F814 vs F850

From: Marek Kowalski (MPKowalski@lbl.gov)
Date: Fri Jan 13 2006 - 15:29:43 PST

  • Next message: Saul Perlmutter: "Re: color plot"

    Hello proposal writers,

    I want to share my thoughts on the issue of F814 vs F850 from the SN
    perspective in order to to start a discussion and make sure we come to
    the optimal solution for both SN and cluster science. In particular I
    think that F850 as the primary search band might have some advantageous
    over F814 and I have the feeling that the SN argumentation has some
    relevance for the clusters.

    The two strategies we want to compare are:

    1) 4/5 of the orbit is spend observing F850LP (four exposures) and the
    rest is spend on a single exposure of F775W.
    2) 4/5 of the orbit is spend in F814W and the rest in F850LP.

    Here are some thoughts which go beyond the fact that it is not yet well
    established that we can fully rely on the U-band for SN cosmology.

    The color measurement is severely degraded from the switch to F814.
    The color obtained from F815-F850 is about sqrt(2) worse than that
    obtained from F775-F850 because of the smaller lever arm. If we search
    in F814 the photometric error in F814 will be much smaller than that
    obtained in F850 both because of the longer exposure time and the larger
    throughput. F850 will completley dominate the error. If we compare
    option 1) and 2), searching in F850 will have about a factor of two
    lower error on the color measurement for SNe with z<1.2. Hence this
    will effect the final measurement of the largest fraction of field and
    cluster SNe and we would have to schedule a number of ToO orbits to
    compensate for this.
     I would think that the galaxy color-magnitude diagram would suffer as
    well from the smaller lever arm and the imbalance of depth.

    Searching in F814 would provide a significantly higher SNR up to a
    redshift of about 1.45-1.5. Above that F850 becomes better. However, the
    higher SNR at z<1.45 will not buy us more SNe, as we are finding them
    anyhow. I think for galaxies the SNR turning-point might be be reached
    at lower redshifts, as they are on average redder than a SN. Hanks
    analysis will show what the gain for weak lensing is once the correct
    weighting is taken into account, but I can imagine it is smaller than
    the simple number count implies.

    I hope we might discuss this further on our next telecon

    Marek

    -- 
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    Marek Kowalski                    Lawrence Berkeley National Laboratory
    mpkowalski@lbl.gov                1 Cyclotron Road, MS 50R5008
    (510) 486-4652                    Berkeley, CA 94720-8160
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