NEWSLETTER OF THE VANCOUVER CENTRE RASC

VOLUME 2002 ISSUE 5 SEPTEMBER/OCTOBER 2002

Lunar 1 The Absolute Minimum You Need

Images 2 to Know About Lunar Libration by Dan Collier President’s Message 3 During the 17th century, J.D. glimpse the forbidden side over its Summertime Observing 6 Cassini published three laws that south (north) pole. This is libration Smudges and Shooting Stars govern the aspect of lunar in latitude. At the same time, its 6 dynamics known as libration. The axis is canted 1° 32' 30" out of word was coined from the same perpendicularity with the ecliptic. Upcoming Events 7 root as Libra, and is applied to The reader might conclude that this For Sale 7 phenomena of chaotic cant adds to the libration in oscillation—as suggested by the latitude, which is true. But while Scope Stuff 10 waggling of a merchant’s balances. the magnitude of it is constant, Cassini’s Laws were educated Cassini’s first two laws say nothing Looking Ahead guesses insofar as the theory was about its direction, so it is tempting Remember, you are always not worked out until the 18th to consider it fixed relative to the welcome to attend meetings of century. Libration lets us see about stars. In reality, the ’s axis Council, held on the first Tues- 9% more of the Moon’s surface precesses to stay aligned with the day of every at 7:30pm in the G.S.O. than the 50% we would be inclination. otherwise limited to. In effect, the This is addressed by the Sept. 10: Our Copernicus. Man in the Moon ceaselessly nods puzzling Third Law, whose terms UBC English professor Dr. up and down, left and right. will now be defined. It is well Dennis Danielson presents a detailed look at the man and The and diurnal known that the Sun’s attraction his accomplishments. terms of libration are well induces disturbances in the Moon’s explained in textbooks, but the orbit, the most important being the Oct. 8: Lee’s Amateur Hour. other terms are almost always regression of the nodes. Because Nov. 12: TBA treated incorrectly. Readers are led the Sun is constantly pulling on the to believe that the tiny physical Moon, its orbit wobbles like a libration is the only nontrivial spinning coin coming to rest on a Next Issue Deadline factor in the orientation of the table. One cycle of regression takes Material for the November Moon. 18.61 years. Like the Moon itself, Nova should submitted by Consider Cassini’s Second the Moon’s orbit has a north pole, Monday, Nov. 4, 2002. Please send submissions to: Law. The plane of the Moon’s orbit defined as the point on the celestial is inclined 5° 8' 43.4" from the sphere 90° north of the orbit’s Gordon Farrell ([email protected]) ecliptic. When the Moon is above plane. As the nodes regress, this (below) the ecliptic, we can continued on page 8 Images by Marc Verschueren The Hubble photographs will presume Hubble does best. The Once again, new fantastic always be exceedingly beautiful raw data of this research do not images from the Hubble Space but when I hear about the Throne look aesthetic at all, just some Telescope appear like glowing of God I do shake my head a little black spots on a white background mysteries from far away on the bit. This expression was used or some graphs representing pages of the astronomical recently in one of the astronomical spectra. But the result of the magazines. The new Advanced magazines in connection with the interpretation is impressive. We Camera for Surveys gives us a new Hubble photographs of the find stars almost as old as the better vision of the far away Cone nebula. Robert Burnham universe. Stars that just stayed universe than ever before. There called it that in his Celestial alive since almost the beginning is probably no astronomer who is Handbook, and he only saw black and gently still spread some light; not impressed by this imagery. and white pictures! The aesthetic witnesses to the age of the What makes it so beautiful? Their excitement is pushed a little bit too universe. And Hubble did not look scientific meaning is far from clear far here. Even if Burnham puts to the edge of the cosmos—only a from what you see. They look these words in the mouth of few thousand light years away in more like abstract paintings than hypothetical ancient observers, the environment of our own Milky astronomical objects, but the view calling for religious connections Way. is stunning. There is a grandiose while commenting on astronomy When I open an astronomical beauty in them. They seem to have is not the thing to do except when magazine, it is hard not to some mysterious meaning. referring to personal beliefs. immediately start looking at the I heard a talk once, illustrated However, there are some very latest parade of glorious galaxies with majestic Hubble images, famous people in science who and nebulae. They are great. But where the speaker wondered why have fallen for the temptation. If I during this glorious summer this we, humans on earth, have such a turn the magazine upside down, year I had the opportunity to look sensitivity for these unearthly which I am allowed to do since the many times through the eyepiece objects. It was clearly suggested universe has no up or down, does of small telescopes at Messier that we have some cosmic it still look like the Throne of God? objects or NGC members. The awareness built into us that gives The Eagle nebula, also a Hubble sensation of a real observation far us this sensation of beauty. I would marvel, was referred to once as the exceeds the impact of a like to suggest we come down back Pillars of Creation. There is no photograph. I can really see an to earth. We find these objects more creation in there than in our object with my own eyes (well yes, beautiful because they have an Sun—no less, either. with the help of some mirrors and intrinsic beauty in an abstract All this glamour, and I will lenses, but essentially with my own sense, just like a piece of art. Just admit I sometimes am somewhat eyes). Standing for hours under a like we can admire the shape of saturated by it, obscures the fact dark sky on a warm summer an arm in a sculpture or a curve in that the Hubble telescope is so evening with a telescope, that is a an abstract painting. And more so, much more than the top fashion true awareness of our universe. these images remind us of purely photographer of outer space. We That is the beginning of my beautiful earthly objects such as had a fascinating example of this curiosity to go and look further in mountain scenes or cloud a few ago when Professor more abstract forms of our formations, especially cloud Richer introduced us to his work knowledge, such as photographs formations with special light on white dwarfs in M4. That is also or scientific theories and effects created by the sun. work that Hubble does and what I continued on page 7 2 President’s 2002 Vancouver About RASC Message Centre Officers The Vancouver Centre, RASC This summer has been one of meets at 7:30 PM in the auditorium President of the H.R. MacMillan Space Cen- the busiest that we have had in Craig Breckenridge 604-437-3103 tre at 1100 Chestnut St., Vancou- quite some time. Some of the [email protected] ver, on the second Tuesday of every highlights include a very Vice-Pres./Webmaster month. Guests are always wel- successful Perseid Meteor shower Bill Ronald 604-733-7036 come. In addition, the Centre has event with the GVRD at [email protected] an observing site where star par- Aldergrove Lake Park. We had Secretary ties are regularly scheduled. Marcellus Redmond604-533-1830 Membership is currently $51.00 over 300 attendees and the Park Treasurer per year ($26.00 for persons under Interpreters were extremely happy. Marc Verschueren 604-986-1485 21 years of age) and can be ob- We had 11 scopes out and thanks [email protected] tained by writing to the Treasurer to all the volunteers. at the address below. Annual Librarian membership includes the invalu- We also represented the William Fearon 604-939-1895 able Observer’s Handbook, six is- Centre well for the two Rotary [email protected] sues of the RASC Journal, and, of Club events. The recipients were National Representatives course, access to all of the club quite pleased with the number of Pomponia Martinez 604-215-8844 events and projects. [email protected] For more information regarding objects they got to see and we were Bob Parry 604-215-8844 the Centre and its activities, please blessed with good sky conditions [email protected] contact our P.R. Director. both nights. This is a new method Membership NOVA, the newsletter of the of spreading the word about our Sean Roddick 604-946-1137 Vancouver Centre, RASC, is pub- [email protected] lished on odd numbered . society and it seems to be working Opinions expressed herein are not quite well. I am sure more of these Chair, CARO Committee Eric Fuller 604-540-2373 necessarily those of the Vancouver will be arranged. [email protected] Centre. Upcoming events include a Material on any aspect of as- Director of Telescopes tronomy should be e-mailed to the neighbourhood star party that will Phil Morris 604-734-8708 editor, mailed to the address on be held either September 21st or Public Relations page 5, or uploaded to 28th. There is a bit of a problem Norman Song 604-299-7924 SpaceBase™ at 604-473-9358, 59. [email protected] with that event as the Vancouver Advertising school board says the original Speakers Barry Shanko 604-271-0615 location is undergoing Nova encourages free use of its [email protected] classified ads for members with construction even though it will Merchandising items for sale or swap. Notify the not be starting until October. A Doug Montgomery 604-596-7058 editor if you wish your ad to run in change of venue is being made [email protected] more than one issue. right now and hopefully the event Nova Editor will be held at Windermere School Gordon Farrell 604-734-0326 Commerical [email protected] on one of those nights. We would Rates Councillors like to get a good number of Dan Collier 604-732-6046 1/2 Page: $25.00 per issue scopes out for the event, so if you Jason Rickerby 604-502-8158 Full Page: $40.00 per issue are interested please contact me. Rates are for camera-ready, or Trustees electronic files. Payment, by We will also be holding a Sally Baker 604-324-3309 cheque, must accompany ad ma- Sidewalk Astronomy on October Lee Johnson 604-941-5364 terial. Make cheque payable to: 4th or 5th at the Inuit statue to the RASC Vancouver Centre. continued on page 5 3 Observing Sites

Dale McNabb Observatory in Aldergrove Lake Park (RASC Van- couver Centre's regular viewing site) Contact Mike Penndelton (604- 888-1505) or Howard Morgan (604-856-9186)

Site of the annual star party organ- ized by the RASC Vancouver Cen- tre

Site of the annual Mt. Kobau Star Party organized by the Mount Kobau Astronomical Society

Site of the regular Thursday night star party. On the dike at the foot of 72nd St.

4 continuted from page 3 ASTROCOMPUTING MEMBERSHIP east of English Bay Beach. In HAS ITS June, we parked right on the grass SpaceBase™ (604-473-9358,59). Affiliated since 1992 with RASC PRIVILEGES! beside our scopes and had quite a Vancouver, our link to RASC Net, few people stopping by. We like RASC Members only chat area. New members, did you know? the new location for the absence Future data distribution hub for The Vancouver Centre has 6 tel- escopes available for loan free of of direct lights so plan on joining CARO Project. Features include latest HST images, current world charge! We have telescopes rang- us. If you are interested in helping space news and astronomy pro- ing from 3" to 10" diameter. For out, please give me a call. grams. Provides a file uploading more information call Phil Morris, The 2003 General Assembly facility for submitting articles and Director of Telescopes at 604-734- 8708, or see him in the lobby of the imagery to Nova. is less than a year away now and GSO after the members meeting. plans are starting to ramp up. We The loaner period is for one month LIBRARY could use some more volunteers to only. All telescopes are to be picked help out on committees or to head The centre has a large library of up and returned after the members books, magazines and old Nova’s meeting. No telescope will be al- them. There are a lot of tasks to for your enjoyment at the GSO. lowed to circulate outside of these perform and we could use Please take advantage of this club meetings! committee chairs for things service and visit often to check out Your greatest opportunity as a ranging from observing the new purchases. Suggestions member of the R.A.S.C. is to take for future library acquisitions are coordinators to door prize advantage of the company of other appreciated. enthusiasts to increase your knowl- organizers. We will meet at 1:00 edge, enjoyment and skill in as- on the Saturday following the RASCVC on the Internet tronomy. regular meeting every month from http://members.shaw.ca/rascvan/ The best thing you can do to now until the event. We meet at the or http://www.rasc.ca/vancouver gain the most from your member- ship is to get active! Take in the Billy Bishop Legion on Laburnum H.R. MACMILLAN club meetings; engage other mem- just a couple of blocks west of the SPACE CENTRE bers with questions; come out to ob- Space Centre and ½ a block north serving sessions (also known as of 2nd. Please join us, as many The Pacific Space Centre Society “star parties”), and, by all means, volunteer to take part in our many hands will help to make the is a non-profit organization which operates the H.R. MacMillan Space public events. workload easier. Centre and Gordon M. Southam Observing takes place at the Last but far from least is the Observatory. Annual Membership Dale McNabb Observatory in the approaching end of the year. The ($30 Individual, $65 Family) in- Aldergrove Lake Park, located in Nominating Committee is being cludes a newsletter, Discounts on Langley, on 8th Avenue, just east Space Camps, special programs of 272nd Street. We are there most formed now and will start and lectures, Vancouver Museum clear nights. Contact Mike soliciting people to hold positions Discounts, and free admission to Penndelton at 604-888-1505 or on Council for the coming year. I the Space Centre. Admission to the Howard Morgan at 604-856-9186. will be concentrating on the Space Centre includes: Astronomy shows, Motion Simulator rides, RASC General Assembly and will not multimedia shows in GroundStation 1100 Chestnut Street hold a position next year. If you Canada, and access to the Cosmic Vancouver, B.C. are interested in deciding the Courtyard Exhibit Gallery. For Mem- V6J 3J9 course of the Centre over the next bership information, call Mahi 604-738-2855 Jordao at 604-738-7827, local 237 year then please contact a current for information. You can also reach member of Council and they will them on the Internet at http:// point you to the members of the www.hrmacmillanspacecentre.com/ Nominating Committee. !

5 Summertime Observing by Doug Montgomery several trips to the Delta site with a slide show and brought our scopes It seems I spend all winter the usual suspects, Jason Rickerby, to Derby Reach for the Boy Scouts’ finding excuses to not go observing. Mike Jensen, and Greg Price. We Heritage adventure. From there, I It’s too cold; it might get cloudy; so also did a night with the Young went to Mt. Kobau where I camped on and so on. But this summer has Naturalists—Sean Roddick and beside Craig and his family. Many been superb for clear nights. myself had a short night that night. of our members were there I found myself going out In July, we had the first including Lee Johnson, Nelson several times a to several Manning Park star party. It was well Teskey, Randy Klassen, Jason different locations almost all attended by the regulars, Mike Rickerby, Riyad Abu-Ladan, summer long. My travels took me Pendelton, John Sherrett, Gary Wallace Helter and many others. to English Bay, Boundary Bay, Wolanski, Hoon Hui, and many There was also lots of members Aldergrove Lake, Macdonald Park, others. One name from the past that from the Fraser Valley group. When Derby Reach, Manning Park, and showed up was Frank Marino; it we got back, we did a display at Mt. Kobau. Oh yes, also my Surrey was very nice to see him back. Later Aldergrove Lake Park for the backyard. that month Mike, Hoon and myself GVRD with a slide show and many June started with a sidewalk went to Macdonald Park and scopes. The regulars—Mike Jensen, astronomy night at English Bay. observed with Victor Pollic, Paul Jason, Greg, Craig, and myself— Craig Breckenridge, Marcellus Greenhalgh, Dennis Dening and the were helped by Karl Miller, Howard Redmond, Bill Ronald and myself rest of the Fraser Valley Morgan, and Hugh Gregory. were parked on the lawn right by the Astronomers. I also spent several In closing, I would like to urge Inuit statue. We could only get nights at Delta and a night at everyone to take advantage of the bright objects, but a good night Aldergrove Lake. weather this year and attend the nonetheless. On the day of the In August I started at Delta upcoming events we have planned annular , I could not make looking at the southern horizon. The for the rest of the year. Clear skies! Spanish Banks, but still watched next night Bob, Pomponia, ! from my backyard. I also made Marcellus, Angela, and myself gave

Smudges and Shooting Stars: A Neophyte on Kobau by Val Stevenson mathematically unlikely. Ahhh— cloud cover. Mike is exultant with I only come up to the star party but the view from my kitchen sink: his view of Andromeda. “A on Mount Kobau every two years sage-covered hills breaking away to smudge,” I sniff, and settle in to for the same reason I only had a the snow-dusted (fresh—in watch the “bodies in motion” that I baby every two years. You forget the August!!) extinct volcano across the prefer: satellites and shooting stars. discomfort and remember the valley. There is still room for me to set up beauty of the experience. Our spot is a good one, near the a lounge chair and, bundled up in a We push the elderly 18-foot summit with enough space to set up heavy hunter’s sleeping bag, I lie on Frontier RV up the twelve vertical, the 8-inch Dobsonian, borrowed for my back and watch the beginning twisting miles to the summit. No the occasion from the RASC, and a of the Perseid meteor shower. They problem there. But the fresh-water set of 10x50 binoculars on a tripod. do not disappoint—one every few lines that produced an occasional These are for Mike, the serious minutes until they, and we, are drip last night in Keremeos become astronomy buff. For the first time defeated by another heavy cloud a rivulet, then a cascade on Kobau. except on an airplane, we watch the cover. And so, to bed. At the end of the first day, the clouds roll in around and below us. The next twenty-four hours are holding tank and water tank both Off to bedlam then until after 10:00 glorious and clear. Our experiences read empty. Alarming, and p.m., when there is a break in the continued on page 7 6 Upcoming Events September starting at 8:30. See President’s November 6-7 – Manning Park Star Party #2 message for details. 18 – Leonid meteor shower peaks 21 – East Vancouver Neighbourhood Party (rain October December day: Sept. 28) at Windermere 4 – Sidewalk Astronomy #2 (rain 10 – Annual General Meeting School at Rupert and 22nd. day: Oct. 5)

FOR SALE Miscellaneous telescope parts for FOR SALE sale: The perfect stargazers vehicle! 1984 GMC 2500 Vandura 1 8" mirror in cell $15.00 'Starcraft'. Newish 350 engine - 1 1.25" rack and pinion $15.00 24,000 miles. Cruise control, tilt steering, power windows & locks. 1 6" mirror in cell $10.00 Excellent sound system/Pioneer 1 3" secondary holder and spider radio/cassette. New front tyres, $10.00 towbar. 4 Captains seats, bench 1 1.5" secondary mirror $5.00 seat converts to bed, rear telescope 1 1.5" secondary holder $5.00 ary noted above. storage space/2nd bed. No auto- 1 barn door camera mount $2.00 matic lights - you are in control! 1 14.75" mirror-unfinished If any or all of these items interest Only selling as bought motorhome. $30.00 you, call David Dodge, 604-739- $6000. 604 734-9726 Angela The 8" mirror is at the bottom of a 3281, or email me at: Squires. poor quality sono-tube and is of [email protected] need of some love. The rack and WANTED pinion are attached to the tube. 10 - 12 inch solid tube Dobsonian There is no secondary but may with excellent optics. 604 734- have once taken the 1.5" second- 9726 Angela Squires. continuted from page 2 they describe the complex observations, but there is a lot of astronomical experiments. It puts techniques used to prepare these creativity in it as well. And it is a it all in the right perspective. Hubble pictures for the way they creativity we all deeply appreciate When I had basically written appear eventually to us. These and that inspires us—inspires us this piece, I came across the article images are indeed creations in to put the telescope out tonight and by Ray Villard and Zoltan Levay their own right. Some strict rules look for ourselves. ! in the August issue of Sky and are followed and the basic black Telescope. It is so appropriate that and white images are very real continuted from page 6 viewing and the enjoyment of from now. ! are shared with all the others at the everything the mountain holds every star party that night—optimal summer. We shall return—two years 7 continuted from page 1 attraction keeps one side in pole sweeps around the ecliptic constant view. Astronomers have Cassini’s Three Laws north pole, defined similarly. At the further confirmed it by timing I. The Moon’s rate of same time, the Moon’s axis and measuring the rotation is constant, and precesses around the ecliptic pole. libration with lasers. But as the its period is equal to the By the Third Law, the orbital and Moon looks as round as a billiard sidereal month such that axial poles remain on opposite ball, the symmetry violation must the same hemisphere sides of the ecliptic pole as they be hidden inside. Why does the constantly faces the sweep around it. The key to the precession follow the regressing Earth. phenomenon lies here. Most nodes? Roughly speaking, if it did textbooks not, the II. The tilt of the Moon’s axis leave out moment of rotation with respect to the vital would not the ecliptic is a constant. insight vanish that the when the III.The pole of the Moon’s Moon’s Moon rotation, the ecliptic pole, axis crosses and the pole of the Moon’s precesses the orbit always lie in the in step ecliptic. with the It same plane, and nodes. turns out therefore on the same Any that great circle on the attempt to Cassini’s celestial sphere, and work out Laws are always in that order. the ebb not exact. and flow The The Four Terms of Libration • Libration in longitude: ofFigure 1. Lunar libration over 30 days. system can be caused by the varying libration The curve shows the motion of the over time likened to speed of the Moon in its Moon’s 0° longitude–0° latitude point will go a orbit (maximum 7° 57') relative to the centre of the disk with awry pendulum • Libration in latitude: all terms included. Grid interval is 1°. unless that caused by the inclination Diurnal libration produces the small lunar receives a of the Moon’s orbit out of loops. The radius of the circle is 9°, precession regular the plane of the Earth’s the maximum value of libration in this is taken impulse orbit around the Sun (the period. into (the ecliptic) (maximum 6° 50') account. attraction • Diurnal libration: caused Why would the Moon of the Earth for the Moon’s visible by the offset of the precess? In general, a rotating body face is analogous to the pendulum observer from the centre will precess if subject to a moment, action.). Although the free-running of the Earth (maximum or off-centre force. Since gravity period of a pendulum may differ 57') acts through the centre of a body from that of the impulses, its • Physical libration: with spherical symmetry, it is oscillations come to be governed wobbles in the Moon’s by them. But if the impulses come correct to conclude that the Moon rotation (maximum 2') lacks that symmetry. This is already irregularly, the pendulum will evident from how the Earth’s undergo transients in which its

8 natural period resurfaces. represents this favoured point for appearance of the rest of the Moon Similarly, the nodal moments that each day of the month. If the dot is too. Imagine the tip of the nose of govern the Moon’s libration are located on a bright limb, a little the Man in the Moon lying at 0° applied in a constantly evolving slice of farside comes into view. If latitude, 0° longitude in the system pattern. As it tries to follow them, the dot is larger, the libration is of lunar coordinates. Lunar the Moon quivers slightly out of closer to the maximum of 9.5°. libration moves this point up, compliance with the Laws. The tiny The beginner is eager to look down, left and right from its mean physical comprise the into the ‘libration zones’ of the position at the centre of the disk. difference. farside, but soon tires of the Tycho, the rayed crater that marks The sum of the inclination and novelty. What is more interesting the Man’s chin at , is axial tilt, 6° 41', is sometimes carried up and down quoted as the angle between substantially by librations in the Moon’s equator and its latitude. Libration also affects orbit. If you’ve followed this the time of sunrise for a given argument closely, you might feature, as well as the profile conclude that the maximum of the limb presented to the libration in latitude is also 6° last rays of the Sun at the onset 41', contradicting the value in of a . the sidebar. The reason is that Perhaps the lunar feature the Moon’s orbital inclination most obviously effected by is not a constant. Over a libration is Mare Crisium, the period of 173 days it varies roundish ‘sea’ at the upper about nine minutes of arc right limb of the Moon (the around its mean. Thus the northeast limb, in current libration in latitude can be as parlance). Mare Crisium is high as 6° 50'. almost round, but our So exactly what do we Figure 2. Foreshortening effect of perspective foreshortens it see? Think of the Moon libration. Figure 1 is reproduced at the into an oval. Moreover, the mounted on a spring, and centre of the lunar disk to scale. North foreshortening is highly imagine pulling at some point is up, east to right. A ‘crater’ is depicted dependent on libration. When on its limb to bring some of the northeast limb is favoured, at the dates of greatest libration. This the farside into view. On the Mare Crisium is less feature is 30° from the mean limb. (i) real Moon, this point migrates foreshortened than usual. Libration 9° at NW limb; feature is less around the limb, completing When the opposite limb is foreshortened. (ii) Libration 9° at SE a revolution in about a month. favoured, Mare Crisium is During that period, the limb; more foreshortened. more foreshortened and closer favoured point may move at to the edge of the Moon. Try a constant rate, speed up in spurts, is that libration reduces the observing this effect without a or slow and dwell. The maximum foreshortening of features on our telescope. Unfortunately, it is amount of ‘pull’ that libration can side of the limb, and greatly difficult to make comparisons on impart varies around the limb. At improves our view of them. For dates two weeks apart because the the poles the limit is 6° 50', at the example, Mare Orientale, Mare advancing phase of the Moon equator 7° 57', but in between it can Australe and craters Bailly and brings darkness to Mare Crisium! be as much as 9.5°. Grimaldi are best observed when Imagine attaching a pencil to Looking at the calendar in Sky S&T’s red dot comes close to them. the nose of the Man in the Moon and Telescope, we see a red dot that Libration affects the continued on page 11 9 Scope Stuff by Steve Whitehouse in your telescope. The first step involves aiming The Schmidt Cassegrain The typical SCT has an f/2 the scope at a bright star, say Telescope has garnered an unfair spherical primary mirror and a magnitude 2 or brighter. Polaris reputation as a mediocre instrument. convex secondary mirror that works very well, and also has the There was a time in the early to mid magnifies the image of the primary advantage of being relatively 1980s that the two biggest mirror five times. The corrector stationary. Some people feel that the producers of these scopes did in fact plate has no role except to correct star diagonal should be removed turn out their share of lemons. This for spherical aberrations and to hold during collimation, however I feel was largely due to the rush to the secondary mirror. Just a minor that if you plan to observe with this produce large quantities of miscollimation of the secondary star diagonal in place you should telescopes for the return of Halley’s mirror will prevent the telescope leave it in, as long as it does not comet. Since then, optical from performing at its best. Small introduce any additional production has seen some major festoons on the equatorial belts of aberrations. changes, resulting in tighter optical Jupiter would fade into non- Use an eyepiece that would control and a better product for the existence with only the smallest give you about 100 to 125 times end user. misalignment of the secondary magnification. Centre Polaris in the True, there are better telescopes mirror. This is why collimation middle, and defocus the star. As for deep sky work (large should be checked every time you long as Polaris is very well centred Dobsonions), and there are better observe. You would not want to play in your eyepiece, you should see an telescopes for planetary work a musical instrument that was not image like a doughnut—a bright set (apochromatic refractors), but the in tune, so why observe with a of concentric rings with a black spot Schmidt Cassegrain Telescope has telescope that cannot perform at its in the middle. This “black spot” is held on as the best selling telescope best? For many SCT owners, the your secondary mirror. It should be in the world. The reason for this is collimation of these instruments has perfectly centred. Follow the rings simple: the Schmidt Cassegrain long been an issue avoided, and this all of the way around. Do they seem Telescope is capable of doing many is why the mediocre reputation of even all the way around? If not, you things very well. Having eight theses modern classics has will require some adjustment to generous inches of aperture persisted. However, the collimation your secondary mirror. Remember combined with an f/10 focal ratio procedure is very easy. to use very small increments when will give you enough power to The secondary mirror holder of you make the adjustments. A cruise through the solar system and your SCT will have one screw in the fraction of a turn on a collimation beyond to the outer galactic realms centre and three screws around the screw is all that is required. You may of the universe. outside. Never touch the centre have to experiment to find out The SCT works well in typical screw. This screw holds the which screw adjusts in which suburban locations due to the long secondary mirror in place and will direction. Moving your finger in focal length and the well designed have no effect on the collimation front of your scope near the part of baffle systems that provide images procedure. Only make adjustments the collimation ring that does not of celestial objects with nice dark to one of the three outside screws. seem concentric will give you an backgrounds. The real key to Never over-tighten any of the idea as to where to start to make the performance on the SCT (as with all screws. If one of the screws is too adjustments. Always remember to telescopes) is collimation. Schmidt tight, loosen the opposite screw. re-centre the star after you make an Cassegrain Telescopes are Make sure your telescope has adjustment. extremely sensitive to cooled to the ambient outside air The second step requires an misalignment. In fact, just a simple temperature. Internal heat plumes eyepiece that will provide about 250 car trip to an observing site may be can provide false readings and make enough to degrade the images seen collimation much more difficult. continued on page 11 10 continuted from page 10 will need an eyepiece (or Barlow third step will be very small. In fact, to 300 times magnification. Don’t combination) that will give you 350 even just some pressure on the be afraid to defocus the star large to 400 times magnification. adjustment screw is all that may be enough to discern any errors in the However, this time you will not needed. If you have trouble seeing collimation pattern. Once again, defocus the star. You will want to the diffraction rings around the centre the star in the eyepiece and focus the star as best as possible. focussed star, you may need to wait defocus. Look at the rings again. You will be looking for the first for a night with better seeing Are they uniformly spaced all the diffraction ring on the outside of the conditions. way around the secondary mirror? focussed star image. Does this With practice, this procedure If not, more adjustments are diffraction ring appear to be will become very easy, and a well necessary. Remember to make uniform all the way around the airy collimated SCT will reveal many minute adjustments, and re-centre disk? If not, a few more tweaks to more details on celestial objects that the star after each adjustment. Once your secondary mirror maybe in you may have previously missed. this step is completed you should be order. Clear skies. ! fairly well collimated. Please note that if you have The third step will require gotten the first two steps correct, steady seeing to accomplish. You then the adjustments required at the continuted from page 9 is more eccentric. This is indirectly and maximum librations in latitude to let him draw his librational due to the fact that can and longitude. These correspond to motion. Such an exercise can occur at any time between lunar dates when the curve is farthest be carried out with an from the horizontal (or ephemeris, and Figures 1–3 vertical) axis in Figure 3. The are typical outcomes. Figure Handbook uses the term 1 shows the behaviour of the ‘minimum’ to mean maximum total libration over 30 days. in the negative direction. For During this period, the Moon example, a ‘minimum’ ‘nods’ in every direction, libration in longitude of –6° although the motion peaks exposes the west limb to view. toward the upper right and In Sky & Telescope, the format lower left. Note how the curve is somewhat different. The almost returns to its starting dates when the curve is either point after 27.3 days, the farthest from or closest to the sidereal period of the Moon centre of the graph are given, (count the loops to see this). along with the name of a Figure 2 shows how the prominent crater. In both appearance of a hypothetical publications the diurnal term ‘crater’ (representing Mare Figure 3. Lunar libration for two is ignored. Take care to Crisium) varies with libration. years, diurnal term suppressed. (All understand the format in use! Two years of libration are figures plotted with data from GSO plotted in Figure 3. At times Further readings from your Moon.) the curve describes roughly Centre library: circular arcs, with the V.A. Firsoff, The Old libration remaining close to 6° at a perigee and apogee. Moon and the New. CCN:66 point that moves smoothly around The RASC Observer’s Zdenek Kopal, The Moon. the limb. At other times the curve Handbook lists dates of minimum CCN:43 !

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