6 inch reflector telescpe

7/30/2019 6 inch reflector telescpe

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THE 6-inch f/8 Newtonian reflectingtelescope is probably the most popular

star gazing instrument in use by amateurastro nomer s. This is easily under stoo dwhen we examine the instrument in moredetail.

The "6-inch" refers to the diameter ofthe objective or main mirror of the telescope and thus affects the diameter of themain telescope tube. The figure "f/ 8" is anoptical expression derived from the ratiobetween the focal length and the diameterof the telescope mirr or. In this case, thefocal length is 48 inches or 8 times themirror diameter of 6 inches.

The name "Newtonian" refers to theover-all design of this type of reflectingtelescope in which rays from the objectiveor main mirror are focused onto a smallermirror known as the "diagonal." The diagonal is set further up in the telescope tubeat a 45 angle to the main mirror and theeyepie ce. Its function is to reflect th e focused rays from the objective into the eye

piece where they are refocused and

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magnified for close viewing of the image.It is enough to say that the Newtonian

design is simply the most practical andleast expensive for nonprofessional instruments . The wor d "reflecting," of course,distinguishes this scope from a refractor inwhich the light rays are "bent" throughand focused behind a lens instead of being"bounced" away from and focused in frontof a mirror.

Since the above description pretty wellfixes the size of. ou r reflector we c an se eone reason for its popu larit y. It's abo utthe largest telescope you can comfortablycarry. Large r reflecting instrumen tsupto 8 inches in diameterare almost alwayssemiportable, with usually a permanentmount and perhaps a transportable maintube. Much larg er reflectorsthose over8 inches in mirror diameterrequire theconstruction of a permanent observatory.

Another obvious reason for the popularity of Newtonian reflectors this size orsmaller is price. To bu y a completely

finished and assembled 6-inch reflector of


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this type would cost you about $200. Pricesof larger instruments soar drastically forthe finished produ ct. You can, howev er,own a high quality instrument for a fraction of the price if you build or assembleyou r own telescope. The object of thischapter is to help you do just that.

REFLECTOR TELESCOPE KITSYou

will note that there are two lists of partsprinted on these pages. Th ere is an

economy kit of parts for around $27 andanother kit, illustrated here, for $150. Whythe larg e difference? Pri mar ily , it is thedifference between buying a finished parabolic mirror and grinding your own.Grinding astronomical mirrors is a fascinating hobby in itself. It does, however, require a healthy degree of skill and patience.We have chosen, therefore, to illustrate theassembly of scope parts that requires onlythe use of simple hand tools and a minimu m of time. If you elect to grind you r

own mirror, the telescope assembly instructions shown here will still apply.


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Another price factor is the inclusion ofa good equat orial moun t. You can buildyour own simple mount or purchase a lessexpensive altazimuth mount. But wehighly recommend an equatorial mount forthe serious star gazer. As you becomemore and more advanced in the science ofastronomy, you will need an equatorial

mount and you can save a considerableamount of money if you purchase one atthe start.

With an equatorial mount you canuse setting circlesaccessories that allowyou to "dial" the exact sky object you wantto see by checking its location in a staratlas. A clock driveessent ial for anytype of asti'ophotography more complicated than snapping a picture of the moonalso requires an equatorial mount.

Having acquired your telescope parts,you are ready to go to work . First, let's

Mousetail hack saw blade with pistol grip can

be used to cut hole. Aluminum is easy to work.

Circle is notched for rack on eyepiece tube and

holes are drilled ior bolts holding the mount.

examine our assemblage of optical equipment.

Basically, we have a big mirror and itsmount that will occupy the lower end of a

large aluminum tube and a small mirrorand eyepiece holder that will occupy theupp er end. The corre ct plac ement of thes eparts in the tube is our only major assembly problem and the key to this problem isthe focal length of the main mirror.

Our large mirror, being concave, willconverge light rays from the sky to a pointin front of itthe focal point. A small,concentrated image will form at this point,which your eyepiece, in turn, will enlargeto the view you see when you actually obser ve the heave ns. The small diagonal

Alter rough cutting, smooth to perfect circle with

semiiound file. Check with the eyepiece rube.

Finished cutout for eyepiece mount. The hole at

middle left is for the diagonal mirror holder.


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A S T R O N O M Y


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mirror catches the image at this point andreflects it into the eyepiece tube . To dothis, the focal point must be placed justoutside the eyepiece tube of the eyepiecemount when this tube is at its furthest "in"position. Th e reason for this location isthat eyepieces, having their own focalpoints of varying lengths, must have roomto focus on the image or focal point formed

by the big mirro r. A sho rt focal lengt h(high po wer ) eyepie ce will focus close tothis "in" position, but longer focal length(low pow er) eyepieces will need more"out" travel and will focus when the eyepiece tube is extended to nearly its fulllength or furthest "out" position.

Now we know where we want the focalpoint of the main mirror but have to figureout how to get it the re. Before proceeding,it should be mentioned that manufacturedmirrors may vary slightly in focal lengthfrom the given figure. A tolerance of about

Although the location oi the finder scope is notcritical, it miut be parallel to the main tube.

Line up mount and tube holes. Mount must be onside of tube opposite eyepiece for best viewing.

an inch and a half is gen eral ly allowe d. A48-inch focal length mirror may actuallybe of 49% -in ch f.L, the refo re. A smallsticker on the back of the mirror usually

gives the exact figure, which you shouldcheck carefully. The main tube should beslight ly long er than this focal leng th. Weused a 5-foot tube, but this could have beencut to 50 inches.

BEFORE DRILLING into your tube, placethe main mirror in its mount. Now measurethe distance from any one mounting holeon the mirror-mount to the front edge ofthe mirro r. Jo t this figure down for futurereference. You may now pu t you r mirr oraway in a safe place as you will not need it

Locate and drill holes for finder bracket at pointout oi the way oi the main scope eyepiece.

Main tube is tightly attached to the head oiequatorial mount using %" dia.. 2" stove bolts.


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Equatorial mount (parts shown below) Is betterior observing than an altazimuth or other type.

Finder telescope is simply slipped into mounting bracket and adjusted for the best operation.

A sturdy tripod is absolutely essential lor observing; vibration makes any viewing impossible.

Adjusting knob on legs of tripod makes it possible to level the head on a variety oi terrain.


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I.VHil'i;i'hiM

The equatorial mount is bolted to tripod. It permits scope to be set parallel to the earth's axis.

Completed equatorial mount looks like this. Thescope carriage must be parallel to polar axis.

Polar axis must be set for latitude of observer.Check the mount head to see that it is level first. Set a protractor at your local angle of latitudeand clamp it to an ordinary carpenter's level-

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A S T R O N O M Y

Completed equatorial mount on tripod with scope

attached gives sturdy platform for best viewing.

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Secure the telescope mirror in its mount before

bolting it into position at lower end oi tube.

again until ready for the final assembly.Next, drill the holes in the big tube that

will hold your mi rro r-m ou nt Th e exactdistance from the lower end of the tubeis not critical; one of two inches is sufficient. It is essential, however, that themirror sits squarely in the tube. Now, fromany one of these mounting holes, measurealong the tube the distance you have jotteddown. Mark this point with great care.It shows you the position the front surfaceof your main mirror will occupy within thetube and it is the point from which youwill measure to locate the position of youreyepiece mount.

Now for some simple, but vital, arithmetic. Meas ure the outsi de dia mete r ofyour main tube (in this case, 7 inches)and divide by 2. Let 's call this figure "A. "Next, place the rack-and-pinion eyepiecemount on the exterior of the main tubeand measure the distance from the surfaceof the main tube to a point about a quarterof an inch above the end of the eyepiecetu be at its fullest "in " position. Let's callthis measu red num ber figure "B. " Addfigure "A" and figure "B ." The n subt ract

the sum of these two figures from the focallength of your main mirror. That's theend of you r arit hmet ic. Ta ke th e finalfigure and measure along your main tubefrom the point you jotted down and mark edwith such great care as described in thepreceding paragr aph. This point is theexact center of a circular hole you will cutout of the main tube to receive the movingtube of your eyepiece rack-and-pinionmount.

Attach the eyepiece holder to the tube. From inside the tube, next insert the diagonal mirror.

Read all of the above paragraph overbefore doing any of the work on your telescope tube . It is the most critical stepin the whole assembly of your instrument.

Further assembly is relatively simpleand can be done by closely following theste p-b y-s tep photograph s. Her e ar e a fewpointers:

Remember that when drilling holes toattach the main tube to the equatorialmount that these holes should be on justthe opposite side of the main tube from theeyepiece rack-and-pinion mount holes.

This will give you the most comfortableposition when observing with your eyepieces for most sect ions of the sky. Youmay center the main tube on the carriageof your equatorial mount or find a moredelicate balance by assembling all opticalparts and locating a fulcrum where the entire assembly balances.

Remember to level the head of the equatorial mount before setting the polar axisto an angle equal to your own latitude.

Paint the exterior of your scope anycolor but the interior color must be the

flattest optical black possible. Exterior isperhaps most often a clear white, which iseasiest to see on a dark night.

Collimation of your optical system whencompletely assembled can be done in a variety of ways. Sketches in this chapte rshow a simple, effective method.

Weights on the declination axis of theequatorial mount may be adjusted to balance the main tube assembly so that itswings freely on the polar axis.


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Photo shows one possible

location for finder where

it won't interfere 'with

operation of regular eye

piece. Finder bracket is

left, the eyepiece at right.

Finder scope is inserted in

place parallel to main scope

tube as shown here. Finder

is centered with the main

scope optics by means of

the three setscrews shown.

Final step in adjusting the

telescope finder is to focus

it on an object at infinity.

The finder is indispensable

in aiming the main scope

for observation purposes.


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EHS

Final steps include insertion oi eyepiece (aborejand adiiutment oi counterbalance weights (right)to offset weight of main tube and optics. Entireassembly should swing freely on the polar cads.

6 inch reflector telescpe

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