From: Lifan Wang (lifan@panisse.lbl.gov)
Date: Thu Apr 01 2004 - 16:11:44 PST
On Thu, 1 Apr 2004, Robert A. Knop Jr. wrote:
> On Thu, Apr 01, 2004 at 03:29:09PM -0800, Lifan Wang wrote:
> > I might have found the reason for the difference between my K-corr and
> > the Serena's. In my fit, I only normalize the spectra to the observed
> > magnitudes close to the region of interest, and use a smooth function for
> > the normalization. When I try to include all the colors, I do end up
> > something closer to what I can get from Serena's template, but I think
> > this is unphysical.
>
> ???
>
> How can that be unphysical?
>
> What's happening is that your smooth function is unanchored when you
> don't use the other colors, and you're getting a different continuum
> from what Serena is getting. I don't see how that could be unphysical;
> it simply sounds like a matter of whether or not you're consistent with
> the observed data.
>
Rob, by unphysical, I mean there should be no wiggles in the
normalization function. Spline, by definition, might be too none-linear
with too many ups and downs.
> > Note that contrary to what's in Rob's email, that in my approach, I
did
> > not "applying the function that pushes it a little up here, a little down
> > there, in comparison to what Serena did".
> >
> > The normalization function I use has NO wiggles, and as it seems, we all
> > agree that this is important.
>
> That's not what I meant. *ANY* normalization that isn't a single
> constant is going to push the spectrum up a little in one place, down a
> little in another. This isn't wiggles, this is just what you have to do
> if you're normalizing more than one color.
>
Put it in a mathematical term, by no wiggles I mean the second
order derivatives should be positive or negative in all places.
> It's probably possible that there are degeneracies-- that two different
> normalization functions could yield the same photometry. This is
> another reason to have as many photometry poitns as possible. (Indeed,
> we'd be better off if we had Z and J photometry to anchor the spectrum
> to the red of the I-band.)
>
> -Rob
>
>
-- Lifan Wang (510) 495 2733 (o) (510) 704 0119 (h)
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