Practical limiting magnitude.

In reality, say from a clear location, what are the actual limiting mags for various scopes.
For example, a 10" should be able to see past mag 14 in theory, but for those with a 10" scope, would you be able to pick up a mag 13 galaxy. Im just curious.

In reality, say from a clear location, what are the actual limiting mags for various scopes.
For example, a 10" should be able to see past mag 14 in theory, but for those with a 10" scope, would you be able to pick up a mag 13 galaxy. Im just curious.

The complication with Galaxies and other extended objects is that a mag 13 galaxy does not have a photographic magnitude of 13!

When we say a galaxy is mag 13 what we mean is that if all the light from that galaxy were to be compressed to a point that point would have a magnitude of 13, or in other words, the galaxy is as bright as a magnitude 13 star turned out of focus till it was the size of teh galaxy. The bigger the galaxy the more dramatic the effect, M33 has a magnitude of between 3 and 4 (I think) yet it is exceptionally difficult to see even in a large scoope becase it is so massive in the sky.

Hope that makes some sense,

Bart.

Ah,
So if I had a 10" or 12" newt, would I even bother searching for mag 13+ galaxies, or are they beyond reach?

Ah,
So if I had a 10" or 12" newt, would I even bother searching for mag 13+ galaxies, or are they beyond reach?

With 10 I'd imagine they would be really at the limit of the scope. I've never bothered to look for galaxies that fain in my 10" to be honest.

It would be equivalent of looking at a 13th magnitude star out of focus. Deep breaths and averted vision might help!

Cheers,
~Al

Stephen,

There are a lot of varibale factors which determine what you can and cant see in a particular sized scope. Factors include but obviously not limited to (and not in any order of importance)

Your location (altitude, air pressure temperature)
Your age (i.e your eyesight) I have excellent eyesight and have regularly seen objects/detail which other observers simply could not see. For example I remember looking for the Horsehead nebula with a 12,5 inch scope and also a 16 inch scope, using a h-beta filter in both. In the 12,5 inch scope I could just about see it. Only one other person in the group (of 5) could see it too. In the 16 inch scope 4 out of 5 could see it.

Size of the object as Bart mentioned this is crucial. However as you can image the fainter galaxies (being generally further away) are generally very small in angular size. This makes it easier to pick em out as they tend to be more condensed.

Magnification. Upping the power oif the eyepiece has the effect of darkening the sky background and increasing the contrast of the faint fuzzies. The problem here is without seeing them at low magnifications its difficult to be sure you are pointing at the right spot

Location of the object; Obviously you stand a much better chance of seeing an object at the zenith than an pbject close to the horizon. That goes without saying.

Sky Transparancy; This is crucial. You may see objects on one night and then not be able to see them the next night. Obviously light pollution is even more crucial.

To really answer your question I would not be put off looking for a 13th mag galaxy even in a 10 inch scope given the right set of circumstances you might well have success assuming good eyesight etc.

Have a read of this and see if it helps;

www.bpccs.com/lcas/Articles/limits.htm

Regards,

Dave.

Stephen,
It'll depend alot on the atmosphere. On a typical night I can see down to just below magnitude 13 with my 8" scope. On clearer nights I can see down to mag 14. On one exceptional night I got down to mag 15.3, but that was an exceptional night.
Also, something that is often overlooked is the observers experience and visual acuity. Different people will see down to different magnitudes. This can be in part to the ability of their eye to observe fain detail, but also, with more experience you'll see fainter and fainter stars.

Clear skies,