A PDF required for this How To is available here.
Small refractors are light and portable.
What’s more, their optics need little or no adjustment so they are ideal scopes if you’re a beginner.
Building your own small refractor from simple components will help you understand how they work.
We guarantee you’ll get a great sense of achievement when you glimpse Jupiter’s moons for the first time, and the whole project shouldn’t cost you more than £50.
The heart of any refractor is its convex glass objective lens, which is mounted close to the front of the hollow telescope tube.
Rays of light from distant objects like the Moon pass through this and are bent or ‘refracted’ (hence the name) towards a point just inside the far end of the tube.
This point is the objective lens’s ‘principal focus’ and the distance from here to the centre of the lens is called the ‘focal length’.
What you’ll need to get started
Eyepiece and tripod – To use your telescope you need an eyepiece. A medium focal length (say 20mm) 1.25-inch Plössl type is a good choice. With a little ingenuity you can mount your telescope on a standard camera tripod.
Focuser – For the sliding focuser you need an offcut of 43mm diameter plastic waste pipe about 160mm long. You also need a small piece of flat timber (plywood, MDF or softwood) for the end of the main tube.
Main tube – Find some tube with a diameter that’s larger than your objective lens. It needs to be at least as long as the objective’s focal length – find out how to measure this in the main article. You also need about 125mm for the dew shield. We used some 110mm plastic drainpipe.
Objective lens – The exact specification is up to you, but a 60-80mm objective lens won’t cost you much to buy from a surplus store. Alternatively, you may be able to extract your objective lens from an old projector, camera, photocopier, damaged binoculars or even a broken telescope.
Tools – You’ll need a handsaw or hacksaw to cut the tube, and a coping saw for the round wooden end. Alternatively, use power tools, but take care.
Delivering the view
At the far end of the tube from the objective lens is the eyepiece, arranged so that its principal focus coincides with that of the objective lens.
If you don’t have one, a 1.25-inch Plössl eyepiece will cost you around £25.
The eyepiece refracts rays of light into your eye.
A telescope’s focuser allows small adjustments to be made to the distance between the eyepiece and objective lens to accommodate different sizes of eyepiece and the standard of your eyesight.
The eyepiece usually has a much shorter focal length, which means that the final image is magnified.
To calculate the magnification, divide the objective’s focal length by the eyepiece’s: an objective with a focal length of 400mm and an eyepiece of 10mm magnify 40x.
The aperture of the objective is larger than that of your naked eye, which means it can capture more light.
The telescope’s ability to magnify faint objects changed the face of modern astronomy and, even today, a small 70mm refractor is still a useful tool.
The first telescopes consisted of a single convex objective lens with a concave eyepiece lens at the other end.
Unfortunately, not all wavelengths of light are refracted by the same amounts, so single lenses cause the coloured components of the white-light spectrum to focus at different points.
The image of a bright star, for example, may appear blurred and suffer from a distracting coloured halo around it.
To overcome this, modern telescopes have ‘doublet’ lenses that use two different glass elements to bring most of the colours to focus at the same point.
These lenses, which can be cemented together or separated by a small gap, are called ‘achromatic’ objectives.
This is the kind of lens we will be building for our refractor.
Very expensive objectives, which often employ three specialist glass elements (triplets), focus all the visual wavelengths at one point and are called ‘apochromatic’, but they’re more expensive.
Unlike mirror making, grinding and polishing an achromatic doublet lens is not something most amateurs would consider.
Thankfully, lenses are relatively easy to come by and can be surprisingly cheap at around £20 for a 60mm lens.
There are a number of surplus companies offering mounted telescope objectives – see Surplus Shed, for example.
You may even have a pair of broken binoculars with one salvageable objective lens.
The other parts of the scope are made from plastic pipe that can be bought from a builder’s merchant for around £20, or donated by a friendly plumber.
The first stage of the build requires you to measure the focal length of your lens.
The easiest method is to make a simple wooden stand for the lens and another with a sheet of white card fixed vertically to it.
Align these on a flat surface so the view through the lens is projected onto the card.
Then aim the lens at a distant object – the view through a window works well – and move the lens back and forth until a focused, upside down image of the outside world appears.
Measuring the distance between the glass of the lens and the card gives you an approximate value for its focal length, which will help you design the rest of the telescope.
At this stage it’s a good idea to make a temporary stand for your eyepiece and replace the card with it to confirm that the optical arrangement works effectively.
Use the measurement with the plan on the PDF in the link at the top of the article to calculate the required tube lengths and get sawing!
Aim for the heavens
You can hold the finished telescope for terrestrial use (slide the focuser in and out to achieve good focus), but for astronomical observations, a tripod is desirable.
We adapted the wooden test stand we made earlier with foam and a luggage strap in order to mount ours. You may eventually want to get a small altaz or equatorial mount for your telescope.
With the latter, you’ll be able to guide it precisely by small movements of the slow motion controls.
To identify the principal focus of your lens use wooden stands and some card, on which to project an image of a distant object.
Measure its focal length.
Use this measurement with the plans on the coverdisc to work out the tube length and construction.
Carefully cut the main tube with a suitable saw.
Wrapping a sheet of paper around the tube is a good way to mark a straight line.
Take care here; perhaps get someone to help hold it.
Cut a second piece and split it lengthways.
Cut three lengths of 43mm pipe to make the sliding focuser.
Split the shorter sections and remove a strip from one so it fits inside the larger piece (a couple of matchstick spacers help the eyepiece fit snugly); the other will stretch to fit around the outside.
Draw round the inside of the main tube on a suitable piece of wood.
Cut out with a coping saw for a tight fit in the tube.
Mark the central hole by drawing round the assembled focuser, then drill and cut out the hole for the focuser tube.
Paint the inside of the tube and dew shield black with paint.
Apply some mastic glue around the focuser and between the wooden end and the main tube when assembling.
Decide what packing is needed for the objective to fit tightly inside the tube.
Cut a ring of plywood or make three packing strips and position these between the objective and the inside of the tube.
Push into position with a little tape or mastic glue.
Fit the dew shield and cover gaps with tape.
Gold paint is optional!
This How To originally appeared in the April 2009 issue of BBC Sky at Night Magazine