Deep Seeing

Jack Kramer

Every now and then, deep sky observers are rewarded with glimpses of faint objects that push the limits of what we expect our telescopes can deliver. Case in point: In April, 1993, I was working the Coma galaxy cluster from the observing site in Hebron. Coma is an area so rich in galaxies that it's possible to hop between galaxies using the Uranometria atlas as a reference. There were several very faint objects that were just barely visible with averted vision, but they could be identified because they were right where the atlas said they should be. The next day while recording the observations, I was amazed to find that some of these galaxies were much fainter than I had any right to see. For example, NGC 4468 was listed at magnitude 14.2, but NGC 4446 and 4447 were a real surprise - they're each shown as 15th magnitude! In all honesty, I'm sure these galaxies would have been overlooked had they been out in the middle of nowhere, but since they were in proximity to brighter galaxies it was as though there were a beacon pointing to them. This certainly wasn't the only time I've had such good fortune, but most surprising was that this observation was from Hebron, which is a decent site, but certainly not what you'd classify as really dark. What causes these windows to open?

Obviously, one factor had to be the weather. This night was shortly after the passage of a cool front. The sky was very clear and light pollution was not being scattered. Moreover, the sky conditions were stable; that is, there were no passing cells that noticeably affected the seeing from one minute to the next. Greg Lutes, who was observing galaxies in Ursa Major, reported the same impression.

The next question revolves around the issue of the theoretical limits of the telescope. Just how faint should one be able to see with a 10" scope? There's a variety of opinions on this, depending on which formula is used. I've seen everything from a limit of 14th magnitude to 15.7. This same lack of unanimity applies to instruments of other sizes, both larger and smaller. Several years ago, there was an article by Lee Cain of the Houston Astronomical Society in which he pointed out why the generally-listed limiting magnitudes may be too conservative.

The eye's pupil is a somewhat imperfect circle and produces diffraction that cuts contrast and thus its limiting magnitude...When the magnification is such that the telescope's exit pupil is smaller than the eye's pupil, the irregular edge of the eye's pupil is not in the light path. So it's as though the eye suddenly has a perfectly circular (if somewhat smaller) pupil and one can see fainter stars.

Using this formula, Lee derived that an observer with a dark-adapted eye pupil open to .23" (6mm) and using a 10" telescope with an eyepiece that has an exit pupil of .23" or smaller, should be able to see as faint as magnitude 15.7. I may have confirmed this thesis, especially since Lee is referring to a point source (a star), while I was observing extended objects (galaxies), which would be more difficult. What this tells us is that we should probably take with a grain of salt those theoretical limiting magnitude figures for our optics of various diameters.

Possibly another factor was that the mirror had been recoated the previous year and was in pristine condition. This may have helped a bit, though a check of my observing notes indicates that prior to the recoating, the mirror was still working at very faint magnitudes on certain nights.

These experiences have shown that we shouldn't be reluctant to tackle faint objects, especially when they're positioned near brighter objects that "point the way".

Published in the July 1993 issue of the NightTimes