NOTE: Most of the High Level Science Products are unavailable while
unscheduled maintenance is being performed. They will be incrementally
restored over the course of this week. We apologize for any inconvenience.
"Is there a way to retrieve DSS images in batches?"
No. We know this would be a convenient service, and we would like
to do it, but our jukebox is currently getting over 1500 hits a day
(more like a thousand, if you count internal accesses and accesses to
the Medium Deep Survey),
and potential abuse of a batch service could easily put the jukebox
into a perpetually busy state, denying the DSS to everyone.
"What are SIMBAD & NED?"
SIMBAD (in Strasbourg, France)
NED (in Pasadena, California)
are centralized astronomical databases that provide services like
taking an object name and returning its coordinates. That's how they're used here:
to redraw the DSS form with your object's RA and Dec in place,
so you don't have to go look them up.
Note: SIMBAD and NED only catalog fixed objects, like stars and galaxies.
These databases don't track moving targets, like planets, comets, and asteroids.
The Sky Surveys were designed to avoid bright solar-system objects anyway,
so there won't be any images in the DSS of Saturn or Comet Hale-Bopp.
Also, remember that NED only catalogs objects outside our own galaxy, like
external galaxies, quasars, etc. NED doesn't catalog individual stars or nebulae.
(For you acronym fans, SIMBAD stands for
NED stands for
"I tried to use SIMBAD, but it never came back."
SIMBAD is a wonderful service, but unfortunately, the network connection between Baltimore and Strasbourg
isn't the best it could be. Sometimes, the network hangs; other times, SIMBAD is just too busy to answer right away.
If you're looking for extragalactic objects, and SIMBAD isn't working,
Ned is in California, and our network connection is a bit better there.
"How can I tell what telescope/filter was used for a given image?"
The data are either from the UK Schmidt or the Oschin Schmidt (Palomar)
telescopes. You can tell which survey the data come from by looking at
the REGION keyword in the FITS header. You can also get the
observatory ID from the header keyword TELESCOP.
For the First Generation DSS, the emulsion/filter combinations are:
XE - POSS-E RED PLATE,
XV - SERC-V Equatorial extension,
S - SERC-J Survey
(Also, see below about the short-exposure plates near M31 and the Magellanic Clouds.)
For the Second Generation, these are:
ER - 'Equatorial Red' survey (UK Schmidt) IIIaF + RG610
GR - 'Galactic Red' survey (UK Schmidt) IIIaF + RG610
SHORT exposure in galactic plane
XP - POSS-II Red IIIaF + RG610
XJ - POSS-II Blue IIIaJ + GG385
XF - POSS-II Near-IR IVN + RG9
(To get information about individual images, you'll need to get them in
FITS format and look at the header. This information is not preserved
in the GIF image.)
"What do the pixel values represent?"
Scans in the DSS are digitizations of photographic plates.
The pixel values in the scans are a measure of the photographic
density of the original plate, which is non-linear with the intensity.
"How about the exposure time?"
That's in the EXPOSURE keyword in the FITS header.
(Note that the exposure time there is in minutes.)
All exposures are between 2400 and 4200 seconds.
"What's the plate scale?"
For the First Generation scans, the plate scale is 1.7 arcsec per pixel;
for the Second Generation scans, it's 1.0 arcsec per pixel.
(The plate scale is derivable from the FITS keywords
PLTSCALE and XPIXELSZ / YPIXELSZ).
"What Second-Generation survey plates are currently available?"
These images were scanned from photographic plates, so every once in
a while, you will encounter a scratch, internal telescope reflection,
fingerprint, etc. in your image. So far, none have turned out to be aliens.
I'm compiling an informal catalog of regions with plate anomalies,
so if you run across one, let us know.
"I have a field near M31 that I know should have a lot of stars in it, but I only see a few."
This field was probably drawn from the special short-exposure plate of M31.
There are three such plates, for the Large Magellenic Cloud,
the Small Magellenic Cloud, and M31, exposed for 5 minutes instead of
the usual 50 or so:
XX001 - The Large Magellanic Cloud
XX002 - The Small Magellanic Cloud
XX005 - M31 (Andromeda Galaxy)
These plates are in the V band; specifically, they were taken with IIaD emulsion and
GG 495 filter.
"I tried to get an image of the Orion Nebula, but all I got was a big white spot!"
It worked; you just need to extract a wider image to see it. The Orion Nebula is so big
that all a 15'x15' extraction shows you is the very center, which in these scans
is a big white spot. Go to a bigger scan size (if you don't mind handling
the correspondingly bigger image file).
"Can I use images from the DSS in a CD-ROM or some software that I'm writing?"
If you're using images from the DSS for research, teaching purposes and
other non-profit activities, you may use them freely, and we only
request that you acknowledge the source.
Commercial applications require a license. For information about licensing,
contact the STScI Business Office:
Contract & Business Services
Space Telescope Science Institute
3700 San Martin Drive
Baltimore MD 21218