WFI Direct Imaging Simulations in Support of Weak Lensing

Summary

The SDT 2015 report proposed a high-latitude survey (HLS) to image 2227 deg² in 4 NIR bands (Y, J, H, and F184) spanning the range 0.92 — 2.00 μm to predicted AB magnitudes of 25.8 — 26.7, depending on band. This page is a repository of simulations of the HLS to help the community plan related Roman Space Telescope observations. The focus is on weak-lensing observations due to their importance in setting Roman Space Telescope imaging requirements. Photometry obtained with the Wide-Field Instrument (WFI) will be used to obtain shapes and photometric redshifts (when combined with ground based photometry) for ≅ 45 galaxies per square arcminute. Deeper images with the WFI, for instance in the supernova detection fields, will be able to obtain weak lensing measurements of well over 200 galaxies per square arcminute.

An example output image from the GalSim demo script. The source galaxy distribution comes from COSMOS observations with HST using separate SEDs for the bulge and the disc components. Stars follow a log normal distribution, and the distinctive Roman PSF diffraction spikes are evident in the the brightest ones. This image is a cutout from SCA 7, but the demo script can simulate the full focal plane with the correct (per Cycle 7 specifications) Roman WCS and sensor positions. The rgb function in ds9 was used to put the images from different filters together. The colors seen in the images are therefore not meant to reproduce the real galaxy color distribution. This image has a linear flux scaling, and filters are color coded so that F158 is red, F129 is green, and F106 is blue.

Generating the Sky Scene

The user selects which objects to include in the simulation. For example, the user could use all galaxies in the COSMOS catalog down to some magnitude limit, or the user may assemble a list of stars at random locations, etc. For a detailed explanation of how to generate a sky scene, see Realistic Scene in GalSim documentation.

Reference Parameters (SDT 2015 Report)

Further parameters are listed in the GalSim Roman module docstring, which users can obtain within GalSim (after importing the Roman module via 'import(galsim.roman)') by typing help(galsim.roman), or by reading the init.py file. For details about bandpasses, WCS, etc. users will need to read the docstrings about the routines in the module (or, they can be accessed via the Python interpreter help command). The bandpasses and zodiacal values are taken from Chris Hirata's Roman Exposure Time Calculator, version 14.

GalSim and IMCOM

The simulations posted here are being produced using the open-source Galsim (ref: Rowe et al., 2015) image simulation suite. The GalSim Roman module for simulating WFI images is currently available (after installing GalSim from the master branch), including the use of realistic Hawaii-4RG detector effects (read noise, nonlinearity, interpixel capacitance, charge spreading among pixels, reciprocity failure), a Roman Space Telescope specific PSF, and throughput and filter curves from the current design reference mission. The simulated images can then be dithered/combined with IMCOM (ref: Rowe, Hirata & Rhodes, 2011) that is optimized for weak lensing and specifically designed with the Roman Space Telescope as a primary application.

There is a demo script available that demonstrates how the GalSim Roman module works.