STEP 4: simple simulations

The aim of STEP 4 is to separate the effects of varying galaxy and PSF properties to understand and remove the dependence of shear measurement accuracy on; galaxy size, galaxy profile, signal-to-noise or magnitude, PSF type, PSF ellipticity and PSF size.

In this first stage, Konrad has developed a suite of simulations that can be downloaded from http://vn70.physics.ubc.ca/kuijken/. You will need 57GB of disk space to download the full set. To see what is proposed for future stages, read the STEP 4 discussion wiki

Details of the first stage STEP4 simulations.

These simulations test the accuracy of shear measurement using very well-controlled simulations. The galaxies are at (actually near) well-known positions, do not overlap, and intrinsically average to exactly random orientation. In these simulations all galaxies are either perfectly exponential or de Vaucouleur. PSFs have constant-ellipticity isophotes and are the same for all galaxies in a simulation. PSF FWHM is 3.5 pixels, galaxy FWHM is 7 pixels after PSF smearing.

There are six simulations, each consisting of a number of shearsets of fits images with 3600 galaxies each:

galtype SN=15 SN=50 SN=200
exp 32 sets of 10 32 sets of 5 32 sets of 2 images
dev 32 sets of 10 32 sets of 5 32 sets of 2 images

Within each shearset the PSF anisotropy and shear are constant, and the galaxies in the images have a uniform spread in intrinsic position angle, at a discrete set of intrinsic axis ratios drawn from realistic distributions. All images in a set need to be analysed to derive a shear.

The filenames are of the form

STEP4.sn50.dev.shear8.im4.fits

indicating S/N (sn15, sn50, sn200)
galaxy type (exp or dev)
shearset (shear1 to shear32)
image number (im1 to im10 for SN=15, fewer for other S/N as per table above)

Each image contains 3600 galaxies and 240 stars. The stars are 10x brighter than the galaxies. The galaxies are placed on a rectangular grid of spacing 60, near x and y coordinates 90, 150, ..., 3630. The actual centroid of each galaxy is randomly offset from the pixel grid. The stars are arranged around the edge, near columns x=30 and 3690, and lines y=30 and 3690.

Each of the 32 shearsets has a different shear value and PSF anisotropy. These values differ from simulation to simulation, so eg set 10 of the exponential, SN=50 simulation has a different shear from set 10 in the devauc, SN=15 simulation. The PSF anisotropies will also be different.

In total there are 3600x32x10 ~ 1E6 galaxies at S/N 15 (33Gb), for exp and dev each, 3600x32x5 ~ 5E5 galaxies at S/N 50 (17Gb) and 3600x32x2 ~ 2E5 at S/N 200 (7Gb).

The input shear and PSF anisotropy of sets 1-8 is public, and IS the same for each simulation: these values are

set id g1 g2 e1psf e2psf
1 0.00 0.02 0.02 0.00
2 0.00 0.02 0.00 0.02
3 0.02 0.00 0.02 0.00
4 0.02 0.00 0.00 0.02
5 0.00 0.02 0.00 0.00
6 0.02 0.00 0.00 0.00
7 0.00 0.00 0.02 0.00
8 0.00 0.00 0.00 0.02

Feel free to contact Konrad with further questions: (kuijken[at]strw.leidenuniv.nl).

Rules for the analysis

In contrast to previous STEPs, we encourage you to use information about the simulations to aid your analysis. This might involve the following;

• Use your knowledge about the location of object centroids to remove any false detections from your source catalogues and separate stars from galaxies.
• Make a PSF model for each image from the average properties of all 240 stars.
• Correct for the PSF as if it is constant across the image.
• Modify and improve your shear measurement pipeline until it can achieve <1% accuracy on the first 8 image sets for which the input shear is public (as listed above).

Once you can demonstrate an accuracy of <1%, i.e m<0.01 on the first 8 image sets for a particular SN, you can send your results for the full SN simulation set to Konrad in the following format

• Filename: STEP4_SN***_\$\$\$_##.dat, where *** is 200, 50 or 15, \$\$\$ is exp or dev and ## is and ID for you.
• File format: setid, g1, g1_error, g2, g2_error

A piece of code will be made available shortly to compute these values from your shear catalogues. Konrad will return STEP m and c results to you. Repeat analyses are permitted, but please limit yourself to as few as possible. In the long term Tim Schrabback has offered to create a web-tool to perform this analysis automatically. Note that the input shears for STEP4 simulations will remain blind for the foreseeable future.