From: Vitaliy Fadeyev (VAFadeyev@lbl.gov)
Date: Sat Apr 03 2004 - 23:04:50 PST
Rob,
here are the differences between the zeropoints we were getting
and the "bogus" ones you've tabulated. I.e. you should ADD
the differences shown below to your tabulated list to obtain
the tuned values.
Chip => Real-Bogus ZP
A => -1.236219071101
B => -1.092696356979
C => -0.468094545286
D => 0.037388301725
E => -1.107980666448
F => -0.804061929121
G => -0.442444634600
H => -0.084504534718
I => -0.717173521800
J => 0.386483423399
Note that, as mentioned before, there was a roughly factor of 2 scatter
in the fluxes ratio. From this, and from statistical speculations,
it looks like the error on the zeropoint values is large, up to 0.3 mag.
The fields covered by chips A, G, I, and J are outside GOODs tiles.
Their zeropoints were mocked up as the averages of the rest.
Greg mentioned an interesting point -- do you have the zeropoint for
the year-old Subaru reference? Should we try to obtain one?
Cheers,
vitaliy
"Robert A. Knop Jr." wrote:
> At some point (probably not until late late tonight) Subaru I-band
> subtractions will start showing up.
>
> These are the zeropoints that will be used for them:
>
> sub04mar/mar172004suprimea28956sum.fts | 38.1048017998
> sub04mar/mar172004suprimeb28956sum.fts | 38.1206147516
> sub04mar/mar172004suprimec28956sum.fts | 37.131587848
> sub04mar/mar172004suprimed28956sum.fts | 36.8684151492
> sub04mar/mar172004suprimef28956sum.fts | 37.7517776011
> sub04mar/mar172004suprimee28956sum.fts | 37.5677277862
> sub04mar/mar172004suprimeg28956sum.fts | 37.3110273633
> sub04mar/mar172004suprimeh28956sum.fts | 37.2026060372
> sub04mar/mar172004suprimei28956sum.fts | 37.5857562505
> sub04mar/mar172004suprimej28956sum.fts | 36.4820993053
> ^
> |
>
> The indicated letter is the chip of the subtraction.
>
> These zeropoints are completely bogus. (They are based on the R
> magnitudes from the USNO catalog and should not be trusted to better
> than a magnitude or two.) I need somebody to figure out a reasonable
> zeropoint for these images. At that point, we will need to adjust any
> magnitudes that come off of the Subaru subtractions by the *difference*
> between this zeropoint and the real zeropoint determined for these
> images.
>
> These are I-band images. You can get at them with
>
> finddbfile -g <filename>
>
> or
>
> imview -scp <filename>
>
> Note that the night was decidedly not photometric, so just taking a
> published Suprime-Cam zeropoint won't work. (Plus, I'm not sure about
> the units of this sum.) We have to do something empirical to get a
> vega-based zeropoint for these imgaes. Perhaps match and compare to
> GODOS images (whose zeropoints *should* be good), or perhaps match and
> compare to pre-existing calibrated data.
>
> This will be a painful hand-work task, and I don't have time to do it.
>
> The same may eventually need to be done for the subaru z-band images.
>
> -Rob
>
> --
> --Prof. Robert Knop
> Department of Physics & Astronomy, Vanderbilt University
> robert.a.knop@vanderbilt.edu
-- ------------------------------------------------- Vitaliy Fadeyev SCP group at LBNL phone: 510-486-4169 web: http://www-physics.lbl.gov/~fadeyev/ -------------------------------------------------
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