DIY Table Top Binocular Collimator - Modifications to Method/Apparatus

As I started setting up I observed that true to the laws of physics, once the collimator lens was set at a distance from the cross wire reticle equivalent to the lens’ focal length and the light was turned on, the image of the wires were in focus in the binocular eyepiece. This, even though they were less than 40” (1m) from the object and that image, through the eyepiece, could be viewed through the spotting scope. BUT I was aware that I had not ensured that the collimated image was hitting the center of the objective nor that the binoculars’ optical axes were parallel to the centre line of the collimated beam both important criteria had been omitted from the book. Nor had I established that the binoculars were parallel to the table top.

From the above I deduced that the only point of reference that I had was that everything had to centre on the axis of the collimated light beam. Both the binoculars and the spotting scope would need to be on or parallel with that axis. On this basis I concluded that I needed a method to determine that the collimated image was coincident with the centre of the objective lens and that I needed to use this same image for establishing the second criteria.

To center the image, I cut two thin cardboard discs to fit inside the dew cap of the objective lenses. To make these, I measured the dew cap internal diameter with Vernier calipers and used a pair of compass dividers to scribe a circle to that diameter in the card. By continuously scribing with the dividers, the card was eventually cut through producing an exact card circle with a centre hole marked within.. To these two discs I glued a piece of twine to facilitate easy removal from the dew cap. With the binoculars in place on the support I inserted the discs. I found that I could see the shadow of the cross wires on the objective by moving the target within the collimator box away from the collimator lens until a sharp image could be seen on the disc. Thus I was able to use the collimator as a crude but effective projector.

Moving the cross wire target also highlighted the image shift if the target was not kept vertical or if it twisted so that it did not remain perpendicular to the walls of the light box. Before I could proceed I decided to fix the position of the reticle so that it remained vertical and square to the lens. Having done this I needed to make the collimator lens position adjustable so that I could obtain:

The collimated cross wire image as before
A focused image of the cross wires on the discs in the dew caps.
Additional adjustment to increase the distance of the focused image by approximately two. (This to be explained in the solution to establishing that the binoculars’ optical axis was collinear with that of the collimator).

I decided to fabricate a cylindrical housing for the collimator lens and for this I used a cleaned plastic tube from a silicone mastic dispenser. I made a lens cell to hold the lens out of the plastic piston within the mastic tube and fixed this in the end of the tube using the glue gun. The end wall of the collimator box was modified to accommodate this new lens arrangement with the addition of further guides/supports attached to both the inner and outer faces of the wall.

INSTRUMENTS: Vernier Calipers, Precision Compass Dividers

In the next article we look at the experiment proper

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Until next time and from a cold day in Normandie,

Cheers, Andy

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