What's new
What's new

Autocollimator mirror alignment tool


Hot Rolled
Feb 21, 2022
Tokyo Japan
As I mentioned in my Japanese Finds thread, I recently picked up a couple of Nikon 6B autocollimators and some neat accessories. There are some interesting design choices made with these units, and one of the things I found was it is a pain to get the mirror initially zeroed out on the reticule. I have both a large 70mm factory mirror that is on a fixed sled, and a Newport precision gimbal with a 1" dielectric mirror. Nikon makes a robust stand for the AC, and their approach is to align it to the mirror rather than aligning the mirror to the AC, which makes a lot of sense as their mirror sleds are fixed geometry and it is a lot easier to align things when looking through the device. But even when I created a mounting system with similar adjustability, it is still quite challenging to get the mirror roughly aligned so even a small part of the reflection is visible, at which point getting it zeroed becomes trivial.

There is a YouTube guy in Ukraine, Gena Bazarko who has some really interesting old Soviet stuff, and the AC mirror sled he has is really interesting as it has its own optics that look back through a hole in the center of the mirror and assists in getting it pointing back at the AC. I also saw a clever solution made by Trioptics that basically is a laser pointer that sits on the barrel of the AC and holds a little laser in front that is concentric to the device. A variation of this seemed the better solution...

So I bought a little green* laser spot generator from Amazon, and 3D printed a simple tilt/nod flexure for it with a rechargeable LiIon coin cell to power it. Rather than making it mount on the barrel, a simple lens cap design with three internal contact pins provided enough alignment accuracy to make getting the mirror in view really easy.

Laser Alignment Tool.jpegLaser Alignment beam.jpeg

The initial alignment of the laser should only need to be done once. I first got a mirror properly zeroed using the manual method, which reenforced my belief that it is a royal pain, and then put the laser alignment cap on the AC and adjusted the two knobs until the laser was reflected directly back upon itself. That ensures it is axially aligned with the optics of the AC. Then to test it I relocated the mirror and moved the AC, and using the laser easily located the new position and adjusted the stand until the laser reflected off the mirror and pointed back to the AC cap. Then removing the lens cap and turning on the light showed the AC almost perfectly zeroed, although my real expectation is just to get it in the ballpark so it can be aligned perfectly through the optics.

I also made a mount so my old iPhone 8 can photo capture through the eyepiece, the 'Camera Remote' feature of my Apple Watch means I can remotely view while repositioning the mirror and trigger the shutter without bumping anything. It likely was more trouble than it was worth, but it would be useful to someone doing a lot of measurements and a very high resolution photo can give more precision than the built in micrometer can provide (1/2 an arc second.) The old iPhone only captures ~2400 pixels² in the readable range (72px/arc min), so I'd need a better camera to really improve on the manual method, but ImageJ (Fiji) can at least make calculating the measurements easy from a bunch of photos.


Speaking of the micrometer, they use a single unit and dial to calculate both horizontal and vertical deviation, which I thought was really clever once I understood it. Instead of using two micrometers that directly move in each axis, it moves both scales at a 45° angle to the X and Y lines of the crosshairs, so you measure each axis independently. They suggest zeroing to the 15 minute marks in the center with the micrometer at zero. Then you move the mirror, check the deviation of one of the crosshair lines, move the micrometer to center it between the ticks, and read the seconds value. Then do the same for the other one. The entire scale reticle moves diagonally while the crosshairs remain fixed (assuming you have a sturdy stand.) Quite a smart way to do it if you ask me, and a wonderful lesson in efficient design.

*They make native Green solid state lasers these days, the first ones commonly available were actually infrared lasers that were frequency doubled with a crystal, but enough IR leaks through some of them that they can be dangerous in some circumstances. Native ones are longer lived and a lot more compact.