DIY Table Top Binocular Collimator

I had already established the approximate distances between the components so I screwed the binocular support to the table such that the binocular objectives would be about 90cm (36”) from the collimator lens.

The collimation scope was fitted to its mount which was on the eyepiece side of the binoculars. As I had previously mentioned, the support for this scope had a base which fitted between the table laths and could mimic the collimator's movement from side to side.

Only the collimator had a fixed vertical height, the other two components in the layout could both have their heights adjusted.

The first stage was to ensure that the collimated light source was centred on the centre line of the binocular. To do this, I inserted the cardboard discs into the dew cap rings of the objective lenses and adjusted the collimator lens so that the image of the target was visible on the one disc.

Binocular collimation with homemade collimator

It was easy to see if the cross wire centre was coincident with that of the disc centre and any discrepancies were easily rectified viz, horizontal difference slide the collimator in its guides, vertically; slacken the binocular mount locknut and move the binoculars up or down. It was also possible to slide the collimator to the other objective to confirm or otherwise that the second objective lens centre was also coincident with the collimated beam centre. The main reason why this would not be so would be that the binoculars were not level on their mount. Obviously any correction to this parameter would require the checking of the other objectives’ position until the two objectives were both centred on the centre of the light beam.

The next parameter to check was that the binoculars were square to the light beam i.e., they were not off-axis (tilted or skewed). This posed a problem as there was not a flat surface on the binocular body which I could use as a reference. I finally realised that the binocular dew cap rings were the only machined flat surface on the front end of the binoculars. I used this feature by securing a flat mirror over the objective lens, held flat against the dew cap ring with an elastic band stretched from the eyepiece end of the binoculars. The reflected light was then visible on the front wall of the collimator on which I had drawn a vertical and horizontal line with their intersection coincident with the lens centre. The lens was refocused. A perfectly squared binocular and hence mirror would superimpose the cross image onto the drawn lines. The arrangement was an optical lever very similar to that used in a ballistic galvanometer. The image was brought into alignment with the drawn cross by tilting and/or skewing the binocular mount. Once this was achieved, I removed the mirror and refocussed the lens so that I could check that the cross centre was still in the centre of the cardboard disc, any shift was corrected as previously described and the mirror replaced to check that squareness to the light beam had not been compromised.

Once this was satisfactorily achieved, the card discs were removed and the lens refocused once again but to the third position i.e., the one that produced the collimated beam. This was confirmed by looking into the binocular eyepiece.

That being so the spotter scope was slid into position, looking through the binocular eyepiece and the image of the cross wire studied through it.

I had set the distance between the spotter scope and the eyepiece of the binoculars such that the image from the eyepiece was slightly smaller than the view in the scope. This was so that I could ensure the scope was looking straight into the centre of the eyepiece-the image was surrounded by an annular dark ring.

Properly aligned binoculars would give an image of the cross wires of the spotting scope superimposed on the target image.

I had decided to correct any misalignment by adjusting the position of the objective lens within its eccentric retainer ring. To do this I had to unscrew and remove the dew cap ring. This I was able to do firstly on the right hand half of the binoculars. There was sufficient adjustment to attain the alignment of the two sets of cross wires. If there hadn’t had been enough I would have had to resort to prism adjustment.

I was also able to ensure the binoculars were still aligned to the collimator by repeating the operation with the mirror only this time the mirror was resting against the machined flat face of the binocular body and not the dew cap ring.

I then shifted the collimator to centre on the left-hand objective lens of the binoculars and moved the finder scope into position behind the eyepiece. The image in the left-hand half of the binoculars was coincident with the cross wire in the finder scope, so no adjustment was necessary in this case.

I could now remove the binoculars from their mount and look through them to see if the quality of the image had improved, which it had! The realignment process was a success.

INSTRUMENTS: Flat Double Sided Mirror

Precautions

Whilst I was checking the alignment with the mirror I leant on the table and saw the cross-wire shadow move on the front wall of the collimator by about ¼” the original position resumed as soon as I removed my weight. Don’t lean on the apparatus!

...and if you're listening Santa

..and now sit back and watch the film:

In Conclusion

I was pleased with the end results that I obtained by using this 'kitchen table' arrangement and once the initial 'teething problems' had been overcome, I found the apparatus easy to use. It would be interesting to develop an optical array that could produce two collimated images so that both sides of the binoculars could be tested simultaneously, obviating the need to move the collimator.

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Until next time and from a cold day in Normandie,

Cheers, Andy

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