A "Starter Set" of Oculars
Recommended Eyepiece Focal Lengths for
Medium Focal Ratio (f/9-f/11) Telescopes

By Chuck Hawks


For the purposes of this article, I am considering "medium focal ratio telescopes" to be those with focal ratios between f/9 and f/11. This focal ratio range encompasses a wide range of popular consumer telescopes, including most Schmidt-Cassegrain catadioptrics, medium priced refractors and many Newtonian reflectors. Common apertures and focal lengths that result in these medium focal ratios include 3.1"/800mm (f/10), 3.15"/900mm (f/11), 3.5"/1000mm (f/11), 4"/900mm (f/9), 4"/1000 (f/10), 4.5"/1200mm (f/10.4), 5"/1143mm (f/9), 5"/1250mm (f/9.8), 6"/1524mm (f/10), 8"/2000mm (f/10), 9.25"/2350mm (f/10), 10"/2540mm (f/10), 11"/2800mm (f/10), 12"/3048mm (f/10) and 14"/3910mm (f/11).

When purchasing oculars, do not exceed the 50 power-per-inch of clear aperture rule. The fifty power/inch of clear aperture rule implies that the maximum usable power for any telescope is determined by the light grasp of that telescope. Magnification beyond 50 power per inch is called "empty magnification," because it produces dark, fuzzy, useless views. The telescope does not have enough light grasp to illuminate the dim field of view. The theoretical maximum is 105 power for a 2.4" (60mm) clear aperture telescope (department store telescopes this size are often advertised as "375x"!), 175 power for a 3.5" (90mm) aperture telescope, 200 power for a 4" (100mm) telescope, 225 power for a 4.5" (114mm) telescope, 300 power for a 6" (150mm) telescope and 400 power for an 8" (200mm) clear aperture telescope.

In the real world of less than ideal seeing, powers well below 50 power/inch are usually the practical maximum. I recommend something closer to 30 power/inch. Due to their lack of a secondary obstruction and consequent high contrast, refractors can usually handle a little more magnification than reflectors of equivalent aperture and quality.

For most beginners, four oculars will suffice as a starter set for general viewing. The first will usually be a high magnification ocular delivering somewhere around 30 power/inch for detailed planetary and lunar viewing, as well as splitting double stars on nights of good seeing. The second would be a medium-high magnification ocular delivering about 15-20 power/inch for detailed views of deep space objects like globular clusters, splitting double stars and planetary and lunar viewing on nights of less than optimum seeing. The third will be a medium magnification ocular delivering around 10 power/inch, which is probably the most versatile ocular of all. This is also the ocular to choose for most terrestrial viewing. The fourth will be a relatively long focal length (low power) ocular for increased true field of view and maximum brightness at about 5-8 power/inch. This ocular is also very handy for aiming your telescope. The actual focal lengths of these oculars will depend on the prime focal length of the telescope itself. For those on a particularly tight budget, there is a possible alternative to a four fixed focal length ocular set. More on that at the end of this article.

Be aware that, no matter how tight the budget, it is important to choose oculars wisely. Most amateur astronomers replace their telescope several times as their experience grows and their observing needs change. However, if you choose your eyepieces wisely from the start, they will serve not only your first telescope, but telescopes well into the future. Conversely, if you buy low grade "starter" oculars for your beginner telescope, you will want to replace them as soon as you upgrade your telescope. Buying premium eyepieces will probably save you money in the long run.

The magnification and field of view figures will be identical for all telescopes of the same focal length using the same eyepiece, regardless of clear aperture. However, the power/inch and exit pupil figures apply only to the specific clear aperture. Divide the magnification produced by the telescope and eyepiece combination by the clear aperture of the telescope to derive the power/inch figure. For example, with a 10mm eyepiece, a telescope with a focal length of 1000mm and a 4" clear aperture is operating at 100 power and 25 power/inch.

Eyepiece design determines eye relief. Eyeglass wearers will need more eye relief than non-eyeglass wearers. Plossl type eyepieces are very popular and use four lens elements in two symmetrical groups. They have generous eye relief (around 17-22mm or more) from about the 25mm focal length and longer. Good Plossl eyepiece lines with which I have had enough experience to recommend include Celestron Omni, Meade Series 4000, Tele Vue Plossl and Vixen NLP. Good Plossl oculars are all that is needed in the longer focal lengths.

Plossl eyepieces from about 15-22mm are generally satisfactory for non-eyeglass wearers, with 13mm eye relief in the case of a 15mm Celestron Omni Plossl, but eyeglass wearers will want oculars designed for more eye relief. Everyone will appreciate the larger ocular lens (compared to a Plossl) and wider AFOV provided by the upscale, six-element oculars. These include the Burgess/TMB Planetary, Celestron X-Cel LX, Tele Vue Radian and Vixen NLV lines, among others. These also provide greater eye relief than similar focal length Plossls, between 16-20mm. Unfortunately, such eyepieces are more expensive, but they are generally more comfortable to look through and a wise investment in the medium and shorter focal lengths.

Since it would be impractical to suggest specific eyepiece focal lengths for all of the apertures and focal lengths that result in a telescope with a medium focal ratio, I have chosen two common combinations to serve as examples in this article. These are a 4" aperture scope with a 900mm focal length (f/9 focal ratio) and an 8" aperture scope with a 2000mm focal length (f/10 focal ratio). Even if your telescope is not one of the two focal lengths chosen as examples, and the odds are it isn't, the following recommendations should give you some idea of how to choose a basic set of oculars.

I have used apparent field and eye relief figures published by TeleVue and Celestron. For simplicity, I have rounded-off some of the figures given below.

The short focal length (high magnification) ocular

Beware: Plossl and Ortho eyepeices with single digit focal lengths, although they can provide sharp images, have very little eye relief and the ocular lens (the closest lens to your eye) can be like looking through a pinhole. For example, Plossls in the 7mm-8mm focal length range only have about 5mm-6mm of eye relief and I find them uncomfortable to look through. Shorter focal length Plossls are so hard to see through that they are virtually unusable. This would be a good time to splurge for a Vixen NLV, TeleVue Radian, Celestron X-Cel LX, Burgess/TMB Planetary, or other eyepiece designed for generous eye relief and a wider AFOV (generally around 60-degrees). You will not regret spending the extra money.

For a 900mm focal length telescope with a 4" clear aperture (f/9 focal ratio), a 7mm ocular will give 129x magnification at 32 power/inch.

In a 2000mm focal length telescope with an 8" clear aperture (f/10 focal ratio), an 8mm ocular will yield 250x at 31 power/inch.

The medium-short focal length (medium-high magnification) ocular

Our goal here is between 15-20 power/inch. Such oculars are versatile and ordinarily get much use. Globular clusters, planetary nebula and the like are common deep sky subjects for this class of ocular. A Plossl design will provide clear images in this focal length range for those who don't wear eyeglasses and don't mind short eye relief (only 8mm for a 12.5mm Celestron Omni Plossl, for example). However, eyepieces from one of the lines mentioned above that are designed for longer eye relief and a wider AFOV will be more comfortable to view through. I recommend such eyepieces in these focal lengths.

Our 4"/900mm telescope will require a shorter focal length ocular than the 8"/2000mm telescope to perform the same duties. In this scope, a 12.5mm ocular produces 72x at 18 power/inch.

A 15mm ocular is appropriate in an 8"/2000mm f/10 telescope. With a 15mm ocular the magnification is 133x at 16.7 power/inch.

The medium focal length (medium magnification) ocular

This is the ocular most commonly supplied with new telescopes. For high quality telescopes with medium focal ratios, such as our examples, it is very often a 25mm or 26mm Plossl. This merely shows that the manufacturers know what they are doing, because this combination is perhaps the most versatile of all. A good quality Plossl is useful for a variety of deep space and terrestrial viewing in telescopes from about 900mm to 2000mm. If your telescope came with an economy ocular, or none at all, I would recommend purchasing a Vixen NLP, Tele Vue Plossl, or Celestron Omni Plossl. Upscale eyepieces with more elements and a wider apparent field of view include the Meade Series 5000 (a 5-element Erfle design) and Celestron X-Cel LX.

For short focal length telescopes (750mm or less), a somewhat shorter focal length ocular of around 18mm-20mm is required to get similar magnification and field of view. In that case, the Celestron X-Cel LX and Tele Vue Radian oculars are excellent choices.

Our 900mm and 2000mm telescopes will both do well with a 25mm-26mm Plossl. The eye relief of these oculars is around 17-22mm. A 25mm eyepiece gives a magnification of 36x in a 900mm telescope at 9 power/inch and 80x in a 2000mm telescope at 10 power/inch.

The long focal length (low magnification) ocular

32mm-40mm oculars are a good choice when you are trying to locate objects in the night sky. They are also an excellent choice for wide sky views of open clusters and star fields. These eyepieces get a lot of use in my telescope. Because the AFOV of the 32mm Plossl is 50-52 degrees and the AFOV of the 40mm Plossl is only about 43-degrees, the actual field of the two focal lengths in a given telescope is essentially identical.

A 32mm ocular will provide excellent wide sky views. You get 28x magnification in a 4"/900mm prime focal length scope at 8 power/inch. A 32mm Plossl has a 3.6mm exit pupil (with a 4" clear aperture) and the eye relief is 22mm, so it is pleasant to look through. The actual field of view of this combination is approximately 1.8 degrees. Train a 32mm eyepiece on the famous "double cluster" for a breathtaking view.

A 2000mm f/10 telescope does well with a 40mm Plossl. The 1.25", 40mm Plossl gives a magnification of 50x at 6.25 power/inch. The exit pupil is 4mm in diameter. In this case, the telescope's long focal length reduces the actual angle of view, which is down to 0.92 degree. Nevertheless, there are many beautiful subjects that can be examined with this combination.

A two eyepiece alternative to the basic four ocular set

As I mentioned earlier in this article, for those who cannot afford to purchase four oculars (or three, if their telescope came with a good 25mm Plossl), or are reluctant to do so, there is a possible alternative that allows us to reduce the total number of starter eyepieces to two. My first suggestion is to purchase a good 32mm Plossl ocular for its brightness and wide field of view. The second ocular in our two ocular set would be a modern 8-24mm zoom eyepiece, replacing the short, medium-short and (possibly) medium focal length oculars previously recommended. (If your telescope came with a 25mm Plossl, it will still be useful.)

Zoom oculars, once considered dramatically inferior to good fixed focal length oculars, have come a long way and are now capable of providing good views, particularly at the high magnification end. The Celestron 8-24mm Zoom Eyepiece, using four optical elements, is the least expensive of the commonly available zoom oculars that I would recommend; call it an "intermediate" level eyepiece. For more money you can have an "advanced" level Meade Series 4000 8-24mm Zoom with six optical elements, but be aware that it costs about twice as much as the Celestron zoom. The Vixen NLV and discontinued Tele Vue 8-24mm Click-Stop zooms are the most highly rated of the zoom eyepieces, but they are almost three times the price of the Celestron 8-24mm zoom. If you are buying oculars with an eye to the future, these smooth operating Click-Stop zooms are designed with six elements, including rare earth ED glass elements. They will complement even the finest "expert" level telescopes and are a good long term investment.

These 8-24mm zooms have good eye relief, but their field of view decreases from 50-degrees at 8mm to 40-degrees at 24mm, the opposite of what you would expect. This is a characteristic of all zoom oculars. However, the fact remains that with a 32mm Plossl and an 8-24mm zoom you can do a heck of a lot of observing with maximum convenience and minimum hassle.




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Copyright 2009, 2011 by Chuck Hawks. All rights reserved.


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