From: klaes@verga.dnet.dec.com (Larry Klaes)
Subject: Electronic Journal of the ASA (EJASA) - November 1992
Apparently-To: sci-space-news@ames.arc.nasa.gov
Followup-To: sci.space
Originator: yee@atlas.arc.nasa.gov
Keywords: stars, binary, double, dew, telescopes, CAT, computers, EJASA
Organization: Digital Equipment Corporation
Date: Mon, 9 Nov 1992 07:20:01 GMT

                          THE ELECTRONIC JOURNAL OF
                  THE ASTRONOMICAL SOCIETY OF THE ATLANTIC

                      Volume 4, Number 4 - November 1992

                         ###########################

                              TABLE OF CONTENTS

                         ###########################

          * ASA Membership and Article Submission Information

          * Tales of the Double Stars - Don Barry

          * Coping with Dew - Charlie Manahan

          * What Does a Computer Aided Telescope (CAT) Do? - Bob Weaver

                         ###########################

                         ASA MEMBERSHIP INFORMATION

        The Electronic Journal of the Astronomical Society of the Atlantic
    (EJASA) is published monthly by the Astronomical Society of the
    Atlantic, Incorporated.  The ASA is a non-profit organization dedicated
    to the advancement of amateur and professional astronomy and space
    exploration, as well as the social and educational needs of its members.

        ASA membership application is open to all with an interest in
    astronomy and space exploration.  Members receive the Journal of the
    ASA (hardcopy sent through United States Mail - Not a duplicate of this
    Electronic Journal) and the Astronomical League's REFLECTOR magazine.
    Members may also purchase discount subscriptions to ASTRONOMY and
    SKY & TELESCOPE magazines.

        For information on membership, you may contact the Society at any
    of the following addresses:

        Astronomical Society of the Atlantic (ASA)
        c/o Center for High Angular Resolution Astronomy (CHARA)
        Georgia State University (GSU)
        Atlanta, Georgia  30303
        U.S.A.

        asa@chara.gsu.edu

        ASA BBS: (404) 564-9623, 300/1200/2400 Baud.

        or telephone the Society Recording at (404) 264-0451 to leave your
    address and/or receive the latest Society news.

        ASA Officers and Council -

        President - Don Barry
        Vice President - Nils Turner
        Secretary - Ingrid Siegert-Tanghe
        Treasurer - Mike Burkhead
        Directors - Bill Bagnuolo, Eric Greene, Tano Scigliano
        Council - Bill Bagnuolo, Bill Black, Mike Burkhead, Frank Guyton, 
                  Larry Klaes, Ken Poshedly, Jim Rouse, Tano Scigliano,
                  John Stauter, Wess Stuckey, Harry Taylor, Gary Thompson, 
                  Cindy Weaver, Bob Vickers

        ARTICLE SUBMISSIONS -

        Article submissions to the EJASA on astronomy and space exploration
    are most welcome.  Please send your on-line articles in ASCII format to
    Larry Klaes, EJASA Editor, at the following net addresses or the above
    Society addresses:

        klaes@verga.enet.dec.com
        or - ...!decwrl!verga.enet.dec.com!klaes
        or - klaes%verga.dec@decwrl.enet.dec.com
        or - klaes%verga.enet.dec.com@uunet.uu.net

        You may also use the above addresses for EJASA back issue requests,
    letters to the editor, and ASA membership information.

        When sending your article submissions, please be certain to include
    either a network or regular mail address where you can be reached, a
    telephone number, and a brief biographical sketch.

        Back issues of the EJASA are also available from anonymous FTP
    at chara.gsu.edu (131.96.5.29)

        DISCLAIMER -

        Submissions are welcome for consideration.  Articles submitted,
    unless otherwise stated, become the property of the Astronomical
    Society of the Atlantic, Incorporated.  Though the articles will not
    be used for profit, they are subject to editing, abridgment, and other
    changes.  Copying or reprinting of the EJASA, in part or in whole, is
    encouraged, provided clear attribution is made to the Astronomical
    Society of the Atlantic, the Electronic Journal, and the author(s).
    Opinions expressed in the EJASA are those of the authors' and not
    necessarily those of the ASA.  This Journal is Copyright (c) 1992 
    by the Astronomical Society of the Atlantic, Incorporated.


                          TALES OF THE DOUBLE STARS

                                by Don Barry

        In the Second Century A.D., Ptolemy noted that the star Nu
    Sagittariae appeared to keen eyesight as a close pair.  Twenty
    centuries later, over one-half million measures of over seventy-five
    thousand star systems have been accumulated by observers around Earth,
    using telescopes from a few centimeters in size to the multi-meter
    behemoths atop high mountain perches. 

        The first telescopically discovered multiple system was the Orion
    Trapezium, noted by Johannes Cysat of Ingolstadt in 1619.  Mizar was
    split by Riccioli in 1650.  Soon after Alpha Centauri, Alpha Crucis,
    Alpha Geminorum (Castor), and Alpha Virginis (Spica) were resolved by
    observers across Europe. 

        Today the cataloguing and measurement of double stars are often
    regarded as an astronomical cliche.  This is certainly not the case,
    with advances in technique and instrumentation making present
    measurements one hundredfold more useful than those of one century
    ago.  Seventeenth Century observers had an additional compulsion to
    search the night sky for stellar pairs.  Just as Galileo Galilei's
    discovery of the orbital motion of Jupiter's major moons demonstrated
    the existence of motion other than heliocentric, Sir Isaac Newton's
    published theory of Universal Gravitation predicted the existence of
    solar systems outside our own, subject to the same planetary motions.
    These early observers were extending the application of new theories
    to the Universe as a whole, at least as it was then known. 

        Mitchell, Mayer, and Herschel separately argued the case for
    orbital motion in several systems, though the lack of accurate tools
    made comparison of measures over time difficult.  By the first decade
    of the Nineteenth Century, rapidly moving wide systems such as Xi
    Ursae Majoris had spun sufficiently in their orbits that the proof was
    clear.  Double stars did move and the motion followed an elliptical
    course, thus extending Newton's reach from the Sol system to the Milky
    Way galaxy.

        Double stars move in elliptical orbits, with both stars moving in
    space about the center of gravity of the system.  However, when we
    observe double stars, we normally assume one star to be fixed and draw
    the motion of the other star in reference to it.  This produces much
    simpler diagrams.  Although in the orbit itself, each star occupies 
    the focus of an ellipse in which the other star moves, we see an orbit
    projected against the plane of the sky:  A circular orbit may appear
    with simple back-and-forth motion because we see the system from the
    edge.  As a result, observed orbits do not have one star at the focus
    of the orbital ellipse.  The distance of the focus from the position 
    of the reference star reveals the angle with which we view the orbit. 

       During each orbit, the two stars reach a most distant point
    (apastron) and a closest approach (periastron).  These do not
    necessarily correspond to what we view as closest approach and widest
    separation, however, because of our vantage point, viewing the orbit
    from a side angle.  For highly elliptical systems, a pair may seem
    immobile for decades or centuries, only to begin rapidly moving as 
    the system moves together for a quick gravitational fling. 

        In many cases, after a rapid periastron passage, two stars may
    appear in roughly the same orientation and separation as before, only
    the stars are now reversed.  Since it is often difficult to tell one
    star apart from another, some periastron passages may go unnoticed. 
    There are many double stars and only a few observers.  It is a vast
    sky and there is always room for more measurement. 

        The Great Star Catalogues

        Herschel began the race for stellar discovery and measurement. 
    His all-sky survey in search of nebulae also revealed over seven
    hundred double stars, which he crudely measured without clock drive 
    or instrumentation.  F. G. W. Struve conducted the first systematic
    program with the 22.5-centimeter (nine-inch) Fraunhofer refractor at
    the Dorpat Observatory, now in Estonia.  This telescope was the first
    "modern" research instrument.  It was outfitted with a clock drive to
    track the sky, precision optics and eyepieces, and for double star
    measurement an instrument to permit far more accurate measurement 
    than the pure guesswork of earlier surveys. 

        The double star instrument was fitted with a bifilar micrometer,
    which superimposed a set of crosshairs on the visual field.  The
    observer could merely line up the crosshairs on a double star - easier
    said than done for a very close pair - and read the results from an
    indicator dial.  Using this system, Struve discovered 3,134 pairs. 
    Otto Struve, his son, continued this work throughout much of the
    Nineteenth Century.  The great grandson Struve continued the line,
    although no longer in double star work, well into the Twentieth
    Century. 

        Sherburne Wesley Burnham was the premier United States contributor
    of the Nineteenth Century, discovering 1,336 doubles, many with the
    65-centimeter (26-inch) and one hundred-centimeter (forty-inch)
    refractors near Chicago, Illinois.  His book, A GENERAL CATALOGUE OF
    DOUBLE STARS WITHIN 120 DEGREES OF THE NORTH POLE, contained notes 
    and measures for 13,665 stars. 

        Aitken continued Burnham's work.  In 1932 he published THE NEW
    GENERAL CATALOGUE OF DOUBLE STARS WITHIN 120 DEGREES OF THE NORTH POLE
    with 17,180 measurements.  Jeffers, van den Bos, and Greeby continued
    the effort, and in 1961 issued perhaps the last catalogue of double
    stars to be issued in bound and printed form.  Their INDEX CATALOGUE
    OF VISUAL DOUBLE STARS 1961.0 contained identification and the oldest
    and most recent measures of 64,247 double stars. 

        Today, Charles Worley of the U. S. Naval Observatory (U.S.N.O.)
    maintains the world database of double star measures, the WASHINGTON
    DOUBLE STAR CATALOGUE, which exists as a computer file constantly
    being updated.  It currently contains over one-half million measures
    of about 75,000 systems. 

        Orbits

        Of these seventy-five thousand systems known to be visibly double,
    orbits have been computed for only about 850.  The reason is that less 
    than two centuries of accurate measurements are available.  Most systems 
    with measurements of several arcseconds, representing stars separated by 
    many tens or hundreds of times the distance between Earth and the Sun, 
    will take many thousands of years to complete one revolution.  Accurate 
    orbits, often called "Grade 1", only exist for about eighty systems. 

        Methods of Detection

        Only a minority of double stars appear as such through a
    telescope.  Earth's atmosphere limits easy resolution to stars with
    separations of one arcsecond or more, though experienced visual
    observers can resolve companions to 0.3 arcsecond.  For very close
    separations, only stars of nearly equal brightness are resolvable.  
    At greater separations, very large differences in brightness may be
    detected.  The net effect is that relatively few true doubles appear
    as "visual doubles". 

        The advent of stellar spectroscopy revolutionized binary research,
    providing a complementary technique capable of resolving the closest
    binaries, including those actually in contact with one another.  Since
    very close stars orbit much more rapidly than widely spaced pairs do,
    the Doppler motion of these rapidly spinning twins shows up much more
    clearly in the stellar spectrum.  These pairs are dubbed "spectroscopic 
    binaries". 

        Still other very close pairs have orbits in which one star
    obscures the other during part of their orbit as seen from Earth.
    Hundreds of these "eclipsing binaries" are known.  It is estimated
    that about one star out of one thousand falls in this class.  The
    brightest are Beta Persei (Algol) and Gamma Persei. 

        Some stars reveal their double nature not through their visual
    appearance or their motions in the spectrum but by the composite
    nature of the spectrum itself.  They show features common to hot and
    cool stars, which is possible only from a dual source.  These stars
    are called "spectrum binaries". 

        A few other stars, lying near the ecliptic, are resolvable during
    lunar occultations during which their light is extinguished in stages.
    About nine percent of the sky lies within the reach of Earth's Moon
    during its 18.6 year precessional cycle, limiting the usefulness of
    this technique. 

        The statistics from these techniques yield an astonishing
    conclusion:  Over half the stars in the sky are double!  Well over ten
    percent seem to be triple systems.  We also know of systems containing
    up to six stars, such as Castor (Alpha Geminorum). 

        New Techniques

        One exciting new technique, speckle interferometry, has extended
    the useful resolution limit of "visual double stars" from one-half
    arcsecond to a few hundredths of an arcsecond, a factor of twenty-
    five improvement.  In addition, this technique yields measurements of
    double star positions which are about one hundred times as precise
    (the errors in the measurements themselves are much smaller) as with 
    a micrometer. 

        Speckle interferometry works by taking rapid photographs of a
    double star, usually using a video camera, and thereby freezing the
    boiling motion which the atmosphere imparts to an image before it can
    blur the view beyond repair. Mathematical techniques, programmed on
    fast computers, then sift thousands of such images and reconstruct the
    double star within.  The technique lets a quality telescope be used at
    its true resolution limit, set only by the size of its mirror and the
    wavelength of light used, rather than limited by the atmosphere to a
    resolution equal to that of a small amateur instrument. 

        Sixteen years of work with this technique has resulted in the
    discovery of over three hundred new binary stars.  All of these
    systems are very close, therefore having potential orbits which are
    much shorter than the very wide pairs which make up the bulk of known
    double stars.  Several dozen new orbits have resulted from this work. 
    Many of these systems are rapid enough "movers" that their motions
    show up in their spectra as well. 

        These "double visual/spectroscopic" binaries are the most exciting
    stars of all.  Although only a few percent of all double stars fit the
    niche - they must be close enough to move sufficiently fast for the
    spectrum to change and wide enough to be resolvable - the combination
    of spectroscopic and visual information yields a very rich harvest of
    information. 

        The velocities of the stars and the period of the orbit gives the
    size of the orbit in space.  The visual orbit gives us the apparent
    size of the orbit as viewed from Earth.  The combination of the two of
    true size and apparent size gives the distance to the double star.  Of
    all methods of determining distance, this is one of the most precise,
    and errors of only a few percent are possible. 

        Once we know the distance, it is easy to calculate the mass of the
    stars and the total amount of energy that they produce.  Imagine, the
    orbit of a stellar-pair lets us place them on an imaginary balance and
    actually weigh them! 

        Modern Observers

        Only a few observers today use the old-fashioned technique of
    filar micrometry.  The last of the breed, as it were, are Charles
    Worley of the U.S.N.O., Wulff Heintz of Swarthmore College, and Paul
    Couteau of the Universite de la Cote d'Azur in France.  Worley
    converted to the speckle technique, using a clone of the Georgia State
    University (GSU) speckle system, just two years ago.  Using an Alvan
    Clark 65-centimeter (26-inch) instrument, Worley is doing modern
    astronomy on a telescope 130 years old.  The Smithsonian Institution
    already wants his micrometer!  Worley is not quite done with it, he
    reports. 

        Our own team at GSU has now accumulated over fifteen thousand
    measures with the speckle technique over sixteen years.  Other groups
    at Harvard, Caltech, University of Wyoming, University of Hawaii, and
    teams in France, Australia, Germany, and the former Soviet Union have
    also made speckle measurements.  Still, over eighty percent of the
    existing measures are by GSU astronomers, primarily using the four-
    meter telescopes at Kitt Peak and Cerro Tololo, Chile. 

        Some Famous Doubles

        Some famous doubles have a few extra companions thrown in, many
    just recently discovered by the Center for High Angular Resolution
    Astronomy (CHARA) astronomers.  In 1989, we discovered a fifth star in
    the famous double-double system, Epsilon Lyrae.  This companion is in
    the fainter of the two pairs, although it is not yet known to which of
    the two stars it belongs.  The separation is a tiny 0.044 arcseconds,
    at position angle 29.4 degrees.  Try seeing that in a typical backyard
    telescope! 

        Charles Worley bitterly complained when he catalogued our
    measurement, "You ruined my favorite star!"  What do we call it now, 
    the double-triple?

        Other discoveries are a companion to the golden component of
    Albireo, found 0.406 arcseconds away at position angle 159.0 degrees. 
    A large brightness difference should make this component almost
    impossible visually, though the separation is not too terribly close. 

        An astrophysically important double discovery is Pleione, the 
    star at the tip of the handle in the Pleiades (Seven Sisters) dipper
    pattern in the constellation of Taurus the Bull.  This very hot star
    is a favorite of GSU astronomer Doug Gies.  His many spectra of the
    star's light reveal pulsations on the stellar surface and the presence
    of orbiting gas just above.  In 1987, we found a companion, just 0.217
    arcseconds away at position angle 54.9 degrees.  Doug has subsequently
    found evidence for the companion in a lunar occultation which he
    observed at the University of Texas a few years earlier.  Every thirty
    years, Pleione seems to acquire a fresh envelope of gas:  Is the
    companion part of the explanation?  We do not know yet, but there is
    exciting work to be done here. 

        The star Capella (Alpha Aurigae), the fifth brightest star in
    Earth's night sky, is a signature star for the CHARA group.  First
    split by an optical interferometer using two separate telescopes in
    1921, over fifty years went by before speckle interferometry was used
    to resolve Capella again in the 1970s.  Capella has now revolved about
    two hundred times since its first observation, making its orbital
    period the most accurately determined of all visual binaries.  With a
    three-month period and maximum separation of only six hundredths of
    one arcsecond, this star requires at least a two-meter (eighty-inch)
    telescope for any hope at resolution. 

        Two interesting stars have been found in orbits which are viewed
    nearly edge-on from Earth.  From this angle, it is possible that one
    star will pass in front of the other during the orbit, causing the
    system to temporarily dim as the light of one star is shadowed. 

        One of these systems, Alpha Comae Berenices (Diadem), might have
    eclipsed in early 1990.  Despite a call for observers, no one reported
    anything unusual.  We will have to wait thirty years to check again. 

        The second system, Gamma Persei, was predicted by our group to 
    be a favorable candidate for eclipse in late 1991.  Last-minute
    spectroscopic observations by British astronomer Roger Griffin
    narrowed the possible time of eclipse to a period of a few weeks. 
    This eclipse was actually observed, lasting almost ten days, in
    September of 1991.  It will be twenty-eight years before another
    eclipse will be visible from Earth. 

        Castor (Alpha Geminorum) consists of six stars, but only two are
    visible through a small telescope.  Each of the two resolvable stars
    is a spectroscopic binary itself.  The entire system is orbited by yet
    another very dim close double at extreme distance. 

        During the last thirty years, Castor has rapidly moved through its
    orbit.  With a period estimated at nearly five hundred years, Castor
    swung through closest approach in the middle 1970s with amazing 
    rapidity.  Coming to about 1.8 arcseconds at closest approach, Castor 
    is already wider than three arcseconds.  During thirty years, the two 
    components have moved by more than ninety degrees, although the exciting 
    period is now over.  During the next decade, Castor will widen by almost 
    one additional arcsecond, although the position angle will change by 
    less than fifteen degrees. 

        A soon-to-be-famous double was just measured by CHARA astronomer
    Brian Mason from 1988 data taken with the CFHT telescope on Mauna Kea
    in Hawaii.  It is a white-dwarf binary, shining at a faint magnitude
    12.3 in the handle of the Big Dipper.  This object is well within the
    reach of amateur telescopes, with a separation of 2.6 arcseconds in
    position angle 125.  Let us know if you can confirm it!  The
    coordinates are RA 12h 50m 05s, +55d06.0m declination (2000). 

        The Difficult Ones

        The brightest star in the sky, Sirius, had its own surprises for
    the latter Nineteenth Century.  Observations of the slow proper motion
    of Sirius throughout the early part of that century showed that Sirius
    does not quite move in a straight line through space.  The luminous
    star is tugged from side to side by an unseen companion.  Many
    observers tested their instruments in vain to find the culprit. 

        Success came with a new refractor, the 65-centimeter (26-inch)
    Dearborn telescope, funded by the Chicago Astronomical Society during
    the late 1860s.  During the telescope's first checkout, the companion
    to Sirius was plainly visible, about ten arcseconds away.  Fortunately, 
    the observers peeked at the right time:  Sirius' companion was near its 
    most distant position.  At present, the Chicago Astronomical Society's 
    logo contains the orbit of Sirius B in its design. 

        Sirius' friend is an 8.5 magnitude white dwarf star, the ember 
    of a companion once mightier than Sirius itself.  With a fifty-year
    orbital period, this star moves from 2.5 to ten arcseconds away from
    Sirius.  With almost ten magnitudes difference in brightness (ten
    thousand times difference!) the companion is visually undetectable 
    in even the largest instruments near closest approach. 

        Sirius B most recently reached widest separation in 1971.  It  
    has been moving towards closest approach, to occur in early 1993.  It 
    will probably not become easily visible in even the finest telescopes 
    until the year 2000, when it will approach five arcseconds separation. 
    However, former GSU astronomer Wean-Shun Tsay, using a 35-centimeter
    (fourteen-inch) Celestron atop the observing mountain of Taiwan's
    National Central University, last year resolved Sirius B with a CCD 
    video camera, averaging many frames together to get a clear image.

        This year, Sirius B moves from position angle 330 to 300,
    presenting one of the ultimate challenges to small instruments and
    acute vision. 

        An even tougher challenge is presented by Procyon, another bright
    star with an even fainter white dwarf companion.  Procyon's secondary
    is magnitude 10.3 and never farther than 5.2 arcseconds distance. 
    Orbiting every forty years, it achieved widest separation just last
    year, although no measurements have been made in decades.  Perhaps no
    one has bothered to look.  

        Charles Worley claims that he is perhaps the last person to have
    viewed the companion and perhaps one of the few people alive to have
    ever seen it.  Procyon B is currently at position angle 30 and not
    moving very fast.  It should maintain its current separation for the
    remainder of the decade, sliding in towards the next close approach 
    in the year 2009. 

        Related EJASA Articles -

        "The CHARA Multi-Telescope Telescope", by Hal McAlister - August 1989

        "A View from Taiwan: Wean-Shun Tsay", an interview by Edmund G. 
            Dombrowski - August 1989

        "Our Closest Neighbors in the Milky Way Subdivision", by Ingemar 
            Furenlid and Tom Meylan - September 1989

        "Long-Term Trends in Ground-Based Astronomy", an interview with 
            Dr. Hal McAlister by Edmund G. Dombrowski - January 1990

        "Stellar Spectroscopy: At the Heart of Astrophysics", an interview 
            with Dr. Ingemar Furenlid by Edmund G. Dombrowski - March 1990

        "Sir William Herschel and the Natural History of the Heavens",
            by Keith M. Parsons - June 1991

        "The Hyades: A Star Cluster Rich in Myth and Astronomy", by Ken 
            Poshedly and Don Barry - June 1992

        About the Author -

        Don Barry, ASA President and Charter Member, is an astronomer 
    with the Center for High Angular Resolution Astronomy (CHARA).  Don is 
    currently writing his Ph.D. thesis involving measuring the relative 
    luminosity of very close double stars.  Don's professional interests 
    include optical interferometry, binary astrometry and photometry, and 
    innovative instrumentation.  An active amateur as well, Don's interests 
    include telescope making, antique instruments, and fostering amateur-
    professional collaborations. 

        Don is the author of the following EJASA articles:  

        "Astronomy Week in Georgia" - August 1989

        "Profiles in Astronomy: Albert Whitford" - September 1989; an
    interview with Edmund Dombrowski and Sethanne Howard

        "Alar Toomre: Galactic Spirals, Bridges, and Tails" - October 
    1989; an interview with Edmund Dombrowski and Sethanne Howard

        "Observing the Wreaths of Winter" - December 1989

        "The Mayall Four-Meter Telescope" - May 1990

        "A Southern Travel Diary: An Observer's Tale" - August 1990

        "Saturn's Great White Spot" - February 1991

        "The Hyades: A Star Cluster Rich in Myth and Astronomy" - June 1992;
    with Ken Poshedly


                               COPING WITH DEW

                              by Charlie Manahan

        How many amateur astronomers have been out happily observing, 
    only to find their celestial targets slowly fade into the background?  
    The culprit is dew, usually covering the front optical element of their
    telescopes.  What is dew?  Where does it come from?  Most importantly,
    what can we "dew" about it? 

        The most popular amateur telescope design is the Schmidt-Casse-
    grain, which is particularly susceptible to dewing on its exposed 
    front corrector.  However, any optical element exposed to the open air 
    can (and usually will) dew, given sufficient time and a humid night. 

        Dew does not fall from the sky.  It is the same phenomenon that
    causes the glass holding your iced drink to get wet.  Any object that 
    is colder than the condensation point (dew point) gets moisture on it. 
    If the object is cold enough, the condensate is ice, otherwise liquid.

        Why do Schmidt-Cassegrain corrector plates seem to be the first to
    fog?  For any given set of atmospheric conditions, dew formation is a
    function of temperature.  Schmidt-Cassegrain correctors are usually
    colder than other things around an observing site.  Why?  The
    temperature of different objects at an observing site can vary
    considerably, depending on the heat flow into and out of the objects. 

        All things in the Universe give heat to objects that are colder
    and accept heat from objects that are warmer.  The temperature of any
    object that does not generate its own heat is solely dependent on the
    net heat flow to and from that object. 

        The corrector of an SC is no exception.  The corrector is at a
    steady state temperature and heat flow achieved by radiating heat to
    the night sky and accepting heat from the terrestrial surroundings. 
    Deep space is at the temperature of the microwave background, a
    frigid 2.73 degrees Kelvin (-454 degrees Fahrenheit).  An unshielded
    corrector plate pointing at the zenith "sees" a hemisphere of extreme
    cold above it and only the telescope below it.  Since the night sky is
    so much colder than the corrector, the corrector radiates heat to the
    sky.  As long as the heat flow out of the corrector is greater than
    the heat flow into it from the air and surrounding objects, the
    corrector gets colder.  The colder the corrector becomes, the more
    heat flows into it and the less heat flows out.  The temperature
    continues to drop until the heat flow in matches the heat flow out. 

        If the steady state temperature of the corrector is lower than the
    dew point, the corrector gets covered with dew.  Wiping it will not
    help; it will only redistribute the dew.  The one act that will prevent 
    dew formation is keeping the corrector plate temperature above the dew 
    point. 

        There are two ways to keep your optics warm enough to prevent 
    dew.  One is to reduce the heat flow out by reducing the area of night 
    sky that the corrector "sees".  This is how dew shields work.  By
    narrowing the circle of sky to which the corrector is exposed, 
    the heat loss is reduced and the steady state temperature of the
    corrector increases.  Dew shields have an additional benefit of
    increasing image contrast by reducing optical flare caused by stray
    light.  The longer and deeper the dew shield, the better it works. 
    This is why Newtonians have less trouble with dew than Schmidt-
    Cassegrains.  Their mirrors are only exposed to a small portion of 
    the sky. 

        With humid nights in such places as Georgia, dew shields are
    rarely sufficient to keep the dew away, although they slow dew
    formation.  In addition, as the environment cools throughout the
    night, the ambient temperature can drop to or below the dew point,
    making dew shields useless.  At this point, the only way to prevent
    dew is by artificially heating the corrector. 

        Hair dryers work, but dew clearing with a hair dryer requires a
    120V power supply and is a repetitive chore.  On a night with heavy
    dew formation the corrector may require a "blow dry" every ten
    minutes.  Corrector plate heaters are a step up from hair dryers but
    can cause convection currents that generate bad "seeing" through the
    telescope.  A combination dew shield and corrector plate heater with a
    temperature control that allows the heater to work at a temperature
    just barely warm enough to keep the corrector above the dew point is 
    a better solution than either alone. 

        Amateurs can save money on dew shields without much effort. 
    Commercial dew shields are expensive and usually too short.  A rolled
    poster board tube painted with the polyester resin used for automotive
    fiberglass makes an acceptable dew shield.  Untreated poster board
    sags out of shape as it gets wet with dew during the night.  Painted
    aluminum flashing (easily joined with pop rivets) makes an inexpensive, 
    durable dew shield. 

        For those who like to solder, resistors for heating are cheap.  As
    a starting value for twenty-centimeter (eight-inch) SCTs, twelve three
    ohm resistors in series (36 ohms total) should give plenty of heat to
    keep dew away when attached to an automotive battery (13.6V).  This
    gives about four-tenths of one watt per resistor.  Allow a safety
    margin of 5x on the power factor for resistors.  Just be sure to mount
    them close to the corrector and give your construction the "smoke test" 
    before you put it on your telescope.  Do NOT plug this into a household 
    outlet - it may cause an electrical fire! 

        The ideal solution maintains the optical element temperature
    slightly above the dew point, while minimizing temperature differences
    and air currents in the optical path. 

        Related EJASA Articles -

        "Amateur Telescopes, Yesterday and Today", by Bill Bagnuolo -
         September 1989

        "A Comparison of Optical and Radio Astronomy", by David J. 
         Babulski - June 1990

        "Low-Budget Astronomy", by Tony Murray - October 1990

        "Aperture Arrogance", by Eric Greene - March 1991

        "Astronomy and the Family", by Larry Klaes - May 1991

        "An Introduction to Celestial Coordinates", by Nils Turner - 
         October 1991

        "Astrophotography the Easy Way", by Harry Taylor - October 1991

        "How to Make a High-Quality Fifty-Millimeter Finderscope",
         by Robert Bunge - December 1991

        "The Elusive Dot", by John Stauter - December 1991

        "Telescopes: A Novice's Guide", by Steven M. Willows - March 1992

        About the Author -

        Charlie Manahan is a familiar ASA member to fellow observers 
    at Hard Labor Creek Observatory (HLCO) activities.  Charlie pursues
    astrophotography and deep sky observing through his custom modified
    Celestron 11 telescope.  His hardware attests to the value of a 
    little "do it yourself" magic, which always seems to make things
    work better.


                 WHAT DOES A COMPUTER AIDED TELESCOPE (CAT) DO?

                        by Bob Weaver (From the ASA BBS)

        Here is a summary of what a Computer Aided Telescope (CAT)
    accessory can and cannot do for you. 

        Misconceptions

        Contrary to popular belief, a CAT will not polar align your
    telescope.  It can greatly simplify the process if you follow a
    relatively difficult "first time" start up procedure.  Personally, 
    I found it simpler to polar align at each observing session.  The
    procedure takes about ten minutes. 

        If you are having trouble with polar alignment and cannot locate
    celestial objects with setting circles, the CAT will not be able to,
    either.  You must have very precise polar alignment.  I highly
    recommend the polar axis finder by Rodger Tuthill.  It aligns the
    mount, not just the telescope tube assembly! 

        What Can a CAT Do?

        1.  Keeps track of the sky coordinates (RA and Dec) that your 
            telescope is pointed at.

        2.  Keeps track of the coordinates and characteristics of thousands 
            of astronomical objects.  The CAT serves the same function as 
            hardcopy star charts plus reference books, but in a much more 
            convenient form.

        3.  Keeps track of miscellaneous information that is necessary or 
            useful during observing, like time, date, site location, etc.
            (This is optional.)

        4.  Identifies the object closest to the center of the field and 
            tells how many other NGCs are in the same field. 
            (This is optional.)

        5.  Automatically selects objects to view based upon characteristics 
            that you specify (type, brightness, size, visual quality, minimum 
            altitude) in a sequence that minimizes the distance between 
            objects.  A display may describe them and an indicator may lead 
            you to them, one after another. (This is optional.)

        Popular Data Bases

         * The NGC database of 8,163 objects.

         * Bright stars (as many as 351) for polar/coordinate setup.

         * Double and multiple stars.

         * Messier objects M1 through M110.

         * Planets in their current locations.

         * Bright planetoids.

        Common Features

         * LED display of telescope RA/DEC

         * Display of object description, catalogue number

         * Command keys with audible, visual, and tactile feedback for 
           entering commands and information to a CAT.

         * Bar LEDs to indicate direction and distance to an object of 
           interest.

         * Star map with different resolution views of a region and finder 
           chart of object. (For sophisticated models.)   

         * Most consume less than ten watts of power.

         * Most offer encoder resolution of at least 1/8 degree in RA 
           and 1/20 degree in Dec.

        Installation

        Most CAT brands require installation of two encoders on the axes
    of the telescope.  They often contain special brackets for mounting to
    popular brands.  Cables run from the encoders to the CAT control box. 

        If you are not mechanically inclined, do yourself a favor and ask
    for help from a dealer or a knowledgeable ASA member.  If you break an
    encoder, it will cost from 125 to 200 dollars to replace it. 

        Operation

        When searching for a particular object, I use a 32-millimeter (mm)
    eyepiece, which delivers a 0.86 degree field with my telescope.  If
    the telescope is polar aligned properly, the object will always be
    somewhere in the field of view.  No other accessory has enhanced my
    enjoyment of astronomy more that the CAT.  I find myself spending time
    observing rather than searching.  Also, since I do not need to fumble
    through star charts and manuals looking for object coordinates, I take
    more chances looking for extremely faint objects. 

        This overview was based on the Meade CAT.  There are a variety of
    CATs to chose from.  Some have more features than the Meade and others
    have less.  If you have any questions about my system, please contact
    Bob Weaver with a message on the ASA BBS. 


      THE ELECTRONIC JOURNAL OF THE ASTRONOMICAL SOCIETY OF THE ATLANTIC

                         November 1992 - Vol. 4, No. 4

                           Copyright (c) 1992 - ASA

Newsgroups: sci.space.news