Pete wrote: ↑
Tue Dec 11, 2018 4:01 pm
Believe that comet magnitudes are figured same as galaxies. Magnitude is the brightness you'd get if all of the photons in the coma were stuck back in a star-like pinpoint.
It's interesting in that there is very little
information out there on exactly how the visual magnitudes of diffuse objects are figured, but your explanation is correct. In this day and age of instant access to information, I was expecting it to be easier to find an answer than it was. The best one I found put it like this - Let's take the galaxy M33 as an example. Listed at a visual magnitude of 5.72, M33 is notoriously difficult to see in the eyepiece under even moderately light-polluted skies, whereas a magnitude 5.72 star is no problem. To gain a better understanding of how the number 5.72 was determined for the much harder to see galaxy, find a mag 5.72 star and defocus it to the same size as the galaxy, that is spread out to about 1* in diameter. Reading that brought immediate clarity to my mind about how the visual magnitude of diffuse objects is stated. Anyone who has hunted objects in a daytime sky understands how critical it is to have your optics at least near to tight focus, because if they're not the light of the object will be spread out and you won't see it - plain and simple.
That's all well and good, but it doesn't really help when it comes to comet-hype, unrealistic expectations, and letdown at the eyepiece. And there's no easy answer. The problem is light pollution and the infinite degrees of variation of it. Light pollution has a dramatic effect on diffuse objects, and light pollution levels vary so much that there is no good way to state the magnitude of a diffuse object in a way that everyone who wants to observe it can know what to expect.
We often hear of the term 'surface brightness' and it's typically a good indicator of how challenging an object will be to observe visually. We use it all the time when looking at galaxies, and again, M33 presents a good example. Possessing a surface brightness rating of magnitude 14.2, an experienced observer can surmise upon seeing that figure that the galaxy will be challenging to observe under suburban skies. But here's the kicker - under dark skies that very same galaxy can be seen naked eye
. So in one way the galaxy presents itself as visible without optical aid, but in other ways it's nearly invisible with even sizeable optics? It's true. In my 4.5" reflector under the skies of Springfield, VT, M33 shows spiral structure, whereas with that same scope from my driveway in Bridgewater, MA, I'd be hard pressed to see anything at all. The difference is that dramatic.
We can't very well go around saying 'well if you live here it will be mag6, but here it will be mag10, and there it will be mag12 - it's just too complicated to figure for every variable. With galaxies it's typically not a problem. They're fixed objects that don't vary and quite frankly don't garner much attention from the public at large. Visual magnitudes and surface brightness are fine for those of us that pursue seeing them.
Comets though, are a problem. On one hand it's now clear how the brightness has been determined and on the other we can see how difficult it is to factor in the sky variables when publicizing them. It's different for everyone
. 46p/Wirtanen may have achieved naked-eye visibility from the darkest parts of the world, but Pete wrote this after viewing the comet through his 14" telescope from Barrington, RI:
Comet brightness and visibility is almost always exaggerated. While Wirtanen may brighten significantly Spaceweather.com is as bad as Popular Science when it comes to promises. We could see no tail and the coma was dim.
So I guess we're just stuck with it. The real problem isn't the system - apparently the magnitudes are being factored consistently - it's the light pollution problem. Because of that, those of us that like to view and share these objects need to understand our local conditions, reconcile them in our own heads, and be prepared to explain the situation when a guest observer turns from the eyepiece and says, 'wow, that's dimmer than I thought it would be'. Difficult words to hear, for sure, but then perhaps that can be turned around when we respond with 'yes it is, now maybe you could go home and turn off some lights, eh?