From: Pierre Astier (pierre.astier@in2p3.fr)
Date: Thu May 02 2002 - 09:46:36 PDT
On Wed, 1 May 2002, Ariel Goobar wrote:
>
> Hi Julien, Reynald and Pierre,
> at the teleconference yesterday we concluded that a likely
> scenario for next week is to do cross-telescope subtractions
> CTIO-CFHT both in R and I. Rob therefore needs immediate
> access to the CFHT references, primarily the april data.
> Please, contact him urgentely, if you haven't done so already.
>
> At the same time, I'd like to insist on the benefit of having
> both the R and I subtractions for the upcoming search.
> Are you guys ready to add that info in the discovery tiles?
We did that once for a shallow search (z=0.5) but unfortunately we did
not get a single redshift. From what I can remember, R-I were widely
spread, even with S/N better than 10 in both colors. I also remember
having plotted the same kind of thing than your plot shows, and
the correlation was not as great as the one you show. Your plot displays
a sigma(R-I) of the order of 0.05 at z=0.5 where R-I is B-V restframe.
Are you sure this makes sense? Or the error bars are not the spread, but
something else?
>
> I added a figure 'R-I color for SNIa's vs z' in our Stockholm webpage
> (http://www.physto.se/~snova/private/2002Arun/2002Arun.html)
> that shows the R-I at the discovery epoch and the sharp correlation
> with redshift, differing by about 1 mag between z=0.5 and z=1.
The problem is that R is fainter by about 0.7 mags at z=0.9, The cfh12k
limiting mags are similar in R and I, and R has 10mn when I has 30. I doubt that
our faint candidates are even detected in R.
Your plot shows a nice correlation, but if I add an optimistic
0.3 mags (sigma) uncertainty on R-I (from the measurement), I doubt
that we get a far better redshift guess than from the I brightness.
Then there is a trivial issue: somebody has to take care of doing
the actual work. Concerning the (very few) people under my control, I am
totally sure that they have more important things to do to actually secure
detections in I. This is of course doable, but it should have been
anticipated.
Cheers,
Pierre.
>
> Cheers,
> Ariel
>
>
--------------------- Pierre Astier , LPNHE, 4 place Jussieu F 75252 Paris Cedex 05 tel (33) 1 44 27 48 42 ---- fax (33) 1 44 27 46 38
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