Telescope Buying Guide: Focal Ratio & Focal Length

Video Transcript

"In this clip we're going to cover those F numbers, the FR and the FL listed on the side of the telescope. That's important to know both before and after you buy your telescope, and it's clearly defined on each telescope as you're looking through it. The shorter focal ratio telescopes, for instance this guy here is an F5.4, this focal ratio is 9.8. The smaller, the shorter, the focal ratio, the lower the usable magnification. That's an important feature to consider for daytime use, for terrestrial viewing. Also, it determines the power of the magnification of the eyepiece that I select to use. And that's where the second number, the FL--the focal length--of the telescope comes into play as well. All of the eyepieces... or, excuse me, the eyepieces that I have here are compatible in just about every telescope we have. But they'll give you a different magnification in each telescope depending on the focal ratio and the focal length. The focal length of this refractor here is 540 millimeters. That means that the moment the light comes into the cone up here down to the eyepiece is 540 millimeters. So when I put a 22 millimeter eyepiece in this telescope, it gives me approximately 25 magnification. Now, as we move to this refractor here, this refractor has a thousand millimeter focal length. The 22 millimeter eyepiece I put in this telescope is going to give me approximately 45 magnification. That's a real nice power to start astronomy, enough to see the rings of Saturn, might be a little bit high end for daytime use, depending on how cold how hot it is outside. So, 25 magnification here, 45 magnification here, and even higher power in some of the longer length focal length telescopes. Another word about focal ratios. You may hear that shorter length telescopes--those that have shorter focal ratios, or faster scopes will produce brighter images for you visually. I've had a chance to... fortunate enough to travel the country and talk to telescope people a lot smarter than me, and I've learned that that is only true for astral photography. So generally speaking, a telescope of relative size--let's say a six inch telescope, an F10 or six inch telescope, an F6.3--will give you similarly bright images at the same magnification. So don't let that myth confuse you either."