Choosing a Set of Celestron Oculars
By Chuck Hawks
Photo courtesy of Celestron International.
Maybe you own a Celestron astronomical telescope and you trust the brand, so you want to build a new Celestron 1.25" ocular set, including a Barlow lens and a "catch all" zoom ocular. Celestron markets several ocular lines, so some decisions will have to be made to get the oculars that best suit your individual needs and preferences.
Alternatively, maybe you're just starting out and can only afford a couple of eyepieces right now. Celestron is a good ocular brand to start with, as they have multiple good quality lines and are generally an excellent value. By choosing wisely you can minimize the expense of getting started and still get great views of both planetary and deep space objects. Keep reading, I'll have some specific recommendations for you as we go along.
Celestron offers a pre-packaged Telescope Eyepiece - Filter Set that is an easy way to get most of what you need. Unfortunately, the oculars are Celestron's lower line Plossls (not the top grade Omni Plossls); ditto the included two-element Barlow lens. The five oculars supplied in the set are fully multi-coated and feature a 52-degree AFOV. Included are two short focal length Plossls, 6mm and 8mm, which are difficult to view through because of their minimal eye relief and tiny field lenses. The other focal lengths include 13mm, 17mm and 32mm. The Set lacks a 40mm ocular and a zoom ocular. So, while the Set is a great value for the bargain minded novice, choosing eyepieces individually from Celestron's more upscale Omni Plossl and X-Cel LX lines will result in a superior collection.
Celestron's top of the line oculars are the Ultima LX series. The 32mm and 22mm Ultima LX come with 2" diameter mounting barrels. The 17mm, 13mm, 8mm and 5mm Ultimas come with dual diameter (2"/1.25") mounting barrels, but are essentially 2" oculars and very large for use in 1.25" star diagonals. They give the person using a mix of 2" and 1.25" oculars a bit of extra convenience, but Celestron's X-Cel and Omni oculars are probably a better choice for the person building a set of dedicated 1.25" oculars. Unless you want or need 2" diameter mounting barrels, the Ultimas are probably best avoided due to their large size and weight. They will not be included in the rest of this article. For more on the Ultima LX oculars, see "Celestron Ultima LX 2" Eyepieces."
If you don't buy the Celestron Telescope Eyepiece - Filter Set, which includes a fitted hard case, you will need to buy a case to hold your oculars. There are many possible sources, but two of the best are Zero Halliburton and Pelican. Be sure to buy a case large enough to hold not only your oculars, but any other small accessories that you need or might remove from your telescope during transportation (finder scope, red flashlight, motor drive hand controller, etc.) A foam-filled case about the dimensions of the Pelican #1520 will usually do the job.
I was a Celestron dealer for many years and I have used their oculars for over 20 years in my personal telescopes. Here is how I would put together a complete set of Celestron eyepieces, using my needs and preferences as a guide. Of course, these may be different than your priorities, but I hope that the basic process will be instructive and makes reading this article worthwhile.
Long focal length oculars
Celestron's Omni Plossl 32mm and 40mm oculars offer very good sharpness, a flat field and plenty of eye relief. Celestron Omni's are fully multi-coated, edge blackened and baffled. The apparent field of view (AFOV) of the 32mm Omni is 52-degrees and the AFOV of the 40mm is 43-degrees, due to vignetting caused by the 1.25" mounting barrel. This result of this difference in AFOV is that in any given telescope the 40mm and 32mm Plossls have essentially the same actual field of view. The 40mm is brighter because it has a larger exit pupil, while the 32mm makes everything in the field of view larger.
These are high grade Plossls and good low magnification eyepieces. If you want to stay with Celestron 1.25" eyepieces, they are really the only choice, since X-Cel's are not offered beyond 25mm. Fortunately, the 32mm and 40mm Omni Plossls do a good job.
Because the actual field of view is the same, the beginner on a budget can get by with just one of these long focal length oculars. If your telescope has a medium to short focal length, say around 1000mm or less, I'd suggest the 32mm Plossl. Folks with long focal length scopes (1500mm and greater) will probably appreciate the greater exit pupil and therefore brightness of the 40mm Plossl.
24mm to 26mm oculars
Chances are that a 25mm Plossl eyepiece came with your telescope. It is probably an adequate ocular and I can see no crying need to replace it. If you did not get a Plossl in the 24mm-26mm focal length range and are looking to purchase one, Celestron's 25mm Omni Plossl is the way to go. It is far superior to inexpensive Plossls. Like all Plossl oculars, its symmetrical optical formula calls for four elements in two groups. It is a good general-purpose, high performance eyepiece. The 25mm Omni's AFOV is 52-degrees and its eye relief is 22mm. Because its performance/price ratio is so favorable, it is the natural choice of beginners on a tight budget.
The 25mm X-Cel LX is a step up from the Omni Plossl. It is a six-element ocular that offers 20mm of eye relief, a 55 degree AFOV and excellent all-around performance.
Celestron's offers four eyepieces in this focal length range. These include the 18mm X-Cel LX, 15mm Omni Plossl, 12.5mm Omni Plossl and 12mm X-Cel LX.
The 18mm and 12mm X-Cel LX oculars offer a 55-degree AFOV with a 20mm eye relief. These are sharp, flat field designs and the best choice for eyeglass wearers. Although they are somewhat more expensive than Omni Plossls, I prefer X-Cel eyepieces in all available focal lengths.
The 15mm Omni Plossl has a 52-degree AFOV and 13mm eye relief; it is also a sharp and contrasty ocular and it weighs less than the X-Cel. It is an acceptable choice for those who do not wear eyeglasses when viewing, but the X-Cel is nicer to look through. The 12.5mm Omni Plossl also has a 52-degree AFOV, but a more restricted 8mm eye relief, which borders on being uncomfortably short.
The beginner on a tight budget can temporarily skip 8mm to 24mm eyepieces in favor of the Celestron 8-24mm Zoom Eyepiece. You'll probably want to add a fixed focal length ocular in this range later on, but for now go directly to the Zoom Eyepiece, which is described below.
Short focal length oculars (9mm-2.3mm)
The selection widens in this focal length range. Below the 12mm focal length, Celestron offers focal lengths as short as 2.3mm, which is much too short to be practical in most Celestron telescopes. The majority of Celestron telescopes have a medium focal ratio (f/9 to f/11) and adequate focal length, so they do not to need very short eyepieces for high magnification. They will be beyond their useful magnification with oculars shorter than about 5mm to 7mm in most seeing conditions. All of these short focal length oculars are adequate optical performers, but the two lines have their advantages and disadvantages and there is variation in the specific focal lengths available.
The entire X-Cel LX line is designed for a 55-degree AFOV and 20mm eye relief, so it is excellent for eyeglass wearers. In the shorter focal lengths, X-Cel's come in focal lengths of 9mm, 7mm, 5mm and 2.3mm. They combine sharp, wide-field views without noticeable lateral color error and a reasonable price. Internal light scatter is well suppressed, important for planetary and lunar observing. In the short focal lengths, the X-Cel LX series is the way to go.
The short focal length Omni Plossls have a 52-degree AFOV. Focal lengths include 9mm, 6mm and 4mm. As the focal length decreases, so does the eye relief and exit pupil of Plossl oculars, which makes short focal length Plossls distinctly less enjoyable to view through. The 4mm, 6mm and 9mm Plossls are difficult to see through. The 4mm and 9mm Omnis have only 6mm of eye relief and the 6mm Omni has 5mm of eye relief. None of these are suitable for eyeglass wearers. I suggest spending the extra money for upscale X-Cel LX oculars in these short focal lengths.
This choice is easy, since Celestron only offers one variable focal length ocular. It is their four element, fully multi-coated 8-24mm Zoom Eyepiece that has an AFOV of 40-60 degrees (40 degrees at 24mm and 60 degrees at 8mm, the opposite of what you would expect) and offers 15-18mm of eye relief. This means the zoom is at its best at the shorter focal length settings.
Zoom oculars have improved tremendously during the years that I have been telescoping. Once regarded as an optically inferior gimmick, the performance of the better zoom eyepieces has been upgraded to the point that they must be taken seriously. This is the same evolution that has occurred during my lifetime in zoom camera lenses and zoom riflescopes and it has been made possible by tremendous advances in computer aided lens design and improved anti-reflection lens coatings (multi-coatings). The user reviews of Celestron's 8-24mm eyepiece have been positive and it is relatively inexpensive.
Zooms are handy for determining the correct eyepiece focal length that the subject and seeing conditions will allow. They are also useful when several people are using the same telescope and want to see the subject at various magnifications or fields of view. A zoom ocular is a useful addition to any eyepiece set.
For the beginner on a tight budget or the traveling astronomer who must travel light, the 8-24mm Zoom can be a worthwhile alternative to initially buying specialized short focal length and medium focal length oculars. If you have Omni Plossl 32mm and 25mm oculars, an 8-24mm Zoom can complete your starter set. You can head into the field for some dark sky observing without being at any great disadvantage or breaking the family budget.
A 2x Barlow effectively doubles the focal length of whatever ocular you put in it. (Technically, it doubles the focal length of the telescope, not the ocular, but in terms of magnification it is the same thing.) This superficially seems like a good deal, but the price for this increase in magnification is making the light pass through more glass, which is never good, and doubling the focal ratio. Your f/8 scope becomes an f/16 scope when you use a 2x Barlow lens!
If you have a complete set of oculars you really don't need a Barlow lens and the beginner on a tight budget would be well advised to forgo purchasing a Barlow lens, at least until he or she has acquired a viable eyepiece collection. In other words, I am suggesting that a Barlow is not a necessity, but rather a luxury item that you can delay acquiring. However, everyone seems to have a Barlow lens and eventually you probably will, too. Celestron offers three 1.25" Barlows, in the Omni, X-Cel and Ultima lines.
The Omni Barlow is the least expensive. It is fully multi-coated and has blackened lens edges, but it is a standard two-element design. This means that it will introduce lateral color error, even if your telescope and oculars are apochromatic. For this reason, I avoid two-element Barlows whenever possible.
The X-Cel Barlow, as near as I can tell without disassembling one, is also a two-element, fully multi-coated Barlow, but it uses an ED glass element to reduce chromatic aberrations. This is a real improvement and it is a good Barlow, particularly for those with a full set of X-Cel eyepieces.
The Ultima 1.25" Barlow is a three-element, fully multi-coated, apochromatic Barlow lens. It is Celestron's best Barlow lens and it only costs a few dollars more than the X-Cel Barlow. For most users, it is the best choice. Remember, the astronomer on a budget can skip the Barlow lens altogether.
Here is a suggested complete set of 1.25" Celestron oculars:
Here is a starter set of Celestron oculars for someone on a budget:
Copyright 2009, 2011 by Chuck Hawks. All rights reserved.