An Essay on High Magnification



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Date: 7-18-2001
Abstract: This article explains some of the basic concepts of magnification as well as dispells various myths about high magnification.

Perhaps the most overstated feature of any telescope system.  When most people think about telescopes they first think of the magnification being used.  In the world of the non astronomer and the novice the magnification of an object is all that matters.  However as most experienced astronomers know magnification does not mean much at all.  This article is geared towards the beginners out there who are looking into the fascinating hobby of amateur astronomy. 

The first factor to be considered when thinking about the magnification of a telescope is quite simply its size.  Not the size of the telescope however but the size of the lens or mirror.  This is called the size of the objective and is usually measured in inches or millimeters.  This is important because the size of the objective determines what can be seen and how bright the image will be.

The second factor to consider is the focal length of the telescope.  The focal length is the distance it takes the objective (mirror or lens) to bring the light from what the telescope is pointed at into focus.  This is usually measured in millimeters.  To make matters more complicated eyepieces also have a focal length.  This also plays a major part in figuring the magnification in use at a particular time.  This also is measured in millimeters. 

Limiting Magnification

Every telescope has a limited range of useful magnifications.  This is where the common bystander or novice observer are usually wrong.  The limiting magnification is quite simply based on the size of the objective lens or mirror.  As a rule of thumb the average telescope can handle between 50x and 70x per inch of aperture.  So for example a small refractor purchased in a department store can really only handle about 100x to 144x magnification. 

Only 100x!  But my telescope says it will work up to 500x.  Tuff.  At 500x in a small telescope you would be lucky to see anything.  The problem is that the telescope does not collect enough light to magnify an image that much.  As the powers increase the brightness of the image seen in the telescope will decrease.  This means that in order to get a telescope to work at 500x it has to be nearly five times larger than its smaller department store counterparts.  That is a big telescope somewhere in the 14 inch (objective) range.  On top of the brightness problem there are also other problems associated with high magnifications that I will explain later.


An average telescope with a 60mm (2.4 inch) objective will work up to about 144x2.4 inch objective x 60 magnification per inch = 144x magnification

An average telescope with a 150mm (6 inch) objective will work up to about 360x.6.0 inch objective x 60 magnification per inch = 360x magnification

So as you see a larger telescope that collects more light allows for a higher magnification. 

How to Find the Current Magnification

As I have previously stated telescopes and eyepieces both have a focal length.  This is all of the information you will need to find out what magnification you are currently using.  The focal length of the telescope should be listed somewhere either on it or in the instruction book.  Most telescopes have focal lengths shorter that 2000mm.  Similarly each eyepiece has a specific focal length.  This is usually somewhere on the eyepiece of with the literature that came with it.  Most common eyepieces have focal lengths between 40mm and 4mm.  If you know both of these numbers you can figure out your magnification.

In order to find the current magnification take the larger number (telescope focal length) and divide it by the smaller number (eyepiece focal length).  This gives you the exact magnification.


A telescope with a focal length of 1000mm and a 10mm eyepiece operates at 100x.

1000mm telescope focal length / 10mm eyepiece focal length = 100x

A telescope with a focal length of 600mm and a 25mm eyepiece operates at 24x.

600mm telescope focal length / 25mm eyepiece focal length = 24x

This shows us another important thing.  Telescope with long focal lengths can more easily reach higher magnifications.  But there is a catch.  As the magnification increases not only does the image get dimmer but the area you can see gets smaller.  This is quite a simple idea.  The higher the magnification the smaller the area you can see.  Most amateur astronomers prefer the lower powers because they not only get brighter and sharper views but the can see a much larger area.

Problems with High Magnification

At first it may seem exciting to push your telescope as high as it can go.  The problem is that this gets old quickly and leads to great dissapointment.  Along with the previously mentioned problems with high magnifications there are quite a few more.  The largest problem is atmospheric turbulance.  When looking into a telescope you must also look through a great deal of our atmosphere.  At the same time the atmosphere is not a calm place.  Upper level winds, hot and cold pockets, as well as warm and cold fronts produce a very unstable atmosphere.  At high powers all you can see is a mushy wavy image similar to what the surface of a road looks like on a hot summer day.

Another problem is with the telescope and mount.  The stars move slowly across the night sky, at least you thought that they moved slowly.  If you focus on a star at super high powers it will simply drift out of your field of view in a matter of seconds.  On top of this when you move your telescope to chase it you will probably move it too much or knock it off line.  Every movement of your telescope will be magnified hundreds of times.  To make matters worse every time you touch your telescope you will have to fight with the vibrations that you create.  At high powers focusing can be impossible because of the vibrations caused when you touch your telescope.

Why Lower Powers are Better

For a much better view of the night sky you will find that magnifications between 20x and 50x tend to offer the best views.  The image is sharp and crisp and stars will form nice small points of light.  On top of this you will see a much larger area and decrease your chances of getting lost.

The only time you should use powers higher than 50x is when you are looking at the Moon or planets.  If this is the case you must still follow the limiting magnification guidelines for the best image quality. 

Submitted by Curt Irwin - - Grand Rapids, MI

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