The biggest virtue of cheap reflector telescopes is undoubtedly those lovely big apertures. These give them the x-factor many people prize above all, as the effective magnification of a telescope depends most of all on how big its objective mirror or lens is.
In all the excitement of choosing the best telescope, you may totally overlook their drawbacks.
The otherwise excellent PowerSeeker 127EQ, for instance, has a focal ratio of under f/8. It’s not terrible, but something to keep in mind if you want to use it to take pictures of the night sky.
Reflector telescopes of this type are also not especially user-friendly. They’re hard to transport and most require frequent calibration. Maybe it’s time to give refractors a second look.
At the same time, a picture-perfect but tiny dot in the sky isn’t really worth looking at, so the magnification of a telescope can’t be ignored.
How the Magnification of a Telescope Is Calculated
Theoretically, the magnification of any telescope is found by dividing the objective focal length (that of the telescope, in other words) by the eyepiece focal length. It really is that simple – in theory.
Now, eyepieces are commonly available in focal lengths from about 5 mm to 50 mm. The Orion SkyQuest XT8 has a focal distance of 1200 mm, leading to a maximum magnification of 240x using a short eyepiece.
Couldn’t they just make one of these with a much smaller focal length, say 0.05 mm, and jack that up to 24,000x?
To begin with, designing and manufacturing that kind of optics would be pretty tricky. More importantly, the air gets in the way: any earth-based telescope you can afford would be lucky to pick up decent images at anything greater than about 300x even under perfect conditions.
Finding the (Real) Magnification of a Telescope
What will truly limit your viewing pleasure, though, is the amount of light that actually enters the business end of any given telescope. This is why objective diameter (or aperture size; they’re exactly the same number) matters so much.
Unless you have the eyes of a cat, you’ll be able to make out objects only up to a magnification of twice the aperture size in millimeters (roughly speaking). What you’ll be able to see will also be much clearer the larger the aperture gets.
The dinky little Meade Infinity 50 mm, in other words, will give you “only” 100x, under perfect conditions in the hands of a skilled operator. This is still pretty useful, though:
A pair of binoculars may magnify views by 50x and is pretty cool for casual stargazing…BUT will generally show you little more than what you can see with the naked eye, just up close.
What’s the difference?
For viewing deep-sky objects, you absolutely need a telescope.
The distinction comes down almost entirely to aperture. A lens that’s large enough to produce a clear image of a daytime landscape just doesn’t cut it when stargazing. If you want to see planets, nebulae and other natural wonders, you need an objective that’s big enough to suck in as much light as possible and direct it to your eye – illumination is in very short supply once you get past Mars’s orbit.