Choosing a Set of Vixen Eyepieces
By Chuck Hawks
Maybe you own a Vixen telescope and you trust the brand, or you've had experience with Vixen oculars and know they are excellent, so you want to standardize on one premium brand. In any case, you want to build a new Vixen 1.25" ocular set, including a Barlow lens and a "catch all" zoom ocular. Vixen markets several ocular lines, so some decisions will have to be made to get the oculars that best suit your individual needs and preferences.
Vixen offers a set of three, top quality, NPL Plossl eyepieces packaged with a convenient, multi-pocketed fanny pack that is an easy way to get started. NPL Plossl oculars use 4-elements in 2-groups and provide sharp, well corrected, flat field views. Included are in the package are NPL 10mm, 15mm and 40mm Plossls. These are all useful focal lengths and, taken with the 24-26mm Plossl that probably came with your telescope, makes a pretty good start on a set of oculars. (If you didn't get a good 25mm Plossl with your telescope, buy a Vixen NPL; it is a handy eyepiece.) All you really need to add later is the Vixen 2x, three element, multi-coated Barlow lens and the excellent Vixen NLV 8-24mm Click Stop Zoom eyepiece and you are ready to observe practically anything in the night sky.
With a 2009 MSRP on only $139, the Vixen NPL set is a great value, as long as you do not have to wear eyeglasses while observing. That qualifier is necessary because short focal length Plossls (in this case the 10mm and 15mm) do not have sufficient eye relief for eyeglass wearers. The eye relief of Plossl design oculars has a positive correlation with focal length. Longer focal length Plossls, from about 25mm on up, offer excellent eye relief.
If you do wear eyeglasses, or simply prefer more eye relief for comfortable viewing, Vixen has you covered with their other ocular lines. They also offer oculars with a wider apparent field of view (AFOV).
If you don't buy the Vixen NPL set with its fanny pack, 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.
Here is how I would put together a complete set of Vixen 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
Vixen's NPL 30mm and 40mm Plossl oculars offer very good sharpness, a flat field and plenty of eye relief (24mm and 36mm respectively). The apparent field of view of the 30mm is 50-degrees and the AFOV of the 40mm is 40-degrees. This result of this difference in AFOV is that in any given telescope the 40mm and 30mm Plossls have essentially the same actual field of view. The 40mm is brighter because it has a larger exit pupil, while the 30mm makes everything in the field of view appear larger. These are high grade Plossls and good low magnification eyepieces.
An alternative to the NPL 40mm Plossl would be the NLV 40mm Lanthanum eyepiece. For about three times the price of the Plossl, this ocular incorporates a twist-up eyecup and a rare earth glass element for enhanced contrast; it provides a slightly wider 42-degree AFOV. If you want to stay with Vixen 1.25" eyepieces, these are the only choices.
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 fast focal ratio in the f/5 to f/9 range, I'd suggest the NPL 30mm Plossl. Folks with slower focal ratios in the f/10 to f/16 range will probably appreciate the greater exit pupil and therefore brightness of a 40mm ocular.
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 NPL Plossl is a good one.
A step up from the NPL Plossl is the 25mm NLV. All Vixen NLV eyepieces offer a 20mm eye relief, twist-up eyecup and Lanthanum element. The 25mm NLV is a 5-element in 3-group design. The AFOV is 50-degrees, the same as the 25mm NPL Plossl.
Vixen has six eyepieces in this focal length range. They are the 15mm and 20mm NPL's, 15mm and 20mm NLV's, and the 17mm and 22mm LVW's. The 15mm NPL Plossl has a 50-degree AFOV and 11mm eye relief; it is also a sharp and contrasty ocular and it weighs less than the others. The 20mm NPL has the same 50-degree AFOV and 15mm eye relief; this is satisfactory eye relief for most non-eyeglass wearers.
The 15mm and 20mm NLV's offer a 50-degree AFOV with 20mm's of eye relief. These are the preferred oculars of eyeglass wearers and, of course, they use a Lanthanum element in their design to minimize aberrations and have the twist-up eyecup feature common to all NLV series oculars. I find the 50-degree AFOV adequate and I like the twist-up eyecup and long eye relief, even though I do not ordinarily wear eyeglasses when observing.
The 17mm LVW is one of my favorite focal lengths. It has a wide 65-degree AFOV with 20mm eye relief. It is a sharp, contrasty ocular with 8-elements (several of which are Lanthanum glass) in 5 groups. The 22mm LVW shares the same basic specifications, but its longer focal length gives a wider true field of view at the cost of some magnification. Choosing between these six eyepieces really comes down to what focal length fits better in the ocular system you are building and your budget.
Short focal length oculars (13mm-2.5mm)
This is where the selection really increases. Below the 15mm focal length, Vixen offers four LVW series eyepieces (13mm, 8mm, 5mm and 3.5mm), seven NLV series oculars (12mm, 10mm, 9mm, 6mm, 5mm, 4mm, and 2.5mm) and two NPL Plossls (10mm and 6mm). All of the Vixen short focal length oculars are adequate performers in terms of providing sharp, contrasty views, but the various lines have their advantages and disadvantages.
I've already indicated that short focal length Plossls lack the requisite eye relief for my purposes. The 10mm Plossl has a 50-degree AFOV, but only 6.5mm eye relief, so your eyelashes will be brushing the ocular when you look through it and the 6mm is worse. Better to invest in the NLV or LVW series eyepieces.
2.5mm, 3.5mm and 4mm are such short focal lengths that they are inappropriate for use in most amateur (and most Vixen) telescopes. The result is useless magnification, which produced dim, fuzzy views. 5mm oculars are too short for most telescopes under normal viewing conditions, but might find an application in fast focal ratio, short focal length scopes. For most telescopes and most users, depending on their telescope's prime focal length and focal ratio, 6-8mm is about the shortest eyepiece they will probably ever need.
The most useful short LVW focal lengths are 8mm and 13mm. If you feel you like their 65-degree AFOV and these focal lengths fit your ocular needs, your high magnification ocular decision has been made. Like all LVW eyepieces, the eye relief is a generous 20mm.
I find the 50-degree AFOV of the NLV series adequate for high magnification observing and 12mm is a handy focal length. For high magnification views, a 10mm or 9mm eyepiece works well and for nights with excellent seeing conditions, when very high magnification may be useable, the 6mm may be worth a try.
Vixen offers two variable focal length oculars. They are both 8-24mm (3:1 zoom ratio) eyepieces. The somewhat less expensive LV 8-24mm zoom has an AFOV of 40-60 degrees (40 degrees at 24mm and 60 degrees at 8mm) and offers 19mm of eye relief. The premium NLV 8-24mm Click Stop Zoom has an AFOV of 40-55 degrees and between 20-15mm of eye relief (20mm at the 24mm focal length). These are both good zoom oculars and neither is inexpensive.
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 has been made possible by tremendous advances in computer aided lens design and improved anti-reflection lens coatings (multi-coatings). We have reviewed the Vixen 8-24mm Click Stop Zoom (see the Astronomy and Photography Online index page) and it is the best of its breed. Since the price difference is not marked, I'd definitely buy the NLV Click Stop zoom.
Zooms are handy for determining the correct eyepiece focal length and exit pupil 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.
The NLV zoom has click stops at the 8mm, 12mm, 16mm and 24mm focal length settings. This is a very handy way to tell where your zoom is set in the dark, since you cannot read the focal length markings on the lens barrel of most zoom oculars. Naturally, the zoom ring can be set anywhere between the clicks, they are just handy references. I have come to appreciate click stop zooms and the NLV's twist-up eyecup is another good feature.
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 should forgo purchasing a Barlow lens 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.
Vixen offers two 2x Barlows. The standard 2-element Barlow lens is the least expensive. Its lens elements are coated, but not multi-coated. It will introduce lateral color error, even if your telescope and oculars are apochromatic, and it is more subject to internal lens flare.
The Vixen Barlow DX is a three-element, apochromatic, fully multi-coated, 2x Barlow lens. It is Vixen's top Barlow lens and, for most users, it is the best choice.
Here are my focal length choices for a complete set of 1.25" Vixen oculars:
Copyright 2009, 2013 by Chuck Hawks. All rights reserved.