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A Guide to Eyepieces Part 2

Last time we used some eyepiece math and got familiar with terminology associated with eyepieces. In the same way that there are several different designs of telescope optics, the lenses within an eyepiece are arranged in a variety of ways to provide the observer with magnification and a focused field of view to observe. This article will look into the types of eyepieces available to modern observers and the strengths and weaknesses of each design.

Basically, as lens makers got better at making consistent glass with the same curves each time, they realized that adding more lenses generally reduced the aberrations in the eyepiece. Many of the designs I will discuss are named for the person who invented the combinations of lenses which make up these eyepieces.

So, the first eyepiece designs, the Ramsden and Huygenian, only contain two lenses and are very poor eyepieces by modern standards. They have very narrow fields of view, short eye relief and many aberrations. Cheap telescopes often include these inexpensive eyepieces.

The Kellner is the best of the inexpensive eyepieces. This style of lens has been around for many years and it contains one doublet (two lenses together) and one singlet lens for a total of three pieces of glass inside. The Kellner does not have any excellent characteristics, but it also has few real flaws. Kellner eyepieces have decent eye relief, a fair field of view (45 degrees) and little curvature of field.

The Plossl eyepiece is composed of two doublets, which are identical to each other. For this reason, you will also hear it called a symmetrical eyepiece. It is an excellent eyepiece and many observers look no farther than a good set of Plossls. They have a wide field of view (55 degrees), very good eye relief and are well corrected for aberrations. They cost more than Kellners, but they are worth it.

Orthoscopic eyepieces are generally not named for their inventors, Mittenzwey and Abbe, and I think you can see why. The “Orthos” have one outstanding characteristic, the aberrations and distortions in these eyepieces are virtually non-existent. These flat field eyepieces have a fair amount of eye relief and field of view (50 degrees). This design contains a triplet lens with one singlet nearest your eye.

The Erfle eyepiece was invented to provide a wide apparent field of view and they do that (65 degrees). What the Erfle design gives up is some sharpness of the image at the edge of the field of view. Also, if there is a very bright star nearby where you are observing, some ghost images can appear within that wide field. Inside the Erfle is a combination of three doublet lenses.

This is where the eyepiece world stood for many years. Then the advent of computerized lens designs changed the standards for eyepiece manufacturers.

Enter the Nagler and Ultra Wide designs. These computer- designed eyepieces contain either seven or eight lenses, some with curves ground into them which would have been impossible before modern grinding machines were constructed. These designs provide an extremely wide field of view (82 degrees) and very distortion-free fields at those wide angles. They all have two disadvantages: cost and weight. All that glass is going cost more to grind and put together, also, once it is assembled these eyepieces weigh nearly two pounds in long focal lengths.

Along with eyepieces themselves, there is a device which will change the magnification of your system. It is called the Barlow lens. Just slide your eyepiece into the Barlow and put the whole thing into the eyepiece focuser and you have raised the magnification. The good news is that the eye relief of the system is the eye relief of the eyepiece alone. So, for high power it is much easier to use a 10mm eyepiece and a 2X Barlow than it is to use a 5mm eyepiece and its’ short eye relief.

This is a great idea and I have owned a Barlow since my first scope, but there are limits. I find that Barlows which more than double the power are also introducing too many optical aberrations to the viewing system to allow me to believe I am seeing more detail than I saw without the Barlow in place. So, use your Barlow in moderation and purchase a Barlow that magnifies either 1.8X or 2X and it will prove a very useful device.

Now that you have acquired all this knowledge about eyepieces, I’ll bet you are still left with the same question “which ones do I buy?”. That is a tough query, but I will give you my opinion. If you are just getting started, purchase three eyepieces and a Barlow lens in the beginning. Buy one low power, wide field eyepiece which has a focal length between 25mm and 20mm. Get one medium power eyepiece, from 16mm to 12mm. Buy one high power eyepiece, from 9mm to 6mm focal length. Get a Barlow lens with a magnification of either 1.8X or 2X. With those four things you will be prepared to observe a wide variety of what there is to see in the sky.

As time goes by you can fill in as much as your budget will allow. You might choose a really wide field 32mm to 35mm eyepiece. Or maybe something between the medium and the high power. I know that if you are just getting started, you might be thinking about a very high power eyepiece in the range of 4mm focal length. Even though it seems nifty to have a scope that can go to 600X, the number of evenings steady enough to use extreme magnifications is rare. You can make use of very high power occasionally, but not often.

What design of eyepiece to purchase is the subject of much talk while astronomers compare eyepieces and determine how much money they have to spent. If you can afford it, at least start with a medium power Plossl, a high power Orthoscopic, and a wide field Erfle. If you are really in a pinch for money then Kellners will suffice. However, if you go out and observe with other folks who have better eyepieces than yours, it can be an expensive trip. One spectacular view through someone’s brand new pride and joy eyepiece can have you looking through catalogs and checking the limit on your credit card.

I have had a variety of different eyepieces in the 20 years that I have been using telescopes. My first scope was an 8 inch f/6 and it had a 1.25″ focuser, so all my eyepieces where that size. I used three Erfles: 20mm, 16mm and 12mm for medium power viewing. When I first got the scope, I did what I have told you not to do, I ordered it with a 4mm eyepiece and never saw a clear view in it. I was able to trade the 4mm for a 6mm Orthoscopic that became a prized eyepiece for looking at fine detail on the Moon and the planets. Once I added a 2X Barlow, I was set and my eyepiece collection changed little for several years.

When I sold the 8″ to finance a 17.5″ Dobsonian (yes, aperture fever got me too), I decided I needed a 2″ focuser and an eyepiece to fit. Luckily, I found a war surplus 38mm Erfle that only needed some machining to make a sleeve that fit the 2″ focuser. A friend with a lathe made the part and I was in business. Again, my eyepiece collection seemed complete for a while.

In the 80’s the Nagler revolution hit. The first 13mm Nagler eyepieces I used had a problem that was serious for some observers, including me. Some folks see a “kidney bean”, a dark marking withing the field of view, which will not go away regardless how the observer moves their eye or head. I did not view this as a problem, because it prevented me from spending the money for these expensive eyepieces.

However, Meade decided to release its Ultra Wide series and I got a chance to use the 14mm at Riverside. That was the final straw. The wide field of view, generous eye relief and excellent contrast of these eyepieces sold me. I found someone to purchase my old eyepieces and I have recently completed the set of Ultra Wide eyepieces. In the same time, I also used and then bought a 22mm Panoptic eyepiece that is excellent in my 13″ f/5.6 Newtonian. The wide, flat, contrasty field of view of the Panoptic is stunning and it has become one of my favorite eyepieces. Now that I know how good the Panoptic design is, I plan to upgrade my widest-field eyepiece to a 35mm Panoptic. There is one drawback, it is one of the most expensive eyepieces available. Don’t say you weren’t warned.

Once you have purchased the eyepieces which will serve you, make certain they are well cared for. Get some foam padding and cut out cavities to fit the eyepieces and protect them. Be careful when cleaning your eyepieces. Never rub them with any real force, always gently, or you will scratch the coating. Use a squeeze bulb or canned air to blow off any dust before cleaning. I use a cleaning rag I purchased at a camera store which does an excellent job removing greasy fingerprints which inevitably happen.

If you are thoughtful in purchasing good eyepieces and then protecting them from the elements, they will last many years and provide you with spectacular views of the heavens.

Clear Skies,

Steve Coe