OAG - Off-axis Guider - a device used to allow manual
guiding of photos being taken through a telescope.
Objective The principle optical element of a telescope
that collects light and brings it to a focus.
Occultation An event that occurs when one celestial
body conceals or obscures another. For example, a solar eclipse is an
occultation of the Sun by the Moon.
Off-axis: used most frequently among amateur astronomers
to denote something that is not at the center of the field of view. In
optics, the "axis" is the axis of (symmetrical) rotation of
an optical surface, and therefore "off-axis" refers to something
not along this line.
Off-Axis Guider - An astrophotographic accessory featuring
an off-axis prism that intercepts unused light and uses this light to
transmit images of guide stars.
Off-Axis Masks - Masks that stop down the aperture of
a reflecting or catadioptric telescope. Their apertures are located in
an off-axis position to avoid obstruction by the telescope's secondary
Offsetting - the procedure of finding objects in the sky
by noting the movements necessary in an equatorial mount to find the object
(e.g.2 degrees north and sweep to the west)
Oort Cloud A theoretical shell of icy bodies (comets)
that is believed to exist at the outermost regions of our solar system.
The Oort cloud was named after the Dutch astronomer who first proposed
Open Cluster A collection of young stars that formed
together. They may or may not be still bound by gravity. Some of the youngest
open clusters are still embedded in the gas and dust from which they formed.
Opposition The position of a planet when it is exactly
opposite the Sun as seen from Earth. A planet at opposition is at its
closest approach to the Earth and is best suitable for observing.
Optically-ground - a description referring to any optical
surface ground or polished to a high standard of accuracy. In a filter,
it refers to having the two sides of the optical window polished to a
flatness not found in normally-created glass.
Orbit The path of a celestial body as it moves through
space, in relation to a much larger body around which it moves.
Orthoscopic Free of distortion, a type of eyepiece designed
by Ernst Abbe in 1880. This eyepiece employs a triplet field lens and
a singlet eye lens. It excels in planetary observing: a lens that is free
of spherical aberration, and magnifies an image uniformly throughout the
field (i.e., the lens satisfies the tangent condition in which the ratio
of the tangent of a' to the tangent of a is a constant for every ray on
the lens). For a description of the "orthoscopic" eyepiece,
see Abbe orthoscopic. Usually limited to a very small apparent field of
OTA - Optical Tube Assembly - a telescope minus the mount.
Oxygen III (O-III) Filter - A type of nebular filter
that isolates just the two doubly ionized oxygen lines at 496 and 501nm
Paraboloid The shape given to the primary mirror of
a Newtonian or Classical Cassegrain telescope.
Paracorr: a coma-correcting lens by TeleVue. It is not
the same as a Ross coma corrector. It is composed of two doublets, and
is designed for good axial and lateral color correction and coma correction,
with reasonable control of spherical aberration. See also coma.
Park's Gold Series Plossl: A Japanese-made version of
the five-element Pl?ssl, with a 52 degree apparent field. Excellent.
Parfocal: two eyepieces, the focal planes of which are
the same distance from the barrel lip on which the eyepiece rests when
in a focuser, are said to be parfocal. Many people note that, for example,
a 12mm and an 8mm eyepiece in a supposedly "parfocal" line focus
at slightly different positions for them. The reason is that the eye's
lens exhibits spherical and other aberrations. As a result, pupil size
affects the position of best focus for the eye. When reducing the pupil
size by going to the shorter eyepiece, less aberration is encountered,
and the focus point shifts accordingly. In addition, there is greater
focus accommodation with longer focal length eyepieces, further obscuring
an eyepiece set's true parfocality.
Parsec The distance an object lies from the Earth to
exhibit a parallax of one arc-second from one end of the Earth's orbit
to the other. A parsec is equal to 3.26 light years.
PE - Periodic Error (for Equatorial Drives). Tracking
errors caused by the imperfections inherent in any worm gear system. These
errors repeat every rotation of the worm, hence Periodic. See also PEC.
PEC - Periodic Error Control or Correction. Records
corrections made during RA tracking, either by hand or autoguider; when
activated, PEC replays the corrections, reducing periodic error (PE).
Mount either does or does not retain the adjustments after powering down.
Perigee The point in the orbit of the Moon or other
satellite at which it is closest to the Earth.
Perihelion The point in the orbit of a planet or other
body where it is closest to the Sun.
Phase The apparent change in shape of the Moon and inferior
planets as seen from Earth as they move in their orbits.
Photopic - daytime vision. The cones in the eye are turned
on and we have color vision.
Photosphere The bright visible surface of the Sun.
Pickering Scale of Seeing - a ten point scale of degrees
of flickering of the star image in a telescope due to turbulence in the
Piggyback Photography - Wide Field astrophotography where
the camera is piggybacked on top of a telescope, and the photo is taken
through the camera's normal lens.
Plages - a bright and intensely hot spot in the Sun's
chromosphere, usually associated with a sunspot.
Planet A large body in orbit around a star. Planets
can be composed mainly of rock, ices, or of dense gases. Hydrostatic equilibrium
has been overcome by gravity and the object has been pulled into a nearly
spherical shape. Additionally, the body must have swept up most of the
debris in its orbital zone so that all that is left is a minor amount
of mass not exceeding 1% of the mass of the planet.
Planetary Filters - Color Filters used to enhance light
and dark features of the moon and planets.
Planetary Nebula A shell of gas surrounding a small,
white star. The gas is usually illuminated by the star, producing a variety
of colors and shapes.
Planisphere - a circular device that allows one to tell
at a glance which constellations are above the horizon at a specific day
Plossl A popular, high-performance eyepiece employing
two cemented doublet lenses. The original Plossl eyepiece was symmetrical
in design. The most traditional design seems to be one in which all air-glass
surfaces are convex and the crown elements face one another, which minimizes
aberrations at the exit pupil and minimizes distortion. The design is
easy to make, as the spacing between the groups is not critical. As the
spacing of the groups increases, focal plane curvature decreases and astigmatism
increases. It is therefore possible to make an orthoscopic Plossl, but
only at the expense of pronounced off-axis aberrations. There are also
designs in which the external flint surfaces are plano. The asymmetrical
Plossl designs, assumed to be variants on the traditional symmetrical
design, may be better corrected than the symmetrical, depending on the
design and execution, but there is no inherent advantage to an asymmetrical
Plossl. This TeleVue design reduces astigmatism at the edge of the field
and provides some correction for coma, at the expense of slight distortion
in the field and exit pupil aberrations. Plossls, including variants,
offer eye relief about 0.7x their focal length.
Plossl, George Simon: born September 19, 1794 near Vienna.
In May 1812 he began work at the Voigtlaender optical firm. Supported
by J.F. Jacquin and J.J. Littrow he studied mathematics and optics, founding
his own firm in 1823 in his parent's house in Vienna. He encountered severe
capital problems starting his firm but moved to much larger shops in 1831
and again in 1835. He made telescopes of the dialytic type, selling a
10.5" in 1850 and several other instruments of 6" to 8"
aperture. He died as the result of an injury incurred dropping a sheet
of glass, which severed an artery in his hand, causing blood loss and
gangrene. He died January 30 1868 in the same house where he was born.
His business was carried on as Fa. Pl?ssl & Co. by M. Wagner until
the last shop was closed in 1905. In 1875 there was an unrelated shop
named Pl?ssl in Vienna. George Pl?ssl is best known today as the designer
of the Pl?ssl eyepiece.
Polar Alignment - The alignment of an equatorial mounting's
polar axis parallel with the axis of rotation of the Earth.
Polar Axis - The axis of an equatorial mounting intended
for alignment with the North Celestial Pole and thus with the rotational
axis of the Earth, enabling a telescope to track celestial objects without
rotating the field of view.
Polaris - In the northern hemisphere, the bright star
that is nearest to the north celestial pole and whose altitude above the
horizon describes ones latitude.
Polarizing Filters - A Neutral Density Filter using polarized
glass. Some polarizing filters use two polar filters that can be rotated
to produce variable amounts of filtering - these are known as Variable Polarizing
Porro Prism - A erecting prism system that corrects the
inverted image of binoculars and refracting and catadioptric telescopes
by simply rotating the field 180ˇă and leaving its left-to-right image
Positive Lens Projection Photography Projection photography
in which a positive lens magnifies the image formed by the telescope objective
and projects the image into the camera. Also called Eyepiece Projection
Power A measurement of magnification. Power can be calculated
by dividing the focal length of a telescope by the focal length of an
Powermate: a telenegative lens by TeleVue. They bear
some similarity to Galilean telescopes. The Powermate eliminates (or nearly
eliminates) the effect that a traditional Barlow lens has on eye relief
(see Barlow lens). The distance between the element and the ocular does
not increase image scale in the same way as with traditional Barlows.
Presbyopia - "Old Vision". What happens when, as we
age, the lens in the eye stiffens up and the muscles in the eye can no
longer force the lens into a shape allowing focus on near objects. People
who hear this word for the first time are usually getting their first
reading glasses. It's a sign of wisdom
Primary - the large mirror or lens in a telescope that
forms the primary agent for the gathering of light.
Prime Focus Photography A photographic technique that
substitutes the telescope objective for the lens of the camera, focusing
the image formed by the objective directly onto the film. Also called
Direct Objective Photography.
Prism Diagonal: a diagonal assembly where the light
is diverted 90 degrees by the use of a prism, rather than a mirror. The
feat is achieved by taking advantage of the critical angle to induce total
internal reflection. Some prism types can transmit as much light as a
good mirror diagonal, but all introduce some chromatic aberration and
other aberrations depending on the focal ratio of the telescope. These
effects are more pronounced in fast (short f/ratio) optical systems. In
addition, in fast systems, the critical angle may not be met for positions
on the prism that are inside the observable field of view. A tilt error
in a prism diagonal (i.e., miscollimation of the diagonal) will result
in a ray deviation of twice the tilt, as is expected. Additional problems
arise because a prism diagonal has 3 optical surfaces. Typically, only
found in lower-priced diagonals.
Prominence An explosion of hot gas that erupts from
the Sun's surface. Solar prominences are usually associated with sunspot
activity and can cause interference with communications on Earth due to
their electromagnetic effects on the atmosphere as the charged gas streams
past the Earth.
Protostar Dense regions of molecular clouds where stars
Pulsar A spinning neutron star that emits energy from
a spinning part of its surface. This energy is received as pulses as the
star rotates, often thousandths of a second.
Pyrex A Trade Name for a glass having a low coefficient
of expansion due to high borosilicate content.
Quadrature A point in the orbit of a superior planet
where it appears at right angles to the Sun as seem from Earth.
Quasar An unusually bright object found in the remote
areas of the universe. Quasars release incredible amounts of energy and
are among the oldest and farthest objects in the known universe. They
may be the nuclei of ancient, active galaxies. There is some evidence
this is a phase in the creation of super-massive black holes that form
in the centers of galaxies.