Subject:

From: Tony Tyson

Submitted: Thu, 26 Dec 2002 17:00:28 -0500 (EST)

Message number: 45 (previous: 44, next: 46 up: Index)

Subject: comments on minutes

Michael had some questions in his writeup.  Here are my answers.
I have been tardy in sending out my simulation of seeing on LSST
WL shear measurement.  I will send that in the next email to
lsst-general.

Happy New Year to all -

Tony

 

<  There was some discussion of whether a decrease in systematic
< effects by a factor of 10 necessarily translates in one's ability to
< measure shear improving by a factor of 10.  For example, what is the
< difference between measuring the PSF shear as 10%+/-1%, and 0%+/-1%?
< The point is that if the PSF shear is 10%, it is unlikely to be
< accurately spatially uniform, and there will be some scale below which
< one can't measure it, giving an irreducible systematic error.  If one
< has faith that the systematics are closer to zero, these irreducible
< errors are likely to be much smaller (Tony, is that a correct way to
< explain it?).

Yes.  The uncorrected shear bias errors scale with amplitude of
shear bias in individual exposures.

  

<  Tony also claimed (for reasons I, Michael, am still a bit unclear
< on) that it is important to measure the PSF shear bias on individual
< exposures (as opposed to the final stacked image).  This requires a
< large A Omega, so you go deep enough to get enough PSF stars to
< measure this.

The reason is that the shear bias pattern can be different on each
new exposure.  Stars and galaxies within each isoplanatic patch move
around relative to those in other patches.  Non-atmospheric effects
can also produce spatially dependent PSFs which change with time
(such as changes in telescope optics).  Once these PSF(x,y) effects
are modeled and removed in each exposure, all good exposures can be
stacked.  Stacking them first makes it impossible to disentangle the
component PSF variations.


  

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