Any discussion about observing galaxies must inevitably come round to the subject of the galaxy's brightness or lack of it in the eyepiece. Galaxies that I could barely see in my previous 8" scope are now much easier to locate, identify and discuss using my 12" mirror. However, an entire new spectrum of much fainter galaxies is now available, going well beyond what the 8" might have seen. If I suddenly started using a 16" scope, then the faintest ones now seen with the 12" would appear brighter, and a whole slew of new and fainter galaxies would be available to challenge my view. This process would literally go on and on with each larger mirror. There is no end to the faintness of distant galaxies, as evidenced by Hubble's deep field explorations.
I am still trying to determine the faintest galaxy that I can view in my nearby dark sky with the 12" scope. So far it has to be NGC 2873 in Leo, with a visual magnitude of 15.4 and a surface brightness of mag. 13.6. While there are several fine explanations on the web as to exactly what visual mag. is, and surface brightness, I will give my own version here without equations, based on my current understanding and experience.
Visual magnitude refers to how much actual light is coming from the galaxy to the observer's eye. How it reaches the eye is greatly affected by the object's size. Some bright galaxies (Messier 66, for example) have a bright visual magnitude, making them seem relatively easy to see in a wide range of small telescopes. The visual mag. for M 66 is given as 8.9. If that object was a star, it would be considered somewhat faint but still easily observable in a small telescope. NGC 2873, by comparison, has a visual mag. of 15.4, making it nearly impossible to see if it was a star, except with a moderately large scope. If 2873 was as large an object in the eyepiece as M66, it would never be seen in amateur scopes. And yet the surface brightness levels of the two galaxies are not that far apart. For M66 it is given as mag. 12.7, and for 2873 it is mag. 13.6. There is less than a magnitude difference between these two vastly differing objects! And yet in fact there is more than six magnitudes of difference between them in actual visual light. A difference of one magnitude is 2 1/2 times, with mag. 10 being 2 1/2 times fainter than mag. 9, for example. This is most interesting.
The size of a galaxy, then, is important in determining its brightness, and in making it easier for the eye to see it. Without getting mathematical (not my strength), let us take 3 hypothetical galaxies as our examples. Our first galaxy is going to be sized at 1'.1 x 1'.0 (arc minutes). This is a somewhat small object, and virtually round. If the visual mag, was, say, 13 (from all the light coming from it), then the surface brightness would be the same mag. as the visual one. That exact size of object has the same visual mag. as its surface brightness.
However, if we now expand the galaxy to 2'.2 x 2'.0, then the surface brightness decreases (there is an easy formula for this). The same amount of visual light is now spread over an area 4x as large. A galaxy of mag 13 might now become one with a surface brightness of 14.7 (not an exact answer, just an example, but it will be 4x fainter).
If we now shrink the galaxy to something like 0'.55 x 0'.5, then the same amount of light now arrives in a more compact, star-like package. The surface brightness now increases, so that our imaginary galaxy with a visual mag. of 13 now seems more like a 12 mag. galaxy (again, just a rough answer--the formula is available on-line if you want exact answers).
The upshot of all this is that a large galaxy that looks bright on paper might actually be very hard to see at the eyepiece, because its light is spread out over a large area (NGC 45 in Cetus is a good example--that one also has a bright star attached that interferes with seeing the fainter haze of the galaxy)). And a tiny galaxy like 2873 might seem at first to be impossible to see, but is actually a bit easier because of its very small size (its actual size is 0'.8 x 0'.2). Which magnitude is the one that observers should pay attention to, visual or surface brightness?
In my experience, neither one should be used apart from the other. Even taken together, there are still so many other factors to consider when hunting galaxies. Some of these factors include how high above the horizon it lies, if it is near a bright star, the transparency of the sky and overall seeing conditions, and even how tired you are. And if the galaxy is the same brightness as the sky background, it will be invisible. When galaxies get too far west I stop observing them, as that area of sky is very light polluted. I take both figures with me outside on the clipboard, along with the size. If the galaxy is very large and bright, then the surface brightness formula only works to a certain degree. A combination of the two magnitudes is often a better way to go. M66 lists a visual mag. of 8.9 and a surface brightness of mag. 12.7. You can't see a 12.7 mag. galaxy in a 4" telescope, but you can see M66. Halfway between 8.9 and 12.7 is magnitude 10.8, which seems closer to the truth in this case. A 4" scope can now work very hard and show the galaxy pretty well on a decent night. My 'split the difference' method is unscientific, but seems to work for me in the field.
M66--is it really only 0.9 magnitude brighter than
NGC 2873? M66's vast dimensions (9'.1 x 4'.1) makes
its bright magnitude appear dimmer to the eye.
Both photos from
NGC 2872 on right. NGC 2874 on left. Both are easy
to locate in a 12" scope. Much trickier is tiny NGC 2873,
lying just north of the two larger Leo galaxies,
in the upper center of the photograph. The photos
show true relative size of all galaxies depicted, including M 66, above.
The same goes for NGC 2873. I could never hope to see a galaxy of 15.4 magnitude in my skies with a 12" scope, and the galaxy certainly doesn't reach its given surface brightness mag of 13.6 either. Halfway between the two figures lies mag. 14.5, which gives a more reasonable ballpark estimate as to what my observing companion and I observed a few nights back.
So for now, I am assuming that I can see a tiny mag. 14.5 galaxy in a good sky with my scope. This will aid me in knowing which faint galaxies to attempt and which ones to pass up.
Of course the companion volume to Uranometria 2000 is also an awesome source of this information.
Of course the companion volume to Uranometria 2000 is also an awesome source of this information.
As ever, I am curious to hear what you think.
Mapman Mike
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