Orwell Astronomical Society (Ipswich)
Construction Of A 50 mm Finderscope
Most telescopes have a narrow field of view which makes it difficult to find a faint object in an unfamiliar part of the sky. By fitting a finderscope with a relatively wide field of view to the telescope, it becomes comparatively easy to "star-hop" from a well-known star to the object of interest.
What follows is a description of a finderscope that I constructed in the late 1980s. The construction was dictated almost entirely by the availability of materials that I had accumulated over the years. For the optics I used a lens and eyepiece combination from an old pair of binoculars that I bought from a second hand shop in Ipswich. The binoculars were badly out of alignment and quite cheap, but nevertheless provided two good lenses and eyepieces.
There are two parts to the finderscope: the tube assembly itself and its supporting rings which fix it to the main telescope and allow it to be adjusted.
The lens from the binoculars was 50 mm in diameter and fitted nicely into a 50 mm internal diameter (ID) steel tube that I bought from a scrapyard in Norwich. I cut a piece of this tube so that, with the eyepiece fixed at one end, the lens would be recessed by about 25 mm at the other end. This acts as a lens hood to exclude stray light. To fix the lens in place I cut two rings from a 70 mm plastic drainage tube. I cut a section from each ring leaving a C-shaped piece which, when compressed, fitted the ID of the steel tube. The elasticity of the plastic ensured that it forced itself against the inside of the steel tube to give a tight fit. I sandwiched the binocular lens between two of these "rings", no glue or fastenings being required.
As this is a low power finderscope it is not necessary to have a focusing system so I fixed the eyepiece rigidly at the other end of the tube. I cut a plywood disc with diameter equal to the outside diameter of the steel tube and then drilled a hole centrally to take the eyepiece. I glued the ring to the steel tube and glued the eyepiece into the hole. I experimented with moving the objective lens up and down the tube until I achieved focus at infinity, and then left the objective in this position. The finder is now finished apart from a coat of paint.
It is necessary to fix the finderscope rigidly to the main telescope but also to enable adjustment of its alignment with the optical axis of the main instrument. To achieve this I constructed two adjustable rings by cutting 10 mm wide slices from the end of a 100 mm diameter thick-walled steel tube. I drilled holes in the slices and tapped them to take bolts at three points separated by 120°: the bolts hold and centralise the finder tube (see the photograph below). By adjusting the bolts the finder can be aligned with the main instrument. I then bolted each ring to a 10x80 mm strip of steel having a hole near each end. These fix the whole finder assembly to the main telescope - it would be just as easy to fit curved pieces of steel to fit any diameter of telescope tube. When the finder is fitted to the telescope it can be aligned either during the day on a distant object or at night on a bright celestial object.
A finderscope made as described above from binocular optics typically has a field of view of approximately 5° with an inverted image matching the view in the main instrument (a consequence of omitting the prisms from the binoculars).
One refinement that I've yet to make is to fit crosshairs into the eyepiece. This is a little difficult in some eyepieces as the focal plane of the objective lens has to lie outside the eyepiece so that it has to be extended before crosshairs can be fitted. However, any difficulty is worthwhile as crosshairs are very useful for accurate pointing.
Safety note: the photograph of the finderscope shows that the three retaining bolts stick out some distance from the rings. This is potentially dangerous if trying to peer through the finder at odd angles in the dark (as is generally the case!) I will shorten the bolts and cover their ends with rounded plastic caps to prevent injury.