From: Andrew Fruchter (fruchter@stsci.edu)
Date: Mon Dec 08 2003 - 22:14:43 PST
Hi All,
I believe Vallery's analysis misses a main point of dithering. By
sub-pixel shifts you can get both better spatial and spectral
resolution out of the grism images. Correlated noise is not really a
problem.
Unfortunately, the software to really take advantage of this is on the
grism is not fully implemented, but it is nearly ready. The software
is a conceptually simple extension/combination of the multidrizzle and
aXe software already developed for ACS. I expect by the time it is
needed for this program (in 3 months time or so?) the software will be
pretty well developed, and ready for use by people outside of STScI and
the ST-ECF.
In any event, I strongly feel we should be planning sub-pixel dithers
in the ACS grism imaging, and I hope that was included in the Phase II.
Andy
P.S. for those interested in more info on the Drizzle approach to
handling sub-pixel dithered images, have a look at
http://adsabs.harvard.edu/cgi-bin/nph-
bib_query?bibcode=2002PASP..114..144F&db_key=AST&high=3fc23c5625
06835
On Monday, December 8, 2003, at 11:04 PM, Rachel G. wrote:
>
> Looking more closely at your write-up, there are
> many details missing, so it's not clear how useful my
> comment #2 is, if at all.
>
>> 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).
>
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