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SHASSA
The Southern H-Alpha Sky Survey Atlas

Arcminute-Resolution Digital Images of Interstellar H-alpha Emission

John E. Gaustad
(Swarthmore College)
Peter R. McCullough
(University of Illinois)
Wayne Rosing
(Las Cumbres Observatory)
Dave Van Buren
(Extrasolar Research Corporation)

Introduction | Sky Coverage and Image Specifications | Download Images | Mosaic Map | Personnel | Publications | Registration/Feedback

This page was last updated on 19 February 2003. (For a chronology of past changes, click here).

SHASSA is now included in NASA's Skyview Virtual Observatory.

Introduction

The Southern H-Alpha Sky Survey Atlas is the product of a wide-angle digital imaging survey of the H-alpha emission from the warm ionized interstellar gas of our Galaxy. (Details of the survey are contained in the accompanying paper.) This atlas covers the southern hemisphere sky (declinations less than +15 degrees). The observations were taken with a robotic camera operating at Cerro Tololo Inter-American Observatory (CTIO) in Chile. A similar survey (VTSS) of the northern hemisphere sky is being carried out at the Virginia Polytechnic Institute. (The WHAM survey also covers the northern hemisphere, but at higher sensitivity and lower angular resolution.) At low galactic latitudes, these images show the structure of the diffuse, warm, ionized component of the interstellar medium. It is hoped that their study will lead to a better understanding of the dynamics and evolutionary history of the interstellar gas. At high galactic latitudes, where little or no H-alpha emission is seen, the images are still scientifically useful. The intensity of the free-free emission from Galactic hydrogen is directly proportional to the brightness at H-alpha, so careful analysis of these images should reveal any anisotropies in the Galactic free-free emission at microwave wavelengths (or proof that these are negligible), emission which must be subtracted from satellite or ground-based measurements to obtain the true cosmic background fluctuations. Survey images are available for downloading from this site. The images are in FITS format (Flexible Image Transport System). If you use any images for research or other purposes, we request that you use this acknowledgement.

Sky Coverage and Image Specifications

The atlas consists of 2168 images covering 542 fields south of +15o declination. There are four images available for each field: H-alpha, Continuum, Continuum-Corrected (the difference of the H-alpha and Continuum images), and Smoothed (median filtered to 5 pixel, or 4.0 arcminute, resolution to remove star residuals better). The images have the following specifications: The fields are labeled with a field number.
Field numbers 002-269 have the same centers as the correspondingly numbered fields in the IRAS Sky Survey Atlas (ISSA).
Field numbers 502-769 are displaced from the above centers by +5o in RA and -5o in Dec.
Fields 435-437 and 935-937 cover the South Polar region.

Download Images

The coordinates for field centers provided in the following lists and image maps are approximate. (An accurate coordinate specification is included in the FITS file header for each image).

The images are provided in compressed (gzip) form. If you cannot decompress the files, and you wish access to them in uncompressed form, please contact us via the form below.

If you use any images for research or other purposes, we request that you use this acknowledgement.

You may download images from a:
or search for the correct field for a specific position of interest:
Enter an RA and Dec 2000.0 (hh mm ss.s... ±dd mm ss.s...), or a galactic longitude and latitude (xxx.xx... ±yy.yy...):

RA Dec Long. lat.

The entered coordinate position is degrees from the (approximate) center of field .
(If the separation is less than 7 degrees the position is probably in the named field. If the separation is greater 7 degrees, the position is not in the survey range.)

Users wishing to download a large number of images may find it more convenient to do so via the anonymous ftp server amundsen.swarthmore.edu/SHASSA/ or the alternative server ftp.lco.org/SHASSA/.

The full Atlas can also be obtained as a set of three CD-ROMs. Contact Wayne Rosing (wrosing@lco.org) for details.

Mosaic Map

The following clickable mosaic map (in galactic coordinates) can be used to find images corresponding to features seen. Click on any point within the mosiac to display the image which has its center nearest that point.