Telescope Reviews: Whats the faintest star you've glimpsed?

A side note to David's comments (and observers should always take magnitude figures for galaxies with a grain of salt) involves the calculation of surface brightness.
Surface brightness on some small galaxies may be HIGHER than the total integrated magnitudes for those galaxies.

Here's how it works:

Our hypothetical galaxy has a total integrated magnitude of 16.0, and its size is 1' x 1'. The calculated surface brightness is 16.0 also.
But let's say the galaxy is 1' x 0.5' The surface brightness is a factor of 2 (about .6 magnitudes, IIRC) brighter than the total integrated magnitude.
So, though the galaxy's TIM (total integrated magnitude) is 16, it's easier to see than the TIM would suggest.

On my observing log, I have calculated SBs for all the galaxies based on the size of each galaxy. Many have SBs brighter than their TIMs.

And another factor to take into account is the brightness gradient. This is how the galaxy goes from its faint edge to the center. In many galaxies, the brightness is faint until the very center, where it spikes several magnitudes brighter than the rest of the galaxy. In such galaxies, though the TIM is noted as quite faint in lists, the core of the galaxy may be visible in very small scopes.

Guides such as the "Night Sky Observer's Guide" by Kepple and Sanner and "Observing Handbook and Catalogue of Deep-Sky Objects" by Luginbuhl and Skiff have entries for each object that explain the differences in appearance that the objects have in scopes of different sizes. Most objects are visible in quite small scopes, but, where galaxies are concerned, it is usually the cores that are visible in small scopes.

For example, I first observed NGC7331 in Pegasus with a 6" scope. My 8" showed 4 companions and some mottling in 7331. My 12.5" shows the galaxy easily 4X as large as in the 8", with most of the size being faint, diffuse, spiral arms.
But NGC7331 is visible in my 50mm finder at 8X, so I guess we'd have to say that detectable is not the same as observed with details.

And for the more mathematically-minded, the Third Reference Catalogue of Galaxies by Vaucouleur (available for download on the web) contains, for about 3000 of its over 30000 galaxies, a figure (m'_e if I recall correctly) that describes the average surface brightness of only the brightest 50% of each galaxy. This is an interesting figure, and, with many exceptions unfortunately, may prove interesting to see if you are trying to predetermine whether a galaxy will be visible in your scope.

But, as David points out, the magnitude figures for many galaxies, especially the fainter ones, may be way off. I recommend going by the empirical rule: look first, then decide whether or not it's visible. This has served me in good stead through the years, and I have recorded some exceedingly "faint" DSOs that weren't really as faint as the figures in books suggested.

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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie