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Contents

  • Using the Web Service
    • Is there a way to retrieve DSS images in batches?
    • What are SIMBAD & NED?
    • I tried to use SIMBAD, but it never came back.
    • Are there other sites that offer DSS services?

  • Exposure and Survey Information
    • How can I tell what telescope/filter was used for a given image?
    • What do the pixel values represent?
    • How about the exposure time?
    • What's the plate scale?
    • How complete is the POSS-2?
    • What Second-Generation survey plates are currently available?

  • Image Anomalies
    • What's this funny line/feature/UFO in my scan?
    • I have a field near M31 that I know should have a lot of stars in it, but I only see a few.
    • I tried to get an image of the Orion Nebula, but all I got was a big white spot!

  • Using the Images
    • If I use the DSS in my published research, do I need to acknowledge it?
    • Can I use images from the DSS in a CD-ROM or some software that I'm writing?

Using the Web Service

"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) and 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 Set of Identifications, Measurements, and Bibliography for Astronomical Data; NED stands for Nasa Extragalactic Database.)

"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, try NED. Ned is in California, and our network connection is a bit better there.

"Are there other sites that offer DSS services?"
Yes, there are. Here's a partial list.

Exposure and Survey Information

"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
  • XS - 'Second Epoch Southern' 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?"
A complete list of the plates currently available for the Second Generation survey is available at http://archive.stsci.edu/cgi-bin/dss2list.

Image Anomalies

"What's this funny line/feature/UFO in my scan?"
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.

The Quick V version may look better; you can access it through the Phase II proposal preparation form.

"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).

Using the Images

"If I use the DSS in my published research, do I need to acknowledge it?"
We request that you do. The Catalogs & Surveys Branch (CASB) has a recommended acknowledgment.

"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

DSS FAQ