Using the Digitized Sky Survey (DSS) for serious deep sky observing

I am sure in some way or another many of us are using computer technology, which has really changed our hobby. Be it a desktop planetarium or venturing into CCD photography, it is all about increasing our knowledge and getting more out of what we do. In an age of being on-line – we have everything at our fingertips. Amateurs now have access to the same information the professionals have. It is only a matter of finding the right tools and learning how to use them.

Since my interest in astronomy and introduction to deep sky observing, I have been enjoying observing articles from past greats like the late Walter Scott Houston to today’s enthusiasts in our hobby like Tom Polakis and Steve Gottlieb. Most of these articles use images from sky survey plates to accompany their articles. Only recently did I realise how easily accessible these sky surveys were – a digital photograph of the entire sky.

With all the technology available to me, I am still of the old school and rely on star charts for my observing sessions. I find the charts made using desktop planetariums awkward to use and difficult to get an accurate printout of the area of interest. The stars are too bloated and objects are not quite to scale or positioned accurately. With a Digitized Sky Survey (DSS) image, you get more detail in the star fields and galaxies as they actually appear.

My first experience using DSS images at the eyepiece was hunting down the Fornax galaxy cluster, provided by the "Observer’s Page" (S&T Jan ’98, Pg 109). Finding many of the brighter galaxy clusters (Abell, Hickson etc.) are not too difficult, but identifying which faint smudges are which is far more challenging. Seeing several galaxies in the same field of view and matching them against a finder chart will enable you to determine a benchmark for your telescope aperture, eyesight acuity and sky conditions.

You no longer have to obtain the over 100 CD’s of the full DSS archive, since it is now available on-line. For the first time, amateur astronomers have access to the actual sky survey plates used for more than thirty years by research astronomers. The unprecedented level of telescopic detail, especially of galaxies, clusters and nebulae is invaluable.

There are several sky archives though, which you will soon find out when you to search the net. One of the latest sky surveys is the Sloan Digital Sky Survey (SDSS), which is busy systematically mapping one-quarter of the entire sky, producing a detailed image of it and determining the positions and absolute brightness of more than 100 million celestial objects! The total quantity of information produced is about 15 terabytes (trillion bytes) and rivals the information content of the Library of Congress.

History of Digitised Sky Survey

DSS images are based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain.

The National Geographic Society's Palomar Observatory Sky Survey project was a seven-year effort to construct a photographic atlas of the sky. Completed in 1958, this project produced thousands of 14 inch square glass plates, each of which encompassed a 6.5 degree square area. Photographed in two wavelength ranges (red and blue), nebulas and stars down to about magnitude 20 were recorded. These images became a basic research tool for astronomers because of the vast amount of information that they contained.

The Space Telescope Science Institute (STScI) digitized the Palomar Observatory Sky Survey as part of an intensive eight-year effort to create the Guide Star Catalogue (GSC) for the Hubble Space Telescope. It took five years to accurately scan the plates and convert them into a database. The resulting scans represented a huge quantity of data, which were later compressed by a ratio of ten to one using a custom algorithm to minimize information loss and make it more accessible to researchers. In 1994, the compressed data, dubbed the Digitized Sky Survey, was released to researchers on 102 CD-ROM's. The CD ROMS were then placed into jukeboxes for rapid access. Users can then easily retrieve image information for any part of the sky. The jukebox apparently gets over 1500 hits a day!

A Powerful Research Tool

DSS images may be retrieved through the Web.

• Go to the DSS Digitized Sky Survey website - http://archive.stsci.edu/dss/
• Take the "Simple Retrieval Form" link on the left navigation.
• On the retrieval page are several links, which provide help files on specific topics.
• If you don’t know your object coordinates, you can retrieve them from either the linked SIMBAD or NED databases.
  
  SIMBAD stands for Set of Identifications, Measurements, and Bibliography for Astronomical Data; NED stands for NASA Extragalactic Database. .
  
  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 comets.
• If you only want to download an image, make sure you change the selection from "FITS" to "GIF". .
  
  The Flexible Image Transport System, or FITS, is the format adopted by the astronomical community for data interchange and archival storage. A data file in FITS format consists of a series of Header Data Units (HDUs), each containing two components: an ASCII text header and the binary data. The header contains a series of header keywords that describe the data in a particular HDU and the data component immediately follows the header).
• Experiment with the image size by entering a height and width in arcminutes. Requesting a larger image size significantly influences the downloaded image byte size. The default size is 15' x 15', which yields a GIF image of usually around 160 kilobytes. Images from the Second Generation scans will be larger than from the First (not all of the Second Generation DSS scans have been loaded).
  
  The First Generation comprises the older surveys, scanned at about 1.7 arcseconds per pixel. The Second Generation is from a newer ongoing survey, scanned at about 1 arcseconds per pixel.
• Before printing the image for use at the eyepiece, use your favourite graphics-editing program to "invert" it (white page with black stars and deep sky objects). This makes it easier to see in the dark and saves on printer inks.

Make sure you visit the "Gallery" section, which has interesting object outtakes from the survey. The images were collected while going through the scans of the photographic plates.

Acknowledgements

If you're using images from the DSS for research, teaching purposes and other non-profit activities, you may use them freely, and only have to acknowledge the source. For commercial applications you require a license.

Success!

On a recent dark sky trip, we were able to positively identify eight galaxies of Abell 3574 galaxy cluster in Centaurus (near M83) using a 12inch f/7. Appearing almost in the same field of view: IC 4329 (M11.3); IC 4329A (M13.9); NGC 5298 (M13.1); PCG 48950 (M14); NGC 5302 (M12.1); NGC 5304 (M 12.6); PCG 48991 (M14.2); NGC 5291 (M 14.1 – Seashell Galaxy).

Eric Brindeau