RA - Right Ascension - The number of hours East of the
Vernal Equinox in the sky, with 24hrs = 0 hrs. 1 hour = 15 degrees.
RACI - Right-Angle Correct-Image finder - flips image
to correspond to what the eye sees unaided. Aids in locating objects when
using charts (stars are oriented correctly - not flipped left-right and
up-down as in regular finders).
Radian: a TeleVue eyepiece. Exhibits inherently less
kidney bean than earlier Nagler designs.
Ramsden - a simple two-lens eyepiece design invented in
the 18th century by Jesse Ramsden. It is a poorly corrected design of
exceptionally narrow field of view, and is not recommended for use in
Rayleigh Limit - In optical systems, the aberration tolerance
of 0.25-wave peak-to-valley and applied to path differences in the converging
RC - Ritchey-Chr¨¦tien Telescope
RDF - Red Dot Finder [Form of Unity or Unit-power (1x
magnification) finder], so-called because it creates the illusion of a
red dot floating among the stars.
Rectilinear distortion: a condition in which straight
objects appear to be curved in the eyepiece. For zero rectilinear distortion
the eyepiece must satisfy the y=f*tan(b) relationship, in which y is the
off-axis distance in the focal plane, b is the image angle from the optical
axis, and f is the focal length of the eyepiece. The curve is not as important
in astronomical observation as the angular magnification distortion which
Red Giant A stage in the evolution of a star when the
fuel begins to exhaust and the star expands to about many times its normal
size. The temperature of its surface cools because of the larger size,
which gives the star a reddish appearance.
Reflecting Telescope An optical system where light is
bent with a curved mirror.
Reflector (reflecting telescope) A telescope that uses
a concave mirror to gather light and form an image at a focal plane.
Refractive Index: the ratio of the velocity of propagation
of an electromagnetic wave in a vacuum, to its velocity in a medium. Simply
put, it is a measure of how much a particular substance bends light.
Refractor (refracting telescope) A telescope that uses
a transparent objective lens to refract, or bend, light that passes through
it in order to form an image at the focal plane.
Resolution - The ability of an optical instrument to
show fine detail.
Reticle Eyepiece - An eyepiece with cross hairs of any
of a number of patterns, used for guiding on stars in long-exposure astrophotography.
Retina - The back surface of the interior of the eye where
light is received and sent on to the brain. It contains rods (sensitive,
black and white, sensors), and cones (less sensitive, color, sensors).
Retrograde Motion The phenomenon where a celestial body
appears to slow down, stop, then move in the opposite direction. This
motion is caused when the Earth overtakes the body in its orbit.
Rich Field Telescope A low-magnification telescope designed
to reveal as many Milky Way stars in the field of view as possible.
Right Ascension - The equivalent to longitude in the
Celestial Sphere locating an object in the east-west directions. . Celestial
coordinates are listed in terms of Right Ascension and Declination.
Ring Galaxy A galaxy that has a ring-like appearance.
The ring usually contains luminous blue stars. Ring galaxies are believed
to have been formed by collisions with other galaxies.
Ritchey-Chretien - A Cassegrain telescope variation
incorporating a hyperbolic primary mirror and a strongly hyperbolic secondary
RKE: an eyepiece that is derived from the Kellner design,
with the difference that the field group is of two elements instead of
the eye lens, and by the addition of low dispersion glasses. Exhibits
a moderately curved field and moderate distortion, and less off axis astigmatism
than the traditional Kellner. The eyepiece was designed by David Rank,
and the name RKE is variously explained as being "Rank-Kellner Eyepiece,"
"Revised Kellner Eyepiece," etc. Kellner actually described
the design, and it is sometimes called a Kellner Type II.
RMS Wavefront: a reference to the root-mean-square of
the wavefront error of an optical system. Since measuring protocols for
the RMS wavefront weight the severity of the wavefront error areally,
this measurement is much more useful than a peak-to-valley wavefront error.
Roof Prism - A compact prism with 5 internal reflections
used to produce a correctly oriented image, usually collinear with the
Ross Corrector: a coma correcting lens system for Newtonian
telescopes. It was invented by Frank Ross in 1935 at Mt. Wilson Observatory.
The Ross corrector is composed of three lenses placed close to the focal
plane. It has no optical power; i.e. it does not change the focal length
of the telescope. It makes the coma-free field of view about a half degree
in diameter at f/5. It is known for an increased spherical aberration,
not important photographically, but not the best visually.
Rotation The spin of a body about its axis.
Satellite A natural or artificial body in orbit around
SB - Surface brightness
Schmidt-Cassegrain telescope: a telescope design comprising
a spherical primary mirror, a full-aperture corrector plate, and a negative
secondary in a Cassegrain configuration. The common commercial design
is one of several possible configurations. SCT's exhibit curved focal
planes, and this can become evident in very wide-angle eyepieces; when
the eyepiece is focused for the center, the extreme edges of the field
may show small defocus donuts. Most commercial SCT's use mirror movement
for focusing; this accommodates terrestrial use and allows the indiscriminate
addition of accessories onto the back of the telescope. For object distances
of less than about 60 feet, moving the mirror is preferable to moving
the eyepiece to focus, but the side effects of moving the mirror include
a change in the focal length of the system and a change in its correction.
Schmidt-Newtonian Telescope - a Newtonian telescope containing
spherical mirrors and a correcting, full-aperture, lens at the front of
Scotopic - The human eye's vision when dark adapted.
The pupil is dilated and color detection is traded for light sensitivity.
The eye gains a sensitivity over 90,000 as great as the daytime (scotopic)
Scotopic Shift - the movement of the frequency of greatest
eye sensitivity from the green to the blue as our eyes adjust to the dark.
SCT - Schmidt-Cassegrain Telescope
Secondary - the second, smaller, mirror in a multiple-mirror
optical system. Usually, the small flat mirror that reflects the light
from the large mirror in a Newtonian telescope out the side of the tube
to the focuser.
Secondary Spectrum Residual chromatic aberration present
in ordinary doublet lenses.
Seconds of Arc - see Arc-second
Seeing - a word used to describe the steadiness, or lack
of turbulence, of the air. This impacts the telescope's ability to clearly
focus on small details. It limits the resolution of larger telescopes.
Semi-apo: a refractor that approaches apochromatic performance.
Typically a semi-apo refractor has a doublet objective comprising at least
one exotic glass, often ED glass. The performance will at least be considerably
better than that of an achromat and in longer focal ratios can approach
the correction of a faster apochromat, assuming both are good designs,
Setting Circles - A pair of graduated disks on telescope
mountings that simplify locating celestial objects by giving a readout
of where the telescope is pointed.
Sharpness - that quality of focus in an optical system
that makes it possible to see finer details and more levels of contrast.
Often related to the optical quality of the optical system.
Sidereal Of, relating to, or concerned with the stars.
Sidereal rotation is that measured with respect to the stars rather than
with respect to the Sun or the primary of a satellite.
Sidereal Month The average period of revolution of the
Moon around the Earth in reference to a fixed star, equal to 27 days,
7 hours, 43 minutes in units of mean solar time.
Sidereal Period The period of revolution of a planet
around the Sun or a satellite around its primary.
Sidereal Time Time measured by the diurnal motion of
Sight Tube - a collimation tool consisting of a long tube
with crosshairs at one end and a small centered peep hole at the other.
This tool is used to align a secondary mirror in a Newtonian telescope.
Singlet: slang for "single-element," in other
words, a single lens, as opposed to a multi-element group.
SN - Schmidt-Newtonian Telescope
Sol Originally a Roman God of the Sun, it is the name
given to our sun.
Solar Cycle The approximately 11-year quasi-periodic
variation in frequency or number of solar active events.
Solar Eclipse A phenomenon that occurs when the Earth
passes into the shadow of the Moon. A total solar eclipse occurs when
the Moon is close enough to completely block the Sun's light at some point
on the surface of the Earth. An annular solar eclipse occurs when the
Moon is farther away and is not able to completely block the light. This
results in a ring of light around the Moon.
Solar Filters Filters that allow safe viewing of the
sun through a telescope.
Solar Flare A bright eruption of hot gas in the Sun's
photosphere. Solar prominences are usually only detectable by specialized
instruments but can be visible during a total solar eclipse.
Solar Prominence - An eruption of relatively cool, high-density
gas from the solar chromosphere into the corona.
Solar Prominence Filters - A specialized solar filter
that reveals prominences by selecting the light of the hydrogen-alpha
line in the spectrum. This is the frequency given off by hydrogen when
its one electron falls back from its excited orbital into its lowest energy
Solar System - The objects that are all in orbit around
out sun, all the way from dust to planets.
Solar Wind A flow of charged particles that travels
from the Sun out into the solar system.
Solstice The time of the year when the Sun appears furthest
north or south of the celestial equator. The solstices mark the beginning
of the Summer and Winter seasons.
Spectrohelioscope A spectroscope equipped with a synthesizer
that produces a narrow-bandpass image of the sun's disk in hydrogen-alpha,
hydrogen-beta, sodium, calcium or other selected wavelengths of light.
Spectrometer The instrument connected to a telescope
that separates the light signals into different frequencies, producing
Spectrum Light spread out into individually visible
frequencies by use of a prism. Or, the continuum of all frequencies of
Spherical Aberration The failure of an optical system
to focus light of a given wavelength from all parts of the objective to
the same point on the optical axis: the inability to focus axial and paraxial
ray bundles that are parallel to the axis at a single point in the image
plane. Peripheral rays focus closer than more central rays. This creates
a fuzzy image that never snaps into a sharp, clean focus.
Spherical Aberration of the Exit Pupil: it is worth
noting that this problem does not affect the sharpness of the image, but
may lead to the kidney-bean effect. See Kidney-Bean Effect.
Spicule - A short-lived spike-like solar prominence.
Spider Diffraction - Diffraction of light by the secondary
support vanes of a reflecting telescope, resulting in bright spikes that
radiate from the center of small, bright images. The brightness of the
diffraction is directly related to the area of obscuration of the incoming
light by the vanes themselves.
Spiral Galaxy A galaxy that contains a prominent central
bulge and luminous arms of gas, dust, and young stars that wind out from
the central nucleus in a spiral formation. Our galaxy, the Milky Way,
is a spiral galaxy. Apparently, the orientation of the ovals of the stars'
orbits around the center determines whether we see the spiral as a normal
pinwheel or a Barred spiral. During the life of a galaxy, precession of
the nodes of those orbits will change a normal spiral into a barred and
Spot Diagram: a two-dimensional representation of where
a collection of rays focused by the objective cross the plane of best
focus. The spot diagram is the "head on view" given by ray tracing
(rather than the side view, which shows a cross-section of the instrument).
Spot diagrams ignore the effects of diffraction and usually seek only
to provide a graphical representation of monochromatic aberrations at
Spotting Scope - a small telescope optimized for the viewing
of land and sea objects in the daytime. Usually a small refractor, and
usually possessed of a zoom eyepiece that allows the changing of magnifications
without changing eyepieces.
Stack - An image created from several frames (sub-exposures),
which have been aligned with image processing software. The process is
Star A giant ball of hot gas that creates and emits
its own radiation through nuclear fusion in its core.
Star Cluster A large grouping of stars, from a few dozen
to a few hundred thousand, which are bound together by their mutual gravitational
Star Diagonal An accessory that is often used on refracting
and Cassegrain telescopes to bend the light cone at right angles for more
Star Hopping - a procedure of telescope use wherein new
objects are found by hopping from a known star to another star nearer
the target, and so on, until the object is located.
Stationary Photography - taking photographs of the night
sky with a non-moving camera in order to see the movement of stars in
the night sky.
Stellar Having to do with Stars
Stellar Wind The ejection of gas from the surface of
a star. Many different types of stars, including our Sun, have stellar
winds. The stellar wind of our Sun is also known as the Solar wind. A
star's stellar wind is strongest near the end of its life when it has
consumed most of its fuel. The core is hotter, the atmosphere larger and
more tenuous, thus more easily blown away from the star.
Steinheil doublet: an achromatic telescope objective
in which the negative element comes first. Stronger curves than the Fraunhoffer
doublet are needed to make such an objective.
Strehl Ratio: According to Suiter, "the Strehl
ratio is defined as the intensity of the image spot at its central brightest
point divided by the same image intensity without aberration." (Star
Testing Astronomical Telescopes, pg. 9). Put more simply, it is the fractional
degradation of the peak of the theoretical diffraction intensity. Put
even more simply, it is a measurement of the amount of light put into
the peak of the image spot in an actual telescope, compared to that put
in the spot of a perfect telescope. Therefore a Strehl ratio of 100% would
constitute a perfect telescope; 98.5% or so constitutes an extremely good
telescope, perhaps as perfect as can be made; 94% begins to be a mundane
telescope. The measurement is more meaningful for very high quality telescopes
than for mediocre ones. For example, since a perfect Airy Disk contains
83.8% of the diffraction pattern's total energy, then if an Airy Disk
contains only 79.6% of the total energy, the Strehl Ratio is 0.95. Also
called Strehl Intensity or Encircled Energy Ratio. A Strehl ratio of 0.80
meets the +/- ? wave criterion.
Strained Optics - Optics that have become pinched in
Sundogs - reflected images of the sun from high-altitude
ice crystals that precede and follow the Sun in the sky. This is usually
a sign of moisture in the high atmosphere, as well as extremely cold temperatures
Sunspot Areas of the Sun's surface that are cooler than
surrounding areas. The usually appear black on visible light photographs
of the Sun. Sunspots are usually associated disturbances in the Sun's
electromagnetic field. There is always a + and a ¨C magnetic polarization
in every pair of sunspots, as they are created by the ¡°puncturing¡± of
the surface of the sun by magnetic field lines.
Super Plossl: generically, a version of the five-element
Supergiant The stage in a giant star's evolution where
the core contracts and the star swells to about five hundreds times its
Superior Conjunction A conjunction that occurs when
a superior planet passes behind the Sun and is on the opposite side of
the Sun from the Earth.
Superior Planet A planet that exists outside the orbit
of the Earth. All of the planets in our solar system are superior except
for Mercury and Venus. These two planets are inferior planets.
Supernova - the explosion of a very massive star as the
material in its core burns out in the end of its life, or the explosion
of a white dwarf star caused by the infall of huge amounts of material
from a nearby giant star. These are the most violent explosions in our
universe. A single star can briefly be as bright or brighter than hundreds
of billions of normal stars. We can detect these at the edge of the visible
Universe, in places where the galaxies in which they reside are not visible.
All of the heavy elements were created in supernova explosions.
Supernova Remnant An expanding shell of gas ejected
at high speeds by a supernova explosion. Supernova remnants are often
visible as diffuse gaseous nebulae usually with a shell-like structure.
Many resemble "bubbles" in space. They usually only exist for
a few thousand years (or a few tens of thousands) before the materials
ejected merge with the galactic milieu.
Surface Accuracy: a reference to how true to its design
an optical surface is. When light is reflected, the amplitude of surface
inaccuracies is doubled. For refracting surfaces, the error is proportional
to its index such that more highly refracting materials have smaller surface
tolerances. In general, the tolerances for refracting surfaces are more
forgiving, often far more so, than that for reflecting surfaces. The more
optical elements present, the tighter the surface accuracy tolerances
must be to provide acceptable performance. For a surface that light passes
through, like a lens, the surface error only creates that amount of error
in the wave passing through. But a surface that reflects light doubles
the error of its surface in the wavefront of the light reflected.
Surface Brightness - the calculated brightness of an extended
deep-sky object in square arc-seconds or square arc-minutes. It is based
on the overall size of the object and its overall total integrated magnitude.
This figure is important to determine how long an exposure to take of
the object, and to determine its visibility in the telescope. The surface
brightness figure is almost always lower than the total integrated magnitude
of the object.
Surface Reflection: in lenses, refers to how much light
is reflected from an air-glass surface. The higher the index, the higher
the reflection. Reflection increases rapidly as the angle of incidence
approaches and exceeds 35 or so. Glass is nearly 100% reflective at a
45 degree angle.
Swan Band - a swath of the electromagnetic spectrum wherein
most comets emit their light.
Symmetrical: an eyepiece exhibiting symmetry in its
element configuration. The most common symmetrical are Pl?ssl eyepieces,
which comprise two identical achromats facing one another.
T-Ring Adapter - A camera accessory for SLR type cameras.
It replaces the removable camera lens assemble so that the camera can
be attached to the optical line of the telescope.
T-Thread - A photographic industry standard screw thread
to attach a telescope's T-Ring to a camera's T-Ring Adapter. The "T"
stands for telescope.
Telescope An optical instrument with lenses, mirrors,
or a combination thereof used to collect large amounts of light from far
away objects and increase their visibility to the naked eye. Telescopes
can also enlarge objects that are relatively close to the Earth or on
Terminator The boundary between the light side and the
dark side of a planet or other body.
Terrestrial A term used to describe anything originating
on the planet Earth.
Terrestrial Planet A name given to a planet composed
mainly of rock and iron, similar to that of Earth.
TFoV - True Field of View. The actual angular measurement
of sky seen through an eyepiece. See also AFoV.
Thermals - In reflecting telescopes, thermal convection
currents originating from the primary mirror that are set in motion within
the open main tube.
Tidal Force The differential gravitational pull exerted
on any extended body within the gravitational field of another body.
TIM - see Total Integrated Magnitude
TOT - Tool On Top. ATM term for mirror grinding, where
the grinding tool is on top of the mirror. See also MOT.
Third Order Aberrations - Spherical aberration, coma,
distortion, astigmatism and field curvature. These are optical, not mechanical,
Total Integrated Magnitude - The apparent magnitude of
a celestial object if it were reduced to a point. Essentially, it is the
total brightness of all of the object added up.
Transit, or Culmination The passage of a celestial body
across an observer's meridian; also the passage of a celestial body across
the disk of a larger one.
Transmission - the percentage of light passing through
an optical device, like a telescope.
Transparency - the term usually used to describe how dark
the sky is, in reference to what can be seen in a telescope. It really
refers to two characteristics - darkness of the sky, and clarity of the
air. Those do not necessarily go together.
Transverse Chromatic Aberration: the separation between
the shortest and longest measured wavelengths at the focal plane of the
Trojan An object orbiting in the Lagrange points of
another (larger) object. This name derives from a generalization of the
names of some of the largest asteroids in Jupiter's Lagrangian points.
Saturn's moons Helene, Calypso and Telesto are also sometimes called Trojans.
True Cassegrain - The classical form of Cassegrain telescope
incorporating a parabolic primary mirror and hyperbolic secondary mirror.
True Field of View (TFOV) The angular size (in degrees)
of the actual area of the sky that you can view through a particular telescope
with a particular eyepiece. (The TFOV depends on both, and is calculated
by dividing the AFOV by the Magnification.) another, more accurate, way
is to multiply 57.3 x the quantity determined by dividing the telescope
focal length by the eyepiece's fieldstop diameter.
Two or Three Star Alignment - A procedure used by computerized
telescope mounts to orient the computer to the celestial sphere.