Two versions of a mosaic map of the SHASSA images are available for downloading,
high resolution (3600x1800 pixels, 0.1 degrees/pixel,
2.9 Mbytes compressed, 25.9 Mbytes uncompressed)
and the other at
medium resolution (1200x600 pixels, 0.3 degrees/pixel,
0.34 Mbytes compressed, 2.9 Mbytes uncompressed). Both are on a linear intensity scale with
intensity unit decirayleighs (dR). To download these, click on
the appropriate highlighted link above with your right mouse button and select
"Save Link As..." or
"Save Target As...", depending on your browser.
They may also be obtained directly by anonymous ftp from amundsen.swarthmore.edu/SHASSA/
by downloading the files bigmosaic.fits.gz or smallmosaic.fits.gz for the high resolution
map and low resolution map respectively.
The maps were made by first mapping and averaging the original SHASSA images to a Hammer-Aitoff projection with 1.1' per pixel. Bad data were clipped from the original images, as were the corners (pixels beyond 555 pixels of the center of each image were discarded). Also, a quadratic weighting scheme was used to give greater weight to the centers of the original images, so that for each point on the sky, the original image that was nearest to that point has greatest weight in the average. We then median-filtered the 1.1' per pixel map with a 5 pixel by 5 pixel boxcar kernel, and then mapped and averaged those pixels upon the 0.1 degree pixels of the final high-resolution mosaic. The medium-resolution mosaic was made by binning down the high-resolution mosaic by a factor of three with neighborhood averaging. Obviously, different mapping schemes will give somewhat different results. The goal of the above procedure was that each output pixel would equal approximately the median of the halpha intensity within its solid angle on the sky, although for practical reasons a simple median was not performed.
Warning to IDL users: These maps were made using the formulae for the Hamar-Aitoff projection given by Calabretta and Greisen in their Paper II and are NOT normalized to x = ±180o, y = ±90o, as is assumed in some IDL procedures, such as aitoff.pro in the Goddard IDL Users Library. Users should carefully check that their procedures produce the correct coordinates for known objects. For example, the pixel with coordinates (2103,1398) on the high resolution map, or (701,466) on the medium resolution map, marks the position of Alpha Vir at l = 316.1o, b = 50.8o. [Both coordinate pairs use the IDL convention, where the lower left corner is pixel (0,0).]