Astronomical Observing and Photography


Making a finderscope from a binocular lens and PVC Plumbing fittings

20 February 2019

I made a finder from old binoculars for my first scope built through a telescope makers club. Everybody in the club used old 50mm binocular objectives to make finders. Having a foundry at the club, we used to cast the finder brackets from aluminium. A few years ago I made another finder almost entirely from off-the-shelf hardware store materials. The tube piece of 50mm PVC pipe. There are many different sizes around 50mm so bring your objective and try it out. The lens hood is a plumbing pipe coupling and the bracket is a piece of 75mm PVC. See photo's attached. I did use a lathe to machine the focuser but I'm sure you could use PVC step down adaptors to get it close to 1 1/4". If step down adaptors are not available, then cut a piece of tube, cut a piece out of the circumference to make an open split ring. It will fit inside the 50 mm tube. A series of these split rings made from reducing sizes of tube, glue them together with epoxy and you'll have your adaptor.

The binocular objective is held between a piece of 50mm PVC drain pipe and a 50mm join coupling
which also acts as a lens hood. On a very dewy night, another piece of 50 mm PVC can be inserted in
the front  side of the coupling to act as a dew cone or extended lens hood.

The length of the tube  is governed by whether you want the finder to be a straight through or a right
angle finder. A binocular prism can be used as a right angle finder.  You can re-use the binocular eyepiece
but I find that a cheap GSO or Skywatcher plossl eyepiece is much better than the binocular eyepiece

The adjustment bracket has 6 x M3 threads tapped straight into the 75mm PVC pipe. You need to tighten
with care to avoid stripping the threads.  I have been using this one for 4 years without issue.  If you prefer
something sturdier, you can use steel thread inserts available at any hardware straight in the 75mm PVC.

I did machine the focus tube from aluminium. It should be possible to find step down PVC adaptors that
can act as a defacto focuser.

The foot is a piece of 25 x 50mm aluminium U-channel extrusion. The dovetail foot is cut with a hand
mitre saw from a piece of delrin, but with the light weight of the finder a piece of hardwood - maple or
ash would suffice. A standard finder bracket has a 2 x 60 degree mitre 1.25 inch (31.75mm) on the base.