Re: Vallery's analysis of grism spectra

From: Andrew Fruchter (fruchter@stsci.edu)
Date: Mon Dec 08 2003 - 22:14:43 PST

  • Next message: Vallery Stanishev: "Re: Vallery's analysis of grism spectra"

    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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