Expert: Paul Wagner - 10/19/2007

Question
You'll know how stupid about optics I am after this question. I know we don't use the term 'power' in telescope jargon. But all I get about eyepieces is their (I think) diameter in mm. How do I know about the magnification I can get? Is there such a thing? I have a big scope but the image in the eyepiece is no larger than on my smaller scope that I just sold. I know it's mostly about light gathering ( I have a 14.5" truss Antares dob, 4.5 focal length )but some images are barely larger than others. I have 4 eyepieces by Orion - 6, 9, 15 & 20mm Expanse Plossls. Is the diameter directly or inversely proprtional to the 'magnification'? It has to mean something, or else why have different diameters? I can tell the difference in a couple, but not all. And does FOV have much to do with a larger-appearing image? If you could give me the basic relationships it would help me a lot. Thanks.

Answer
Hi Tom

Boy, are you close.  You have all the pieces, you just haven't put it all together. Those numbers on the eyepieces are not their diameter, but their focal length.  And the magnification (or power) of a telescope is the focal length of your primary divided by the focal length of your eyepiece.

Your scope has a primary f/length of about 60 inches.  And those eyepieces are about .8 (20mm) .6 (15mm) .4 (9mm) and .25 (6mm).  The power would then be, in order, 75X, 100X,m 150X and 240X.  As the power goes up, the field of view gets smaller--you can't get the larger magnified image in the same field of view.  

It's easiest to see this while looking at the moon.  With your lowest power eyepiece, the 20mm, you should just about get the whole moon in the field.  With the 6mm, you won't be able to do that, because it's too magnified.

This is all made a bit more complicated when looking at dim fuzzy galaxies.  Because as they are magnified, the light is spread out over a larger image...and so they actually appear dimmer.  It takes some time and experience to get a sense of which eyepiece will give you the best view of each object.

Hope that helps.  Write back it you need more.

Paul Wagner