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How to select an eyepiece ?

The brand argument (VI)

Incidentally, many medium price eyepieces hold their own against so-called high-ends, to name e.g. University Optics Konig Orthos vs Tele Vue Plössl for example. For many amateurs the first brand sells eyepieces supporting easily the comparison with models available to the second, which price is 2 or 3 times higher. This example is significant. Take an extreme example.

For an identical barrel, the 32 mm Tele Vue is cheaper than the UO equivalent model. Is this a good argument to buy the first and ignore the second ? Surely not.  Are they not both equivalent ? In the field it appears that the 32mm 2" from UO yield a better image than its 1.25" model. But worse, the 1.25" is less good than Erfle's. Even if the 32mm from Tele Vue displays the largest frontal lens in its category it looses some contrast and sharpness at the edges. Both yield in fact less good images than a 30mm Celestron Ultima and it even seems that the 32mm Celestron Nexstar and the 32mm serie 4000 from Meade are all better. So this comparison demonstrates that the fame of a brand or a short comparison between two models is insufficient to appreciate the quality of an eyepiece. You have to test as many similar models as you can to really appreciate their value.

What prevails, the eyepiece or the scope quality ?

Is it more important to use a medium quality eyepiece in a scope of high quality or to use an excellent eyepiece in a standard scope of commercial quality ?

One can say in a few words that the quality of the optical tube assembly (OTA) matters a whole lot more than any high-tech eyepiece. It arrived in the past that an experienced observer forgets his eyepieces collection and ended up observing the Moon through a large dobsonian using the magnifying glass on his swiss army knife as an eyepiece ! He reported excellent results what astonished quite a lot the newsgroup readers. But it is a fact, as we told before, that your system is as good as its weakest part : using an excellent eyepiece in a scope showing aberrations or distorsions will never improve its intrinsec quality excepting if a master optician think to create a corrective relay lens specifically designed for that scope, as they did for the Hubble Space Telescope. But I don't think that many of us could bought that patent...

Now, on the other hand, eyepieces are much cheaper than telescopes and the 100 dollars question arises to know if some money invested in eyepieces is more profitable and will gain you more than if you invest this amount in a telescope of better quality.

You are the only person able to appreciate what prevails between the quality of a scope, which has a direct impact on the price, and the aperture. Here are some models between 5" and 16" : from left to right a Takahashi fluorite doublet FS 128 mm f/8.1 ($5300), a Celestron C8 CPC Deluxe EdgeHD ($2400) and a Meade 16" f/4.5 Starfinder Dobsonian ($1200) among hundreds of other models. In each category (refractor, catadioptric and Dobs) prices vary from a factor 5 to 10 depending on the accessories and their quality (mount, GoTo, eyepieces, ...). Documents from the manufacturers.

Knowing the range of prices of both optics, whatever your point of view, the answer is in favor of eyepieces. A quality aprochromat refractor costs over $5000 and maybe 50% more abroad. For a larger scope the price increases from 50 to 300%. In this example for a difference of $1000 you can get 2 to 4 high-ends eyepieces from Tele Vue, Zeiss or Leitz providing an exceptional field of view over 80° wide. The difference is still more important if you change from a 6" newtonian to a 7" APO for example ; with the difference you could buy in once nearly all mighty eyepieces !

But there is one thing we have to remember. A large 300 mm dobsonian for example costs about the same as a small 80 mm refactor from Astro-Physics. It is obvious that what you gain in luminosity and resolution could never be equalled with a refractor four times smaller which image will be totally blown out by the large scope, which picture is 16 times more bright. Knowing that gaining 25% of luminosity is already visually appreciable you will understand that 1600% more light is so huge that is almost useless to even think to compare both scopes.

The "best" eyepiece does it exist ?

By design all eyepieces are sensible to optical aberrations due to the nature of light and their design, in particular the refractive index of the optical glass which varyies with the wavelength and the light beam position entering the eyepiece (incidence angle and off-axis position). All these variables may produce well-known aberrations as chromatic, coma, spherical, astigmatism, field curvature and geometrical distortions.

Knowing the resources needed in terms of time, knowledge and money to reduce or suppress these aberrations, it is not a surprise if dozen of manufacturers, most of them from Taiwan prefer investing in the immediate profit than in quality eyepieces. European and US manufacturers are also involved in this practice as sleeping parter of these inventions. They also balance their profit and the time consumming that requests a specific design. Quality control is also a major factor they consider when creating an object.

Amateur astronomy represents a very narrow market for professionnals whose accessories interest probably a few hundreds of thousands amateurs, a straw for the commercial people who have to convince you they sale the best accessories and eyepieces.

To help you selecting your scope and its accessories, here are two useful links : Cloudy Nights Telescope reviews and Sky & Telescope that list or have tested a great number of astronomical items for both casual or advanced amateurs. Scopereviews and Astronomy-Mall are also two other useful sources of information very appreciated.

To convince is the correct word. A beginner or a casual observer doesn't necessary know how many brands exist and the diversity of eyepieces available on the place, and still lesser the quality of each of them.

My own inventory lists over 530 eyepieces but their number is probably till higher and most of them were only tested at only one f/ ratio and not necessary on all celestial objects. A casual amateur is forced to trust his dealer and believe usually what he says as the words of the Holy Bible. If I willingly encourage dealers to advice their customers, we have to be critical too and buy our accessories knowing their potential and their limits.

The best way to know the quality of eyepieces and before you invest your savings in any optical system, I warmly suggest you to participate in star parties and discuss with specialists to check if the amount of money you want to invest worth well the gain you are expecting. Another way is to read all test reports you can in magazines or forums in order to get a critic as complete as possible of each model under various conditions of use, both in laboratory and in situ.

If you reached this page you understand now that famous brands do not guarantee that the eyepiece you want to use is really suited to your scope. You have to evaluate its performance according to your own parameters : scope f/ratio, sky brightness, eye pupil, need of long eye relief, accessories in the ray path, subjects of interest, casual or advanced observer, etc. At last whatever your overall appreciation it will be maybe determined by the price factor.

For eyepieces, in most cases the image quality is worth well the expense. Nobody will never tell you that you must search for the high-ends on your dealer shelves but it is also sure that low-end eyepieces are suited to low-end scopes, and it should be disastrous to destroy your image by using an guenine eyepiece badly corrected.

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