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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
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