MAKING A TELESCOPE

Updated March 16th 2004

Simple plywood construction: The following designs are typical of the types of telescopes made by some of our Club members.

Prints 9 pages letter size.

First of all - some words on overall design

The main (or Primary) mirror can be either made - or you can buy a ready-made mirror of the size and focal length you would like. The following is based on a 6" or 8" diameter mirror although the Construction and other details may apply equally to larger mirrors.

The size of the diagonal (secondary) mirror is important. ( These are usually purchased, as they are relatively inexpensive - perhaps \$15 or so.) You must decide on how much fully-illuminated sky you are going to see at the lowest magnification of your Telescope. This is usually only about 1/2 to 3/4 of a degree. If you choose more than that it will lead to a large and 'obstructive' size of the 2nd mirror, which should not be more than about 20% of the diameter of the primary mirror. If you make it larger then it causes a major change in the 'diffraction pattern' of the stars you see.

We have a formula to calculate the size needed :

Size of diagonal = { ( D - d ) L /F } + d

where D = diameter of primary mirror,

L = distance inside of the focal distance of primary mirror.

F = focus (of the primary mirror)

and d = the size of the image-field reaching the eyepiece.(That is the inside of the focus-mount on the side of the tube.)

These measurements are illustrated below:

As you will see this is exaggerated a bit to show the diagonal placed inside of the main mirror focus. We could simply place a screen at the focus of the mirror and look at the image. But if we stick our head in front of the 'scope we will block all the light reaching it! So we place a diagonal mirror some distance inside to project the image out of the side of our telescope tube. There we put a 'focus-mount' which can hold various eyepieces of differing strengths. This determines the magnification of our image.

The size of our diagonal, which is an elliptical shape because it will be mounted at a 45 degree angle, is always specified as the narrower dimension. Now the whole thing is going to depend on the 'field of view' which we decide to have. It is usually about 1 to 1-1/2 times the diameter of the full moon. That is about 1/2 to 3/4 of a degree.

The size of the moon's image at the focus of the mirror, depends only on the focal length of whatever size mirror we use. (Only the focal length - the diameter of the mirror makes no difference.) It is calculated by multiplying the focal-length by .009 ! So with an 8" mirror at 64" focus the image will be .576" Now if you compare that with the image of the moon formed at the back of your eye - which has an average focus of about 1" the image in your eye will be a mere .009" . Wow !! So the 'magnification' inherent in the telescope is .576" / .009" = 64 times ! We must use an eyepiece of 1" focus to match the focus of our eye-lens, then we will see an image of the moon 64 times larger than when we look at it directly.

I'm digressing here - so lets get back to the image size that we figured our mirror is going to give us. (.576") This will be the factor 'd' in our calculation above. Let's say too that our tube will have an outside diameter of 10" To project our image outside of the tube we must 'shoot' it at least half of that diameter. Also we are going to have a focus mount and we want our image to form up inside that where our shortest focus(Highest magnification) eyepieces can view it. So we add say another 1-1/2" to 2" for that - depending on the height of the fully collapsed focusser. So we arrive at a distance of 5" + 2" = 7" from the center of our diagonal mirror. This will be the factor "L" in our calculation. The focal length is 64" so our mirror must be mounted 64' - 7" (57") from the face of the main mirror.

So in the case of our 8" mirror, and designing for a field of 3/4 degree (almost 1-1/2 Moon diameters, i.e. 1.4 x .576 = .81 ) we would have: Diagonal = { (8 - .81 ) x 7 / 64 ) } + .81 = 1.6" (approximately) The preferred size would be 1.6" across the 'minor axis'. However these items usually come in a series of 'standard sizes' and we might have to settle for a 1.5" mirror. This would decrease our field of view slightly, but not enough to worry about. It is only 0.1" smaller than our calculated size. We could also use a 1.8" size which would increase the design field a little.

One important note here -- the diameter (minor axis) of the diagonal mirror is normally held to be about 16 to 20 per-cent of the diameter of the main mirror, for visual use. (20% is the maximum we recommend) If the diagonal is much larger than 20% of the main mirror diameter - then the "diffraction disc" is adversly affected. This reduces detail and causes a more objectionable 'flare' on brighter images. It may also cause some reduction in contrast and an increase in 'coma' at the edge of the field of view. Even though this may (at 16% especially,) reduce the 'fully illuminated field' to perhaps less than half the field seen by the lower magnification eyepieces you use, for visual use this is hardly noticable by most people. Remember that the 'magnitude level' assigned to the stars we see is based on a change of some two and a half times in brightness before the eye notices that one star is definitely brighter than another. (In 5 'magnitudes' a given star is 100 times brighter than another)

Also note that with a smaller diagonal (16%) you may need to move the diagonal and focus-mount closer to the main mirror to get enough 'sideways throw' to put the image in the right place outside the tube, in order to reach focus with your eyepieces - this means a reduced field of view, although this can still be quite enough for visual work. Or you may find you can do ir by using a very 'low profile' focus-mount.

We usually make the inside diameter of the telescope tube a bit larger than the mirror, firstly to accomodate the mirror's mechanical support, and also to allow space at the front end for the 'spider' which holds the diagonal mirror. There are a other reasons for making the tube's inside diameter larger : One is that air currents (due to temperature changes) tend to run along the outer walls of the tube and can interfere with 'seeing.' The other reason is that the edges of the mirror must be able to 'see' at least a half to three quarters of a degree, otherwise the outer areas of the mirror will not see past the front edges of the tube. For our 8" mirror I would suggest about 9-1/2" to 10."

So now you've got all the numbers you can start building the tube, along with the mirror support, and the diagonal mount, as well as the focus mount.

If you want to be really 'basic' the tube can be made of plywood, square in section - or you can make or buy a round metal or fibreglass tube of the size and length you need. As to the length - it is usually about the same as your mirror's focal length or a few inches longer. Allowing perhaps a couple of inches for the mirror and its mount at the back of the tube, and taking into account that the diagonal is going to be 7" inside the focus - then its best to add a couple of inches to the tube length. Otherwise the diagonal will be quite close to the front end. This can allow 'dewing' of the diagonal on colder, damp nights. So its best to add a little to the tube length so the diagonal is further inside the end.

The inside of the tube should be painted a very flat black to prevent other external light from 'bouncing around' inside.

On the left is a drawing of a simple plywood design, based on the "Dobsonian" mount. There are probably as many variations in the Dobson-mount as there are home-built versions of it. It is a simple ALT-AZIMUTH type of mount. That means that to follow the motions of stars etc., you must track in two directions - up and-down, and left and right along the horizon. For general 'stargazing' this is no real inconvenience. The 'rocker box' swivels on a pivot at the center of the base-plate which has three small Teflon pads (about an inch square or less) These are placed at 120 degree intervals on a circle of about 10" or 12" diameter, and the upper face of the bottom of the box is covered with a layer of Formica (best) or Arborite, which allows enough friction against the teflon pads to give a very smooth, but not jerky movement. The front-board is made low enough that the tube can be placed horizontal, and another brace can be put across the back, if the tube does not strike it when completely vertical.

If this type of unit is made with a round tube - then you need to make a small frame around the tube where the 'rocker wheels' are fastened. Under the base plate you need three small blocks arranged like the bottom of a tripod so that it stands evenly on any kind of surface. (We have used 3 hockey-pucks in some cases, they are ideal - they don't rot and it doesn't matter if you are standing it on wet or dewey grass in your backyard.)

On top of the tube I have shown a small 'sighting scope' This can be as simple as a hollow tube or can be a couple of small rings (like a gun-sight) which allows you to aim the scope at the various objects in the sky. When you are ready to attach the rocker-wheels, have everything mounted on the tube - including whatever eyepiece you have (the heaviest one) and balance it across a small round dowel to find the point of balance. The centers of your wheels should coincide with this point. Perhaps make it slightly ahead of this - so that the tube is if anything, a little back-heavy. You only need to add a small weight at the front to balance it afterwards, but if you add anything in the future such as a heavier focusser or sight scope, it will need a lot more weight at the back to re-balance it.

On each of the sides of the rocker-box section where the wheels on the tube rest, put two small teflon pads about 3/4" x 1" towards the outside of the 'arc' these will act the same way as the pads underneath to allow slight friction when the tube is tilted, but again will be quite smooth and not jerky. The wheels are made from 3/4" plywood - as are the rocker-box and the base-plate at the bottom. The tube can be 1/4" ply and will be rigid enough to hold the optics without sagging.

MOUNTING THE MIRRORS

The mirror should be mounted in such a way that there are no stresses placed on it. A very simple way to do this is to put three blocks or supports in a circle just slightly larger than the mirror, spaced at 120 degrees, with two of these positioned at the lower part of the tube section. (see diagram) Behind the mirror and screwed through the back plate of the tube are three 1/4 x 20 machine screws - about 1/2" inside the edge of the circle, on which the back of the mirror rests.

As you can see from the diagram - the mirror rests on the two lower blocks. In normal use this is enough to prevent it from moving sideways, but puts no stress on the mirror. The support blocks should be slightly tapered downward toward the rear - this makes the mirror lean back and rest against the three screws coming through the back-plate. The screws should be tipped with a tiny piece of thin plastic or leather to prevent metal to glass contact. In the front of each block is fastened a safety clip - to prevent the mirror from tilting forward. The end of these clips should overlap the edge of the mirror by perhaps 1/8" or 3/16"

This way of mounting the mirror is easy to do, and is the way we recommend - unless you have bought a metal (usually aluminum) mount specially made for the purpose. There are also some designs for wood versions of mirror-mounts - but we'll leave you to investigate those on your own !

Mounting the diagonal mirror

This mirror must be supported as carefully as the main mirror. A poorly mounted diagonal can easily ruin the quality of image which the primary mirror can produce. Like the first mirror, it must be supported without stress - and it must also be made adjustable so as to align the reflected image exactly with the center of the focus mount. There are diagonal mounts available commercially from various astronomy suppliers which satisfy these conditions.

You can however save some expense by making your own, so I'll describe a method which we have used which is not difficult to make. See diagram : The main support is an Aluminum disc about 1 to 1-1/4" diameter and some 5/8" thick. Through the center is a 1/4" hole for the screw which holds a small metal fork. In the fork is a hardwood block with the front cut at a 45 degree angle on which is glued a piece of 1/4" plywood cut to an elliptical shape but a little smaller than the diagonal itself. The diagonal is fastened to this flat plate by using three small 'blobs' of silicon caulking compound. These are arranged in a 120 degree pattern so that they hold the diagonal, which is carefully centered to the axis of the 1/4" screw which holds the fork. The diagonal is placed gently onto the 'blobs' with the mounting plate held horizontally - until the silicon dries.The silicon holds the mirror quite firmly but puts no stress on it

Most diagonals are made with the top and bottom areas cut at a 45 deg. angle so that when mounted they produce a circular shadow onto the main mirror, so some care is needed to center them correctly. The 'spider' vanes which hold the complete assembly to the tube can be brass or steel - about 1/2" wide and long enough to reach across the tube and have some left to bend over at the ends, and also to allow for the part which fits across the groove in the aluminum block. When fitting them into the grooves, you can use a center-punch to tighten up the grooves by punching a few spots alongside the blade. If necessary, if they are a bit loose in the groove you may have to 'shim' them with some very thin metal foil.

If the block is some 5/8" thick you can cut grooves perhaps a little more than 3/8" deep - using a hacksaw with a blade that cuts about a 1/16" groove to match the thickness of the vanes. The vanes can be marked where they should bend and be bent at least partially before inserting them. The grooves are cut so that the two blades, when fitted can be bent so as to reach across to each side of the tube - and also cut so that the vanes are 'in-line' at opposite sides of the block.,

When fitted to the tube - the mirror can be moved forwards or back to align it exactly to the center of the focus-mount, By adjusting screw passing through the aluminum block. It can be rotated to align the mirror exactly perpendicular to the focus-tube. The diagonal can also be tilted in the fork to make it exactly 45 deg. with the optical axis of the primary mirror.

eyepieces Choosing your eyepieces (Some guidelines for selecting eyepieces.)

words A few words about other types of tubes. (Typical amateur telescopes section)

The earlier diagram shows a square-section tube. The tube is not fastened to the rocker-base section but simply 'sits there' under its own weight. So it is easy to pack into your car - and head off to a nearby 'dark site' well away from the city lights for better viewing ! Below is another design using the same plywood construction, This has a 2-piece tube section. It is also a round tube which is split into two sections joined together about 1/3rd the way from the back.

This would produce a three-piece telescope which can fit easily in a small car for good portability. It can be assembled in a matter of less than a minute. If you find cutting 5 separate rings from your supply of plywood is wasteful (which it is !) you can do the same thing with a square tube arrangement - except that the 'rings' will now be squares which can be made up of four straight pieces of wood. Also you could use the same 'strut' connections between the sections and make a round paper or card tube inside. We don't recommend an 'open' tube design.The closed tube serves a few purposes : It keeps other light off the optics and also prevents disturbance of the image caused by air currents (or wind) moving through the optical train. It also protects the primary mirror from 'dewing' and helps keep it clean.

THE BASE

the base unit is made from 5/8" or 3/4" plywood. It is basically a 'box' open at the rear and top, with 2 half-circles cut at the top of each side-piece about 1/8" larger than the diameter of the wheels on the tube section. This allows for the addition of the two teflon pads on each side upon which the wheels rest.

The bottom of the box has a 3/8" or 1/2" bolt through the center to join it to the baseboard underneath. When fitted to the base the bolt should be long enough to allow for the teflon pads. When tightened up it should have no play - but should be loose enough to prevent binding against the teflon pads, and a lock nut should be added.

The side supports should be high enough to let the tube be tilted to the vertical position and clear the bolt holding the base-section. It can be made higher if you find that the eyepiece is at too low a level when looking at stars etc., high above. If you extend the height too much for this purpose - you might need to add some extra bracing to keep the sides from spreading.

So far this is about the simplest type of reflector telescope, but there are some disadvantages to making an 'alt-azimuth' type of mount. Firstly to follow the stars as they move across the sky you need to move both up or down (as they rise and set - just like the Sun and Moon) and along the horizon. Secondly if you ever hope to try your hand at taking photographs of stars or nebulae it is virtually impossible to track them this way for more than a second or two (perhaps for a picture of the Moon - which can be done in a fraction of a second with the right film.)

There are methods of 'computerising' the motions with electronic control of small drive motors for each motion, but this still limits the time avalable for photography as the image at the eyepiece will rotate. For simple visual work though this can make viewing much easier. Another possible answer to this is to make a "Poncet" type of mount which allows perhaps a half-hour or so of 'equatorial' drive. If you are likely to want to try astro- photography - then it is better to start with an equatorial-mount. This will be described further down in this page.

Before that - I should tell you how to align the optics once your telescope is completed : You must be sure to put the center of the focus-mount directly over the center of the diagonal mirror. It should also be exactly perpendicular to the length of the tube. (If need be, you may have to 'shim' it to make it so.)

Then extend the focus mount close to its maximum height and with no eyepiece in place, look down at the circle of the bottom of the eyepiece tube. You should see a reflection of the main mirror in the diagonal . You may also see part of the inside of the tube, an off center image of the diagonal and your own eye in the center of that, as shown in "A" The diagonal is adjusted to bring the image of the main mirror's outline (ignore the reflections in that mirror for now.) until the outline of the main mirror is concentric with the outline of the eyepiece tube- as shown in 'B'

It can help to keep your eye located in the center of the eyepiece tube - if you find a plastic 'cap' or insert which fits the tube, and drill a small hole ( about 1/8" or less) in it so as to locate your eye centrally.

Once you have it set-up so that it looks like the drawing 'B' you move to the rear of the telescope. At this point it helps to have someone to assist. Then the 3 screws at the back against which the main mirror is resting are adjusted, one at a time to bring the reflected image of the diagonal and its 'spider' or support, into the center of the field of view.

Finally you should arrive at an adjustment where the image will appear as in 'C' You will also see the reflection of your eye, in the center of the diagonal. When you arrive at this point - your optics will be mechanically aligned as accurately as you can do. Any further refinement of these adjustments can only be done by using special 'alignment-eyepieces' or by adjusting on the slightly out of focus image of a bright star. For normal viewing - the adjustments given above should provide good images when viewing through the telescope.

AN EQUATORIAL MOUNT

If we placed our alt-azimuth telescope at the North or South poles of the Earth we could follow the motions of the stars with a single motion of the telescope. This is because our base pivot would be an extension of the Earth's axis. At any location in between - we can get the same effect if we make the axis around which our telescope turns parallel to the axis of the Earth !

To do this we must tilt the main axis towards the N. or S. pole (depending whether we are North or South of the Equator) See diagram:

As shown if you are located at 40 degrees North latitude, you would tilt the axis towards the North pole at a 40 degree angle. (If you were South of the equator at 40 degrees South latitude you would tilt it at 40 degrees towards the South pole.) Then if you are following a star you need only to turn your 'scope around this axis to keep the star in view continuously. The addition of a 'clock-drive' can make this even easier. Once you have sighted on a star you need only 'start the drive' and concentrate on the view !

The axis which is tilted - around which the telescope now turns is called the "POLAR AXIS" and is also referred to as the R.A.(Right ascension axis) and the new axis around which the tube turns (which must still be at right-angles to the other) will be called the Declination axis These terms describe the positions of the stars and planets etc., on the Celestial-Sphere. The sky is imagined to be a huge sphere which forms the background for the stars. It is extended from the Earth's equator and the ' celestial equator' will appear to be located 40 degrees above your horizon if you are located at 40 degrees latitude. The axis around which the 'celestial sphere' revolves extends from the Earth's poles. The locations of the stars - their Declination for example is their position above and below this imaginary equator in the sky. Their position along the 'celestial equator' is referred to as their Right Ascension. All sky atlases and maps will show these postions as R.A. and DEC.

There are many different types of equatorial mounts but they all follow the principles outlined above. The easiest to make is described next :

The first shaft is the 'polar axis' and the second, which carries the tube is the declination axis. The tube shaft has to have a counterweight to balance it as it is rotated. This can be made of any heavy material, lead, Steel and sometimes concrete which can be cast into a tin - with a wooden centerpiece to make a hole through which the shaft fits. Some plastic, sandfilled weights used for weightlifting may be adapted also to this purpose. The shafts (for an 8" tube") which may weigh some 15 to 20 pounds with the diagonal, focusser and sight-scope etc., should be of fairly large diameter - about 1-1/4" to 1-1/2"it is very easy to underestimate the rigidity of these shafts - which will lead to vibration of the image, especially at higher magnification.

The shafts can be made from plumbing piping (cast iron) and pillow blocks or brass bushings for the bearings. The pipes can be smoothed with emery cloth - and tee-fittings used to join the declination shaft to the polar shaft. Mount the tube support fairly close to the upper end of the other one, as long as it will easily clear the tilted block at the top of the main support. A 'saddle' is fastened to the top of the polar-shaft and the tube can be fixed to it with a pair of flexible metal straps. If possible you should provide for loosening these in order to rotate the tube. In some positions of this type of mount - the eyepiece can assume some awkward positions for viewing. Turning the tube can bring the eyepiece back into a more comfortable position. nother thing - sometimes the bottom of the tube may 'conflict' with the main support, whether it is a tripod type or a pier - in this case you simply rotate the tube around the declination axis by 180 degrees and line it up again.

There are lots of books available from Publishers or your Library on the subject of telescope-making, with plenty of designs for all types of telescopes. This web-page is really written for the beginner - and I would recommend that to start out - make the easily constructed Alt-azimuth type of unit. You can always build an equatorial mount for your tube when you have learned more about this subject.

If you would like plans for a "Dobsonian" telescope, courtesy of the San Francisco Sidewalk Astronomers member "Ray Cash" Click on this link

Good luck with your projects - and HAPPY STARGAZING !