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Eyepieces & Choosing an Eyepiece Set
Whether you're a beginning amateur astronomer or a seasoned veteran, selecting a proper eypiece set is key to extracting the best possible images from your telescope.
Coming in all sizes, shapes, and prices, the shear variety of oculars in today's amateur astronomy market may make the selection of a good set quite difficult. Eventually, many observers end up with a vast number of eyepieces, but trimming down the fat in your eyepiece case can really add to your observing experiences and ease of setup for a night under the stars. The choosing of proper eyepieces is nearly as important as the choosing of a telescope; for a set of well selected oculars may make the difference between seeing or not seeing the vague polar caps on Mars or detecting faint wisps of a dim deep sky treasure.
One of the first, and most important, considerations given in selecting eyepieces should be the effective magnification,
or power. Mistakes such as selecting eyepieces that produce too high of power for good or similar magnification
can be easily avoided by determining the power that a particular eyepiece yields. The calculation of an eyepiece's
magnification is simple: divide the focal length of your telescope by the eyepiece's focal length. For example:
my 80mm F/5 Rich Field Refractor has a focal length of 400mm (80mm * 5). When a 40mm eyepiece is used with this
telescope, the effective magnification is 10x.
With magnification, the goal is to achieve a balance between image amplification and image brightness and sharpness. Remember that the higher the power utilized, the less sharp and dimmer the image becomes. A good general rule to follow in determining the highest practical power for your telescope is that anything above about 50x per inch of aperture will result in poor images and be of little use. This is only a general guideline, however, as some of today's expensive apochromat refractors can accept over 100x per inch of aperture and still produce good images, while some lower quality telescopes may not be able to handle much above 25x per inch of aperture. Theoretically, my 80mm Refractor can handle magnification up to about 150x on a good night (80mm = 3 inches). Another determination of how much power your telescope can handle and a still produce good image is beyond anyone's control: the atmosphere. On a steady, hazy night, high power views of the planets might be stunning, while on a turbulent crisp night following the passage of a cold front, even medium powers might suffer from fluctuating star images due to the rapid movement of air particles high above.
Comfort: Eye Relief
Another aspect to be considered with eyepieces is their eye relief. Usually the eye relief of an eyepiece can be found in literature provided by the manufacturer, but the number advertised can vary. The shorter the eye relief, the closer your eye will have to be placed to the eyepiece in order to see the image. Eyepieces with short eye relief are often uncomfortable to use and may be useless for those who wear eyeglasses. High power eyepieces typically have shorter eye relief than low power eyepieces. Just as short eye relief can be uncomfortable, so can too long of eye relief. I have a fine 28mm Orthoscopic with terrific, long eye relief, but I find it unpleasant to use. This is because the eye relief is so long that positioning the eye to receive the image is difficult. Fortunately, long eye relief can be easily dealt with by the addition of rubber eyecups, which are available for relatively low costs from a variety of retailers. I find that eyepieces with eye relief less than about 5mm are uncomfortable to use, as are those with eye relief larger than about 20mm.
Fit, Finish, and Field of View
The final considerations with eyepieces are found around the overall quality and features of the ocular and its field of view. The field of view is basically the angular coverage of the eyepiece: large fields of view provide images that appear wide and expansive, some say as if looking through the window of a spaceship; short fields of view yield an impression of a narrow image area, with very short fields of view appearing like looking down a drain-pipe. Today's eyepieces are available with fields of view ranging from a terrible 30 degrees to a wonderful 80 degrees or more. The majority of eyepieces today produce fields of view in the 45 to 55 degree range, which is usually good for most observing requirements.
There are many factors to look into concerning the fit and finish of an eyepiece. Parfocal eyepieces are those that can be interchanged and not need refocusing. In almost all cases, eyepieces advertised as parfocal are only so with eyepieces in the same set by the same manufacturer. Optical coatings on eyepieces are also a good consideration before a purchase. A "coated eyepiece" is one that has a single layer of antireflective coating (usually magnesium fluoride) applied on a lens. Far better than coated eyepieces are those that are "fully coated". Nearly all quality eyepieces feature fully coated lenses, which means that every optical surface has received a layer of antireflective coating. Fully coated eyepieces will show a blue or purple color on the lenses when seen under bright light. Finally, some eyepieces are offered as "fully multi-coated", which simply indicates that the eyepiece has had antireflective coatings applied in multiple layers. These eyepieces will reveal hues of red, green, or purple when looked at under bright light. Additional considerations with eyepieces should be whether or not the barrel of the eyepiece has been threaded to accept filters. Often, it is only a matter of time before an observer will wish to apply Wratten filters for study of the planets or a light pollution reduction filter for the enhancement of nebulae. A final good accessory offered on some eyepieces is rubber eye guards and grip rings. If an eyepiece does not offer these, they can be easily added.
Sorting the Types
Of all the eyepieces in the world, there are basically five types: Kellners, Orthoscopics, Plossls, Wide Field, and "Junk". When considering an eyepiece, its design can tell you a lot about important factors of its performance.
"Junk" Eyepieces - Maybe not the nicest of terms, but "Junk" eyepieces are just that. Eyepiece designs falling into this category include the Huygenian and the Ramsden. While these eyepieces do deserve some credit for making way to today's more improved oculars, both are significantly inferior to other designs. These eyepieces often suffer from internal reflections, narrow fields of view, poor sharpness, and low contrast. The most typical place the Huygenian and Ramsden eyepiece design is found is with low quality "department store" telescopes. Most manufacturers do not offer these designs and for good reason.
- Pros: none other than extreme low cost
- Cons: extremely poor image quality when compared to any of the other designs
Kellners - I like Kellner eyepieces myself. While not able to provide the sharpness and wide fields of view found with eyepieces of other designs, there is one major reason for the continued popularity of this eyepiece type in today's market: cost. Many companies offer Kellner eyepieces at under $40.00 per eyepiece. Not all of these are equal in terms of quality, however. Not to mention specific brand names, but personally, I find that the Orion Explorer II series is the best bargain in Kellner-type eyepieces available today. As I mentioned, I find that Kellner eyepieces are good to have. I've used Kellners for most of my observing for nearly three years, and have only recently begun to "modernize" my ocular sets.
- Pros: low cost, good field of view, simply good all-around performance
- Cons: out performed by more modern eyepiece designs, variation in quality between different manufacturers
(ABOVE): Good Kellner eyepieces offer amateur astronomers value, and are an excellent choice for beginning amateurs or observers on a budget.
Orthoscopics - A sad fact is that the venerable Orthoscopic eyepiece design is loosing popularity to some of the more modern designs. I happen to consider myself fortunate to have a high quality set of these eyepieces. During my four years of observing, I have yet to find an eyepiece that can offer a sharper, higher contrast image than those produced in a good orthoscopic. The drawback of these eyepieces: narrow field of view.
- Pros: wonderfully sharp, high contrast images, relatively low cost
- Cons: few companies offer this design, narrow field of view
(ABOVE): Classic Orthoscopic eyepieces are capable of producing high contrast, sharp images that few other designs can rival.
Plossls - Perhaps the "standard" eyepiece of today's amateur astronomy, the Plossl earns this honor for good reason. Eyepieces of this design usually offer good apparent field of view, sharp images, and good contrast. There is also considerable competition between manufacturers concerning this eyepiece, thus making for a vast amount of choices. Personally, I have found that the Televue and Meade products are of excellent quality. Orion's Sirius Plossl series is also a good choice, especially considering the low cost when compared with Plossls produced by other companies.
- Pros: good field of view, good contrast, good sharpness - good all around performance
- Cons: selection - lots of companies make these, price can sometimes be a bit high
Wide Field - The king and queens of eyepieces, wide field oculars are excellent additions to any eyepiece collection. This final type of eyepiece actually encompasses various designs. In general, those of the Erfle design are becoming somewhat dated but are still capable of producing great wide fields of view, especially at lower magnifications. Konigs are also an older design, but usually offer better images than Erfles. The stereotype of wide field eyepieces, however, goes to the classic Panoptic and Nagler Eyepiece, by Televue. In more recent years, other companies have begun to offer wide field eyepieces to include Meade, Vixen, and Pentax. When it comes to wide field eyepieces, I have found that the old rule "you get what you pay for" generally applies.
- Pros (Erfles): relatively low cost when compared with other wide field eyepiece designs, good low to medium power performance
- Cons (Erfles): somewhat outdated in design, suffer more from internal reflections and edge of field problems
than modern designs
- Pros (Modern Wide Field): field of view that ranges from expansive to "Whoa!"
- Cons (Modern Wide Field): price, some of these might cost more than your telescope
(ABOVE): Modern, Wide Field eyepieces often require considerable investment, but can offer a wide apparent field of view that is a pleasure to behold.
Selecting Your Eyepiece Set
Now that you have an overview of the differences between the many eyepieces available on the market and what
factors should be most important in the consideration of your eyepiece set, you can actually spend a few moments
to determine exactly what you need.
For beginning observers and as a core ocular set, three eyepieces, one each in the low, medium, and high power category, would be a great start. For most amateurs, a low power eyepiece is one that has a focal length greater than 20mm, medium power eyepieces typically have a focal length in the 10mm to 20mm range, and high power eyepieces possess focal lengths under 10mm. In my opinion, a good core set includes a 25mm - 28mm low power eyepiece, a 12 - 16mm mid power eyepiece, and a 6 to 9mm high power eyepiece.
Expanding on this core set usually involves consideration of your observing interests. Deep sky fanatics will usually favor a greater number of low and medium power eyepieces for viewing faint objects with minimal amount of contrast loss. Planetary and double star work usually requires higher powers, and thus eyepieces suiting such a role may have greater necessity.
For experiencing rich, wide fields of view, a 40mm Plossl may suit the job. Better than the 40mm Plossl would be a 35mm or 40mm Wide Field eyepiece, or a 55mm Plossl. It is important to consider that wide field eyepieces with focal lengths above 30mm, and Plossls above 40mm usually require a 2 inch focuser barrel.
Resolving the thousands of stars in globular clusters and extracting dim details in galaxies and nebulae usually requires good medium power eyepieces. Those in the 16mm to 20mm range will be well suited for glimpsing details in extended deep space objects, and can be exceptionally well suited to observation of open star clusters. The 12mm to 16mm range, I find, is ideal for observation of globular clusters, as the eyepiece will provide good magnification and still keep the image fairly bright.
With high power observing, eyepieces with focal lengths lower than 6mm can be hard to use due to eye relief issues. Orion offers the Vixen Lanthanum series eyepieces with a fine eye relief at high powers, but these are somewhat pricey. Televue also now offers their excellent Radian eyepieces, but these too are costly. A good accessory to consider if high power observing is routine in during your observing sessions would be a quality Barlow lens. These useful accessories effectively double the power an eyepiece provides. If you will be using a Barlow lens, select your eyepieces so that their magnification when used with the barlow is not duplicated by other eyepieces. For example: a 20mm eyepiece will act as a 10mm eyepiece when coupled with a Barlow, thus a 10mm eyepiece is not necessary for your set. Instead, choose something like 8mm or 12mm.
… For More Advanced Amateurs
Most of the information provided here is probably old hat to advanced amateur astronomers, yet I can attest how hard it can be to select the eyepieces that are to fill the eyepiece case. I have found that observers who own a considerable number of eyepieces do well to arrange their set to have an eyepiece covering the range of focal lengths in about 3 to 5mm steps. For example: in recently deciding on the purchase of a few new eyepieces I wrote down the list of eyepieces I currently own. Disregarding the Kellners (since I am phasing those out to more modern designs), I located the focal lengths where my eyepiece collection had gaps. Here's the list:
40mm, 28mm, 26mm, 18mm, 16.3mm, 14mm, 10.5mm, 9.7mm, 8.8mm, 6mm, 5mm, 4mm
My largest gaps are from 40mm to 28mm, and from 26mm to 18mm. Because of this, I decided to add eyepieces to fill those gaps (32mm and 22mm).
The variation between eyepiece types is also of consideration for the advanced amateur. While my Orthoscopics are great on planets, I'd usually prefer a modern Wide Field design when observing extended star clusters. Having two sets of eyepieces (or even three) can make sense. By looking at what I have, I determined my planetary eyepiece set is well filled:
Orthoscopics: 28mm, 16.3mm, 10.5mm, 6mm, 5mm, 4mm.
For my Wide Field Collection, I located gaps in low and high power areas:
40mm, 18mm, 14mm, 8.8mm.
Adding to the Wide Field eyepieces, to me, makes sense. Note that my two "general, all-purpose" eyepieces: 26mm and 9.7mm Plossls are not mentioned as I find them good to use in all situations.
With all of this information and discussion on eyepieces, perhaps your eager to add one or two new ones to your existing set. The best telescope in the world will perform at poor levels without good eyepieces, but remember: you don't need a set of the most expensive eyepieces on the market to have great observing experiences. Careful consideration of your observing interests and informed buying decisions can combine to allow you to form an ideal eyepiece set for your astronomy interests.
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