How to make a 50mm eyepiece

Turn a redundant SLR lens into a champion accessory.

Written by Simon Dawes.

Image Credit: 
Mark Parrish

Note: some older camera lenses are known to be radioactive and, as such, can cause damage to the eye if used for long periods of time. More information can be found here, but it is worth doing some research to check that your lense is safe to use before adapting for this purpose.

This How to follows in a long tradition of making telescope accessories from parts originally intended for a different use. Here we’re taking old SLR (single lens reflex) lenses that are no longer needed in the digital age and putting them to use as telescope eyepieces.

Eyepieces come in many shapes and sizes, from the simple single-element Kellner to more complex multi-element zoom affairs. However, with little or no optical experience, it is possible to adapt old SLR camera lenses into eyepieces that will perform well – not to mention give you the satisfaction of using a homemade accessory that costs next to nothing to make.

You can use SLR lenses with focal lengths up to about 50mm for this project. The thing you need to watch out for is the size of the exit pupil, which is the size of the beam of light that emerges from the eyepiece. A diameter greater than 7mm will result in a beam of light larger than your eye’s fully dilated pupil – your eye will not be able to collect all of the light being gathered by the eyepiece, so effectively the image will appear dimmer.

Thankfully, the exit pupil is not dependent on the optical design of the telescope or eyepiece. It is easily calculated by dividing the focal length of the lens (in millimetres) by the focal ratio of the telescope. For example, a ‘standard’ 50mm lens on an f/8 telescope will yield an exit pupil of 6.25mm.

Having chosen your lens, you need to modify it for telescope use. When your eyepiece is complete you will be looking through the rear of the lens – the end that connects to the camera. You will need to glue a 1.25-inch diameter tube to the front of the lens to act as the eyepiece barrel, allowing you to slot it into a telescope focuser – a metal one is best. If you don’t have something suitable lying around, they can often be bought for a few pounds from astronomy equipment suppliers or picked up at astro boot fares. If you can find a barrel with a flange, all the better, as this provides a good surface for the glue.

Using an epoxy resin, glue the barrel to the centre of the front of the lens. Once the glue hardens you can continue with the rest of the modifications.

Minor alterations

Start by opening the lens aperture fully. Most old lenses will have a manual aperture control ring – set this to the smallest number. Tape or glue the control ring in place so that it doesn’t move while you are using it. If you don’t have the original rear lens cover, you will need to remove any levers that protrude from the lens with a hacksaw; you don’t want them to poke you in the eye in the dark.

Take the original lens cover, cover it with masking tape and mark the centre. Use a small drill bit to make a pilot hole. Then using a pillar drill, hole cutter and machine vice, cut a hole large enough to look through. This should be at least as large as the exit pupil, but a larger hole looks better.

Refit the rear lens cover and your new eyepiece is ready for use. It has its own focus ring but it doesn’t matter how this is set as you can focus with the telescope focuser. The performance of the eyepiece will depend on the optical design, but most SLR lenses have good eye relief (the distance from the lens to the focus), so are good for spectacle wearers, while the low magnification allows for great views of larger objects that are usually best seen through binoculars, like the Pleiades.

Tools and materials

Glue - Use epoxy resin glue to stick your barrel to the front of the lens.

Masking tape and pen - To mark the centre of the rear lens cover prior to cutting the eyehole.

Optics - You’ll need an old lens with a focal length up to 50mm to convert into an eyepiece. Ideally it will have its original rear lens cover.

Tools - A vice, electric drill, hole cutter and pilot drill bit will allow you to make a suitable hole in the rear lens cover; use a hacksaw to remove any levers.

Tube - Use a piece of tube 1.5 inches long for the barrel for your eyepiece. It needs to be 1.25 inches in diameter so you can slot the finished eyepiece into a focuser.

Step 1

Check your chosen lens for scratches, dirt or mildew. Dust on the outer surfaces should be carefully removed with a lens brush. For stubborn dust grains, apply a little lens cleaning fluid to ensure the optics are clean.

Step 2

Glue the 1.25-inch diameter tube (your eyepiece barrel) to the front of the SLR lens using an epoxy resin, making sure it is centrally placed in relation to the lens. You will need a 1.5-inch length of tube for this.  

Step 3

Open the aperture of the lens to its fullest (it will have a small f/ratio), using the aperture control ring on the lens. If your lens also has automatic aperture control, you will need to set the lens to manual first.

Step 4

Once completed, you will be looking through the eyepiece through a hole in the rear lens cover. To make sure the hole is central, mark the centre of the cover using masking tape and a pen, and drill a small pilot hole. 

Step 5

Cut the hole in the rear lens cover. The lens cover protects the eye from any levers at
the rear of the lens, so if you don’t have it remove the levers with a hacksaw – that way you won’t poke your eye while observing.

Step 6

Refit the rear lens cover. Check that the aperture is fully open and tape or glue the aperture ring in place so that it can’t move. All that’s now left is to try out your new lens at the telescope.

This 'How to' originally appeared in the July 2013 issue of BBC Sky at Night Magazine.


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