From: Alex Kim (agkim@lbl.gov)
Date: Fri Jan 02 2004 - 14:54:52 PST
A small addendum to Alex's e-mail. Interestingly enough, if you
mistakenly assume that the Johnson-Cousins system is energy based AND
you mistakenly take Bessell's curves to be R, you get the correct
Johnson-Cousins (count-based) K-correction. So the K-corrections we
used in the days of yore were "correct"!
Alex
Alex Conley wrote:
> Hello all,
>
> I hope you had a pleasant holiday season.
>
> We've discovered that the filter transmission functions that we have
> used in the past for the Bessell filters are slightly incorrect.
> Basically, the passbands for UBVRI given in Bessell (1990), Table 2, are
> not what we thought they were, and hence our K-corrections have been
> slightly incorrect in some of our previous work (including Knop et al.
> 2003 and possibly the 42 SNe paper). This is NOT a large effect, so
> nobody should get too worked up about it, but it's worth pointing out.
>
> The problem is that the numbers that Bessell gives in his table are
> actually not R (the dimensionless response, or transmission, or whatever
> you feel like calling it), but actually lambda * R (suitably
> renormalized).
>
> Note that this is absolutely not the energy vs. counts issue again. It
> takes a similar form mathematically, but there is no physics in this one,
> just a question of a paper that isn't as clear as it should have been.
> If you actually read Bessell (1990) you will find no explanation of what
> he means by passband, and so it is very natural to (incorrectly) assume
> that the numbers in Table 2 are just the dimensionless response. In
> subsequent papers is he much more clear about this issue, and always
> tabulates lambda*R.
>
> This discovery arose when Lifan pointed out a footnote in Jha's thesis
> that points this out, and references some papers by Suntzeff that have
> also discussed this issue. I ended up contacting Nick Suntzeff about
> this, and after a few email exchanges I contacted Mike Bessell directly.
> He confirms that the numbers in Table 2 of Bessel 1990 are actually
> lambda*R, and not R. Therefore, the real filter is slightly bluer than we
> have been using. Fortunately, as I said before, the size of the effect is
> not large, generally less than 0.02 magnitudes. If you would like to see
> the size of the effect, take a look at
>
> panisse.lbl.gov/~aconley/kdiff.eps
>
> This is the old K correction minus the new K correction (the new one using
> the right form for the Bessell passbands) for a variety of redshifts.
> The blue lines are rest frame B band, the red lines rest frame V band. The
> sign of the effect is such that SNe are actually slightly dimmer and
> redder at higher redshifts than we believed (recall that K-corrections are
> subtracted from observed magnitudes). At z=0.5, a SNe is about 0.015 mag
> dimmer and 0.012 mag redder with the correct filter. With extinction
> correction, these counteract each other to some extent. This makes the
> Omega_Lambda case a little bit weaker (with extinction correction), but by
> an amount much smaller than we care about -- so far.
>
> So -- how do you figure out if this affects you? Well, take a look at
> whatever dimensionless filter passband you have been using. If, for the
> I band it looks something like
>
> lambda R
> 700 0.0
> 710 0.024
> 720 0.232
> 730 0.555
> 740 0.785
> 750 0.910
> 760 0.965
> 770 0.985
> 780 0.990
> 790 0.995
> 800 1.000
>
> etc.
>
> you are probably affected. If this is so, you have to decide if the size
> of the problem is big enough for you to redo whatever you have been using
> them for.
>
> Note that this lambda*R thing does not affect all passband information.
> For example, as far as I can tell (I would appreciate somebody
> cross-checking this) the HST filters that synphot provides are simply R,
> and not lambda*R. I think that the Sloan filters as specified by Fukugita
> et al. are also R. Some other papers (the Y band of Hillenbrand, the Js,
> H, Ks of Persson) don't define their terms clearly enough that I can be
> sure either way.
>
> This issue has given me a new appreciation of why the HST folks decided to
> define their own passband formalism instead of trying to work with the
> very messy and creakily defined standard filter stuff.
>
> Incidentally, the full reference to Bessel 1990:
>
> Bessel, M.S. 1990 Pub. A.S.P. 102, 1191.
>
> Alex
>
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