How to select an eyepiece ?
brand argument (VI)
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.
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.
prevails, the eyepiece
or the scope quality ?
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 ?
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...
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.
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
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.
"best" eyepiece does it exist ?
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.
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
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.
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.
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.
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.
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.