Does something like 4 inch telescope refers to the aperture? Also, tips for choosing eyepiece + useful formula

Answer 1

Yes.
Also, for choosing eyepieces, read the advice on televue.com.
The important point to get is to choose medium and low power eyepieces by field stop diameter and NOT by magnification factors. Then choose your high power eyepieces by magnification factors. Why is explained in the advice by Al Nagler.

Answer 2

Hi Dan -

Yes, the 4 inch designation refers to the aperture. The larger the aperture, the dimmer the objects that you can see (a large scope scoops up more photons from the distant object than a small one). The eyepieces you need depend on the focal length of the scope. The magnification is obtained by dividing the scope focal length by the eyepiece focal length. So, if the focal length (not aperture) of the scope is 1000 mm, then with a 10 mm eyepiece you would obtain a magnification of 1000 / 10 = 100X. I think two eyepieces would be a good start - one about 50-70X and the other about 180 - 230X. Much higher mag than that will be useless - you will not be able to see much that's clear, unless it's an unusually good sky condition.

Eyepieces come in two barrel sizes - 1.25 in and 2.0 in. The 1.25 in. is probably what you are looking for , although you could adapt to a 2.0 in. The larger diameter eyepieces are much more expensive, but easier to use. A 1.25 in. will work just fine.

Apparent field of view is also of interest. An eyepiece with a larger AFoV will provide a more pleasing wide-angle view than one with a smaller AFoV. AFoV numbers range from about 40 to 82 degrees - of course, the larger the AFoV, the more expensive the eyepiece.

Eye relief is sort of important if you wear eyeglasses. Eye relief is a measure of the distance from the eyepiece lens to the pupil of your eye. It ranges from about 12 to 20 mm - 12 is adequate if you don't use glasses; 20 is going to be more comfortable if you do. Hope this helps.

Answer 3

Yes, modern astronomers always refer to telescopes by their aperture. In the 17th and 18th centuries they usually referred to them by their focal length, so if you see "a refractor of fifteen feet" that usually will be focal length. For some odd reason, reflectors' apertures are usually measured in inches and refractors' measured in millimetres, at least by amateurs. Professionals usually use metres (and don't use refractors any more).

Jay Reynolds Freeman has a good rule for choosing eyepieces. Get a low power eyepiece with a magnification equal to your scope's aperture in millimetres divided by 5, and a high power eyepiece with a magnification equal to your scope's aperture in millimetres. So, for an 8" scope (200mm) you'd get eyepieces giving 40x and 200x. Another good rule is to get an low power eyepiece with an actual field equal to one degree. That's because there are only a few objects in the sky bigger than one degree in diameter.