Eyepiece Characteristics: The various eyepiece types we carry are described
below, with our subjective opinions of their performance in the following areas:
Number of lens elements: typically, more lenses
in an eyepiece mean sharper images, and/or a wider field of view, and/or longer
eye relief . . . although often with a loss of contrast compared to simpler eyepieces.
Sharpness: how well the eyepiece
concentrates the light of a star into a sharp point at the center of the field.
Astigmatism: an aberration that
turns stars into fuzzy oblongs instead of points, particularly towards the edge
of the field.
Color correction: how free an eyepiece
is from colored halos around stars at the edge of the field, false color in planetary
images, or stars that change color as they move from the center to the edges of
Ghosting: a flare of unwanted light
around bright objects, or faint multiple images of bright objects, due to internal
Field curvature: an inability to
bring the center and edge of the field into focus at the same time, with the edge
out of focus when the center is sharply focused and vice-versa.
Brandon: four lenses; medium to
high power use with any telescope at focal ratios down to f/4; very sharp at the
center (somewhat less so at the edges) for lunar, planetary, star cluster, and binary
star observing; excellent color correction and contrast; very low astigmatism; the
sharpest design currently available for eyepiece projection photography; very minor
field curvature and ghosting; standard equipment on ultra-premium Questar Maksutovs.
Modified Achromatic, Super Modified Achromatic:
computer-optimized versions of the three-lens design known as a Kellner (or achromatic
Ramsden) that is supplied with many inexpensive scopes; for low to medium power
use on telescopes with focal ratios down to f/6, but best above f/8; for general
observing for the astronomer on a budget; acceptable to good sharpness at the center
of the field with some astigmatism at the edge; good color correction; moderate
ghosting; field curvature is noticeable, emphasizing the coma in fast focal ratio
reflectors; telescope focal plane usually lies on the surface of the eyepiece lens
nearest telescope (the field lens) so that any dust on the lens is in sharp focus,
requiring extra care in keeping the eyepiece clean.
Vixen Lanthanum: six to eight lenses,
using a lanthanum (rare earth glass) field lens; exceptional 20mm eye relief for
eyeglass use, even in the exceptionally short 2.5mm focal length version; very short
focal lengths use a built-in Barlow to achieve high power without reducing eye relief;
optical performance approaches that of a Plössl at the center of the field (less
so at the edges and with generally lower contrast), but with longer eye relief.
Nagler, Nagler 2, Ultra Wide Angle:
Naglers have seven lenses, Nagler 2 and Ultra Wide have eight; up to three times
the field area of a Plössl, but sharper edge to edge; up to ten times sharper at
the edges than older wide angle designs such as Erfles; field is so wide you have
to move your head from side to side to see all of it; more like looking out a window
into space rather than looking through an eyepiece; superb color correction, with
little ghosting or field curvature; some loss of contrast on planets due to many
lens elements; short focal lengths use a built-in Barlow to achieve high power without
reducing eye relief; medium to high power use down to f/4; for star clusters, nebulas,
and galaxies with any scope, but particularly good with fast focal ratio reflectors.
Panoptic: six lenses, with an ED
(extra low dispersion glass) lens for exceptional contrast and a very wide field;
up to five times the field edge correction of older wide field designs; for low
to medium powers down to f/4 with any telescope, although particularly good with
fast focal ratio reflectors when combined with a Paracorr coma corrector; use of
the 27mm or 35mm with a TeleVue 2" Big Barlow requires a Panoptic interface lens
for the best performance, yielding an edge sharpness and lack of astigmatism that
is comparable to a Nagler.
XL: proprietary Pentax wide field
designs using five to seven lenses; very long eye relief; for low to very high power
use down to f/4; all are fully Super Multi-Coated (SMC) and all use an ED (extra
low dispersion glass) lens to minimize chromatic aberrations and astigmatism; usable
for all types of observing, with all telescope types; good contrast and excellent
center to edge sharpness; very little ghosting.
Plössl, Super Plössl: four lenses;
from very low to high power use with all telescope focal ratios down to f/4; for
lunar and planetary observing, nebulas, open clusters, or anywhere edge-to-edge
sharpness and very good contrast is more important than an extra-wide field; excellent
color correction; virtually no field curvature, astigmatism, or ghosting; work very
well with any telescope type.
Wide Wide, Super Wide Angle: six
lenses; 70% greater field area than a Plössl, and a Plössl’s edge to edge sharpness,
but with somewhat less contrast; low to medium power down to f/4; for wide angle
views of open clusters, large nebula remnants, clusters of galaxies, Milky Way star
fields – wherever a very sharp, very wide field is called for; excellent color correction;
virtually no ghosting or field curvature.
Takahashi: proprietary fully-multicoated
five lens design with long eye relief; for low to very high power use to f/4; two
shortest focal lengths use one ED (extra low dispersion glass) lens for exceptional
contrast and color correction; usable with all scope types; suitable for all types
of observing; very good sharpness and contrast; very little ghosting.
four to seven lenses; short focal lengths use a built-in Barlow to achieve high
power without reducing eye relief; for low to very high power use down to f/5, although
with some field curvature at lower focal ratios; usable for all types of observing
with all scope types; very good contrast, sharpness, and color correction, with
only minor ghosting.