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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 modern telescopes.

Rayleigh Limit - In optical systems, the aberration tolerance of 0.25-wave peak-to-valley and applied to path differences in the converging wave front.

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 creates it.

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 mirror.

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 optical path.

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 another body.

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 the tube.

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) vision.

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, well-executed.

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 stars.

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 orbital.

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 a spectrum.

Spectrum Light spread out into individually visible frequencies by use of a prism. Or, the continuum of all frequencies of electromagnetic radiation.

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 back again.

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 the focus.

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 called Stacking.

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 attraction.

Star Diagonal An accessory that is often used on refracting and Cassegrain telescopes to bend the light cone at right angles for more comfortable viewing.

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 their cells.

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 there.

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 Plossl.

Supergiant The stage in a giant star's evolution where the core contracts and the star swells to about five hundreds times its original size.

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 the Earth.

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, errors.

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 lens.

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.

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Agena Astro Products - The Great Atlas of the Sky, Jubilee Edition
Agena Astro Products - The Great Atlas of the Sky, Jubilee Edition
Item No. 786-20000

Agena Astro Products BOOKS The Great Atlas of the Sky by P.Brych
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Sun Observer's Guide, Firefly Books
Sun Observer's Guide, Firefly Books
Item No. 896-23211
Manuf. No. Firefly 978-1-55297-941-9

A practical reference for how and why to view our nearest star. This book can start you making worthwhile observations.
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The History of Astronomy, Firefly Books
The History of Astronomy, Firefly Books
Item No. 896-23210
Manuf. No. Firefly 978-1-55407-537-9

This extraordinary book traces humans' interaction with the endless wonders of the night sky.
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Stargazing with a Telescope, Firefly Books
Stargazing with a Telescope, Firefly Books
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Manuf. No. Firefly 978-1-55407-577-5

Stargazing with a Telescope is a practical guide that demystifies the process of buying and using a telescope.
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Stargazing with Binoculars, Firefly Books
Stargazing with Binoculars, Firefly Books
Item No. 896-23207
Manuf. No. Firefly 978-1-55407-368-9

A practical, concise beginner's guide to viewing the night sky through binoculars
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300 Astronomical Objects: A Visual Reference to the Universe, Firefly Books
300 Astronomical Objects: A Visual Reference to the Universe, Firefly Books
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Manuf. No. Firefly 978-1-55407-175-3

A handy and comprehensive reference to the 300 most interesting celestial objects. This book provides a tour through the galaxy.
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Moon Observer's Guide, Firefly Books
Moon Observer's Guide, Firefly Books
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Manuf. No. Firefly 978-1-55297-888-7

Moon Observer's Guide offers practical guidance to amateur astronomers viewing Earth's only natural satellite.
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Night Sky Atlas: The Moon, Planets, Stars and Deep Sky Objects, Firefly Book
Night Sky Atlas: The Moon, Planets, Stars and Deep Sky Objects, Firefly Book
Item No. 896-10719
Manuf. No. Firefly 978-1-55407-026-8

This Firefly book is a very nice introduction to astronomy for beginners.
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Software Bisque TheSkyX Professional Edition for Mac
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Software Bisque TheSkyX Professional Edition for Mac
Item No. 773-55161
Manuf. No. Software Bisque SKYX_PRO_MAC

TheSkyX Professional Edition for Mac is the next major release in TheSky's 25+ year history
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Software Bisque TheSkyX Professional Edition for Windows
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Software Bisque TheSkyX Professional Edition for Windows
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TheSkyX Professional for Windows will get you charged up about your passion. Explore, engage, enjoy while getting the most from your observing sessions.
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Sky & Telescope - Sky Atlas 2000.0 Companion, 2nd Edition
Sky & Telescope - Sky Atlas 2000.0 Companion, 2nd Edition
Item No. 846-10899
Manuf. No. Sky Publishing 46956

This essential reference features an alphabetical listing of every deep sky object plotted in Sky Atlas 2000.00.
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Sky & Telescope - SKY ATLAS 2000.0 DESK UNLAMINATED
Sky & Telescope - SKY ATLAS 2000.0 DESK UNLAMINATED
Item No. 846-10898
Manuf. No. Sky Publishing 46883

Includes close-up charts of such areas as the celestial poles and the Virgo-Coma galaxy region, as well as an acetate coordinate-grid overlay for determining accurate positions
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Sky & Telescope - SKY ATLAS 2000.0 FIELD LAMINATED
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Item No. 846-10895
Manuf. No. Sky Publishing 46921

The standard against which all other star atlases are measured, each version of Sky Atlas 2000.0 contains 26 charts covering the whole sky and showing 81,312 single, multiple, and variable stars of magnitude of 8.5 and brighter and 2,700 deep-sky objects
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Item No. 846-10890
Manuf. No. Sky Publishing 46891

Field Version: Stars and deep sky objects are white on a black background. Unbound and printed on heavy paper
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Sky & Telescope - Astronomy Library Sky Atlas 2000.0 Deluxe Edition
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Manuf. No. Sky Publishing 46875

Sky Atlas 2000.0 Deluxe Edition contains 26 charts showing close to 85,000 objects (stars and deep sky objects) down to magnitude 8.5
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