A Guide to Eyepieces This time Novice's Corner will take a look at the wide variety of eyepieces that are available to modern amateur astronomers. If you get a group of observers together and let them chat for long enough, the subject will become eyepieces sooner or later. The reason is that eyepieces are the most important accessory that you can purchase for your telescope. Once the scope itself has been purchased, then all the characteristics of that astronomical viewing system are determined by the eyepiece. I always look at an eyepiece as a small magnifying microscope which allows me to inspect the image formed by the mirror in my telescope. If you aim your scope at the Moon, then your optical system will create a tiny image of the Moon in midair; your eyepieces let you observe that image with varying magnifications and fields of view, all by changing the eyepiece. So, let us begin by getting some terminology straight. Here is a list of the definitions of some words used in connection with eyepieces:
Now that we know the meanings of some key phrases,
let us move gently into a little calculation concerning eyepieces. There
are three formulae that apply to using and understanding the
Just remember that one inch equals 2.5 cm or 25
mm and you are ready to figure out these values for your telescope. So,
grab your calculator and we will try a worked example of some scope and
eyepiece combinations. Assume you have a 6 inch f/8 telescope. That means
the scope has 48 inches of focal length from 6" times f/8. Converting
48 inches to millimeters equals 1200 mm of focal length from 48"
X 25 mm/in. 60 X for the 20mm eyepiece from 1200mm / 20mm 100 X for the 12mm eyepiece from 1200mm / 12mm 171 X for the 7mm eyepiece from 1200mm / 7mm Now, here are the exit pupils for those eyepieces. Remember, you had to convert 6 inches to 150mm first. 2.5mm exit pupil for the 20mm eyepiece from 150mm / 60 X 1.5mm exit pupil for the 12mm eyepiece from 150mm / 100 X 0.88mm exit pupil for the 7mm eyepiece from 150mm / 171 X To figure out the True field of view for each eyepiece,
we need to know the apparent field of view for the type of eyepiece used.
Let's assume you are evaluating eyepieces with an apparent field 1 degree FOV for the 20mm eyepiece from 60 degrees / 60 X 0.6 degree FOV for the 12mm eyepiece from 60 degrees / 100 X 0.35 degree FOV for the 7mm eyepiece from 60 degrees / 171 X Because the True FOV is often less than one degree, this value is generally given in arc minutes. There are 60 arc minutes in one degree. So, 0.6 degrees X 60 arcmin per degree equals 36 arc minutes as the True FOV of the 12mm eyepiece. Also, 0.35 X 60 means that the 7mm eyepiece provides a 21 arcmin field. I know that all this math is not particularly fun,
but it does give some useful results. We can draw some general conclusions
from our results. As the power is increased in your telescope, you get
a smaller exit pupil and a narrower field of view. Because the pupil cannot
generally get wider than 7mm, it is not useful to buy an eyepiece that
gives a larger exit pupil than that. The news is worse for those of us
in advanced puberty, if you are over 35 years old, your eye probably does
not open larger than 6mm. At the other end of the scale, magnifications
which yield an exit pupil smaller than 0.5mm are not very useful either.
It So, if you are looking for great, wide angle views of Eta Carina or a Magellanic Cloud, then get an eyepiece with a long focal length and a wide apparent field of view. Are you looking for Encke's division in Saturn's rings? Or maybe trying to spot detail within planetary nebulae? Then look for an eyepiece with a short focal length and still with good eye relief so you don't have to jam your eye into the glass to observe. Maybe you wish to observe star clusters and galaxies at their best, then get an eyepiece that provides that magic 2mm exit pupil. Next time, we will delve inside the eyepiece and get some info on what type of eyepiece will provide the best views for the money. Clear skies until then. |