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17 December 2008 23:22
- perigee is the point in an object's orbit around the earth where it is closest to the earth
- contrast with apogee, which is the point where the object is farthest from the earth
- should only apply to objects, such as the moon, which have elliptical orbits
- the more elliptical the orbit, the greater the difference in distance at perigee and apogee
- the "eccentricity" of an orbit is the degree to which the orbit is elliptical
- periapsis or apoapsis are the corresponding terms for generic objects, for objects circling the sun, the terms are perihelion and aphelion
- we just (on 12/12/2008) experienced a full moon (more or less) at perigee - the closest full moon in 15 years - an excellent image and comparison to an apogee full moon is here
- another full moon will occur within 16 hours of perigee on 1/10/2009, but the closest full moon of 2009 is on 7/7
- an online lunar perigee/apogee calculator is here
17 December 2008 01:19
- the point on the celestial sphere directly above the observer
- opposite the nadir, i.e., the direction of the force of gravity
- the zenith's altitude on an altazimuth scope is 90 degrees
- an object's zenith distance is 90 degrees less the object's altitude in degrees
- when used as a property of a celestial object - for example, the sun's zenith - it means the highest point the object reaches in the sky
15 December 2008 15:01
- the mean distance from the sun to the Earth (approximately)
- the mean is used because the distance is not constant; all the planet's orbits are slightly elliptical
- 149,597,870.691 km or 92,955,807.3 miles
- early estimates of the earth-sun distance were 8 million km (Tycho Brahe), and 24 million km (Johannes Kepler).
- In 1672, Giovanni Cassini measured the distance as 140 million kilometers (87 million miles), which is pretty darn close, considering
- Jupiter is about 4.949 AU from the sun at its closest; and about 5.465 AU from the sun at its farthest
- 1 light year is 63,239.6717 AU
- You can convert distances into AUs using Google, in the Google search field, type "xxx uuu in AU" where uuu is the unit of measure, and xxx is the value - for example, typing "384,403 km in au" gives you the mean earth-moon distance in AU, or 0.00256957312
13 December 2008 00:11
- bodies of ice, gas, dust, and rocks orbiting the sun
- as comets near the sun, material boils off and comets develop a coma, a gaseous ball, and a tail which stretches out behind the comet
- the word comet comes from Greek; Aristotle called them komētēs or "stars with hair"
- comets are described by their orbital period, those with periods of 200 years are less are considered short-period, and greater than 200 years are considered long-period
- some comets only swing by the sun once before being tossed out of the solar system like unruly drunks
- many comets are visible only through telescopes, about one a year is visible to the naked eye
- comets used to be described by the year of their appearance, then they were named for their discoverer, today they are named for two or three independent discoverers and given a coded designation by the IAU
- sometimes comets brighten suddenly - as happened with Comet Holmes in 2007 pictured below
11 December 2008 23:14
- an optical device, typically used just before the eyepiece in the light path of a telescope, that multiplies the magnification of the eyepiece
- usually referred to simply as a Barlow, often (incorrectly) barlow with a lower-case "B"
- Barlows are basically a tube with a lens at the bottom
- named for its creator, the English engineer Peter Barlow
- they come in 2x, 3x, 4x, and other variations, typically (but not always) whole numbers
- using a 2x Barlow is precisely like using an eyepiece of half the focal length, so experts often advise not to buy eyepieces of exact multiples of 2 - so buying an 18mm and a 9mm eyepiece of the same design is somewhat redundant
- using a Barlow increases the eye relief of an eyepiece, the effect is more noticable as the focal length of the eyepiece increases
- Televue's Powermates perform a function similar to a Barlow, but they consist of two doublet lenses that increase magnification without increasing the focal length of the eyepiece
- "stacking" barlows yields even higher magnifications (i.e., a 2x and 3x stack results in 6x magnification) - seldom useful for visual due to seeing limitations; planetary imagers often stack barlows to achieve insane focal lengths
- planetary imagers sometimes put extenders or diagonals after the barlow to further increase the magnification
10 December 2008 17:30
- a device inserted into the optical path that splits the light from a telescope's objective into two eyepieces
- since the light path is being split between two eyepieces, the each image is dimmer than looking through a single eyepiece
- people often say viewing through a binoviewer is more natural and/or easier
- contrast with binoculars, which have two objectives and light paths (typically mirrors/prisms), and allow for true stereo vision
- contrast also with binocular telescopes, which are two full-size telescopes set up for simultaneous viewing
- binoviewers require sets of eyepieces matched in focal length and almost always by design and manufacturer
- some binoviewers have mechanical means of moving filters and/or barlows into the light path without removing the binoviewer and/or eyepieces to make them more versatile
- since the space between peoples' eyes (interocular distance) differs, binoviewers adjust the distance between the eyepieces like binoculars; however, some people still have issues "merging the images" in binoviewers
- a person talking about his Denks is referring to a particular top-of-the-line brand of binoviewer, Denkmeier
- some telescopes may not be able to come to focus with binoviewers due to the additional length of the light path through the binoviewers
- the weight of the binoviewers and the additional eyepiece may need to be taken into account when balancing the telescope on its mount
9 December 2008 15:07
- a telescope mount that looks like a "T" tilted so the base of the "T" is pointed at the celestial pole, so that as the telescope rotates around this axis (the right ascension axis), it counteracts the apparent motion of heavenly bodies due to the rotation of the earth
- the other axis is known as the declination axis, and the telescope is attached to one end, with counterweights at the other end
- also known as a GEM
- favored by amateur astrophotographers, as they allow for long exposures without field rotation
- non-motorized models have knobs (often attached to flexible cables) called slow-motion controls that users turn to keep the telescope aimed at a target
- motorized versions may have motors on one or both axes
- often have polar alignment scopes inside the right ascension axis to aid in aligning the mount with the celestial pole
- many "go-to" mounts have polar alignment routines that align the scope through iteratively aligning the scope and slewing back to Polaris
- counterweights may be added or removed, or adjusted up and down the counterweight shaft as needed to balance the weight of the optical tube(s), camera(s) and other mounted equipment
- as a safety measure, an oversized nut at the end of the counterweight shaft - aptly called a "toe saver" - hopefully prevents loose counterweights from sliding off unexpectedly
- other types of equatorial mounts include open fork, English (or yoke), and cross-axis
8 December 2008 12:33
- a group of stars that create a recognizable pattern
- asterisms are not the same as the 88 standard constellations - some make up a part of the constellation, while some are made up of stars from multiple constellations
- the Big Dipper is the most well-known asterism - it is part of the constellation Ursa Major
- the Summer Triangle of Altair, Deneb, and Vega is made up of the brightest stars in three different constellations - Aquila, Cygnus, and Lyra
- the Coathanger (also known as Ronchi's Cluster) in the constellation Vulpecula is a popular and very striking asterism when viewed in binoculars or a small telescope
- while the 88 official constellations are the product of the International Astronomy Union, no organization is responsible for cataloging asterisms
- author Phil Harrington started a list of asterisms in his book Touring the Universe through Binoculars that is now called the STAR list (small telescope asterism roster)
5 December 2008 16:55
- a large spherical group of stars bound together by gravity containing as few as ten thousand or as many as a million stars
- also known as globular star clusters, or sometimes just globs
- contrast with open clusters (aka, galactic clusters)
- all galaxies above a certain size are believed to be orbited by associated clusters
- the term was first used by William Herschel
- in general, clusters appear to contain stars of roughly the same age, and they appear to be some of the oldest stars known
- the Milky Way is estimated to have as many as 200 associated globular cluster, fewer than 160 have been discovered - the rest may not be visible from Earth
- a visualization of the Milky Way's globular clusters can be found here (scroll to the bottom)
- a few globular clusters are visible to the naked eye; Omega Centauri, for example, is a globular cluster masquerading as a star
4 December 2008 17:00
- using a system to automatically correct the position of a telescope's mount to keep the telescope pointed precisely at a given target
- autoguiding systems work by sending signals to the mount to correct the
mount's position when a given star (the "guide star") moves from its
starting position
- autoguiding systems typically consist of a either a separate guidescope and camera, a camera mounted in an off-axis guider
- some imaging cameras have dedicated autoguiding chips built-in
- most systems utilize a camera, software and a computer, but there are some self-contained autoguiders (the SBIG ST-4 being the most well-known)
- cameras used include repurposed imaging cameras (the Meade DSI series, for example), or webcams like the Philips SPC900
- popular free autoguiding software includes PHD, GuideDog, and MetaGuide
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Orion Telescopes
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