The term magnification or “power” is often used to describe department store telescopes in order to oversimplify the “quality” of the scope. It makes it sound like a telescope with a magnification of 100x is more powerful, and therefore of higher quality, than one with 50x magnification.
This is really far from the truth and a bit misleading. The quality of a telescope does not lie in its ability to magnify objects, but rather in the quality of the optical system of mirrors and lenses, as well as the aperture, or diameter of its opening. As you might expect, a telescope with a larger aperture can capture more light. And since magnification spreads the captured light over a larger area, it dims the object. A small aperture telescope cannot magnify an object as well as a larger aperture scope, because the object becomes too dim, and detail gets blurred.
As a general rule of thumb, the maximum practical magnification of a telescope with very good seeing conditions is about twice the aperture in millimeters or 50-60 times the aperture in inches. For example, a telescope with an aperture of 100 mm will have a maximum magnification of 200x in good seeing conditions. As the seeing deteriorates, this number will decrease.
With astrophotography, the concept of magnification has no real meaning, as the field of view determines the apparent size of an object. Magnification really only applies to visual observation with an eyepiece. Based on these factors, you may ask, “How do you determine magnification?”. Well, it is really quite simple, and you don’t need a telescope magnification calculator. You simply divide the focal length of the telescope by the focal length of the eyepiece. As an example, a telescope of focal length 1000mm paired with an eyepiece of 20 mm focal length will produce an effective magnification of 50x. As the focal length of the eyepiece gets shorter, the magnification will increase. So, the same telescope with an eyepiece of 10 mm will produce a 100x magnification. Keep in mind that a small aperture telescope of less than 50mm will not be able to produce this level of magnification realistically.
How do you choose an eyepiece?
Let’s assume you already have your telescope. When you are purchasing an eyepiece, just like with telescopes, all eyepieces of the same focal length are not the same. The optical quality of the eyepiece is just as important as the telescope. Some companies, like Tele Vue, are well-known for producing eyepieces of excellent quality.
Let’s start with what you want to see with the telescope. If you want to see large swaths of the Milky Way Galaxy or large nebulae, you will want an eyepiece with a longer focal length, since this will produce less magnification and a larger field of view. Also consider that eyepieces come in two barrel sizes to match the standard telescope interfaces: 1.25” and 2”. The 2” barrel size allows more light to pass and can reveal more detail than an eyepiece with a 1.25” barrel. The Tele Vue 2” Ethos 21mm eyepiece checks all of the boxes. It has a longer focal length of 21 mm, a barrel size of 2”, as well as high-quality optics. It will be perfect if you want a larger field of view.
If your interest is in viewing the planets, you will need as much magnification as your telescope will allow, since all of the planets appear very small. This will require an eyepiece with a shorter focal length, such as the Tele Vue Ethos 6mm. Like its 21mm brother, this eyepiece is of excellent quality but reveals a much smaller field of view and much higher magnification.
Keep it simple
Don’t be confused or misled by advertisements that tout the magnification of a telescope. Be reasonable about your expectations and don’t think that higher magnification means a better telescope or eyepiece. As with many products, you get what you pay for. With telescopes and eyepieces, the optical quality is the most important factor, as well as the ratio of telescope focal length to eyepiece focal length and your seeing conditions.