From: Rachel G. (gibbo@panisse.lbl.gov)
Date: Mon Dec 08 2003 - 19:23:07 PST
> As Vallery points out, there is a discrepancy with Adam's spectrum,
> especially on the red end. Please send comments to Vallery and cc to
> this mail list - hstsearch@lbl.gov.
Two immediate comments on Vallery's conclusions
regarding Riess' data:
1. We should be able to handle sub-pixel dithered data. It
isn't an easy problem, but there is existing software that
can properly handle such data. I can look into this.
2. The background subtraction is an important step.
Remember flat-fielding is wavelength dependent, so the
background you subtract has to be flattened properly as
well. This could very well be the source of some of the
differences with Riess' reduction (along with #1).
3. The correlated noise you see in your shift and add
of Riess' data cannot be directly compared to a straight
sum. If you simply sum Riess' data ignoring image shifts,
you should find the data have better SNR than ours (although
they could have been taken when background levels were
higher). But even in the present sum, one can see Riess'
spectrum has more interesting structure (features) than do
ours.
Definitely summing sub-pixel dithered data has to be
handled more carefully.
In addition, Given there looks to be a bit of a lull
in the search planning (for a few days perhaps) I will
finish my write-up of the reductions and analysis I did.
The details of how this stuff is done is important and I
suggest we compare results before moving on.
Rachel
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