Magnitude per square arc seond is a measure of the surface brightness of an object. You add up the light detected in an angular field one arc second on a side to figure the apparent magnitude. 25mag/arcsec2 means that the light detected in one square arc second equals magnitude 25. Astronomers use some measure of surface brightness to differentiate the object being catalog from the sky surrounding it. They can put the data from a digital image through a computer program and the program calculates boundaries of constant brightness. The major axis diameter is the apparent diameter of the widest part of the object. This gives a measure of the size of the object. It doesn't tell you the total brightness of the object because it has no information on the light emitted within the 25 magnitude per square arc second boundary.
Look at the source code for IRAF to see one way of computing this from the data. <a "href=http://iraf.noao.edu/";>IRAF</a> Bud On Wed, 2 Jul 2008, klauslms wrote: > Hello all! > Is someone so kind and explain what major axis diameter at > 25mag/arcsec2 (from PGC catalogue) means and if there is a connection > to magnitude and size of the objekt and how to compute this? > Kind regards > Charly > > > ------------------------------------ > > Yahoo! Groups Links > > >
