[vsnet-chat 2142] Re: The Digitized Sky Survey (DSS)

[bas@lowell.Lowell.Edu (Brian Skiff)]

     The digitzed POSS-I and POSS-II frames are from the "red" plates in
each survey.  More specifically, the POSS-I red plates were taken on 103a-E
behind a deep orange plexiglas filter.  The net result was a passband open
redward of about 5900A, with a fairly sharp peak near H-alpha, and cutting
off sharply after this.  The POSS-II red plates are taken on IIIa-F plates
with an RG610 filter.  This cuts on at 6100A (as per the Schott filter name)
and continues to the end of the 'F' sensitivity somewhere just short of
7000A.  Thus the POSS-II passband is a bit broader than the POSS-I, but with
very similar central wavelength whereby H-alpha emission is shown clearly.
     Thus both sets of plates produce relative magnitudes for stars quite
similar to the standard Cousins R passband.  The main difference one will
notice when using an unfiltered CCD will be on red stars, where the strong
CCD sensitivity redwad of 7000A will make these objects seem brighter (often
dramticially and misleadingly so) than they appear on the the DSS images.
Apart from this, one's unfiltered CCD frames should appear quite similar to
a DSS frame.
     Note that along the southern galactic plane, the DSS-I (at least) 
provides short-exposure V-band images.  These reach only to mag. 15-16 at
best (this was done to avoid severe crowding), and lots of things that are
easy to pick up with a small telescope + CCD simply don't show up.

\Brian

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