRM assumed a read noise floor of 5 e rms and a dark current of 0.05 e/p/s.
• The wavefront error budgets are 0.071 μm rms (imaging) and 0.213 μm rms (spectroscopy); the jitter specification is 0.04 arcsec rms per axis.
• The thermal background calculations in the ETC do not contain any explicit margin factor. For the WFIRST studies, we have added 5 K to the input temperature as margin to cover various issues not modeled in the ETC (e.g. the filter bandpass cutoff is not a perfect step function).
• The IDRM imaging filter set included: F087 (0.76-0.97 μm); F111 (0.97-1.24 μm); F141 (1.24-1.57 μm); F178 (1.57-2.00 μm); and W149 (0.97-2.00 μm). Of these, F111/F141/F178 were used for the extragalactic/cosmology surveys, W149+F087 was used for microlensing, and all the standard width filters were used for the supernova program.
• The in-band filter transmission was assumed to be 0.95.
• The galaxy redshift survey bandpass (spectroscopy channels) was 1.116-2.00 μm.

2012 DRM1/DRM1A/DRM2: These are single channel configurations with a movable prism for spectroscopy (i.e. the same focal plane is used for both imaging and spectroscopy, but in series rather than in parallel). The single channel configuration was motivated by an effort to reduce mission cost.

The DRM1 uses H2RG detectors (TRL6), whereas the DRM1A uses H4RG detectors (requiring development -- not ready to be the baseline option). Changes are described below. The detailed description of the 2012 DRMs will be provided in the forthcoming SDT final report.

• The DRM1 has a 9x4 layout of H2RGs at 0.18"/p (18 μm pixels).
• The DRM1A has a 5x3 layout of H4RGs at 0.17"/p (10 μm pixels).
• The DRM2 has a 7x2 layout of H4RGs at 0.18"/p (10 μm pixels) and has the primary mirror diameter descoped from 1.3 to 1.1 m. This is intended to be a lower cost option driven by current constraints.
• The read noise floor is 5 e rms (note that the ETC separately includes the statistical noise from multiple up-the-ramp samples).
• The dark current for the H2RGs is taken to be 0.05 e/p/s. The H4RGs are not yet mature and for this case we have assumed the same dark current density as for the H2RGs (i.e. 0.015 e/p/s).
• In DRM1A, the trade of 32 versus 64 channel readout has not yet been made.
• In the 2012 DRMs, the detectors were switched back to the 2.5 μm cutoff devices and the telescope temperature has been reduced to 205 K. The filter set thus contains standard width filters, Z+Y+J+H+K, plus a wide filter (W) for the microlensing program. Of these Y+J+H+K are used for the extragalactic/cosmology survey. The filter definitions are:
  • Z: 0.732-0.962 μm
  • Y: 0.920-1.209 μm
  • J: 1.156-1.520 μm
  • H: 1.453-1.910 μm
  • K: 1.826-2.400 μm
  • W: 0.920-2.400 μm
  These do not correspond exactly to the usual ground-based filter definitions because there is no need to avoid the H₂O bands.
• The galaxy redshift survey bandpass (spectroscopy channels) has a wavelength range of 1.5-2.4 μm (DRM1) or 1.7-2.4 μm (DRM2).

The current (June 2012) version of the 2012 DRM configuration files is here: [DRM1 imaging/spectroscopy || DRM1A imaging/spectroscopy || DRM2 imaging/spectroscopy ]. Full details will bein the upcoming SDT report.

• For historical puroposes, the original (April 2012) release of the DRM2 configuration files is here: [ DRM2 imaging/spectroscopy ]. The June 2012 files supersede these.

The weak lensing version of the code requires an input galaxy catalog, which is technically not part of the exposure time calculator and is not included in the above files. The catalog used for the WFIRST forecasts is derived from the COSMOS Mock Catalog by Jouvel et al (2009), which is available here (external link!). The catalogs appropriately formatted for use in the ETC are:

• v2, current: here.
• v1, used for IDRM study: here.