Telescope Mirrors & Prisms

How to Choose Telescope Mirrors and Prisms

There are several different key numerical specifications that you must understand to make shopping for telescope mirrors and prisms simple. Once you know what aperture, focal length, focal ratio, and magnification mean, then it will be easy to select the proper mirror sold on eBay.

Key numbers to know when buying an affordable primary telescope mirror

The aperture of a telescope is the size of the primary mirror. The aperture can be measured in either inches or mm. A 10-inch telescope mirror has a 10-inch or 250mm primary mirror. You can collect more light with a larger aperture. A 20-inch telescope mirror for sale will collect 6.25 times more light than an 8-inch telescope mirror will. However, the price of the mirror increases as the mirror gets larger. A larger telescope primary mirror will also make your telescope heavier and longer.

The curvature of the primary mirror dictates the focal length of the telescope. The focal length also dictates the overall length of the telescope. A longer focal length requires a longer telescope housing. The eyepiece of the telescope also has a focal length. The ratio of these two focal lengths gives you the magnification of the telescope. It is important to note that practically speaking, the maximum magnification you should aim for is 50 times the aperture in inches. A 4-inch primary mirror allows you to reach a magnification of 200x. While you could buy better optics to achieve a higher magnification, your aperture will not collect enough light. Your images will be dim and blurry.

The last important number is the focal ratio. You should choose your focal ratio based on the type of astronomy you wish to do. f/10 is better for observing planets and the moon, while f/7 is better for looking at galaxies.

What brands of telescope mirrors can you buy?

There are many different brands of telescope mirrors you can buy. Many telescope mirror kits are unbranded and work in most telescopes as long as the numerical specifications are correct. Common brands of surplus telescope mirrors include Celestron, Orion, Baader, and Svbony. All of these brands produce high-quality mirrors and can work in your telescope. However, you do need to check all the specifications and tolerances carefully. Each telescope may have a slightly different procedure, so refer to the manufacturer's instructions.

What condition telescope mirrors should you buy?

New telescope mirrors are commonly available but can be more expensive. A refurbished telescope mirror is more affordable while still assuring you of a high-quality optical surface. Another choice for an affordable telescope mirror is to buy an open box item. These mirrors are unused, though the packaging is opened. Used mirrors can be a bargain, but you need to make sure the optical surface is pristine.