I had hoped to send something to this mailing list earlier.
The disk crash we have just recovered from, and a personal
crash due to bronchitis and pneumonia have slowed us down a bit.
Introduction to SkyView.
Since the IPAC group has already given a nice intro to their system
let me just say a paragraph or two about our SkyView. The purpose
of SkyView is to provide a single interface by which astronomers
can get data in any useful coordinate system or projection. SkyView
keeps copies of the data on-line on jukeboxes and dynamically reformats
the data to fit the user's request. There are some reasonably nice
things one can do with the images in the interactive interface, but
the purpose is primarily image generation, not image display or
manipulation. The holdings currently include the compressed optical
Southern Sky Survey as distributed by ST ScI, 3 radio surveys, the
IRAS Sky Atlas, the preliminary EUVE data, a mosaic of all public
ROSAT images (due to be much improved in the next month), the HEAO 1 A2
survey, and the EGRET gamma-ray survey.
A user can, e.g., specify a point in the southern sky and SkyView
will automatically generate a image in a standard coordinate system
from the Southern Sky Survey, eliminating the rotations and distortions
of the Schmidt plates. In this case, since the survey is undersampled,
there may be occasional distortions of up to a pixel of individual
objects (since a nearest neighbor algorithm is used). Images are
typically returned within a minute of the request. Images from
multiple surveys can be requested at the same time.
The actual process is to first determine the coordinates of each pixel
in the user's image in the user's coordinate system and then transform
each of these coordinates to the native coordinate system of the
survey being sampled. These values are then projected using the same
projection as the survey. If the survey involves a number of submaps,
then this is done separately for each submap as needed. This gives
up the central pixel location for each of the user's pixels in terms
of the survey data. The survey is then resampled (using nearest
neighbor for the Mosaic interface, some more sophisticated possibilities
are available in the interactive interface) at the these points
and the user's image is returned.
In the Web version, the image is returned as a GIF on a page with
some descriptive information and pointers to more detailed information.
An anchor allows the user to retrieve the FITS file.
SkyView is not intended for users who need to take a survey to its
limit. Rather the intent is let users get pretty good answers very
quickly.
SkyView and direct postage stamp access.
Currently SkyView returns a page to the user with an image and
context information included. Links to the FITS file allow a
user to retrieve it directly. We shall try to implement Doug
Mink's suggestions for making retrieval of the FITS files by
users more convenient.
During the ADASS meeting in discussions with Joe Pollizzi and Don Wells
it became clear that we could make SkyView work as client for software
as well as for humans. In the next week or so we will also provide a URL
which will directly return the FITS file when it receives a properly
formatted request. Since the HTTP daemon simply receives an encoded
message of the form:
keyword=value&keyword=value&...
we can easily adapt our system to return a unencumbered FITS file
when it receives the appropriate string. A program which needs a FITS
file then will need only to send an appropriately formatted string to
our HTTP server and a FITS file will be returned directly.
Currently some the keywords our server expects are:
VCOORD The coordinate (or object name) string
MAPROJ (values: Gnomonic, Rectangular, Hammer-Aitoff and Orthographic)
SCOORD Equatorial, Galactic, and Ecliptic
EQUINX Equinox of image
SFACTR Size of image in degrees
SURVEY Requested survey
CATLOG Requested catalogs to overlay
ISCALN Image scaling (Linear, Log10, or Hist.Eq.)
GRIDDD Provide grid
PIXELX Number of pixels in X directory
PIXELY Number of pixels in Y
SMOOTH Size of boxcar to smooth image.
These choices were not made for compatibility with anything, so they
aren't particularly obvious. However if we are to provide these kinds
of servers, it behooves us to think about what the standard keywords
will be (and what the legal values will be). Where appropriate the FITS
keywords might be used, but this is long term issue. In the shorter
term, we shall add another keyword, RTFITS, which says return the FITS
file directly without all of the ancillary data. Using our HTTP server,
SkyView can then serve client software as well as users.
We would be very interested in comments by users about how SkyView
might better serve their needs.
Tom McGlynn
Goddard Space Flight Center
mcglynn@grossc.gsfc.nasa.gov