Calibrating a Laser

In order to keep my astronomy gear at the highest quality possible, I need all sorts of specialized tools.  The more specialized my gear gets, the more specialized my tools need to be.  That being said, one of the most common tasks an astronomer needs at a high level of proficiency is the task of collimating optics.

Lenses comprising a lens group need to be square to each other...  lens groups need to be square to the focuser...  and mirrors, if your telescope has any, need to be perfectly aligned so the light focuses at one sharp point after reflecting off of two mirrors.  The solar spectroheliograph I am building also needs each of its four optical elements to be aligned perfectly with respect to each other.  Astronomy isn't just about looking at the sky.  Your gear needs to be properly maintained otherwise your observing sessions will be less-than-stellar (pun intended).  Of course, I'm also not your typical backyard astronomer who is just looking for some "gee whiz" views.  I look at astronomy from a scientific and mathematical standpoint so I demand a bit more precision in fine instruments.  

The most commonly used tool to have on hand for all of this optical alignment is a laser collimator.  Like any other tool or fine instrument, if the laser collimator is not calibrated accurately, the tool will only make your optical alignments worse.  So all your tools need to be properly calibrated, especially a laser collimator.  

I actually had a few close friends in the Air Force who worked in PMEL (Precision Measurement Equipment Laboratory) so I was introduced to the importance of precision instrument standards many decades ago.  This has carried over into my own tools and instruments.

I recently realized that I need a laser collimator to accurately align the optical elements of the solar spectroheliograph I am building so I went searching for one online.  I found a few that were in stock and chose the one I felt would be best of the few available.  Unfortunately, like many products since the beginning of the pandemic, the selection of available products was rather slim so I had to purchase one that was on the lower end.  

Regardless of low end or high end, a laser collimator still needs to be properly calibrated which means actually collimating the laser collimator.  New rulers are calibrated to a standard.  New scales are calibrated to a standard.  New ohmmeters and voltmeters need to be calibrated to a standard.  Altimeters need to be calibrated.  Speedometers needs to be calibrated.  Digital calipers need to be calibrated.  You get the idea.  If the specifications of the tool/instrument are not meeting the standard, the tool/instrument is useless.  The same holds true for lasers and this laser collimator.  

First, I needed to make a stand to hold my laser collimator to keep the laser beam at the same exact angle no matter how much I rotate it in the stand.  The stand clamps to a table or workbench.  I did this small project a few days ago...

In use, this laser collimator emits a laser beam and then the beam is returned after reflecting off glass or a mirror.  The laser beam then hits the circular target in the center of the laser collimator.  If everything is aligned properly, then this return beam should fall exactly in the center of this target.  However, in order to calibrate this collimator, I was not reflecting it off of any surface.  I simply needed to project it toward a far wall onto a target...

On this far wall (from kitchen to the other side of the living room), I placed a piece of white paper...

After each turn of screws to perfectly align this laser beam (collimation), I started over with a new target.  The dots I drew on the paper were too close together to simply continue aligning in the exact same spot because the dots were overlapping each other so I kept moving the paper target.  Well, I kept moving it until the three dots (one dot for each position of the collimating alignment screws at 120° intervals around the perimeter of the laser collimator) were so close that my adjustments were miniscule.  

The first test was in the center of the paper target.  For this first one, I turned the laser collimator 90° each time.  This showed me which direction each dot needed to move so I could get each dot on top of each other.  After this first test, I only turned the laser collimator three times in a full turn...  placing each of the centering screws at the top position for each dot on the target.  This made it easier to know which adjustment screws to turn and by how much. 

Each time, I would get closer and closer to having each 120° turn of the collimator land on top of the previously positioned laser dot.  When all of the dots were placed within just a millimeter or two of each other, I simply corrected on the fly.  At this point, these adjustments were very miniscule and were really just a matter of tightening the right adjustment screws with the right pressure.  

I think that the next time I will make a bullseye target and use just the one target in place without moving it.  I don't think I need to draw all the dots although, admittedly, the dots made it easier to see which direction I needed to go and by how much.  Now that I have a better understanding of how to do this, I think I can do without drawing a dot at each step of the way.  I'll need to start the process this way but I should be able to end with just a bullseye target and adjust on the fly.  

Overall, this took me a little over an hour.  The hardest part was all the walking back and forth.  Each time I turned the laser collimator 120°, I had to go to the wall to mark where the laser beam was hitting the target...  then go back to the laser collimator and turn it another 120°...  then back to the wall to mark it...  back and forth...  slightly adjusting the alignment screws after turning the laser collimator 360°.  In all, I made about 100 trips to and from the living room wall where the paper target was located and this required a little over an hour of time and a lot of patience.

Now my laser collimator is collimated and ready to align optics.  Hopefully I won't need to do this again for quite a while!