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SAC point of view: MegaCam legacy survey
Dear SAC Members
During the past weeks we have been hearing lots of comments regarding
the CFH Legacy Survey, some of them being quite technical.
They'll be probably discussed again at length at the next meetings
(Users and and SAC) in Lyon.
But, it is a fact, that the current document "CFHLS, 28 Feb 2001" by the
MegaCam Survey Working Group is a "report to the CFHT Science Advisory
Committee".
We are thus expected to give our comprehensive opinion on the document -
including feedback from our communities -, which will be forwarded to
the Board for final amendments.
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Below, here are a few points of general interest, I consider as quite
important, and which, to my knowledge, have not been addressed.
(1) It seems very timid to almost consider as possible CFHLSs, only
those being "self-consistent". i.e. those, not requiring substantial
spectroscopic follow-up (this concerns LSS and SN programmes).
Question: what would have been the now famous SIRTF Legacy surveys, if
they would not have implicitly heavily relied on extensive ground based
spectroscopic follow-up?
(2) One understands well that 6 areas with RA uniformly distributed are
optimal for telescope scheduling.
- Are they really necessary, (at least from the weak shear point of
view)? This does not appear in the document.
(3) The purely LSS aspect of the programme, despite the denomination of
Sec. 3.1, is totally absent.
I seems unbelievable that "the" optical Legacy Survey of the next decade
will not probe scaleslarger than some 100 Mpc/h at z=1.
Just think about what SLOAN will be doing at z = 0. We need absolutely
to investigate LSS evolution of large scales ( as given by galaxies or
clusters) out to z=1.
Weak lensing alone cannot drive the entire survey design. There are
several key evolutionary LSS questions, that need to be addressed.
(4) I do not quite understand the exposure times given for bands other
than the I one (page 6) .
What photometric z shall we be able to reach with a U exposure of one
hour???
To get the same depth as in I (1 hour), the U time has to be ~ 5h, i.e.
U=26.1
(5) I propose we seriously consider covering :
- 3 areas of 6x6 sq.deg. (if necessary) and one of at least 100 sq.deg
- with RA separated from ~ 6 hours.
- critically review the usefulness of the present proposed depths in the
various bands, for the "shallow" surveys.
Marguerite Pierre
(I'll be away from May 12 to 29)