Eyepieces and Accessories


Recall that in the previous section I informed you that a telescope’s magnification is varied by switching eyepieces (oculars).  How can you know which eyepiece will produce the particular magnification you want to use?  The answer to this question is very simple.  Just follow these steps: 

1)         If you know your telescope’s focal length, skip to step 5.  Many telescopes have the focal length printed on their focusing tube. This value will be expressed in millimeters.  Should your telescope not show it, you may need to look up the focal length in the telescope’s user’s manual.  If you DO NOT find the telescope’s focal length, you will have to calculate it yourself as described in steps 2 through 4. 

2)         If your objective’s diameter is measured in inches, convert this value to millimeters by multiplying it by 25.4. To illustrate, if your telescope’s objective is 4 inches in diameter the arithmetic is 4 times 25.4 = 101.6 mm or approximately 102 mm. 

3)         Look up your telescope’s f-ratio in the user’s manual. 

4)         Multiply your objective’s diameter by the f-ratio to obtain the focal length of your telescope.  For example, if the diameter of your objective is 102 mm and the f-ratio is 10, your telescope’s focal length will be 1020 mm.  

5)         Find out the focal length of the eyepiece in question.  This is almost always printed on the outside of the eyepiece.  The eyepiece’s focal length will be expressed in millimeters. 

6)         The magnification of the eyepiece will be the focal length of your telescope divided by the focal length of the eyepiece.  For instance, if the telescope has a 1020 mm focal length and its eyepiece has a 20 mm focal length, then 1020 divided by 20 is 51 times.  


Now that you can calculate the magnification of any given eyepiece, let’s discuss a couple of very important characteristics of eyepieces.

Probably the most important characteristic to consider when contemplating the purchase of an ocular is its eye relief.  Most manufacturers will specify an ocular’s eye-relief as a distance in millimeters.  Eye relief can best be defined as the maximum distance between your eye and the eyepiece where the entire field of view of the eyepiece is visible.  Given this definition, it should be obvious that the longer the eye relief the better.  An ocular with a short eye relief may be alright for someone who is not wearing glasses because such a person can place his eye right next to the ocular’s eye lens.  On-the-other-hand,  if you are sharing your telescope with others who are wearing such corrective lenses or you yourself are wearing them, it is best to have an eyepiece with a long eye relief in order for the observer’s eye to be far enough away to where he may leave on his glasses and still see the entire field of view that the ocular offers.  As a rule, the shorter the ocular’s focal length, the shorter the eye relief.  From that fact you can infer that high power eyepieces have uncomfortably short eye relief, though as you will see later, new optical technology can lead to an exception to that rule if you are willing to pay a higher price tag.  Fortunately, there is a cheaper way of getting high power and long eye relief that I shall discuss as well.


copyright 2004 Singularity Scientific