What can u see with a 70mm refracting scope?

Answer 1

The brilliance scale of stars is called magnitudes. It runs backwards (a star of magnitude 1 is brighter than a star of magnitude 3). A difference of 5 on the magnitude scale is a factor of 100x in brightness.

Normally, the faintest stars we can see with the unaided eye is magnitude 6. Some people can see 7 and (in very rare cases) 8. In town, I feel lucky when I can get down to 4.

Let's say that you can see down to magnitude 6 and that your pupil has a diameter of 7 mm (varies with age and other factors).

Then a 70 mm telescope has a diameter 10 times that of your eye's pupil. This means that the area of the telescope's objective lens is 100 times that of your pupil (area varies as the square of the diameter).

The 70 mm telescope then gathers 100 times more light than your unaided eye (and concentrates it in your eye). Since it makes stars appear 100 times brighter, you should gain 5 steps on the magnitude scale. In theory, if you see down to magnitude 6 with your eyes, you should see stars down to magnitude 11 with the telescope. In practice, other factors limit the gain so maybe you'll get to magnitude 9 or 10.

The diameter also affects "discrimination": the ability to separate features that are very close. Discrimination varies as the diameter, so the image seen through a 70 mm telescope should be 10 times "crisper" than with the eye only. For example, some stars that appear as single with the eye, could be seen as double in the telescope.