Mark's Telescope Building Page

This page will contain information about how to build your own telescope.

This page is only temporary while I construct it properly.

Mark's 8.75" Dobsonion Telescope

Building your own telescope isn't that difficult, especially with all the help and information available in books and in various places on the Internet. A couple of years ago I decided to embark on the task of having a go myself.

This is page is the story of how I designed and constructed it. I hope to encourage others to build their own telescopes, share those ideas that worked for me, and those that didn't (!). Hopefully it will be entertaining too.

In future versions of this page I'm going to add some introductory information for beginners to telescopes and astronomy, but for now here's the technical data:

Newtonion reflecting telescope on a Dobsonion type mount
222mm (8.75 inch) main mirror
44mm secondary mirror
Focal length of 1600mm (63 inches)
Focal Ratio of about 7.3
1.25 inch focuser
Bought mirrors and focuser, built virtually everything else
Cost between 400-500 UKP to construct
Took about 3 months of weekends and occasional evenings from start to "first light", but has been continually "tweeked" ever since!
Materials: plywood, chipboard, broken binocculars for finder, paint & screws etc and a few odds and ends scrounged from various places.
Tools: saws, planes, hammers, screwdrivers. Nothing that the average DIY guy doesn't have.

I'll be breaking this page into separate areas and detailing the construction more fully, but for how here's some pictures and "running commentary".

Clicking on any of the pictures will bring up a larger version.

The tube is made from a number of octagonal rings constructed from 1/2 inch softwood. They provide the main strength (and also double as internal baffles). 1/8th inch plywood panels are glued and screwed over this to produce the tube itself. One of these sides is screwed with brass screws and cups and can be removed to gain access to the inside of the tube. This has proved useful for general maintenance and was very handy for working out the initial position of the focuser. The inside of the tube is painted flat black and at the opposite side to the focuser I have glued sandpaper strips which are also painted black. This improves the contrast. A couple of cupboard handles are fastened onto the tube to make handling easier. All wood used in the project was treated with waterproof sealant and was well sanded and painted after construction. Joints are made with "no nails" cement and also screwed. It's probably over-engineered but I wanted something that will last a long time.

Yours truly posing with my pride and joy. You might notice that the rocker box is rather short and squat, and there's a good reason for this. Originally I'd planned on mounting the bottom on an equatorial wedge (photo further down), thinking that this might make finding objects a bit easier. Because of this the whole scope would be tilted at an angle of 34 degrees (as I live around 56 degrees North). This means that the centre of gravity had to be very low down otherwise it would fall outside its own base and the scope would tip over!

What I hadn't realised about an equatorial mount is that unless there is some method for the tube to rotate, the eyepiece will end up at all kinds of strange angles depending where in the sky the telescope is pointing. Okay, you can stop laughing your arses off now, I've learned that lesson!

Notice on this one that there is a fair amount of added weight at the back of the telescope. Most of them are those weights used on old fashioned balance scales, painted gold and bolted to the back. There is also a highly technical counter balance which can be adjusted to cater for different eyepieces and finders. This consists of a swiped "tupperwear" box full of led sheet and sealed with duck tape. The adjustment comes from the fact that it is fastened on with velcro and can be moved up and down the length of the tube within seconds. Yup, extremely low tech and tacky, but simple and effective!

This is the scope on its original equatorial wedge mounting. The idea is to point the axis of rotation at polaris which then means that when observing an object one only needs to nudge the scope in one axis rather than two. Two major problems have made me abandon this idea. The first one, mentioned above, is that the eyepiece ends up in strange places, and the second being that although the weight is very low down the whole mount is too shaky to be practical. Shortly after trying the scope in this configuration I returned to the more conventional "Dobsonion" type mount.

Linda, my beautiful wife, posing with the telescope in the "solar observing configuration". The whole tube and tube box is turned upside down and is replaced in the rocker. This points the focuser downwards where it projects an image of the sun onto the attached white screen. The light plywood sheet sprouting from the front provides a nice shadow for contrast. The finders unclip from the tube which means the tube still balances with all the extra gubbins bolted on. (The tupperwear box can always be moved for fine tuning). This photo was taken at a popular local spot where members of the public were encouraged to meet the sun's features up front and personal.

This is a close up picture of the base. The two outer walls are made from two layers of 1/8th inch plywood sandwiched around 1/2 in softwood battons. This gives a very stiff and strong construction but without too much weight. The visible white discs are "furniture sliders" which are used in leiu of the traditional teflon. (easier to obtain around here).

The white disc with the "V" cut out at the top left is part of a clutch mechanism which is used to stabilise the scope in breezy conditions.

This shot shows the bottom of the rocker lifted from the triangular base plate (1/2 inch chipboard). The same furniture sliders are used for azimuth movement, and are positioned over the three wooden feet for stability.

Originally I used a loose bolt in the centre as the pivot, but this quickly proved a bit sloppy. The scope now uses a tapered roller bearing which was saved when I replaced the steering head bearings on my Suzuki GS750 motorcycle. (nothing like recycling!). The base plate holds the track and the roller bearings are fitted to the bottom of the rocker with a piece of appropriately sized dowel. (okay, a bit of broom handle). The strange shaped holder for the bearing track is simply an offcut from some other part that I couldn't be bothered to shape further.

The circular bottom of the base is painted and glossed which I hoped would provide a nice surface for the teflon sliders to run on. It's not bad, but the bumpy "ebony star" type surface is said to be much better. The white rim is the remains of grease that I occasionally apply to keep it lubricated. My next scope will use an "ebony star" type approach and I wouldn't recommend the glossed paint.

This photo shows how the tube box fits around the tube. The box is constructed in two halves which are fastened together with hinges. The box clamps around the tube and fastens with a couple of brass clips. The tube is kept secure by pieces of foam which are compressed by the tight fit of the box. The idea of this arrangement was to allow the tube to be slid up and down for balance, and also to allow it to be rotated to some extent. In practice I've never done either, so the next scope will probably have the side bearings directly fastened to the tube.

Ah... my scope's bum - an elegant sight! The mirror cell is made from two pieces of pine separated by three springs which are threaded onto bolts. The large wing nuts visible in this picture are threaded onto these bolts and provide the adjustment for collimating the main mirror. Nice bit nuts make for easy adjustment in the dark. Inside each wing nut is a small locking bolt which fixes the collimation in place. The whole cell assembly is bolted to an octagonal frame piece with T nuts. If I were to redo this bit I'd probably leave space for ventilation, but keeping the scope in an outside shed means that cool down is quick anyway.

The side bearings are made from 1 inch sections of drain pipe which were fitted over plywood discs, glued, and then screwed to the rocker box. The rather nice scales were lovingly and painstakingly painted by my son. Thanks Richard!

The clutch mechanism can be seen. A strong spring (value return string from a motorcycle engine head) pushes against a wooden disc which is lined with sandpaper. This pushes against the plywood disc inside the side bearing. It is adjusted with a large wing nut. This has proved to be a much used mechanism to stiffen the movement in wind and when enthusiastic children are around at public gatherings.

The weird looking white box at the top is my red LED light which is velcroed to the tube when not in use. You can see the bottom of a clipboard that also lives on the tube where I fasten observing notes and maps to help star hop. I put clips at either end of the board so the notes don't fly off during windy conditions.

This is a close-up of the side bearings. There is an "inspection hatch" which gives access to the main mirror. This can be useful when collimating, or to allow the mirror to cool down. I tend to keep it closed to keep dust and passing cats out.

I have two finders connected to a single base plate which is clipped to the main tube. The whole assembly can be unclipped for carriage in the car, or to be replaced with different finders.

The near finder is a conventional one built from half of a pair of broken 10x50 binoculars. The objective and eyepiece are mounted in plastic tubes (old drain pipes), and there is a long due shield at the front. The eyepiece was adapted with a "cross-hair" made from thin wire at the focal point. I utilised the screw focus adjuster from the original binoculars. This allows me to focus the finder both with and without my eyeglasses. In practice I tend to observe with glasses or contact lenses as I have astigmatism.

The second is a "red dot" finder which works by projecting the light from an LED, through a small lens. The finder is used by keeping both eyes open and looking at the LED with one eye and the sky with the other eye. The LED appears to be projected into the sky. The lens is required to remove much of the apparent parallax when ones head is moved from side to side. It's like a poor man's Telrad and costs only pennies to make. I found the idea on Stephen Tonkin's excellent site, where you can find out much more. http://www.aegis1.demon.co.uk/gallery/red-dot.htm

The rather unusual looking finder holder is made from aluminium. I tried the traditional two ring arrangement with three bolts at 120 degrees apart, but found them a real pain to adjust. As the scope gets moved about a lot this current arrangement is more practical. There are just two adjustment bolts per finder. The front bolts move each finder laterally and the rear bolts move them up/down.

Secondary and Spider

I've not taken any piccies of these yet so a description will have to suffice. My spider was made from a small circle cut from wood, into which four vanes are attached at 90 degrees. The vanes are made from long hacksaw blades which were ground to remove the teeth. The vanes fasten to the tube walls with bolts. There is sufficient adjustment to allow the spider to be centralised in the tube.

The central disc is drilled and has a bolt which fastens to the back of the secondary holder. The secondary itself is simply a wooden disc cut at 45 degrees using a simple mitre template. Adjustment is made by three further small bolts fastened through the spiders central disc at 120 degrees apart. The whole assembly is painted using the flat black paint that is designed for painting school chalk boards.

Future

I'm currently reworking the scope to computerise it using Mel Bartels excellent software. As this adventure unfolds I'll be adding a new page to this site to explain how things pan out.

I am deeply grateful for the ideas and help and support from other telescope builders around the world. Some of this has come from books, but many ideas have also come from other telescope builder's web sites. I humbly hope that I've added just a little here to the helpful and generous ATM community. When time permits I will be adding a link section to other good sites and the ATM e-list which is a friendly and knowledgeable source of help and encouragement. Thanks folks!

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Page last updated: 11th Nov 2000