Spectrum Online

February 2006 Issue Page Three

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Magnification and Focal Length

As we’ve said, in last months issue, both telescopes and eyepieces have a “focal length”.  Magnification is a measure of the number of times the image is enlarged.  A pair of 7 power binoculars magnifies the image 7 times.

To find the magnification for a particular eyepiece in your telescope, first find the focal length of the eyepiece.  This is probably stamped on the eyepiece, or the available sizes are listed in astronomy catalogs.  Second, find the focal length of the telescope.

Magnification is the focal length of the telescope divided by the focal length of the eyepiece (use the same units, if the eyepiece is specified in mm, then make sure the telescope focal length is also expressed in mm).  If you don’t know the focal length of the telescope, it is equal to the focal ratio times the aperture.

For example, a 26mm eyepiece, placed in a telescope with a focal length of 1200 mm, will have a magnification of about 46x (which is 1200 / 26).  The same eyepiece, placed in a telescope with a focal length of 1430 mm will have a magnification of 55x (which is 1430 / 26).

Notice that the longer the focal length of the telescope, the greater the magnification from a given eyepiece.

You may hear a rule that says that the maximum usable magnification is 50x or 60x per inch of aperture.  This is a rough guideline, and it can vary depending on the kind of telescope you have, the seeing conditions, and what kind of object you are trying to view.  But, in general, extremely high magnifications are not useful.


Exit Pupil

The exit pupil is the diameter of the light beam that exits an eyepiece.

The pupil of a human eye dilates to about 7 mm when accustomed to darkness.  This varies a little from person to person, and shrinks a little with age.

If the exit pupil is greater than 7 mm, then some of the light will be lost, and the eye will not be able to take it all in.  This can be a problem when viewing from an urban or light polluted sight — the eyepiece is not only transmitting star light, but also sky glow.  On the other hand, very small exit pupils are only good for some types of astronomical targets.  For example, an exit pupil of about 1 mm might be OK for splitting a double star, but it wouldn’t be good for viewing a dim galaxy.

To find the exit pupil, divide the aperture of the telescope  (in mm) by the magnification.

Or, the exit pupil is also equal to the focal length of the eyepiece (in mm) divided by the focal ratio of the telescope (the f number).

Next month the Field of view in your telescope will be discussed. Remember if you have a question concerning the operation of your telescope you can go to the Telescope Clinic web page on the main YCAS web site and E-mail YCAS optical expert Paul Leuba.



The month of February finds comet Pojmanski being  reported at magnitude 5.9. The comet is expected to be visible for Northern hemisphere observers in late February at around 5.5 magnitude. It will reach to 5 mag at maximum, brighter than the original prediction by 2 mag. It will be in Capricornus by month’s end.

Here is a free cool Lunar Software Program for maps of the Moon. It does take a bit of memory to run but is very nice and it is free! To check it out go to this link: VIRTUAL MOON ATLAS

This Months Night Sky


See the YCAS Tonight Sky Page for this months Sky Calendar.  for this months Sky Calendar. 

This Months Astronomy Links

Windows To The Universe

The American Museum of Natural History Features Near Earth Objects.

Out of area events

North Museum

Albert Einstein Planetarium

Gettysburg College Hatter Planetarium


IreneJackson Spectrum