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A Telescope Operating Manualfor Celestron 8-inch Schmidt Cassegran Telescopes

Even though these are very good telescopes, somefirst-time observers are disappointed with the view.The images are small and the observations are diffi-cult to make and require practice and effort. Perhapswe have all been spoiled by television images fromspace telescopes or the wonderful color pictures onthe web that are the result of hours-long timeexposures with the largest and best telescopes in theworld.

Still there is a real thrill in following in the footstepsof the early astronomers, in seeing what they saw,and in discovering for oneself what they discovered.In this way beginning astronomers become one withthe ancients and relive one of the greatest adventuresof discovery that the human race has experienced.Here is to many hours of happing observing!

I. Introduction

This booklet is a handbook for the use of the lab tele-scopes. In it you should find all the information nec-essary to become an expert in operating the lab tele-scopes.

Read it more than once and review it each weekbefore lab. Bring copies of any useful pages to labuntil you are comfortable working without them.Remember as you read this document, the telescopeyou will be using is delicate and can become severe-ly damaged unless the proper rules of operation arefollowed. These rules are in bold faced typethroughout this handbook and collected togetheron page 9. Please memorize these rules and followthem as you use the telescope. Also please report anytelescope problems or difficulties to your lab instruc-tor or lab assistant.

2. The Celestron Telescope

There are two models of Celestron telescopes inservice in astronomy labs—the older telescopes,which have orange tubes and newer models withblack tubes. Both types are functionally equivalentand with slight differences between them. Learningon one will teach you to operate the other.

Figure 1 is a photograph of a Celestron assem-bled and ready for use. The photograph names ofmost of the parts used in operating the telescope.Figure 2 shows the tailpiece of an assembled tele-scope. Study these photographs and familiarize your-self with the part names before you first work withthe telescope. In addition it is useful to know a littleabout the purpose of each part and the special cau-tions to be observed when using the telescope.

On the following pages is a glossary ofCelestron telescope part names with a descriptionand a few important facts about each. It is helpfulfor the student to memorize the important cau-

tions that are shown in boldface type. They shouldbe followed or serious telescope damage will cer-tainly result.

Glossary of Celestron parts:

base—the part of the telescope that attaches to thewedge and contains the clock drive.

clock drive—the motor in the base of the telescopethat allows a telescope to follow the stars. Inorder for this function to operate properly, thetelescope must be mounted on an equatorialwedge that is corectly aligned with the Earth’saxis of rotation.

corrector plate—a thin lens of low refractive powerthat is the front optical element of a Schmidt-Cassegrain telescope. Never touch this glass ortry to clean or wipe any condensation off thiscorrector plate.

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Figure 1 A Celestron 8-inch Schmidt-Cassegrain telescope assembled on a pier.

finderscope

tube

fork tine

base

wedge pier

declination slow-motion knob

R.A. lock

R.A. slow-motion knob

declination lock

focus knob

tailpiece adaptor

star diagonal

eyepiece

declination lock—the lock on the declination axis ofthe telescope located at the top of one of the forktines. See Figure 3. This lock must be loosenedbefore slewing the telescope in the direction ofchanging declination or the mount will bedamaged. This lock is operated by turning thelock knob to the left to unlock and to the rightto lock the declination axis. It is important tohold the telescope while unlocking the declina-tion axis and usually on turn or less will loosenthe lock enough. When locking the axis, turnthe know only enough to keep the telescopefrom moving. It is unnecessary and damagingto the mechanism if one overtightens this

knob.

declination setting circle—the scale on the end ofthe fork tines, which shows the declination angleof the telescope. See Figure 4. This angle tellshow far north or south of the the celestial equatorthe telescope is pointed.

declination slow-motion drive arm—the drive armlocated inside one of the fork tines and whichdrives the telescope in the direction of changingdeclination. See Figure 5. It is important tocenter this arm at the beginning of an observ-ing session. Stop immediately and check thisarm if the declination slow-motion knob shows

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tive and make it viewable by the human eye. Donot to touch the glass parts of the eyepiece.Different eyepieces produce different magnifica-tions and different size fields of view.

finderscope—a small telescope with a wider field ofview than the main telescope. It aides in locatingobjects for the main telescope.

focus knob—an adjustment knob on the back of thetelescope that adjusts the sharpness of the imageyou see. There is no need to use this knob unlesssomething is visible in the telescope field of view.No amount of turning this knob will makesomething miraculously appear. One must havean image before that image can be focused.

fork tines—the arms of the telescope mount whichsupport and carry the telescope tube on its decli-nation axis. The declination lock is on one of thefork tines.

lens cap—with the Celestron telescope, the cover forthe front element which is the corrector plate.

the slightest bit of increased resistance toturning. Not doing so will result in the declina-tion slow-motion drive becoming stripped andunusable.

declination slow-motion knob—the knob at thebase of one of the fork tines which is used tomakes slight changes in the declination setting ofthe telescope. Unlike the R.A. slow-motionknob, the declination slow-motion knob isoperated with the declination lock fully locked.However if this knob shows the slightest bit ofincreased resistance to turning, it is importantto stop immediately and check the declinationslow-motion drive arm. It may have reachedthe end of its travel and will need to be reset tothe middle of its track. Not doing so will resultin the declination slow-motion drive becomingstripped and unusable.

eyepiece—an optical element consisting of severalcarefully spaced lenses in a tube. Its purpose is tomagnify the image formed by the telescope objec-

Figure 2 The tailpiece end of the telescope showing the parts used for focusing and viewing. Most of theseparts are stored after an observing session and part of telescope assembly is to properly install these parts.

tailpiece

focus knob

star diagonal

eyepiece

set screwfor

eyepiece

set screwfor

star diagonal

tailpieceadaptor

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right ascension (or R.A.) slow-motion knob—theknob which is used to make slight changes in theright ascension setting of the telescope. It isessential to partially unlock the right ascen-sion lock before using the right ascension slowmotion knob. This knob should never be usedif any force greater than a light finger touch isneeded to operate it. Not following these pro-cedures will always result in the right ascen-sion slow-motion drive becoming stripped andunusable.

star diagonal—an accessory with a mirror or prismand which directs the light from the telescopethrough an angle of 90° thus making overheadobjects easier to view. Usually our labs use oneeven when not viewing overhead. One note ofcaution is in order. This device inverts the image,making it upright from the astronomical tele-scope’s normal upside-down nature, however theimage is also reversed from left to right. Thisfeature is inconvenient when using maps.

tailpiece—the part of a telescope where the focusedlight exits the tube and various accessories (ey-epiece holder, star diagonal, camera, etc.) areattached.

pier—a permanent support for a telescope. With theCelestron telescope it is the support structure towhich the wedge is attached.

rear cap—the cover of the tailpiece opening. With most telescopes it is a yellow rubber cup.

right ascension (or R.A.) lock—the lock on the tele-scope base which controls slewing in the direc-tion of right ascension. See Figure 7. This lockand its associated mechanism is very delicateand damage to it is a perenial problem forbeginners. Proper use of this lock is essential toavoid any damage to the R.A. drive mechan-ism. This lock must be unlocked in order tomove the telescope through various angles ofright ascension. This lock must be partlyloosened before turning the RA slow-motionknob. Not doing so will result in the R.A. slow-motion drive becoming stripped and unusable.

right ascension (or R.A.) setting circle—the scaleon the telescope base which shows the angle ofright ascension (and the hour angle) at which thetelescope is pointed. See Figure 10. The rightascension angle is measured in hours (1 hour =15°) eastward along the celestial equator from thevernal equinox.

Figure 3 A photograph of the modified declinationlock at the top of the fork tine. The lock is operatedby turning the knob to the left to unlock the declina-tion and to the right to lock the declination. If it doesnot operate properly, consult your lab instructor.

lockedposition

unlockedposition

Figure 4 A photograph of the declination settingcircle. The use of this setting circle is detailed in thesection on setting circles.

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is attached the forks will point in the generaldirection of Polaris (for reasons you will learnabout later). The bolt holding the wedge shouldbe fastened as tightly as possible. After this, the

tailpiece adaptor—the accessory which threads ontothe tailpiece and adapts the tailpiece to acceptother accessories. The adaptor shown in thishandbook is for visual work and holds a star diag-onal or an eyepiece.

tube—the telescope itself which contains the mainoptical components. See figure 8 for a diagramshowing the path of light in this tube.

visual back—Celestron’s proprietary name for thetailpiece adaptor that holds an eyepiece or stardiagonal for visual observation.

wedge—the supporting structure of the telescopewhich attaches to a tripod or pier and to which thebase of the telescope is attached. The wedge isadjustable in order to align the telescope with thepolar axis of the earth. For proper alignment thewedge allows the clock drive of the telescope totrack the stars and planets. Figure 5 A photograph of the declination slow-

motion drive arm. (Note: some Celestron telescopeshave only one declination slow-motion knob.)

dec. slow-motion drive arm

dec. slow-motion knob

forktine

3. Telescope Assembly

Before assembling the telescope be certain you arefamiliar with the names and purposes of the parts ofthe telescope in the previous section and especiallyimportant to follow the proper procedures for han-dling the telescope otherwise it can be damaged.Even the way to carry the telescope is important,otherwise it can be seriously damaged simply bypicking it up improperly.

When lifting and carrying the telescope it is best tohold the scope by the fork tines. This approachallows a good grip and puts no strain on any of thelocks. While moving the telescope take care to guardagainst bumping or moving the tube. Whenever thetube is moved to point in a new direction the begin-ner is wise to recite the sutra: “unlock, move,relock.” Whenever the tube is moved, the first thingto do is unlock one or both of the locks, then movethe tube, and finally relock everything after the tele-scope is properly pointed.

The steps in assembling the telescope are listedbelow:

1. Attach the wedge to the pier. The wedgeshould be orientated so that when the telescope

Figure 6 The wedge with the bolt installed on thetelescope base being slid into the slot on its face.

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The design of the star diagonal and tailpieceadaptor is fully idiot proof. They will fit togeth-er only one way. The star diagonal is secured inthe tailpiece adaptor by a small thumb screw.This screw is to be loosened each time the stardiagonal is rotated to change its orientation andthen tightened afterwards.

6. Insert eyepiece in the star diagonal. There aretwo different types of star diagonals in use andthe means of securing the eyepiece is differentin each. If the tube designed to accept the eye-piece is chrome, then the tube holds the eye-piece by friction. If you eyepiece seems tooloose with this type of star diagonal, then youmust call your instructor or the lab assistant toadjust it. If the tube designed to accept the eye-piece is black, then the tube holds the eyepiecewith a small thumb screw. It is wise in using thisscrew to loosen it only enough to remove theocular and no more. This trick guards againstthe thumb screw falling out and being lost in thedark.

7. Remove the lens cap (if present). You are nowready to collimate your finderscope and beginviewing.

wedge should not be moved in the course of anormal observing session.

2. Attach the telescope to the wedge. See Figure6. Thread one of the mounting bolts partiallyinto the base of the telescope and, cradling theinstruments in one arm, slide its base against themounting plate of the wedge so that the boltslips into the slot on the mounting plate. Tightenthis first bolt to make the telescope secure butloose enough for the telescope base to be able tomove slightly against the wedge mounting platein order to align the holes for the threading ofthe other two bolts. While one observing partnercontinues to hold the telescope as insuranceagainst its falling, the other person can insert theother two bolts through the plate of the wedgeand into the base. Secure the bolts–they do nothave to be as tight as the bolt holding the wedgedown.

3. Remove the rear cap.

4. Attach the tailpiece adaptor. Hold the tailpieceadaptor flat against the tailpiece and thread theattachment collar on the tailpiece being certainto guard against cross threading.

5. Insert star diagonal into the tailpiece adaptor.

Figure 7 The parts used in telescope assembly.

Allen wrench

Allen bolts

eyepiece

star diagonaltailpiece adaptor

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6. Do not attempt to clean any dirty glass sur-faces. Tell your instructor or the lab assis-tant. Cleaning requires special fluids andtissues. The antireflection coatings are softand even kleenex tissue is like sandpaper tothese coatings.

7. Do not store mechanical parts such as boltsand wrenches with the eyepieces and the stardiagonal.

8. Since the glass parts are fragile, please bevery careful and guard against breakage.

9. All optical parts (eyepieces and star diagon-al) must be stored in proper containers (pillbottle or ziploc baggie).

10. Don’t take the eyepieces apart.

11. Think first before doing! Or ask.

4. Important Cautions

Note: never attempt to use the Celestron telescopeor its finderscope to observe the sun.

When operating the telescope, technique is every-thing. Give the telescope the greatest of care. If youattend to the following cautions, the telescopes willgive semester after semester of faithful service.

1. Do not force any movements. Unlock, moveand relock to adjust anything on the scope.

2. Always stop turning any slow-motion knob ifyou feel more than the slightest resistance.

3. Lift and carry the telescope by the fork tinesonly.

4. Never unbolt the telescope from the wedgeunless someone is holding it.

5. Do not touch any glass surface. The oils andacids in fingerprints will deteriorate the anti-reflection coating on the lenses.

Figure 8 A photograph of the base of the fork tines of a newer Celestron showing the right ascension lockand associated parts. Notice the warning label on the use of the R.A. slow-motion knob. Failure to follow thiswarning will damage the R.A. slow-motion function and make the telescope unusable. The sidereal time indi-cator is located elsewhere on the base of the older telescopes but its function is similar.

R.A. lock

R.A. slow-motion knob

sidereal time indicator

R.A. indicator

R.A. setting circle

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around it. At night select a object shown againsta lighter background if possible.. A tower withthree red lights is a poor choice because it isdifficult to be sure of which red light is visible inthe main telescope. The student might have thetelescope pointing at one light while the finder-scope is actually centered on another. Theobserver could think that collimation was perfectbut actually the telescope would be cross-eyed.

3. Center the finderscope on the object. Loosenthe telescope locks, grasp the telescope by thetailpiece and the tailpiece adaptor and point thefinderscope at the object. Center the middle ofthat object exactly behind the “X” of thecrosshairs. Retighten the telescope locks.

4. Check the main telescope for the object. Lookthrough the main telescope. If the same object iscentered in the field of view of view, then thefinderscope doesn't need collimation and thenights work can begin. Otherwise see #5.

5. The object is visible but not centered in thetelescope. If the object is visible but isn't in theexact center of the main telescope, then a smallamount of adjustment is needed. See the nextprocedure in this section. Otherwise see #6.

6. The object is not visible in the telescope at all.If the object is not even present in the field ofview of the telescope, then the finderscope mustbe collimated from scratch. See the paragraphbelow.

The goal here is to somehow center the maintelescope on the selected object, and then adjust thefinderscope screws to cause the finderscopecrosshairs to point to the very same object. In thissection when the procedure says exactly centered, itmeans exact. The finderscope’s usefulness as afinding aid is directly dependent on how carefullyand exactly this task is done. The following stepswill lead you through the process. Proceed to theappropriate step below depending on how great themisalignment is as determined above.

1. If your selected object is in the finderscopebut not visible in the main telescope begin the

5. Finderscope Collimation

The field of view of the Celestron telescope isrelatively small even at low powers. Finding evenbright objects without some optical aid is difficult.Finding fainter objects would be a matter of luck,even for the skilled observer. To overcome this limi-tation the telescope is equipped with an excellentfinderscope, a low-power, wide field of view tele-scope similar which facilitates locating an object inthe field of view of the main telescope.

The finderscope eyepiece is fitted with crosshairs composed of fine wire. Their purpose is toprovide an aiming point in the large field of view ofthe finderscope. The idea is that when some object isdead center behind the cross hairs of the finderscope,then it will also be centered in the field of view of themain telescope. In order for this idea to be true, thefinderscope has to be collimated, that is adjusted topoint in exactly the same direction as the main tele-scope

When a finderscope is installed on a telescope,the mounting bracket is designed so that the finder-scope points in approximately the same direction asthe telescope, usually within one or two degrees ofeach other. (The full moon has a diameter of aboutone-half of a degree). Thus the two are almost colli-mated as they come from the factory. All that is leftis final adjustment to bring the two into collimation.In theory the two should remain in collimation, butthe breakdown and reassembly of the telescopesresults in small misalignments which must be cor-rected before a night’s observing can begin.

The first step after assembly is to check finder-scope collimation. The steps in checking finderscopecollimation are listed below:

1. Focus the finderscope. If your finderscope hasa focusing adjustment, focus the finderscope foryour eye. Some finderscopes are fixed focus,and some allow for focusing by turning a rubber-ized collar. In your main telescope use the eye-piece with the longest focal length).

2. Select an object. Select an object. This selectionis important. It should be on the Earth and atleast 500 meters away. It should be easily foundand easily distinguishable from the things

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limation is complete. The telescope is nowpointing at the object and the step is to make thefinderscope point in exactly that same directionby adjusting its aim using the screws which holdit in its bracket.

4. Check the finderscope mount. Check andmake sure that the screws holding the finder-scope in its mount are lightly finger tight. If anypart feels loose, ask your instructor or teachingassistant.

5. Adjust the finderscope screws. Look throughthe finderscope. You should see your collima-tion object but it will not be exactly behind the“X” of the crosshairs. Without moving the tele-scope turn the adjustment screws on the frontring of the finderscope mount and watch whichway the crosshairs move. If you have them, donot use the screws on the rear mounting ring.The adjusting screws are always worked inpairs, loosening one, tightening the other, andalways keeping the screws lightly finger tight onthe finderscope. Select any pair then loosen onescrew a half turn while tightening the other ahalf turn. Evaluate how the crosshairs appearedto move when you turned the screws. Did theyget closer to the object? If not, then reverse whatyou just did and try another pair of screws.Using various pairs of screws you should be ableto move the finderscope so that the point wherethe crosshairs meet is exactly centered on thecollimation object.

6. Check the collimation. Check to make sure theobject is still centered in the main scope. If itisn't, something moved and you must return tostep 3 or earlier depending on how much it wasmoved. If it is centered, collimation is done forthe rest of the evening. You should not touch thefinderscope screws again unless your finder-scope is bumped out of collimation.

It is significant that there is so much space inthis handbook on finderscope collimation. This taskshould be done perfectly or at least very well in orderfor your telescope to be fully functional. It is also themost involved technique a beginning astronomer hastoo learn, and reading about it before trying to do itcan be a great benefit. Be familiar with the task by

collimation process here. As noted above thefinderscope is aimed within a few degrees ofyour main scope because the bracket holding thefinderscope causes the two of them to point inthe same approximate direction.

2a. Find the object in the main telescope method1. Since you are close to the target only smalladjustments are needed. There are two tech-niques. Try them both and use the one whichworks best for you. First, in the finderscope scanthe area around the target object for clues onnearby things to look for in the main telescopethat might lead you to the object. Next, lookthrough the main telescope, loosen only one ofthe locks holding the telescope axes, and movethe telescope very slightly back and forth whilelooking in the main telescope for the selectedobject or recognizable nearby things. Try thesame technique with the other axis until you getthe object in the field of view. Once you havethe object in the field of view of the telescope,go on to step 3. If you have difficulties with thismethod try the technique suggested in step 2bbelow.

2b. Find the object in the main telescope method2. Some observers can aim the telescope bysighting along the tube and place the desiredobject somewhere in the field of view of themain telescope. The idea here is to hold the tail-piece and its adaptor while backing away fromthe telescope by the length of your arm. Theobject in the field of view of the telescopeshould appear to be sitting right on the end ofthe tube when your eye is situated such that youcan see none of the tube. See Figure 12. Onceyou have the selected object in the field of viewof the telescope, go on to step 3.

3. Center the object in the field of view. Usingthe slow motion controls to position the objectexactly in the center of the field of view of thetelescope. (Note that for changes in declination,the declination slow-motion control is used withthe declination lock in the locked position; butfor changes in right ascension, the right ascen-sion lock must be partly loosened before theright ascension slow motion knob is turned.)Once this step is done the locks and the slow-motion controls are not adjusted again until col-

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both and use the method that works best foryou. The first technique, called the two eyemethod, is fast and easy and does a verygood job if you can make it work for you.Grasp the telescope by the tailpiece, loosenthe locks on both axes, and point the tele-scope in the direction of the object you areseeking. Keep the locks undone and lookthrough the telescope with one eye while youkeep the other eye on the object—that iskeep both eyes open. This way you will seetwo images superimposed: one image is ofthe finderscope field of view with thecrosshairs and the other is the object in thesky which you are trying to acquire.

Next move the telescope by the tailpiecein such a way that the finderscope crosshairsseen with one eye appears to move towardthe object seen with the other eye. As youcontinue to move the telescope in thisfashion the object will suddenly jump intothe finderscope field of view after which youshould lock both axes so as not to lose theobject. Next loosen one axis at a time andmove the telescope until the object is closeto the “X” of the crosshairs after which theslow-motion controls will allow it to be cen-tered behind the “X.” (Note that for changesin declination, the declination slow-motioncontrol is used with the declination lock inthe locked position; but for changes in rightascension, the right ascension lock must bepartly loosened before the right ascensionslow motion knob is turned.) Now yourobject should be visible in the main tele-scope.

For those people who have difficultlywith the two eye method, the other approachinvolves sighting along the tube of the maintelescope. To begin grasp the telescope bythe tailpiece, loosen the locks on both axes,

6. Finding Bright Objects andUsing the Telescope

Note: never attempt to use theCelestron telescope or its finderscope toobserve the sun. The Celestron telescopehas a large light gathering power designedfor faint objects and it must be modifiedwith special accessories to convert it tosolar use. Ignoring these fact can result inserious permanent eye damage and blind-ness.

This section details the techniques forusing a telescope. It includes focusing thefinderscope, using the finderscope to findbright objects, working with the locks andslow-motion controls, focusing the maintelescope, and tips on observing.

First check the focus of your finderscope.Some finderscopes are fixed focus, and someallow for focusing by turning a rubberizedcollar or some other arrangement. Ask yourinstructor or lab assistant if you are unsure.If your finderscope has a focusing adjust-ment, then focus the finderscope for youreye using a distant object on the horizon orin the sky.

The major hurtle for the beginning astro-nomer is finding the object to be observed.Various methods are used to locate skyobjects with a telescope, ranging from coarsenaked eye sighting along the tube to precisepinpointing by the use of setting circles.Don’t expect to find much of interest in thesky by random sweeping. The sky is largecompared to the field of view of your tele-scope and you really must know exactlywhat you are looking for and how to getthere.

There are two methods commonly usedto find bright objects and place them in thefield of view of the main telescope. Try them

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astronomy to 25 feet as used in nature study.If you are trying to focus the telescope andyou are turning the focus adjustment in theclockwise direction, then you are moving thefocus in the direction of focusing on verynearby objects. In astronomy all objects areat infinity and theoretically at the same focusadjustment. In practice with groups ofobservers having different eyesight, a turn ortwo of focus adjustment, but no more, mightbe necessary to obtain clear focus. If theperson who used the telescope before youignorantly turned the focus adjustment allthe way to close focus hoping to “getsomething in focus,” then an equal amountof adjustment will be required to reverse themistake. In these situations, it is often wiseto get the help your lab instructor who isexpert at detecting this problem, unless youare certain of the situation.

Unless someone before you has grosslymisadjusted the focus or it has been usedinside for some lab exercise, one or twoturns of the focus knob should be plenty tobring a distant subject into focus. An imagethat is far out of focus will look like a bigwhite light or, if the misadjustment is not toobad, a donut of light, the hole actually beinga silhouette of the secondary mirror and itssupport. An out of focus object will get

and point the telescope in the direction of theobject you are seeking. Keep the locksundone while you hold the telescope at armslength and sight along the telescope tube insuch a way that none of the tube is visibleand the object appears to be sitting on the farend of the telescope tube. If done properlythe object should be in the field of view ofthe finderscope after which you can followthe procedure described at the end of theparagraph above to get the object in the maintelescope. The geometry of this method isdepicted in figure 12.

Once the object is in the main telescopethe telescope will have to be focused foreach different observer. Each person’s eyesare different and these differences can becorrected with the focus adjustment. Anobserver can view and focus with or withoutglasses. Never turn the focus knob unlesssome distinct form of light can be seen in theeyepiece. The verb “focus” means to makesharper, not to make something miraculousappear.

The Celestron telescope design allowsfor focusing from infinity to as close asabout 25 feet. A considerable amount offocus adjustment is required to move thefocus from infinity focus as is required in

Figure 9 A drawingdemonstrating thegeometry of sighting astar along the top ofthe telescope tube.The observer’s eyeshould be as far backas possible and noneof the tube should beseen, but both thefront and rear rings onthe tube should bevisible and the starshould appear to sit onthe front ring.

*

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watch for those moments of good seeing.That is when you will see the views that aremissed by the careless or impatient observer.These brief moments of good seeing are thereason why an artist with a telescope canmake an accurate and more detailed drawingthan a photograph can because turbulencemakes the image move during the course ofthe photographic exposure while the artistcan freeze in his mind an image of the objectduring those split seconds of good or excel-lent seeing that occur even on nights ofaverage or poor observing conditions.

This kind of observing requires practiceto develop. The experienced observer with atrained eye can see subtle colors and detailsthat are missed by the observer new toastronomy. Take every opportunity as abeginning observer to extend your vision andlook for those subtle features on objects thatcan be seen with our telescopes. Look for aminute or two—not the five or ten secondsthat is typical of most beginners. Also keepyour hand on the focus and adjust it slightlyeach way to be sure to obtain that perfectfocus. Remember if there is wind or othersources of vibration to keep one finger onthe back of the telescope applying very slightlateral pressure to dampen some of the vibra-tion.

smaller as the focus improves, with thesmallest image being the one that is in thebest focus. As you focus watch the imagecarefully trying to see that position of perfectfocus. The better your job at focusing, themore you will be able to see.

Once you have focused on an object, takeyour time to observe what you see. Youshould notice that the image may “dance”around due to turbulence in the earth’satmosphere. (Some dancing can be due tovibrations in the telescope mounting or tubeand these can be much reduced by simplyplacing one finger against the back of thetube and applying a slight steady lateral pres-sure.) This turbulence is the same phenome-non which causes the stars to twinkle and itaffects the observing condition we callseeing which is a measure of how steady theimage is. On nights of poor seeing theatmosphere causes the image to look biggerand to dance around more. On nights of goodseeing the image is smaller and moves verylittle; much more detail is visible. But theseeing conditions are not constant and evenon a poor night, the atmosphere can besteady for a split second, allowing the care-fully observer to glimpse small details suchas the Great Red Spot on Jupiter or the polarcap on Mars.

When you look through a telescope