Shallow-Sky list Planetary Eyepiece FAQ

Introduction

At the end of this article are some discussions of specific makes and models.

Information about eye relief and other useful information about specific eyepieces can be found at Telescope Eyepiece Specifications by Manufacturer.

Q. Does the "number of elements" in an eyepiece matter?

A. "Element" is a term used - fairly loosely in most circles - to designate a single optical component of an eyepiece. Most eyepieces are made up of from 2 to 8 lenses, each called elements.

Many (but not all) planetary observers feel that they get a more contrasty image from an eyepiece of simpler design, such as an Orthoscopic, than from more modern eyepieces which use more elements.

Q. What effect do coatings have on contrast?

A. Anti-reflection coatings decrease reflection from air-glass surfaces but these coatings scatter more light than uncoated air-glass surfaces alone do. Ignoring the beneficial effects of suppressed reflections, this scattering reduces contrast in the image by forming a veiling glare in part or all of the field.

Q. What is the effect of reflections and contrast?

A. Generally reflections will reduce the contrast in the image. Reflections that appear on top of the part of the field you are inspecting can dramatically lower contrast, so if for example a reflection of Jupiter in the eyepiece is superposed over Jupiter's image, it can make for a very bad view. Also, reflections from any source cause inherent scatter, so a little more light ends up scattered around the rest of the field as well.

Q. How do I test for reflection ...?

A. Reflections can be easily tested by turning the scope on a bright star (Sirius is ideal, but any bright blue star like Vega works well). Move the star around in the field of view and see if a hazy, out of focus though well-defined blob of the star image is visible elsewhere in the field. This blob might only be slightly larger than the star or it might be bigger than the field of view of the eyepiece, in which case it can only be detected by its edges.

This blob should move whenever you move the star around the field; frequently it will move the opposite of the star's direction. In some cases, moving the star around the field is not necessary; sometimes these images will move every time the observer moves his or her eye. If this kind of reflection moves very quickly when the eye is moved, it is likely that this is a reflection off the observer's cornea, reflecting in turn in the eye lens of the telescope.

If no blobs are seen, obviously there is no internal reflection.

Q. ... and scattering?

A. Scattering is somewhat more difficult to evaluate. One technique is to put the bright star in the middle of the field, and move it around. This time look at the general background brightness of the field. Indications of high levels of scattered light include a hazy patch near the star image, a general brightening of the field background as the star approaches or enters the field of view, or various kinds of halos or brightness gradients in the field.

Such a test only proves there is scattering somewhere in the optical system. The scattering could be that of the telescope's optics, the mechanical supports (tubes and cells, for example, or even in the observer's eyeball. For this reason it is generally good to compare the scattering characteristics of a new eyepiece with that of an eyepiece that is already known.

Q: Why not use an eyepiece with good scattering characteristics and bad reflections, and move the image of the planet being inspected off-axis in order to move it out of the way of the reflection? This way you get the best of both worlds.

A: This is an option, especially with longer-focus refractors and some Schmidt and Maksutov designs in which off-axis astigmatism or other aberrations are held down to reasonably low levels at distances of a few minutes off-axis. However, the fastest growing segment of planetary observers are using optimized Newtonians for their planetary viewing.

In the case of Newtonian reflectors, be aware that a properly corrected reflector exhibits coma exceeding 1/4 wave very quickly as the image moves off axis. For example, the field of view in which coma is less than 1/4 wave in an f/4.5 reflector is not much larger than the angular size of Jupiter! For this reason, eyepieces free from reflections are especially critical. It can also be seen that wide-field eyepieces are no particular advantage (they don't correct coma, at least not without introducing other undesirable aberrations, and they make it harder to know when the planetary image is actually on-axis).

Q. Is off-axis aberration more important for fast Newtonians?

A. There are no great differences on-axis. Conventional wisdom that an ortho is not well suited to a fast Newt or a "steeply converging light cone" is completely wrong, as long as we are talking about inspecting small objects that lie on-axis. They do break down more quickly at the edges in fast newts primarily because coma is added to the eyepiece astigmatism and all conspires to mess up the image, but also the field is narrower than in many eyepieces so this is not as noticeable as one might think.

Pretty much the same goes for refractors - f/5 to f/15, the planetary performance is really not all that difference as long as the refractors have good color correction (which is not always the case). The performance (in both cases) on starfields and the like will appear radically different, of course.

Q. How important is eye relief? A. Some observers find eyepieces with short eye relief difficult to use. This includes orthoscopics, Plossls, and some of the modern wide-field eyepieces such as Naglers and Panoptics. Other observers aren't as bothered by the lack of eye relief.

Eyeglass wearers, especially those with astigmatism who cannot simply remove their glasses to look through the eyepiece, may do best with eyepieces specifically designed for long eye relief, such as the Vixen Lanthanums, or with longer focal length eyepieces combined with a barlow.

Q. What about barlows?

A. This is a subject of much confusion. Some people like them, some think the extra elements reduce contrast unacceptably. Also, there is no consensus about which barlow is the best, though most people agree that Tele-Vue barlows are excellent.

There is a specific controversy over the Klee barlow, and any other air-spaced barlow, and that is the possibility of internal reflections, though some Klee users are very happy with this barlow (which is said to work especially well on fast Newtonians).

There is some question as to whether any of the available barlows are adequately baffled (lack of baffling can reduce contrast). For more information, see Top Ten Ways of Improving Newtonian Telescope Optics.

Q. Okay, this theory is all very well, but I just want to know which brand of eyepiece I should buy ... Which is the best for planetary observing?

A. Unfortunately, there's no easy answer to that question. Opinions of eyepieces varies wildly from one observer to the next.

Rule #1 in buying eyepieces is: if at all possible, get thee to a public star party and try out various types of eyepieces. You may find that the eyepieces people rave about on the net perform quite poorly in your telescope or on the types of objects you like to observe. You may also find that an eyepiece costing much less than the eyepiece you were intending on buying will do a better job.

If you live somewhere where you absolutely can't ever borrow eyepieces, then http://www.astromart.com is your friend. Buy used, sell used, and you'll come out more or less even.

If you must have specifics:

Just about everybody agrees that the Zeiss Abbe Orthoscopics are excellent for planetary observing. Unfortunately, they are no longer sold and and are rare and expensive on the used market.

Some people feel that Brandons are very nearly the equal of the Zeiss Abbes. Others disagree.

Many, but not all, planetary observers like the Takahashi LE series of eyepieces almost as much as the Zeiss Abbe Orthos. The Takahashi .965" orthoscopics are also very well regarded, but are difficult to buy in the US.

Many planetary observers find low-priced orthoscopics (for instance, those sold by University Optics, or used Meade Reasearch-Grade, or Cave Orthostar) to be excellent planetary eyepieces and a very good "bang for the buck".

Opinions on Tele-Vue Plossls, Naglers, and Panoptics vary widely. Some people love them for their wide fields (especially the Naglers), while others feel that in short focal lengths they suffer from internal reflections, have low sharpness and contrast, and are difficult to use due to their lack of eye relief. Try before buying if possible.

Tele-Vue Radians, unlike the other Tele-Vues, are designed specifically as planetary eyepieces. They are still fairly new to the scene, but many who have tried them have been enthusiastic about their sharpness; others are less impressed. They're much heavier than other planetary eyepieces, which may be a factor with some telescopes.

Most people who have tried the Pentax orthoscopics like them very much, but not many people have had the chance.

Vixen Lanthanums are generally considered to be good though not great: a bit better than generic orthoscopics or plossls but not as good as premium eyepieces. Observers who wear eyeglasses for astigmatism may find these the best choice due to their extremely generous eye relief, but some observers are bothered by internal reflections or lower contrast. Lanthanums seem to attract dirt more than other eyepieces, so keeping them clean is especially important.

Orion Ultrascopics seem to do about as well as good orthoscopics; some observers like them better, particularly due to their better eye relief due to a built-in telenegative lens (which acts like a barlow) in the two shortest focal lengths.

Some people say that Celestron Ultimas are the same design as Orion Ultrascopics, but this apparently isn't true, especially at the short focal lengths normally used for planetary observing (Ultimas don't have the built-in barlow that the shortest Ultrascopics do). Few people have reported on Ultimas for planetary observing.

Some people like the Vixen/TeleVue 8-24mm zoom eyepiece for planetary observing; although it's not considered as sharp as fixed focal length eyepieces, it's still fairly sharp, and sometimes it's helpful to be able to choose an in-between focal length based on sky conditions, and adjust it as necessary.