How about a chart of scope size vs magnitude

[chuckinnc]

I have never seen a chart of what magnitude you could see with a certain size scope? It would be good to know the minumum scope you

need if you want to be able to see certian magnitude objects. Like M57

I know you need more than binoculars or small scope but how large will you need?

Sure would be nice to have a chart in inches, say from 2-20" and the minumum magnitude you could expect to see for each size.

[John]

There are charts that show you the theoretical limiting magnitude of scopes but the seeing conditions, light pollution and other factors external to the scope are what really dictate what you can actually see.

Google threw this one up straight away:

Light gathering power and magnitude limit

But to be taken with the "health warning" above !

[chuckinnc]

Thanks that was just what I was looking for, it's good to know what you gain by going larger (all things like seeing being equal). This should help me decide my upper limit in size for a telescope.

[Mr Q]

Not only is aperture size to be considered but physical size/weight of the scope also. If it is too hard to move, transport in a vehicle or set up, it won't get the use it should. I find a 10" f4.5 newt the largest and still useable for the above conditions.

[Double Kick Drum]

Magnitude is only an indicative measure of the total light emitted by an object an can occaisionally be mis-leading.

For example, M33 the Pinwheel galaxy in Triangulum is published as mag. 6.5 but is large and has a low surface brightness and is very difficult to pick up in urban / suburban skies. The same is true of M101.

In contrast, I have managed to find galaxies / planetaries which are as faint as mag. 10.5 where they are small and the light is not spread out so sparsely. Two good examples are the Eskimo nebula and M105 (one of the Leo triplet) which are surprisingly bright.

Surface brightness is an alternative measure although this can also be mis-leading. Galaxies may be brighter in some areas (usually the nucleus) than others but surface brightness being a calculation based on magnitude and size, only gives an average reading.

As a result, the Great Andromeda galaxy M31 has a very low reading due to the fact that a great part of its area is in the dim spiral arms which normally cannot be picked up visually.

As Mr Q has already stated, the seeing conditions also make a massive difference. Sorry it is not all that straight-forward. As with so many things a little research and a little trial and error go a long way.

[Karen]

I am confused as to what those magnitude numbers actually mean because if I am correct they are not a linear scale. So magnitude 6 is not twice as bright as magnitude 12. Confusing!

[swamp thing]

Hi

magnitude

A measure of the brightness of a star. Ancient Greek astronomers defined the brightest stars as being of the first magnitude because they were the first to appear after sunset. The magnitude scale continued in steps of decreasing brightness down to sixth magnitude, for those stars which were visible only in total darkness. From its crude beginnings, the magnitude scale has been extended and is now on a strictly defined footing (see Pogson Scale ) so that a difference of one magnitude corresponds to a difference in brightness of a factor of 2.512, and 5 magnitudes equals a brightness difference of exactly a hundredfold. Ancient magnitude estimates depended solely on the human eye, corresponding roughly to the modern V magnitude . The apparent magnitude of a star is its brightness as seen from Earth, whereas the absolute magnitude is a measure of its actual (i.e. intrinsic) brightness; the two differ because the intensity of light falls off with distance, and because of interstellar absorption . When the brightness is measured over all wavelengths, rather than just visible wavelengths, it is known as the bolometric magnitude .

Pogson scale

The standard scale of magnitude, which was formulated mathematically by N. R. Pogson in 1856 . He proposed that a difference of 5 magnitudes should be defined as corresponding to a difference of exactly 100 in the intensities of the stars concerned. A difference of 1 magnitude therefore corresponds to the fifth root of 100, which is 2.512.

HTH

Regards Steve

[Astro Imp]

Steve, that is a very informative and concise answer, thank you.

[Karen]

Thanks Steve, the mathematics are clearer now.

[cantab]

Is it a reasonable approximation that light pollution will reduce the limiting magnitude by the same amount for all size instruments. Ie, if the NELM is 4.5 instead of the ideal-world 6.0, the limiting magnitude for a 6 inch scope would be about 12 instead of the ideal-world 13.6? Or are things more complicated than that?