The assumption we make all too often is simple: "I'll see objects that are far away much better with higher magnification." That way of thinking is logical but false. The fact of the matter is that magnification effectively reduces the brightness of an object and the light gathering ability of your telescope. The rule is simple. If you double the magnification you reduce the brightness of an object by a factor of 4. That may not be a problem for an intensely bright object like the moon, but the farther we peer into space, the dimmer objects become. There's also a characteristic of every telescope known as the speed of the scope identified by a f. number. This also affects light gathering capability and can be a determining factor when trying to achieve the optimum practical magnification level.
Why size and magnification does not matter
Case in point, the Orion nebula is twice the size of the full moon. The Andromeda galaxy is as much as 6 times the size of the full moon. It's a fact we might not have guessed, but it's not apparent because the brightness of the Orion nebula and the Andromeda galaxy is so dim. Only with long, time-exposures can we gather enough light to make them apparent through a photograph.
What's the key?
We're going to explore two definitions of magnification. Maximum magnification and practical magnification. Maximum magnification is the upper most limit of a telescope to magnify an object with acceptable light gathering ability and resolution. Practical magnification is the ideal magnification that combines the best aspects of light gathering, resolution and magnification. Understanding the point where your scope achieves the best practical magnification and starts to approach or exceed maximum magnification will save you some time and frustration before you buy a telescope and while using it.
It's all about the aperture
The reality is that the true measure of a telescope's power and light gathering ability is determined by its aperture. You may have heard of a 4-inch or 100mm refractor. That means it has a 4-inch/100mm aperture. A reflector with a 6-inch/150mm mirror has a 6-inch/150mm aperture. Knowing your aperture is important for some basic calculations you can make to understand and use your scope better.
Remember the eyepiece
The eyepieces we use ultimately determine magnification, but what's surprising is that an effective magnification of 50x is often the most desirable limit for both viewing and light gathering ability with many telescopes. You can exceed this magnification depending on your scope, but it starts to point to the value of light gathering versus high power magnification. When we think about this limit we're defining the practical magnification for a telescope. When we exceed the maximum magnification we start to diminish both the light we perceive and the overall resolution of an object.
Determining maximum magnification
What's so surprisingly simple is that the millimeters that define the aperture equals the maximum magnification when multiplied by 2. A 100mm aperture has a maximum limit of 200x. A 150mm scope has a maximum limit of 300x. Going beyond those limits will result in dimmed objects, difficulty with focus and significantly reduced light gathering ability. In fact, the maximum limit is not necessarily your goal.
Let's get practical
Practical magnification is not the maximum magnification, but potentially the ideal magnification for your scope. It's where you find the best combination of light gathering capability, resolution and magnification. You may find that the best limit for practical magnification which gives you the best resolution is equal to the aperture. As a result a 100mm scope aperture may perform at its best at 100x while a 150mm aperture on a scope may perform best at 150x. Its maximum magnification may be double that number, but here again -it's not just about magnification. Whether or not the optimum practical magnification is equal to your aperture depends on the quality of your optics, the darkness of the night sky and the speed of your telescope, but it's a good rule of thumb.
Getting up to speed on telescope speed
An equally simple determination is related to the focal length of your eyepiece to achieve maximum magnification or practical magnification. Telescopes are measured in speeds which is usually determined by an f. number. Quite often this speed or f. number is printed on the scope or in the instruction book. This is similar to the f. stops on a camera. If your scope has a speed of f.10 the eyepiece that will give you maximum magnification is 10mm and your best practical magnification may be with an eyepiece that's 20mm or double the f. number. If the speed of your scope is f.5 then 5mm is the eyepiece that will give you maximum magnification and 10mm might give you the best practical magnification.
To determine the speed of your telescope you need to know the focal length and the aperture. The focal length of a telescope is determined by measuring the distance between the primary lens and the eyepiece in a refractor, or the distance between the mirror and the eyepiece in a reflector. If you have a reflector that has a focal length of 2000 millimeters and the aperture of your scope is 6 inches or 150 millimeters, you divide 2000mm by 150mm and get an f. number of 13. That means that the eyepiece that will give you the maximum magnification is 13mm. You may not have a 13mm eyepiece, but 12 or 14mm will get you in the neighbourhood.
Less is more
You should also keep in mind that the lower the focal length of the eye piece in millimeters the higher the magnification power. As a result, an eyepiece that's 20mm might give you the practical magnification you're looking for if your scope has a speed of f.10.
But remember, it's not always about maxing out the magnification but finding the optimum practical magnification which combines great resolution and light gathering with magnification. That's why having a range of eyepieces can be an advantage. You can drop them in and determine when you're achieving the best resolution and light gathering ability with sufficient magnification to discern the object.
Another way to determine magnification is by dividing the focal length of the telescope by the focal length of the eyepiece. If you have a reflector with a focal length of 2000 millimeters and are using an eye piece that is 20 millimeters you have a magnification power of 100x. 2000mm20mm=100x. If the eyepiece is 2mm and you have a focal length of 2000mm you'll have magnification of 1000x. Wow! 1000x sounds impressive but it's not. You'll hardly see a thing through most scopes except a fuzzy, faint blur.
The best approach
The best approach is determining and achieving the practical magnification for your scope. If your aperture is 100mm, figure 100x is your practical limit. If your focal length is 2000mm you'll want a 20mm eyepiece to achieve 100x. However, if your telescope has a speed of f.15 you might want to double the 15 and use an eyepiece that's 30mm. Magnification will be reduced but you'll be compensating for the reduced light gathering ability of an f.15 scope.
Hopefully this isn't too complicated. The key thing to remember is that magnification is only part of the equation. Light gathering ability and resolution are equally important and finding your scopes practical magnification can give you the best combination of those elements.
Related Posts: Observing the Night Sky
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