How to select
an eyepiece ?
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.
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 5mm 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 4mm and 6mm TV Radian eyepieces do not
exhibit this symptom".
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.
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.
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
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.
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.
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
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.
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.
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
RKE series, at left the modified Kellner's (3 lenses), at
right the 32 mm Wide Angle Erfle. Documents Edmund
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
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.
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 24.5mm 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.
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 lenses-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.
3 mm Radian with Instadjust mechanism. Document Tele Vue.
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
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
25mm 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°.
horrible low-end eyepieces display many aberrations and a
too small frontal lens to be useful. Forget them.
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.
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.
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.
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.