|STEP 4: simple simulations|
|STEP home||Simulations||Data||Mailing List|
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:
|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
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
Feel free to contact Konrad with further questions: (kuijken[at]strw.leidenuniv.nl).
In contrast to previous STEPs, we encourage you to use information about the simulations to aid your analysis. This might involve the following;
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
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.
Last modified 4th April.
STEP pages maintained by Catherine Heymans: heymans[at]physics.ubc.ca
Questions regarding the STEP4 simulations should be directed to Konrad Kuijken (kuijken[at]strw.leidenuniv.nl).