DOBSONIANS

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DOBSONIANS150mm/1000mm200mm/1200mm254mm/1200mm

TABLE OF CONTENTSAssembling Your TelescopeParts ListBase AssemblyOptical Tube AssemblyOperating Your TelescopeAligning the FinderscopeFocusingUsing the optional Barlow LensUsing the Tension Control HandlePointing the DobsonianCalculating the Magnification (power)Calculating the Field of ViewCalculating the Exit PupilObserving the SkySky ConditionsSelecting an Observing SiteChoosing the Best Time to ObserveCooling the TelescopeAdopting Your Eyes3345666677888999999Proper Care for Your Telescope10CollimationCleaning Your Telescope1011Technical Support11Before you beginThis instruction manual is applicable to all the models listed on the cover. Read the entiremanual carefully before beginning. Your telescope should be assembled during daylighthours. Choose a large, open area to work to allow room for all parts to be unpacked.Caution!NEVER USE YOUR TELESCOPE TO LOOK DIRECTLY AT THE SUN. PERMANENTEYE DAMAGE WILL RESULT. USE A PROPER SOLAR FILTER FOR VIEWING THESUN. WHEN OBSERVING THE SUN, PLACE A DUST CAP OVER YOURFINDERSCOPE TO PROTECT IT FROM EXPOSURE. NEVER USE AN EYEPIECETYPE SOLAR FILTER AND NEVER USE YOUR TELESCOPE TO PROJECT SUNLIGHTONTO ANOTHER SURFACE, THE INTERNAL HEAT BUILD-UP WILL DAMAGE THETELESCOPE OPTICAL ELEMENTS.

PARTS LIST1. Base (Part 1)A1A2BPackage 2 (1 tube, 1 bolt,2 washers, 1 nut, 1 Teflon pad)Package 1 (14 screws,14 screw caps)Package 4(1 handle)DCPackage 5( 2 screws,1 Alan wrench)Package 3 (1 accessorytray, 3 screws)Package 6(4 cylindrical saddleside bearings,4 screws)Package 7(1 handle, 1 tensioncontrol handle)handletension control handle2. OTA (Optical Tube Assembly) (Part 2)Finderscope and bracketFinderscopemounting slotFocus locking screwRubber O Ring(Remove before installation)Two eyepieces(One 2" eyepiecefor models withPyrex glass)Dust cap/mask(remove before viewing)SecondaryFocusing Knobmirror positionSide bearingPrimary mirrorposition32" Eyepiecesadapter (notincluded in modelswith Pyrex glass)

BASE ASSEMBLY1. Connect the board B to theboard A1 and A2. Make surethe Sky-Watcher logos onthe board A1 and A2 are onthe outside. The side of theboard B with three small holesshould be facing the front.2. Connect the assemblyto the round board C.Sky-Watcherlogo on theother sideSky-Watcherlogo on theother side3. Place the teflon pad in betweenthe assembly from step 2 and theboard D. Insert the black tubefrom package 2 through the holesin the centre of all the above.A2A2A1A1This side onthe outsideCBD4. Take the bolt from pacage 2 with awasher on it and insert it through thetube. Secure with second washer andthe nut using the two wrench provided.Do not over-tighten the bolt. This willprevent the mount base from freelyspin on the bottom round board.5. Position the accessory trayover the 3 holes in the boardB and secure with the 3 smallscrews provided.6. Attach the handle from package4 to the board B with the twoscrews and the Alen wrenchfrom package 5.7. Unassemble the cylindricalsaddle side bearings and thescrews from package 5. Attachthe cylindrical saddle sidebearings to the inside of board Aand A1 using the screws provided.C8. Cover all visible small screws with the screw caps provided4

OPTICAL TUBE ASSEMBLY9. Place the optical tube between the boardA1 and A2. Make sure that the side bearingsof the tube are rested on the 4 cylindricalsaddle side bearings.10. Install the handles from package 7into the holes on the board A1 andA2. Do not over tighten the handles.Make sure that the tension controlhandle is installed on the same sideas the focuser for your convenience.12. Slide the finderscope bracket11. Locate the finderscope bracket13. Loosen the two adjustinginto the mounting slot and tightenand carefully remove the rubberscrews on the bracket. Positionthe screw to hold the bracket ino-ring from it. Position the o-ringthe finderscope into its backetplace.into groove on the finderscope tube.by sliding it backwards untilthe rubber o-ring seats. Alignas described below14. Remove the black plastic end-cap.Loosen the thumbscrews on theend of the focus tube if the end-capcan not be removed.15. Insert the desired eyepieceand lightly tighten the thumbscrews to hold the eyepiecein place.5

Aligning the FinderscopeFig.aThese fixed magnification scopes mounted on the optical tube arevery useful accessories. When they are correctly aligned with thetelescope, objects can be quickly located and brought to the centreof the field. Alignment is best done outdoors in day light when it'seasier to locate objects. If it is necessary to refocus yourfinderscope, sight on an object that is at least 500 yards (metres)away. Loosen the locking ring by unscrewing it back towards thebracket. The front lens holder can now be turned in and out tofocus. When focus is reached, lock it in position with the locking ring(Fig.a).Fig.b1. Choose a distant object that is at least 500 yards away and pointthe main telescope at it. Adjust the telescope so that the objectis in the centre of the view in your eyepiece.2. Check the finderscope to see if the object centred in the maintelescope view is centred on the crosshairs.3. Use the two small alignment screws to centre the finderscopecrosshairs on the object. The screws work in opposition to aspring-loaded knob (Fig.b).FocusingFig.cSlowly turn the focus knobs (Fig.c), one way or the other, until theimage in the eyepiece is sharp. The image usually has to be finelyrefocused over time, due to small variations caused bytemperature changes, flexures, etc. This often happens with shortfocal ratio telescopes, particularly when they haven't yet reachedoutside temperature. Refocusing is almost always necessarywhen you change an eyepiece or add or remove a Barlow lens.Using the Barlow lens (optional)A Barlow is a negative lens which increases the magnifying powerof an eyepiece, while reducing the field of view. It expands thecone of the focussed light before it reaches the focal point, so thatthe telescope's focal length appears longer to the eyepiece. TheBarlow should be inserted between the focuser and the eyepiecein your Dobsonian (Fig.d).In addition to increasing magnification, the benefits of using aBarlow lens include improved eye relief, and reduced sphericalaberration in the eyepiece. For this reason, a Barlow plus a lensoften outperform a single lens producing the same magnification.However, it is greatest value may be that a Barlow can potentiallydouble the number of eyepiece in your collection.6Fig.dEyepieceBarlow

Using the Tension Control HandleFig.etension controlLoosen or tightend the tension control handle to add just enoughfriction to allow the tube to move easily when nudged but to stayin position when not. It may be necessary to re-adjust the tensioncontrol handle when accessories are added to, or removed from,the tube. The tension control handle should be installed on thesame side as the eyepiece for your convenience. Leave thehandle on the other side slightly loose to allow full tension controlfor the tension control handle (Fig.e).Pointing the DobsonianPointing an altitude-azimuth (alt-az) mounted telescope, such as a Dobsonian, is relatively easy. With themount level, you can swivel the telescope around on a plane parallel to your horizon and then tilt it up anddown from there (Fig.f). You can think of it as turning your telescope in azimuth until it is facing the horizonbelow a celestial object and then tilting it up to the object's altitude. However, the Earth rotates and thereforethe stars are constantly moving, so to track with this mount you have to constantly nudge the optical tube inboth azimuth and altitude to keep the object in the field.In reference material for your local position, the altitude will be listed as degrees (minutes, seconds) aboveor below your horizon. Azimuth may be listed by the cardinal compass points such as N, SW, ENE, etc., butit is usually listed in 360 degree (minutes, seconds) steps clockwise from North (0 ), with East, South andWest being 90 , 180 and 270 , respectively (Fig.f).7

Calculating the Magnification (Power)The magnification produced by a telescope is determined by the focal length of the eyepiece that is usedwith it. To determine a magnification for your telescope, divide its focal length by the focal length of theeyepieces you are going to use. For example, a 10mm focal length eyepiece will give 80X magnification withan 800mm focal length telescope.Focal length of the telescopemagnification 800mm Focal length of the eyepiece10mm 80XWhen you are looking at astronomical objects, you are looking through a column of air that reaches to theedge of space and that column seldom stays still. Similarly, when viewing over land you are often lookingthrough heat waves radiating from the ground, house, buildings, etc. Your telescope may be able to givevery high magnification but what you end up magnifying is all the turbulence between the telescope and thesubject. A good rule of thumb is that the usable magnification of a telescope is about 2X per mm of apertureunder good conditions.Calculating the Field of ViewThe size of the view that you see through your telescope is called the true (or actual) field of view and it isdetermined by the design of the eyepiece. Every eyepiece has a value, called the apparent field of view,which is supplied by the manufacturer. Field of view is usually measured in degrees and/or arc-minutes(there are 60 arc-minutes in a degree). The true field of view produced by your telescope is calculated bydividing the eyepiece's apparent field of view by the magnification that you previously calculated for thecombination. Using the figures in the previous magnification example, if your 10mm eyepiece has anapparent field of view of 52 degrees, then the true field of view is 0.65 degrees or 39 arc-minutes.Apparent Field of ViewTrue Field of View Magnification 52 80X 0.65 To put this in perspective, the moon is about 0.5 or 30 arc-minutes in diameter, so this combination wouldbe fine for viewing the whole moon with a little room to spare. Remember, too much magnification and toosmall a field of view can make it very hard to find things. It is usually best to start at a lower magnificationwith its wider field and then increase the magnification when you have found what you are looking for. Firstfind the moon then look at the shadows in the craters!Calculating the Exit PupilThe Exit Pupil is the diameter (in mm) of the narrowest point of the cone of light leaving your telescope.Knowing this value for a telescope-eyepiece combination tells you whether your eye is receiving all of thelight that your primary lens or mirror is providing. The average person has a fully dilated pupil diameter ofabout 7mm. This value varies a bit from person to person, is less until your eyes become fully dark adaptedand decreases as you get older. To determine an exit pupil, you divide the diameter of the primary of yourtelescope (in mm) by the magnification.Diameter of Primary mirror in mmExit Pupil MagnificationFor example, a 200mm f/5 telescope with a 40mm eyepiece produces a magnification of 25x and an exitpupil of 8mm. This combination can probably be used by a young person but would not be of much value toa senior citizen. The same telescope used with a 32mm eyepiece gives a magnification of about 31x and anexit pupil of 6.4mm which should be fine for most dark adapted eyes. In contrast, a 200mm f/10 telescopewith the 40mm eyepiece gives a magnification of 50x and an exit pupil of 4mm, which is fine for everyone.8

Sky conditionsSky conditions are usually defined by two atmospheric characteristics, seeing, or the steadiness of the air,and transparency, light scattering due to the amount of water vapour and particulate material in the air.When you observe the Moon and the planets, and they appear as though water is running over them, youprobably have bad "seeing" because you are observing through turbulent air. In conditions of good "seeing",the stars appear steady, without twinkling, when you look at them with unassisted eyes (without atelescope). Ideal "transparency" is when the sky is inky black and the air is unpolluted.Selecting an observing siteTravel to the best site that is reasonably accessible. It should be away from city lights, and upwind from anysource of air pollution. Always choose as high an elevation as possible; this will get you above some of thelights and pollution and will ensure that you aren't in any ground fog. Sometimes low fog banks help to blocklight pollution if you get above them. Try to have a dark, unobstructed view of the horizon, especially thesouthern horizon if you are in the Northern Hemisphere and vice versa. However, remember that thedarkest sky is usually at the "Zenith", directly above your head. It is the shortest path through theatmosphere. Do not try to observe any object when the light path passes near any protrusion on the ground.Even extremely light winds can cause major air turbulence as they flow over the top of a building or wall. Ifyou try to observe on any structure, or even a sidewalk, movements you make may cause the telescope tovibrate. Pavement and concrete can also radiate stored heat which will affect observing.Observing through a window is not recommended because the window glass will distort imagesconsiderably. And an open window can be even worse, because warmer indoor air will escape out thewindow, causing turbulence which also affects images. Astronomy is an outdoor activity.Choosing the best time to observeThe best conditions will have still air, and obviously, a clear view of the sky. It is not necessary that the sky becloud-free. Often broken cloud conditions provide excellent seeing. Do not view immediately after sunset.After the sun goes down, the Earth is still cooling, causing air turbulence. As the night goes on, not only willseeing improve, but air pollution and ground lights will often diminish. Some