Maginitude with my 'scope

[Jezza94 (Jeremy)]

Hi all,

Just a quick question regarding magnitudes. Some of the nebulae, star clusters etc give me a magnitude of say... 7, which i believe is relatively low? The lower the number the brighter it is? (-3.90 would be brighter than 3.90?)

Just wondering if someone would be able to tell me what sort of magnitude of star i could see up too with my scope? (Celestron Astromaster 70mm refractor)

Jeremy

[mental4astro (Alexander)]

Hi Jeremy,

The following link will give you a table showing the limiting magnitude for a variety of apertures:

http://www.twcac.org/Tutorials/limit...tude_table.htm

The table shows two possible limiting magnitudes, depending on the algorithum used. But in a nutshell, take your 70mm limiting magnitude as close to 13.

But this will in practice be less (that is, a smaller number) as instrument quality and how clean it is will affect things, and a major factor will be the prevailing weather conditions. Magnitude 12/12.5 would be a closer value for you scope.

There's another thing you should note when you read the visual magnitude of an object. This value is the 'aggregate' of the entire object put into a point source. This means that an object can actually appear much, much dimmer in an eyepiece if it is very large in diameter.

Mental.

[Robh (Rob)]

Hi Jeremy,

Most of the figures quoted for limiting magnitude are theoretical and based on certain assumptions. The figures are usually much higher than what you would get in practice.
One of the biggest factors in determining your limiting magnitude is skyglow. For instance, your naked-eye limiting magnitude can vary enormously from inner city (e.g. 3) to outer suburban (e.g. 5) to rural (e.g. 6+). And then, everbody's eyes are different. Some people can see fainter stars under the same sky. Maybe we should eat more carrots!

You will also reach fainter magnitudes directly overhead to somewhere nearer the horizon.
Even the eyepieces you use can affect limiting magnitude. A higher power EP should pick up fainter stars than a low power EP because it improves the star to background contrast. But this is true only to a point as usable light will eventually diminish as power continues to increase.

But why not test your scope out yourself?
Attached is a screen clip from SkySafari of the Jewel Box star cluster with the image inverted (black to white and vice-versa) in PhotoShop. The blue star at centre is actually the red DU Crucis, magnitude 7.5
I've added the star magnitudes in EazyDraw. For example, 86 reads mag 8.6, 114 reads mag 11.4.

Pick the stars out and see how dim your scope can go!
Try different eyepieces.

Regards, Rob

[Jezza94 (Jeremy)]

g'day rob, thankyou very much for going to the effort of making that up for me. I will try it out tonight.

Clear skies finally after a week.

Happy 'Gazing,

Jeremy