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What's Up...Don’t overmagnify a telescope PDF Print E-mail

By Vernon Whetstone

Amateur Astronomer


Okay troops, how about we take one from the mailbag. A reader wrote in to ask about the telescope they received for Christmas. They have it all put together but can’t see anything through it.
Without knowing some of the specifics, it will be a little difficult to find an answer, but let’s go for the basics.
The major function of a telescope is to gather light. Therefore, the larger the primary lens or reflecting mirror the more light will be gathered. That light is then magnified by the eyepiece to give us an object to look at.
If I may quote from a publication by Orion Telescopes and Binoculars, “It is a telescope’s ability to capture light that makes these objects easier to see, not its magnifying power. And a telescope’s light-gathering capability is directly related to the size of its primary lens or mirror. The bigger the size, or aperture, of the main optical element, the more light it will collect, and thus the brighter an object will appear.”
In spite of the claims made on the side of the box, how much light-gathering ability a telescope has is its true measure, not its magnification.
Magnification is measured by taking the focal length of the scope and divide it by the size of the eyepiece (in millimeters) will return the approximate magnification.
For example. My eight-inch Dobsonian telescope when paired with a 25mm lens will give a magnification of 40x while the 9mm lens will render a little over 100x.
It is possible to overpower a telescope, or apply too much magnification.
Magnification limits can be determined by using the “Rule of 60.” Take the diameter of the telescope in inches and multiply it by 60. Thus, my 8-inch telescope will have a maximum magnification limit of 480x, anything beyond that is useless.
If we aren’t careful we can overpower the telescope, put too much magnification on it. The result of that will be, what the reader asks about, not being able to see anything.
If the new telescope comes with what is called a Barlow magnifier, a device that will in effect double the magnification of whatever lens you are using, our first response is thinking that bigger is better and to install the Barlow. In some cases that is the worst possible thing you can do.
It will only push the magnification beyond the telescope’s acceptable limits.
Another consequence of increasing magnification is, everything is magnified, not just the light. Any slight movement of the telescope, a slight nudge, a little breeze will be magnified as well. What was a nice, steady view in lower magnification will be turned into a wild hurricane of movement, again, rendering the scope useless. I hope this helps.
Keep those cards and letters coming in folks.
SKY WATCH: First quarter Moon on Friday, Feb. 11, also Moon and Pleiades in conjunction. Jupiter is still in the evening sky but sinking lower each day.  Dazzling Venus rules the morning sky. You won’t be able to see it, but just so you will know, this morning Pluto was about two degrees to the upper left of Venus.
Also on Feb. 11, use binoculars and Venus to locate three Messier objects. Two degrees above Venus and in the same field of view is the nice little star cluster M25 and also in the same field of view but two degrees to the lower right is the star cluster M22. Move one field of view right of Venus for M8, the Lagoon nebula and M20, the Trifid nebula.
NEXT WEEK: More astronomical blathering.