Build a guidescope using an old camera lens

Upcycle your old camera lens into a neat astronomy accessory with this step-by-step DIY guide to building a guidescope for your telescope.

Build a guidescope using an old camera lens

A guidescope is an important item that helps with autoguiding in astrophotography. It’s needed to correct the small errors that can occur when you are tracking with a motorised mount that has mechanical gears.

An example is the unwanted image trailing that can occur – even though your mount has a good alignment – when you are taking very long exposures of an object on your main camera.

The guidescope and guide camera, along with suitable software, will help to counter this by checking the position of selected stars and communicating with the mount.

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Importantly, the guidescope makes small corrections to keep your scope stable and fixed on a target object.

There are many purpose-built guidescopes that are commercially available for astrophotography, but it’s also possible to make your own – at a reasonably low cost – with the help of some simple DIY skills.

The repurposing, or upcycling, of equipment is always a good idea and if it will help you improve your astronomy so much the better.

William Optics Zenithstar 126 apo refractor
This is what you’re aiming for. Pictured here is the William Optics Zenithstar 126 apo refractor, with guidescope attached. Read our full Zentihstar 126 review.

Build your own guidescope

We are going to look at how a guidescope can be made by repurposing an old camera lens. A guidescope itself is basically just a lens on the end of a tube with a fitting for a camera at the other end.

This description may sound a bit simplistic, but it got us thinking about how easy it might be to come up with a DIY solution.

The aim of this project is to make an adaptor that will allow you to attach a guide camera to any lens that you think is suitable for guiding your setup, plus a bracket to attach the lens securely to your scope, without it being too expensive or complicated to achieve.

Lots of people have old lenses that date from the pre-digital camera era and there are many for sale on online auction sites at relatively low prices. If you have a modern lens that’s going spare you can also use that.

This project won’t damage your lens; it will actually make it more versatile and you’ll be able to easily put it back to use on a regular camera.

Canon EOS-1D X DSLR camera
Our project won’t damage your lens. You’ll be able to easily put it back to use on a regular camera.

What size of lens should I use?

As a low focal ratio is better for picking up stars for your guide camera to track. We wouldn’t recommend using a lens less than 50mm. For our project we opted for an old 135mm M42-fit lens with a focal ratio of f/3.5.

Your lens also needs to be fairly compact, making it easier to construct a bracket to fit it to a standard finderscope shoe.

The DIY bracket could also be used in other areas of astronomy, maybe with a lens as a telescope or as a finderscope (by inserting an eyepiece instead of a guide camera).

Another bonus about our project is that it uses nearly all plastic parts, such as rear lens caps and PVC pipe connectors.

As solvents are used in the guidescope’s construction, we recommend that you work in a well-ventilated area.

Also, be careful to ensure you have got suitable personal protection when you are using hazardous tools and materials.

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You Will Need

  • An old camera lens, 50mm or above
  • Plastic M42-fit rear lens cap
  • PVC pipe coupler with an internal diameter of 32mm
  • Nine M4 x 16mm nylon bolts to hold the guide camera and lens in place
  • Solvent glue
  • Superglue
  • Files
  • Sandpaper
  • 36mm hole cutter
  • 3mm drill bit
  • M4 tap attachment
  • Hand saw
  • Drill
  • Electric screwdriver
  • Plastic chopping board
  • Section of 68mm drainpipe

Step 1

Build a guidescope using an old camera lens

To make the lens-to-camera adaptor, clamp and saw the PVC pipe coupler to the required length; ours was 20mm long to get the correct focal point for the guide camera. Use some fine sandpaper to clean up any rough-cut edges.

Step 2

Build a guidescope using an old camera lens

For the adaptor’s lens collar, first take the lens cap and drill a 3mm hole in its centre. Then remove most of the flat face with the 36mm hole cutter. We used a compass to draw a circle on a sticky label to accurately mark the centre of the lens cap.

Step 3

Build a guidescope using an old camera lens

After sanding any roughness, carefully apply the plastic solvent glue to the cut edges of the lens cap, making sure not to get any solvent on any threads, then insert the PVC pipe by a couple of millimetres ensuring a nice fit. Allow 24 hours for the glue to set.

Step 4

Build a guidescope using an old camera lens

Now drill three 3mm holes, 10mm from the edge of the PVC pipe and 120° apart. Next, create threads on the insides of drilled holes, by slowly using the electric screwdriver with the M4 tap attachment, before screwing in the M4 nylon bolts. The adaptor is now ready and we can move onto making the bracket.

Step 5

Build a guidescope using an old camera lens

Measure your telescope’s finderscope shoe and then saw strips from the plastic chopping board and file them for  a smooth finish, making sure the strip for the base fits the findershoe. To build the pedestal part of the bracket, glue two strips together for the stem and then glue the base and top to it as shown here.

Step 6

Build a guidescope using an old camera lens

For the lens ring part of the bracket we used a 35mm section of 68mm plastic drain pipe. Make three double sets of M4 locking bolt holes to hold the lens in place (using the method set out in Step 4). Finally, the lens ring is both glued and screwed to the pedestal to finish the bracket. The bracket and adaptor are now ready to use with your lens.

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Mark Whalebone is a former builder, and is a keen amateur astronomer and tinkerer who lives in Hertfordshire. This How To originally appeared in the November 2020 issue of BBC Sky at Night Magazine.