Checking your surface brightness depth

A quick way to check this is to use the isophotal diameter, D₂₅, listed in the Third Reference Catalog of Bright Galaxies (or RC3). D₂₅ is the diameter of the blue light isophote having a surface brightness of 25.0 mag arcsec^-2. In RC3, this parameter is given as logD₂₅, the logarithm in units of tenths of an arcminute. Given logD₂₅ and the pixel scale of your image, derive the radius of a circle whose diameter equals D₂₅ in pixels on your image as follows:

radius (pix) = 10^logD₂₅ x 3/pixel size

where "pixel size" equals the number of arcseconds per pixel. Then plot this circle on your image. If you see light from the galaxy outside this circle, then the image is probably deep enough for the atlas. Following are two inclined examples obtained with the University of Hawaii 2.2-m telescope by R. B. Tully.

NGC 2713, where the circle has a radius of 109", or 495 pixels on the scale (0.22 arcsec/pix) of the image

NGC 5253, where the circle has a radius of 150", or 683 pixels on the scale (0.22 arcsec/pix) of the image

The next galaxy, NGC 1291, is a face-on early-type barred spiral obtained with the CTIO 1.5-m telescope by R. Grouchy and R. Buta.

NGC 1291, where the circle has a radius of 293", or 674 pixels on the scale (0.44 arcsec/pix) of the image

In all of these cases, one can see a significant amount of light outside the circles along the major axes of the galaxies. Thus, these images are deep enough for the atlas. All of these images are displayed in units of magnitudes per square arcsecond. However, displays in raw intensity units would be just as effective for the test. Values of logD₂₅ are given for all galaxies on our "wish lists" on the main webpage.