Menu Close

A Guide to Eyepieces Part 1

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:

Apparent field of view — this is the width in degrees of the field as seen through just the eyepiece alone. If I have two eyepieces with the same focal length, the one with the larger apparent field of view will show more of the sky if inserted into the same telescope. This parameter is determined by the design of the lenses inside an eyepiece.

Curvature of field — good eyepieces provide a field of view which is flat. The focused image should be sharp from edge to edge. Star fields are a tough test of this characteristic.

Distortion — good eyepieces also have little distortion, this means if you viewed a piece of lined graph paper that all the lines would be straight and would cross at right angles. Distortion can be a problem for only a small section of the field of view, but curvature generally happens to the entire field of an eyepiece.

Exit Pupil — the lenses in an eyepiece form an image that floats in midair just outside the lens closest to your eye. When you observe you place your eye so that it can see this exit pupil image. If all is going as planned, the image size will fit with room to spare within your eye. The size of this image is the exit pupil.

Eye relief — the distance from the eye lens to your eyeball. This value is important to eyeglass wearers. If you need to have your glasses on to view the sky, there must be plenty of eye relief so that your eyeglasses will fit between the eyepiece and your face. Those of us who don’t wear glasses to observe generally like some eye relief to avoid the feeling that I am jamming my eye lens against the glass lens of the eyepiece.

Focal length — the apparent distance from the lens to the object being viewed, in this case the image formed by your telescope. Long focal length eyepieces show a large portion of the image being viewed and short focal length eyepieces will allow a small section of the image to be inspected. This is how you choose the magnification of your optical system. Pick out a long focal length eyepiece, say 40mm to 24mm, and the system will give a wide field and low power. Select a short focal length eyepiece, around 8mm to 4mm, and you will get a high power, small field of view look at whatever is in the scope.

Ghost images — in poorly made eyepieces some of the light from a bright star can reflect about within an eyepiece and form faint images within the field of view. These ghost images can be subdued by multicoating the lenses in the eyepieces. Only the cheapest eyepieces nowadays are not coated to suppress this problem.

True field of view — this is the field of view of the entire telescope system, including the eyepiece.

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 values associated with eyepieces.

These formulae are:
Magnification = Telescope focal length / Eyepiece focal length

Exit pupil = Telescope aperture / Magnification

True field of view = Apparent field of view / Magnification

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.

Let’s say you have three eyepieces which have focal lengths of 20mm, 12mm and 7mm. Here are the magnifications each will supply:

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 of 60 degrees.

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 turns out that your eye has its’ best resolution if provided with an exit pupil of about 2mm. So, every set of eyepieces should provide a magnification that gets the system close to this value.

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.

Steve Coe