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How to select an eyepiece ?

The sensitivity to moisture (V)

If you experienced moisture on your optics after have observed for a while in automn or winter, the problem can have various sources. First your observation spot is too wet due to the presence of a lake or a river nearby or the sea at a few kilometers far. If the turbulence is low and the sky clear, some haze can even ruin your night session, specially if you are interested in spectroscopy. The second possibility is the coming of a warm front which fill the air with moisture announcing rain in the next days. If the temperature is very low, droplets can even cover your optics.

Takahashi 12.5 mm

The third condition has an indirect relation with the weather. You can be the source of moisture. I explain. When the weather is cold you feel your body radiates like a radiator warmed at 37°C (98.6°F). Regulated by your metabolism this is very convenient to keep you warm but the contrast between the temperature of your optics and the one of your body is very important. In these conditions when you place your eye at the eyepiece, in a few minuts stars and objects you are looking to become fuzzy and soon yourcan no more use your eyepiece because your frontal lens is covered with moisture.

Here is the comment from a user : "On my 5 mm Tak LE the sharp, bright, contrasty view suddenly have a circle of about half the diameter of the AFOV that caused like scattering. The symptom is like it got fogged up all of the sudden. I removed the eyepiece and looked at it and saw nothing.  I put it back on, got clear view for another minute and wham again.  My 4 mm and 6 mm TV Radian eyepieces do not exhibit this symptom."

This problem occurs when you place your eye very near your eyepiece, specially the ones of low eye relief and short frontal lens. The eyeball moisture is transfered to the frontal lens which become unusable. The problem arises regularly in the winter especially on the coastal areas and up to a few kilometers from rivers and lakes.

The solution is to observe under the protection of a dome in which the temperature is in equilibrium with the outside air for one hour or so. If you cannot benefit of such an expensive protected environment, they are two other solutions to consider : buying an ocular resistance that warms your eyepiece to avoid the deposit of moisture or, if your eyepiece is equipped with, to remove (slip down) the rubber eyecup which exascerbates the problem. Indeed if the eyecup prevents you to knock the frontal lens and help you finding the focal point it prevents also the free flow of air around the eyepiece frontal lens. So if your eyecup can preserve you from parasitic light or wind gusts too, in automn and winter it can be useful to not use it. With some times you will easily find the focal point of the ocular light cone.

The price argument

Jack Marling from Lumicon has told "your eyepiece represents the half of your scope", meaning by the way that if you take care to your scope quality, you should take care in selecting your eyepieces too. But know that even the best eyepiece cannot make up for a defective primary optics or a poor wavefront error. As one say in computing or engineering "your system is as good as its weakest part". This is also true about astronomical optics and many other systems.

For some amateurs Jack Marling's sentence means that they can invest as much in their eyepiece as in their scope. Personally I did it once in the past when I bought a small refractor for my girfriend and placed on it a Tele Vue zoom. But now my current scope is much more expensive and hopefully I don't have to buy eyepieces at the same price level...  But be serious and back to our considerations.

Searching some eyepieces to buy, prices start at cheap Paul Rini's and MA at $18 to reach $300 or more for some high-end from Celestron, Leitz, Meade, Tele Vue or Zeiss without to speak about image intensifiers or some Nagler's that exceed $500. In between all brands provide their "best" optics including Lichtenknecker Optics, Pentax, Takahashi, University Optics or Vixen to name a few.

The price ? You have the choice : the very cheap Paul Rini 45 mm Plössl or Celestron 10 mm Kellner SMA up to the expensive Leica 14 mm wild field or the very expensive Apogee 30 mm widescan type II or Tele Vue 17 mm Nagler type IV. Documents from manufacturers.

We can wonder if at some points the entry-level eyepieces sold with cheap scopes of 60 to 80 mm of aperture can rival with their high-end concurrents, some ten times more expensive.

There is a simple rule applicable to all thing : "you get what you pay for" and its corollary " the quality buys". This is also applicable to eyepieces and scopes. Generally speaking the more you pay, the more sophisticated and the more performance you tend to get. No master designer can create a masterpiece in a few hours and its price will be fix consequently. But another factor comes over this one : the fame of the enterprise. All famous optical master designers have skills very specialized and are for example able to design an apochromat doublet of short focal length in a few weeks but this optic will be so hard to calculate and optimize that its price will reserve its usage to universities or millionaires. This fact is true for many scopes and accessories that go from equatorial mounts to interferential filters. So unfortunately we have to find a compromise between the quality and what we accept to pay for.

Any advanced observer will tell you that very low-cost eyepieces like Paul Rini's 3-elements Erfle or modified Kellner/RKE are not really the best choice you can do. Of course at $18 or even $40 if you loose one in the dark, cover it with jam during star parties or if someone let falling it on the cement, the lost has no dramatic consequencies comparing by the loose of a $300 eyepiece. This argument is valid if for example you quite often present your scope and instrumentation to classrooms or clubs. Calculating the potential risks, I also prefer to take with me my cheap eyepieces in such occasions and let my most expensives in the trunk of my car and show them only if someone ask me questions about them.

Edmund RKE series, at left the modified Kellner's (3 lenses), at right the 32 mm Wide Angle Erfle. Documents Edmund Optics.

Excepting this special conditions of use, common low-cost eyepieces like Kellner's or Plössl's are know to provide good to excellent  performances, especially when used in moderate to long focal length telescopes, for example in f/10 SCT's. If 70% of eyepieces cost less than $100, the most appreciated cost between $100-200 with some special interests for some high-ends which price seems not to be a limit for the buyer...

Eyepieces which price is rather high, to say over $200, which represent 20% of my list, tend to better perform as their price increase. The image is crisp, contrasty and the off-axis image is improved without presenting signifiant aberrations, sometimes even in scopes as fast as f/4.5.

This is not always true for medium-cost eyepieces. Many models below $200 tend to show aberrations like astigmatism, coma, colors or curvature of field near the edges, which are reinforced when used at shorter focal ratios, to say below f/6, or when they are designed with a too large field stop. Meade 24.5 mm SWA Plössl serie 4000 for example cost ~$180. This is modified Plössl that yields a field of 67°. This eyepiece yield good images at f/10 but is practically unusable in a f/5.4 dobsonian. 20% cheaper, the 24 mm Plössl LE ED from Takahashi is fine to the edge, luminous and yields crisper images without field curvature. One of the reasons comes from the fact the Meade has tried to circle a field 67° wide whereas the Takahashi limited its field to 52°, preserving the overall quality of the eyepiece.

As we told before, for a lesser price (below ~ $100) opticians cannot achieve complex eyepieces designs without sacrify some residual aberrations off-axis, mainly on wide field eyepieces. If you like exploring the Milky Way or the Moon surface with super wide eyepieces showing no residual aberrations at the edges of the field sometimes exceeding 84°, there is no compromise : you have to pay this quality as such characteristics can not be achieved easily by juxtaposing some lenses in a barrel. To create an excellent super wide eyepiece the optical designers will have to combine up to 6 or 8 lens-elements fully multicoated to correct most of residual aberrations and reduce internal reflections and scattering. The correction will depend on the refraction index of the lenses used, their design, their separation and type of coating, many factors which take time to assemble and create an unique model, proud of its manufacturer.

Tele Vue 3 mm Radian with Instadjust mechanism. Document Tele Vue.

In the lowest-cost range, to say between $30-$80, the quality depends on the manufacturer. In that category the problem is still worst when you try to use these low-costs in fast scopes. All them present aberrations off-axis and are practically usable. In that category the best to do is selecting designs famous for their performances, like Plössl. Models from Koenig University Optics, Atlas, Celestron, Meade, Orion or ProOptics to name a few are considered as "good" eyepieces, which I rate "medium overall merit". We can say these Plössls are excellent eyepieces for their price but an accurate examination in specific conditions reveals their limits (aberration off-axis, out-of-focus at the edges, lost of contrast, colors...). Between them the differences are subtle. For its price, the 20 mm from ProOptic for example is excellent, as good as a 25 mm Meade serie 3000. The 20 mm Atlas is even better than the Meade. But for sure compared to their competitors (Vixen, Tele Vue, Takahashi, Celestron, Clavé or Baader) these eyepieces yield a low contrast and their field is limited to 50°.

The horrible low-end eyepieces display many aberrations and a too small frontal lens to be useful. Forget them.

These considerations explain why some brands are much expensive than others. If you take in hand a 6 mm Tele Vue Radian for example and an ordinary Orthoscopic from Pocono you will appreciate the differences. The first one will present a comfortable eye relief of 20mm, use a large frontal lens which makes the observation easier, the weight of the assembly is not too heavy (380 gr), it uses an eyecup to preserve the observer from parasitic light and wind and at last it has a good looking and use a profiled screw in inox. It is also full serviced by the manufacturer. The Ortho on the other hand is very small and presents both a small frontal lens and eye relief. Looking at the sky the first one yields a crisp and contrasted image offering an apparent field of view 68° wide, the second barely 45°. The first cost $228, the second $50. The choice is quickly done if you like a image of quality and want to keep your eyepiece all your life long.

Of course this example is an extreme. Many eyepieces in the mid-range, below $150 perform very well and yield crisp images and are free of aberrations looking at both stars and planets in long focal scopes to name for example Antares, Vixen and even Tele Vue Plössl's. But remember aberrations become apparent when you place an object  at the edge of the field or when you try to detail a high contrasted object like a bright planet. Internal reflections and aberrations off-axis will more than probably arise.

I can say that I have used both high-end eyepieces (Tele Vue) or the defaut low-costs provided with the smallest scopes offered to a kid as the one displayed at left. In all cases I have appreciated looking the sky through these optics. My only wishes were always the same : be under a darker sky or using a larger scope, but it was never a question linked to my eyepieces, excepting the rare times I had to use the horrible low-ends like the modified Huygens, achromat (MA, MH) and other Plössl that show all a too small frontal lens and many aberrations.

If you invest at least in a mid-range eyepiece to get a quality item, I am sure that you will keep it all your life and you will find a subject to observe for each of them. There are so many eyepieces on the place that even mid-ranges will satisfy the needs of many advanced observers. All depends on your passion for practical astronomy and the time you want to dedicate to this hobby opposite to your other activities.

Next chapter

The brand argument

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