Choosing Eyepieces - Part Two

Rick Blakley

Modern amateurs forget that moderate field eyepieces of considerably less expense can be used very effectively on unguided telescopes, and simple eyepieces with small fields can show one something at high power if the telescope is guided. The very best view of Jupiter I have ever had was with an "antique" Tolles solid eyepiece, probably made in the late Nineteenth Century, with an apparent field of about 12 degrees, used on a 6" Clark refractor. No Nagler can come close to providing the detail that was seen that evening. For example, Tele Vue's Wide Field and Meade's Super Wide Angle eyepieces are both 6-element deigns with apparent fields of 65 degrees to 67 degrees. Both series offer low-to medium-power focal lengths and are excellent choices for deep-sky and general-purpose viewing, especially with f/6 to f/15 telescopes. Another wide-angle variation, Orion's MegaVista, is a 7-element eyepiece available in focal lengths from 40- to 10.5-mm. It provides freedom from ghost images and a 67 degrees to 70 degrees field at an economical price.

I have heard a very bad report on the manufacturing quality of the Orion MegaVista. My experience some years ago with Orion's brand name Kellner has convinced me not to purchase other eyepieces from them that carry their name. Again, quality of manufacture was the issue.

For example, Tele Vue's 6-element Panoptic models provide a 68 degrees field with ghost images completely gone, generous eye relief, and only a trace of astigmatism even at the edge. Even more remarkable are the new generation of extra-wide-angle eyepieces. The most popular are the Ultra Wide Angle models from Meade and Nagler and Nagler II series made by Tele Vue. All provide pinpoint stars virtually right to the edge of an amazing 82 degrees to 84 degrees field. All work well on f/4 to f/6 telescopes and have long, comfortable eye reliefs for their focal lengths.

All eyepieces, including the Naglers, show degradation in the quality of their images away from field center when working with instruments as fast as f/4. Most amateurs feel that as long as they don't "see" the smears of unwanted aberrations they need not be concerned with them. However, even undetected aberrations rob light from the image so that faint stars are progressively dimmed and finally "eliminated" as one peers further out toward the field edge. Practicality of transport usually drives the f-number of the instrument an amateur will have, but as the f-number diminishes one should understand that he will have increasing difficulty with eyepieces. Focal ratios falling in the f/4 to f/6 range will place the greatest demands on eyepieces, forcing most observers to either rely heavily on Barlows or pay large sums for the very expensive designs that better manage these f-numbers.

Coatings. Most eyepieces have optics coated with at least a single layer of magnesium fluoride on all air-to-glass surfaces. These coatings (which give the glass a bluish tint) help increase light transmission and reduce the internal reflections that create ghost images. Many eyepieces offer multicoated optics. Good multicoatings, which can look yellow, red, green, deep blue, or purple, offer even better contrast and light transmission and are a necessity on complex 6- to 8-element eyepieces. However, the effectiveness of any coating depends on how well it is applied. An eyepiece with superb single-layer coatings can be better than an eyepiece with poor multicoatings.

Indeed, with all of the elements some modern eyepiece have, multicoating is desirable. Sometimes, though, one can still notice a "light deficit" when comparing a design with a lot of elements that is multicoated with a simple design that has few elements. I have a beautiful Hastings triplet that has three cemented lenses with magnesium fluoride coatings on the air-exposed surfaces that offers brighter images than any Nagler of the same focal length. As to "reading" the color of coatings, allow me to relate an unfortunate story. Some years ago, a manufacturer of binoculars found that their sales volume for one of their products increased after they had mistakenly undercoated the lenses for that model's run. Apparently the public thought the new coating color they were seeing indicated a new coating technology of greater optical efficiency! If you wish to check the effectiveness of the coatings on an eyepiece you desire, observe the moon or Venus through one you have borrowed. Forget the old coating color rules! Something else to remember: it's not always necessary to coat every element in an eyepiece assembly to keep the reflections down.

0.965-Inch. A good core set for a typical department-store telescope would be a 40-mm, a 25-mm, a 12-mm, and a 9-mm or 7-mm, either Kellners, Modified Achromats, or Orthoscopics.

1.25-Inch. A set of four eyepieces with 28- to 24-mm, 20to 15-mm, 13- to 10-mm, and 9- to 7-mm focal lengths will provide magnifications to handle just about all observing situations with most telescopes. You could also restrict your choice to as few as two, a 28- to 24-mm low-power and a moderate-to high-power 12- to 9-mm, and add others later. All designs will work well, but Orthos and Ploessls are better for high-power use.

In 0.965-inch barrel sizes there are no wide-angle eyepieces available. You can choose from some superb standard-field models from Takahasi and Zeiss but at over $100 each, these eyepieces are for the planet-observing aficionado and are likely to be beyond the budget of most owners of entry-level telescopes. But in the 1.25-inch size, you have several choices for expanding your core set. An Ultra-Low-Power Eyepiece. A 35-mm to 40-mm Ploessl or Modified Ploessl will show you as much sky as you can get out of any 1.25-inch-diameter eyepiece. If you enjoy deep-sky observing, an eyepiece in this range is an excellent choice as your first additional eyepiece beyond the core set, with one proviso - avoid any eyepiece that provides too low a power.

Generally, amateurs do not realize that the barrel sizes of eyepieces limit the size of the maximum apparent fields that they contain. Thus, the 40-mm and 25-mm focal length eyepieces in 0.965" barrels are limited to about 28 degrees and 45 degrees respectively, and the 40-mm, 35-mm, and 28-mm focal length eyepieces in the 1.25" barrel sizes are limited at around 36 degrees, 42 degrees, and 52 degrees respectively. Barrels having expanded diameters like the Nagler can manage larger apparent fields although they can slip into 1.25" adapters.

At too low a power, the eyepiece puts out a cone of light wider than the diameter of the pupil in your dark-adapted eye. Some of the light coming from the telescope will not make it into your eye. In this situation, with reflector telescopes you see the dark shadow of the secondary mirror floating in the center of the field.

Restriction of the exit pupil alone does not explain why the secondary mirror's shadow appears within the eye's view. The true reason is more subtle. The secondary is imaged by an eyepiece at a point different from that of its exit pupil, which is itself the location of the imaging of the primary mirror. Eyepieces with long focal lengths image the secondary nearer their exit pupils than do those with short focal lengths. If the power used is so low that the diameter of the exit pupil is restricted closely around the image of the secondary, the secondary shadow is seen.

An Ultra-High-Power Eyepiece. You might wish to add a 6-mm to 2.5-mm eyepiece, provided such an eyepiece does not give you much more than 50X to 60X per inch of aperture with your telescope. For example, if you have a 4-inch telescope with a focal length of only 500-mm, a 2.5-mm eyepiece will give you 200X, a magnification at the upper limit but usable on nights with steady seeing conditions.

I will always advocate the choice of a good Barlow with a moderate focal length eyepiece over the short focal length eyepiece for achieving high power. The eye relief is lengthened (something that is especially important for eyeglass wearers), and one obtains more power, and a range of more powers if one has more eyepieces, with less cost. Also, a good Barlow will actually improve the performance of a telescope at power. Generally, one should avoid variable power Barlows and variable focal length eyepieces, for that matter.

Extra-Wide-Angle Eyepieces. If your budget will allow it, you could select extra-wide-angle eyepieces in the 14- to 4.8-mm range for all your moderate-to high-power eyepieces. All will fit 1.25-inch focusers. Extra-wides are especially recommended for owners of fast f/ratio telescopes or for observers keen on deep-sky observing. However, purists who shun multi-element eyepieces often like to retain less complex Ploessls or premium Orthoscopics for high-power planetary use.

I have said all one needs to about the virtues of "simpler" eyepieces. For planetary use, I say these eyepieces are better suited for transferring visual detail than are the super sophisticated, multi-element Cadillacs that one can purchase. Planets are objects of small angular diameter, and most observation will take place at or near the field center. An excellent achromatic doublet will provide great imagery here, and is certainly more transparent than any design with more elements. Unfortunately, its field is so small (around 15 degrees) that one will have difficulty using it on any telescope not accurately driven on the pole.

You may choose to spend on a fancy multi-element eyepiece for planetary viewing, but I bet I'll see as much or more than you with one of my "cheapies"!

Published in the September 1996 issue of the NightTimes