Star Magnitude

[Kirscovitch]

In the online owners manuel for my Z10 it claims it has a "Limiting Magnitude" of 14.52. 

I'm assuming that this is the extent of the scopes ability to see "brightness"?

I just ask as on stellarium there are tons of galaxies and other objects that when I try to use the ocular setting there is nothing there. And then I notice that the magnitude is around 10 or higher.

So I'm thinking that if it says my scope has a magnitude of 14 I should be able to see these objects?

Is there some sort of formula to figure out what objects I can and can't see? No point looking around the sky if I wouldn't be able to pick out a target I thought would be neat to see from stellarium.

The dob is a 254mm with a FL of 1250.

[Moonshane]

hi Matt

the limiting magnitude stated is for a single point source (i.e. a single star), it's a bit more complicated but the magnitude of galaxies is measured across the whole scale of the galaxy which is of course much more 'spread out' than a star so as a result they are visually fainter. usually galaxies, nebulae etc have a visual (apparent) magnitude too.

For messier 31 (andromeda) the figures are as follows.

Apparent magnitude (V) 3.44^[9][10]Absolute magnitude (V)

−21.5^[b][4]

http://en.wikipedia.org/wiki/Apparent_magnitude

http://en.wikipedia.org/wiki/Absolute_magnitude

[Moonshane]

ps I'd suggest starting with the galaxies etc with the highest apparent magnitude and work your way up to maybe 10.0 AM

[itmo]

Yes the problem is that for point sources the apparent magnitude equals their surface magnitude. For extended objects surface magnitude is less than their apparent magnitude, because it is more spread out. It is the surface magnitude which matters. Also note that magnitude goes the wrong way round! bigger magnitude number means a less bright object. 

[cantab]

Your scope's actual limiting magnitude will depend on your sky darkness. If you're under urban skies, you won't see as faint. And, as mentioned, surface brightness is also important. You'll have to learn the real-world limits of your scope, skies, and own ability by experience.

As for Stellarium, it only has pictures of the Messiers and a few other objects. Most things it will navigate to the positions, but won't actually show you any graphic other than a standard marker.

[IVAN TAPIA]

Plus, I think light pollution will block out faint objects that are easily viewed from dark sites.

Call me Desdinova.

[bingevader]

It's advisable to take all these "limiting magnitude", "maximum magnification" claims with a pinch of salt anyway unless you live in the Atacama desert.

There are numerous other factors that'll get in the way of the manufacturer's theoretical limits of your 'scope.

[acey]

Formulas for telescopic limiting magnitude usually assume a naked-eye limit of 6. If your actual naked-eye limit is les than 6 then reduce accordingly, e.g. if your limit is 4 then knock off 2. For extended objects the limit will be less - it is hard to give a definite figure. For objects that are very small and almost star-like, the limit may be almost the same as the stellar limit. For large objects the limit is set by surface brightness rather than magnitude, so a galaxy with a total magnitude of 10 may be invisible in a scope with a stellar limit of 12.

In practice, the factors that determine telescopic limits are:

The observer's eyesight

The brightness of the sky

The size of the object being looked at (i.e. point-like or extended)

The instrument being used (how clean are the optics, how much light do they transmit, scatter or absorb)

The light-grasp of the instrument (a reflector has a central obstruction so you can't just take the full aperture as a measure of light grasp, as you would with a refractor)

The magnification being used (higher power shows fainter stars but may make extended objects invisible).

So as others have said, take any stated limit with a big pinch of salt. Find out from experience what your own limits are. If you're interested in galaxies then the most important factor in the list is sky brightness. If your sky is dark enough for the Milky Way to be clearly visible then with a 10" dob you can expect to see many hundreds of galaxies - maybe a thousand or more. If your sky is so light polluted that you can't see all the main stars of Ursa Minor, then you might struggle to see more than a dozen.

For galaxies, start with objects in the Messier list. They're some of the biggest, brightest, easiest targets up there. Don't draw up a list based on magnitude, otherwise you'll waste your time looking for objects that you'll never see.

[Charic]

Kirscovitch..........Hi, The brightness of an astronomical object is measured by its magnitude; the lower the number, the brighter the object. Sirius the brightest Star has a negative magnitude of -1.4

Increasing magnification also dims the brightness of the sky background, increasing the contrast of stellar objects to the background. So the limiting magnitude can vary due to practical reasons.

My understanding is that the theoretical limiting magnitude, generally applies to Stars only, not galaxies. A Star and a Galaxy may have the same magnitude, but because the star is a stellar point of light, it will be easier to see than a galaxy, with its  nebululosity spread across the sky, making it march harder to see( even though they have the same magnitudes) As for Stellarium. I just assumed that some galaxies were just too far away to see, and hence why you dont see them?

The theoretical limit for my scope is 14.2 magnitude.

Something else to research.

EDIT*  If you go into Stellarium "Sky and Viewing options" you can vary the Limiting Magnitudes!.........not sure its made any difference on some of my DSO's. still can't see some of them?