Understanding Magnification
This article reviews some of the absurd claims and outright lies regarding magnification sometimes seen on the packaging or advertising of low-end telescopes. We'll review what magnification is used for, how you calculate it, and what are its practical limits. You'll see it's not a significant factor in distinguishing one telescope from another, and you'll understand how that misleading claim on the toy telescope is justified.
Lies Toy Telescopes Tell You
You've probably seen shiny telescope boxes in department stores, toy stores, camera stores, and on eBay, with colourful space pictures, and loudly proclaiming the "power" of the telescope. 450-power and 675-power are especially common claims.
They're lying.
Ok, they're not lying in a mathematical sense. As we'll see in a moment, they have correctly multiplied two numbers together to get the answer 675.
The lie is the implication that you care; that it means something; that it is a mark of quality; that it is a reason to buy that telescope. It would be like a box containing a clock saying "Buy this clock! It has over 3500 seconds in each hour!"
Customers who do buy such a telescope (often as a gift for a child) will never be able to use the claimed high magnification. And even if they could, they never would, because using such high magnification requires other conditions and equipment not available to most mortals. Most such scopes end up not being used at all, or being used only at the lowest available power.
What's Magnification For Anyway?
Magnification is important, though, right? Your telescope's job is to make things bigger so you can see them?
Actually, no.
Or at least not always. Many of the things we like to look at (like nebulae and galaxies) are actually not small, they are dim. Your telescope's job isn't so much to make them bigger as to make them brighter. You need some magnification - like 20 to 100 power - but you don't need a lot.
An exception is viewing planets, like Saturn and Jupiter. They are already bright, but they are small, and magnification is needed. But what you need and can use is 200 to 300 power, not 600. More on this in a moment.
How To Calculate Magnification
Calculating magnification is simple.
 Your telescope's main tube has a "focal length", which is essentially the distance the light travels from the first lens or mirror to the eyepiece. It's usually measured in millimetres, and a number like 1000mm is common. (Small refractors tend to be in the 400 - 1000mm range; entry-level reflectors in the 1000 - 2000mm range; and Schmidt-Cassegrains in the 1500 - 3000mm range.) It will be specified in your telescope's manual and may be marked on the lens. |
 You also put an eyepiece in your telescope, and the eyepiece also has a focal length. It will be clearly marked on the end or the side of the eyepiece. Numbers between 3mm and about 30mm are common for eyepieces. |
Magnification is just
So, in the examples shown in these photos, a 816mm telescope fitted with a 25mm eyepiece gives a magnification of
816 / 25 = 33 power.
If we replaced the eyepiece with a 4mm eyepiece in the same telescope, we would get
816 / 4 = 204 power.
Change the eyepiece and you change the magnification - it is not a fixed property of the telescope. Note that smaller eyepiece numbers give larger magnifications.
You can also get add-on optical devices called Barlow Lenses that act as amplifiers. They multiply the magnification by an amount marked on the lens. For example, that 816mm telescope, 4mm eyepiece, and a "2x Barlow", gives a magnification of
816 / 4 x 2 = 408 power.
But there's a catch. That telescope would not produce useful results at such high magnification. Its main lens is 102 mm (4 inches) in diameter, and its useful magnification is limited to about 200 to 250 times. Let's discuss that.
Limits of Magnification
As you can see, any telescope can be made to produce any magnification just by mixing eyepieces and Barlow lenses. However, there is a limit to the amount of magnification a given telescope can usefully provide. Beyond this limit, the image will be blurry -- like trying to zoom in a digital photo beyond what the resolution of the camera supports.
This limit is complex, but a simple rule of thumb that works in many circumstances is that the maximum magnification a given telescope can usefully provide is about 2 power per millimetre of aperture (or, for the USA, Liberia, and Myanmar, 50 power per inch of aperture). So a telescope with 152 mm (6") aperture might be able to produce around 300 power magnification before going fuzzy. Note this is aperture - the diameter of the main lens or mirror - not focal length that we are using for this estimate.
There is a second limit, too. The fact that we have air around us, turbulent, churning, and dust- and moisture-filled, also limits magnification. Beyond about 400 to 500 power, no small telescope located on the ground produces clear views. That's why observatories are on mountains, or in space.
The Toy Telescope Revisited
The telescope in the first photo at the top of this page has about 76 mm (3") of aperture, so its maximum useable magnification is about 2 x 76 (= 3 x 50) = 150 power. You might get 200 power out of it under ideal conditions. 675? Not a chance. Only mountaintop observatories, or the dozen-foot-high, multi-thousand-dollar, monster scopes used by very serious amateurs, will do that.
Where did the number 675 come from?
That telescope has a focal length of 900mm. It comes with 20mm and 4mm eyepieces, and a 3x Barlow. The smallest eyepiece produces the highest magnification. 900 / 4 = 225 power. Add the 3x Barlow and you get 225 x 3 = 675 power. This is a very common combination, specifically set up to allow these misleading magnification claims. 450-power is another common one. That's a 900mm telescope, a 4mm eyepiece, and a 2x Barlow.
On the other hand, those telescopes are usually 76 mm (3") or 100 mm (4") aperture. The better one, 100 mm, would be good for a maximum of about 2 x 100 = 200 power. Maybe 250 on rare occasions. Claiming 675 or 450 power is true in the sense that the arithmetic is correct, but it's misleading and, in my opinion, fraudulent.
Needless to say I wouldn't recommend anyone buy such a telescope. More than that -- I tend to have strong feelings about being lied to. Personally, I won't buy from a manufacturer that makes telescopes with such claims (which rules out even some mid- and high- end manufacturers), and I prefer not to buy from a store that sells them, although that's not always practical. (If they're willing to lie to me about that product, why should I trust them on other topics?)
How to Control Magnification
As we've seen, there are 3 ways to control the magnification of your system:
- The focal length of the telescope. Longer gives higher magnification. This is a fixed attribute of the telescope -- you can't vary it. Focal lengths are usually between 480mm and 3000mm. For a beginner, something around 600 to 1500mm is a good choice.
- The focal length of the eyepiece. You will want several eyepieces, to give low, moderate, and high magnification.
- Amplifiers like Barlow lenses. A good quality 2x Barlow is like doubling the population of your eyepiece collection, provided it doesn't push the magnification of your highest-power eyepieces beyond the limits of your telescope.
Depending on the focal length of the telescope, a typical beginner might like to start with something like two eyepieces and a Barlow.
For example, let's imagine you had a 150 mm (6-inch) scope with a focal length of 1200mm. The maximum magnification of your 150 mm (6-inch) scope would be about 2 * 150= 300 power, which you would get with a 4mm eyepiece. However, you might find it better to start with something like a 25mm eyepiece, a 10mm eyepiece, and a 2x Barlow. You then get 4 useful magnifications with only 3 pieces:
| 25mm alone | 1200 / 25 = 48 power | |
| 25mm and 2x Barlow | 48 x 2 = 96 power | |
| 10mm alone | 1200 / 10 = 120 power | |
| 10mm and 2x Barlow | 120 x 2 = 240 power |
The lower powers will be great for large nebulae and clusters, and the higher powers will be good for the Moon, Saturn, and Jupiter.
Magnification versus Brightness
Photographers with single-lens reflex cameras know that, when they double the focal length of their lens, the image gets dimmer so they have to double the exposure too.
The same is true with your telescope. When you raise the magnification, the image gets dimmer. This means that selecting the right magnification to view a given object will often involve making trade-offs.
For example, suppose you are observing M57, the Ring Nebula. It's interesting, but it's quite small. At moderate power, you'll see the tiny wispy smoke ring. Should you raise the magnification to get a better look? Doing so will make the nebula bigger, but it will also make it dimmer. Whether this is better or worse for observing will depend on you and on observing conditions. Personally, I tend to keep such objects smaller and brighter for observing, while others find magnification adds detail and that the dimmer view is still acceptable. |
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Reducing Magnification
Strange as it may seem, you may even find yourself wishing you had less magnification at certain times. For example, certain objects (e.g. M31 - the Andromeda Galaxy; M45 - the Pleiades) may be too large to fit in your field of view, even with your lowest power eyepiece. This is especially a problem with mid-sized and large Schmidt-Cassegrain telescopes, which tend to have a very long focal length. In such circumstances, you may wish to reduce the magnification of your telescope. You can do this by adding an accessory called a focal reducer. This is an extra lens which acts like the opposite of a Barlow lens - it reduces the magnification of the telescope/eyepiece set by a fixed factor, usually something like 1.4 times. |
What Magnification to Use
Which magnification you should use at a given moment depends on many factors including the size of the object you are observing, its brightness, the surface detail, the aperture of your scope, the stability of your mount, and the observing conditions. I can't think of any way to give you specific guidelines, except that I most often find myself using around 100 power when observing star clusters and dim objects like nebulae, and around 200-250 power when observing bright planets. If I had larger telescopes with greater resolving power (greater aperture) I'd probably use more, but not much more.
However, I recommend you should always start with your lowest magnification, or at least something on the low end. This will assist in finding the object and observing it against the context of the background stars. Then you can gradually increase the magnification on objects where you think it will improve the view.
Comments?
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