Two More Observing Issues

Jack Kramer

Searching out deep sky objects can sometimes prove challenging, and during a recent observing session I was reminded of two issues that we sometimes fail to take into account. Both of these came to the forefront while I was trying to locate the galaxy NGC 691 in Aries.

Magnitude vs Surface Brightness
NGC 691 is listed at magnitude 11.4, so I concluded that it should be near the upper limit for my 4-inch refractor. Despite my best efforts, I was unable to find it from my backyard with that scope. A check of the Saguaro database quickly pointed out the root of my problem. Next to the magnitude column is another column called "SUBR", which is short for surface brightness. The SUBR for NGC 691 is 13.7, which clearly places it beyond the capability of a 4-inch scope. I had made the mistake of ignoring the fact that published magnitudes are an integrated figure; that is, they treat extended objects (like galaxies) as though they were point sources. The Saguaro database, and a few other sources, considers the effect of the object's size. In this case, NGC 691 is 3.8 by 2.7 arc minutes in size. When the 11.4 magnitude illumination is spread over that area, the galaxy as a whole appears much fainter than the published figure, with the net effect being closer to magnitude 13.7. On the following night, I again searched for this galaxy, except this time with a 10-inch Newtonian. I did find it, but with considerable effort. So the moral of this story is that when searching for a faint fuzzy, don't consider just the magnitude.

Scattered Light
What made my search for NGC 691 more difficult was that the ground was covered with a layer of snow, which brightens up the nighttime landscape, thereby greatly enhancing the detrimental effect of any light pollution. And because snow is all around you, light is scattered in a myriad of different directions. This scattering brightens the background as we look at the night sky through our telescopes. It's an especially serious problem with truss-tube designs (such as my 10-inch), because the mirrors are literally bathed in scattered light. That's the reason why many owners of truss-tube telescopes cover the open framework with a black cloth shroud. Even at a dark site, the slightest ambient light can have an effect despite the fact that it may not fall directly on your optics. Several years ago while observing in New Mexico with the open tube 10-inch and a Collins image intensifier eyepiece, I was frequently seeing bright flashes. These were caused by the red lights from other observers. Even though they weren't aimed in my direction, the intensifier picked up the scattered light that my eye would not have normally noticed.

Published in the March 2007 issue of the NightTimes