STEP2 simulated images

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STEP2 simulated images
STEP2
simulated
images
Richard Massey with Molly Peeples, Will High,
Catherine Heymans, etc. etc.
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Shapelets basis functions(an empirical approach)
8 6 4 2 0 -2 -4 -6 -8
M
Truncation of high spatial frequencies
http//www.astro.caltech.edu/rjm/shapelets
0 2 4 6
8
N
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STEP2 PSFs
A motley assortment designed to target several
specific problems in shear measurement.
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STEP2 shapelet galaxies

• Shear galaxies by a known amount, then
• apply PSF and add observational noise.

• Place real galaxies into parameter space

• Smooth parameter space and resample
• to generate fake simulated galaxies

• Decompose COSMOS galaxies into
• representative shapelet components

? Obtain galaxies with the same distribution and
shape statistics as that from the input
population ? Contain complex morphologies
COSMOS first year mosaic Hubble Space Telescope
ACS
Massey et al. (2003)
Refregier et al. (2001)
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Example STEP2 images
Real image
Simulated image
Animations show 0-10 shear in 1 steps. Real
signal is only 2.

• Shapelet method has been extended by Molly
  Peeples to simulate Subaru i-band images.
• COSMOS ACS F814W galaxies used as a training
  set.
• Plenty of galaxies to keep populations unique
  without perturbing shapelet coefficients.

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STEP2 noise model

• Modelled on real, 40 minute r exposures with
  Subaru
• Photon shot noise
• Sky background noise (constant level subtracted)
• Correlated between adjacent pixels
• (present in real data due to unresolved
  background
• sources and use of DRIZZLE to stack exposures)

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Available in input catalogues

• Input shear for each image (nb a few images were
  malformed)
• Object photometry
• Object astrometry
• Object star/galaxy classification
• Unweighted object ellipticity (Q11-Q22,2Q12)/(Q11
  Q22)
• Unweighted object rms size2 (Q11Q22)
• Everything quoted before convolution with the PSF.

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STEP2 image ingredients
GALAXIES COSMOS training set automatically incorp
orates the true cosmology, galaxy morphology
distribution (size, mag), etc.
Model galaxy morphology PDF in shapelet space
)
(
Morph and resample PDF to generate new galaxies
Apply shear signal
STARS Modelled from real Subaru data, using
shapelets to nmax8
Convolve with PSF
Add observational noise
?
NOISE Photon shot noise plus sky background
Realise image
Measure shear