From carlberg@moonray.astro.utoronto.ca Sat Jul 10 05:23:22 1999
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From: Ray Carlberg <carlberg@moonray.astro.utoronto.ca>
Message-Id: <199907101521.LAA15271@moonray.astro.utoronto.ca>
Subject: megacam interest
To: pierrec@cfht.hawaii.edu, fahlman@cfht.hawaii.edu
Date: Sat, 10 Jul 1999 11:21:28 -0400 (EDT)
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Megacam brings new opportunities to investigate a number of
fundamental cosmological issues. The various major efforts
to undertake weak lensing analyses and large photometric 
redshift surveys are well known and should be supported. 
My own interest is mainly in the direction of variability which
Megacam will probably allow for the first time to be routinely
done on a proper sampling basis. One of the most interesting possibilities
is to find very distant lensed SNIa which will allow new constraints
to be placed on cosmological models (and likely the SNIa themselves).

The need for something like Megacam is clear. Depending on the details
of the cosmic star formation history and the mechanism for the formation
of SNIa it is easy to work out that there should be hundreds of SNIa per
year per square degree to a depth of I=25 or so. It is a classical 
calculation that the strong lensing due to inviddual galaxies should
produce a strong lens with multiple images for about 1 out of 1000
sightlines to redshift one. The details are of course very dependent
on cosmology which is one of the reasons to make the measurement.
A split lens with a known redshift of source, lens, the lens
time delay from the light curve and the magnification
inferred from the magnitude-fading relation for Ia allows a one step
measurement of the geometric distance. Together these will offer two
new datasets for geometric constraints on cosmology. Such "classical"
tests are essential to do to provide cosmological measurements with minimal
model dependence. 

This type of observation can be done at very little cost, simply by 
appropriately scheduling other very deep observations of a limited
number of sky patches. Moreover, such synoptic data are of immense
interest for SNII, faint white dwarves, outer solar system, etc. 
(Because the ecliptic plane contains dust it might not be compatible
with standard Kuiper belt type searches). For people interested in
spectroscopic followup of SNe with VLT/Gemini it will be important
to promptly reduce the data, which will pose another time constraint 
and could effect the observational design.

Chris Pritchet is the PI of a proposal to undertake a general SN search.
The point of my note is to support the collaborative use of Megacam and
my interest in being involved at the more general level in the 
planning of observations. I have briefly discussed this with Yannick 
Mellier, indicating my interest in supporting collaborative use of Megacam
for increased mutual benefit where appropriate.

--Ray Carlberg