Limiting magnitude of a telescope?

[maw lod qan]

From what I get online, the limit for my 8" F4.9 is around 15.

This morning while out viewing, and except for a slight breeze causing it to dance a bit, visibility was good. I've seen better, but with averted vision I was seeing faint detail on Saturn. One of its moons stood out very good.

Looking around the area of Scorpios I found a faint fuzzy right where comet C2020 f2 atlas was showing, but on line info showed it mag 18.

It would have been slightly above Daschubba.

Think it was possible or was my eyes playing tricks on me?

[vlaiv]

There are numerous magnitudes used and one should really differentiate between them to be able to compare things.

For example - there is integrated magnitude - this is "default" magnitude for stellar sources - means total light emitted from an object. Often, this magnitude is given and it is very useful for comparison of point type sources like stars. However it is next to useless for comparing brightness of extended sources like galaxies.

This is because it represents total amount of light given off by object. Some objects are small and compact (in eyepiece - not necessarily in physical size) while others are large / extended over great surface. If they have same magnitude (integrated) - compact and small object will appear much brighter than large/extended one.

For this reason - there is surface brightness as a measure - or magnitude per unit of "surface" (or rather surface angle) - like magnitude per arc minute squared and magnitude per arc second squared. Since objects are not uniformly bright - these two can be somewhat "wrong" - or rather give wrong impression as they are derived by taking total/integrated magnitude and dividing it with surface.

For example - relatively uniform nebula will have good surface brightness, but galaxy that has large halo and very bright dense core - surface brightness can be misleading - since core will be much brighter than average surface magnitude suggests while outer halo region will be much fainter.

Telescope limiting magnitude is calculated by how much light it collects vs human eye. On one side you have mark I eyeball with 7mm of aperture (on average when dark adapted) - capable of seeing NELM 7.6-8 mag stars at Bortle 1 sky. Now take 8" of aperture and see how much more surface that is compared to human eye. About x841  = (203/7)^2.

Now we use magnitude system to figure out how many magnitudes that is: m = -2.5*log_base_10(ratio) = -2.5*log_base_10(841) = ~7.312 magnitudes of difference.

7.6 - 8  + 7.312 = ~14.9 - 15.3

But we did not take into account central obstruction and reflectivity of the mirrors and we assumed bortle 1 sky.

Yep, forgot to mention - when doing visual observation - make sure you compare V magnitude (and not other bands) as it is close to human visual response (maybe V stands for visual, but I'm not sure).

[John]

With my 12 inch dob, from home, with moderate light pollution, the faintest point source that I've seen is magnitude 14.7. For an extended source (eg: galaxy, comet etc) the limit seems to be a little less - probably around magnitude 14 or so.

 

 

 

 

[maw lod qan]

Thanks, now I'm wondering what I did see.