| Astronomical Images Over the Web
|
Many groups are serving and displaying small sections of the sky
("postage stamps").
Current servers and client browsers mostly deal with simple
single-wavelength FITS images
with
World Coordinate
System (WCS) sky coordinate to pixel transforms encoded in their
headers. See
Mark Calabretta and Eric Greisen's paper
Image Servers
- IR: IPAC's
ISSA Postage Stamp Service currently serves 80x80-pixel WCS FITS
images with selected centers from the IRAS Sky Survey Atlas (ISSA) at IPAC.
If you know what IRAS plate you are looking for, the entire ISSA plate
set for the
southern
hemisphere
and the
northern
hemisphere is online.
- SkyView,
a versatile Mosaic-based, multi-wavelength image server, has been
implemented at NASA/Goddard. It also returns a FITS file with WCS information.
The SkyView Basic
Form lets you select center coordinates, a sky survey, and image
projection, size, and brightness scaling.
The
SkyView Advanced Form adds exact image size specification in pixels
and degrees, catalog overlays, multi-color image overlays, and
contouring.
- DSS: The
STScI Catalogs and Surveys Branch
has a nice form
for retrieving an image from the
Digitized Sky Survey
CDROMs.
- VLA: The
NRAO/VLA Sky Survey,
a survey of the sky north of -40 declination at wavelength of 20 cm,
is being served by NRAO.
Image Browsers
- Revisions have been made to
SAOimage
so it can display FITS images with WCS information, including plate
solutions in the format used by the Digital Sky Survey, as well as the
most common standard FITS WCS formats. It is available by anonymous ftp
from
ftp://cfa0.harvard.edu/pub/gsc/SAOimage/.
- The European Southern Observatory's
SKYCAT tool not only
tracks in WCS, but can overplot catalogs retrieved over the network.
- SAOtng is
Eric Mandel's evolved version of SAOimage based on the IRAF Ximtool.
Use of the X Public Access mechanism makes it easily extensible.
- IPAC's Skyview (no capital V) program can deal with WCS, but needs a
better browsing interface.
- Doug Mink's
Skymap
has extensive overlay capabilities, despite a rudimentary image display,
Browsing adaptations will soon be made.
- For PC's running Microsoft Windows, NRAO has written
FITSview,
a C++ program which displays FITS images with WCS information.
- The Java-based Horizon
Image Data Browser is being developed at NCSA using the NRAO WCSLIB.
A Test
If you have the line
image/x-fits fit fits fts FIT FITS
in your mime.types file and a browser set up to deal with the
image/x-fits mime type by an entry something like this in a mailcap file
image/x-fits; saoimage -fits %s
you should be able to retrieve and display a FITS image of M51 as shown at
the top of this page from
http://tdc-www.harvard.edu/postage_stamp/wpix.fits
Mailing List
A mailing list
was begun in 1994 as a means for discussing standards and procedures
for serving and viewing "postage stamp" images of an arbitrary portion of
the sky. Although it has been defunct for several years, the mailing
list was intended to include both servers and users of public image databases.
Standards for access, such as keywords and coordinate systems, and standards
for provided products, such as compression and dealing with image overlaps
and gaps were discussed. To avoid unnecessary duplication, work in
progress as well as existing products were covered.
Future Goals
Client software is still needed which can be executed from within application
programs, in order to compare survey catalogs, overlay information, or
facilitate astrometric calibration of optical imagery.
A major issue to discuss is the standardization of a compression method
(or methods--that is an additional debate). Other tasks include finding
ways to deliver multiple simultaneous wavelengths and overlays of
catalogued data. It is hoped that any software which is produced be
shared with the rest of the community. Incremental goals will be set,
so that the large databases can be used as soon as possible.
