2006-03-30

SDSS sky

Blanton and I worked for a while on sky. It is interesting that such a simple problem (subtract the smooth, additive foreground) is so hard! We decided that the way you subtract sky is a strong function of the scientific question you are asking, and you make different choices in different cases. So to make a sky determination, you have to have a very specific question in mind. This may seem strange, but it is because the sky has structure on all scales, and sometimes you want to photometer a star in a dense field, and sometimes you want to detect low surface-brightness emission behind a field of Galactic stars. There is no one sky that gives good signal-to-noise for both of these cases. For measuring the properties of huge, well-resolved galaxies, we were using a hack, but now we are getting close. Here's a comparison (before is on top, after is below):

5 comments:

  1. The red swath is a single camcol
    width where the sky was improperly
    subtracted? Why was it so much
    worse in one bandpass?

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  2. My guess is that the g-band and r-band were over subtracted due to the galaxy size, making the i-band look redder. We see this all the time. I just made a nice sky-subtracted image of this guy 2 weeks ago (http://astro.berkeley.edu/~awest/katarina/64.jpeg) for a collaborator that has high res HI data for this system. I have a new technique too that combines my original method and Blanton's all in one. We should really sit doen and talk since I spend time on this too.

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  3. Actually, since we set the zeros hard, it can't be that g and r are low. It has to be that i is high.

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  4. The bad i-band sky subtraction
    has to do with it not tracking
    the variation in the sky on short
    enough position OR time scales.

    Some of the sky is scattered light
    from stars and should be pretty
    constant with time and vary only
    with position in the run. So even
    though the g, r, and i images are
    taken at different times by a few
    minutes, that sort of sky should
    not be bad in only one band.

    But the sky comes from other sources
    than stars fixed in the sky too.
    That it happens in one band and
    not another might be because the
    fluctuations in TIME from such
    sources are large --- the sky is
    not coming from scattered
    light from nearby stars but
    something else, that affects all
    bands simultaneously in TIME, but
    in different positions on the SKY.

    So you can look for that signature.
    In my sky fits to 4623, I find
    exactly that pattern: an upward
    fluctation a few percent of sky
    first in r, then in i, NOT in u,
    then in z, then NOT in g. The
    spacing is exactly what you expect
    it to be (around 2 fields between
    r and i, 4 fields between i in z).

    So that sounds like why it is in
    one band but not another for this
    object.

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