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 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 all E-Lux Plossls, their lowest line oculars; ditto the included two-element Barlow lens. The five oculars supplied in the set include two very short focal length Plossls, 4mm and 6mm, which are difficult to view through because of their minimal eye relief and tiny field lenses and provide more magnification than most Celestron telescopes can use under average seeing conditions. Also, the set lacks a long 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, X-Cel, Ultima LX and Axiom LX lines will result in a superior collection.
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. The apparent field of view (AFOV) of the 32mm Omni is 52-degrees and the AFOV of the 40mm is 43-degrees. 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 1.25" eyepieces, they are really the only choice, since X-Cel's are not offered beyond 25mm and the 31mm-32mm Axiom and Ultima oculars use 2" mounting barrels. 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 a decent 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 such an ocular, the 25mm X-Cel is a step up from the standard Plossl. It is a six-element, ED glass ocular that offers 20mm of eye relief, a 55 degree AFOV and excellent all-around performance. The other option, and somewhat less expensive, is the 25mm Omni Plossl. 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. 15-23mm oculars Celestron's has four offerings in this focal length range. They are the 18mm X-Cel, 17mm Ultima LX, 15mm Axiom LX and 15mm Omni Plossl. The 18mm X-Cel offers a 60-degree AFOV with a 20mm eye relief. It is a sharp, flat field design and the best choice for eyeglass wearers. The 17mm Ultima LX has a wide 70-degree apparent field, but slightly less eye relief at 16mm. Ultimas are known for their high performance. The 15mm Axiom LX features a retractable eyecup, an ultra-wide AFOV of 82-degrees and a 13mm eye relief. It is the most deluxe of the three oculars and also the most expensive. The 15mm Plossl has a 52-degree AFOV and 13mm eye relief; it is also a sharp and contrasty ocular and it weighs less than the others. Choosing between these four really comes down to what focal length fits better in the ocular system you are building and your budget. The beginner on a tight budget can temporarily skip a 15-23mm eyepiece 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 (13mm-2.3mm) This is where the choice widens. Below the 15mm focal length, Celestron offers Omni Plossl, X-Cel, Ultima LX and Axiom LX 1.25" eyepieces. There are plenty of styles and focal lengths available, down to 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. Most will be beyond their useful magnification with oculars shorter than about 7mm in average seeing conditions. All of these short focal length oculars are adequate performers, but the various lines have their advantages and disadvantages and there is some variation in the specific focal lengths offered. Let's look at the choices. The 1.25" Axiom LX short focal lengths are 7mm and 10mm. The premium Axiom LX line is Celestron's most expensive. These seven-element eyepieces offer an ultra-wide 82-degree AFOV, 13mm of eye relief and retractable eyecups. 13mm eye relief is acceptable, as long as you do not wear eyeglasses, and the retractable eyecups are a nice feature. The ultra-wide 82-degree AFOV was pioneered by TeleVue with their Nagler line of oculars. It is beloved by many observers, but I am not among them. The AFOV is so wide that you must move your head around to see the edges, which I find irritating. At 12 ounces apiece in the 7-15mm focal lengths the Axioms are rather heavy, about 50% more than an X-Cel and over twice the weight of similar focal length Omni Plossls. I also do not like the barrel-like shape of the Axiom eyepieces. For these reasons I have avoided the Celestron Axion LX line. If you adore an ultra-wide AFOV and do not mind the weight, then the Axiom's are for you and the 7mm and 10mm focal lengths will complete your ocular collection. The Ultima LX Series is available in short focal lengths of 13mm, 8mm and 5mm. These deluxe eight-element eyepieces have a wide 70-degree AFOV and a 16mm eye relief. They are designed to mount in either 1.25" or 2" star diagonals or focusers, good if you have other 2" oculars and a 2" focuser, but irrelevant if you do not. The 70-degree AFOV is the widest I can appreciate. You will probably not have much occasion to use the 5mm Ultima unless you have a short focal length telescope (the C-6, a 750mm focal length Newtonian, for example) or are frequently blessed with excellent seeing conditions. If you decide to go with Ultima LX short focal length oculars, which I would recommend if you use other oculars in both 1.25" and 2" diameter, the 13mm and 8mm focal lengths will probably suffice. Should the need for very high magnification arise and the seeing conditions permit, the 13mm Ultima can be used with an Ultima 2x Barlow to give magnification equivalent to a 6.5mm ocular and the 8mm Ultima equals a 4mm ocular. An eyepiece used with a Barlow lens retains its original eye relief, a big advantage over very short focal length Ortho and Plossl oculars. X-Cel oculars are made with 1.25" mounting barrels. The entire X-Cel 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 12.5mm, 10mm, 8mm, 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. If you choose Ultima LX short focal length oculars, the 10mm X-Cel may be useful to fill the gap between 13mm and 8mm. The 10mm X-Cel can be used with a 2x Barlow lens to give the equivalent of a 5mm ocular and any eyepiece used with a Barlow lens retains its original eye relief. Omni Plossls are also available in a number of short focal lengths, all of which have a 52-degree AFOV. These include 12.5mm, 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 than Celestron's other ocular lines. I find the 4mm, 6mm and 9mm Plossls difficult to see through. The 6mm Omni has only 5mm of eye relief and the 9mm has 6mm, while the 12.5mm offers an 8mm eye relief. None of these are suitable for eyeglass wearers. An alternative for those on a tight budget is to forego the short focal length oculars in favor of the 8-24mm Zoom Eyepiece (see below). Zoom ocular  This choice is easy, since Celestron only offers one variable focal length ocular. It is their 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, for a zoom ocular, 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 (2.3-13mm) and medium-short (15-23mm) 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. Barlow lens  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 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 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 complete set of 1.25" Celestron oculars chosen without regard for price:
Here is an alternative for those who prefer long eye relief or must wear eyeglasses when viewing:
Here is a starter set of Celestron oculars for someone on a budget:
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