Saturday, January 27, 2018

PFS Upgrade Series, Day 3: Optical Alignment

This is part of a series of posts about upgrading an instrument at Las Campanas Observatory. If you want to start at the beginning, it's here.

One of the best parts of my job is learning new things. I like puzzles and intellectual challenges (as well as some physical ones), and very often that's what my work comes down to -- solving challenging puzzles. This process often consists of a lot of trial and error, and can sometimes be frustrating, but it makes solving the puzzles even more rewarding. Today was a day of puzzles. 

We want to make sure that the light inside our instrument will go where we want it to, namely straight through the lenses, off the grating and through the prism and back to the right spot on our detector, so it can record the light and we can detect planets. To make the light go where we want it to, we have to change the positions of the different components in our instrument so they guide the light path correctly. This is one of the three "upgrades" to PFS we have planned -- a precise (as possible) optical alignment of the lenses inside the instrument. 

To accomplish this optical alignment, we want to use our fancy-shmancy Shack-Hartmann wavefront sensor. It produces a beam of light that we can (eventually) use to shine down all of the lenses in the instrument and make sure the light is reflected off the grating and comes right back to the sensor in the same spot. Then we know the same thing will happen when we replace the sensor with our CCD.

We ended yesterday and started today somewhat confused about how to use it; the manual was originally written in French and translated to English, but unfortunately not very well, so it was a challenge to even know where to start. After several hours of just playing around -- turning knobs, watching how the wavefront changed (using the computer software that came with the sensor), turning knobs again, putting in a pinhole, taking it out, etc. We first wanted to make sure that the wavefront sensor itself was producing a beam perfectly perpendicular to our optical bench, parallel with the desire optical axis inside the instrument. Steve developed his own method for doing this wavefront sensor calibration, but we determined this morning that the method recommended by the sensor manual (as best we could tell) worked just as well, and was easier to replicate. Check.

Next, we needed to make sure the light beam from the sensor would hit the grating in the right spot and come back towards the sensor. Instead of using the grating from the start, we are using a large mirror in its place for now. We kind of got stuck at this part -- how could we make sure the light was leaving the sensor and coming back to the same place, and hitting the mirror on the other side of the instrument? It sounds trivial but it was not. Steve had a great breakthrough after lunch, though, when he rigged up a small aperture in front of the sensor, got the light beam adjusted to go through that aperture, and then we adjusted the sensor so that the light beam that fell on the mirror's surface reflected back nearly perfectly through the same aperture. 

The photos above show Steve's set-up for verifying that the light beam from the sensor was properly aligned with the optical axis of the instrument. The yellow arrows in the top photo represent the (ideal) path of light, except really it's all in one line, not two lines on top of each other  The bottom left photo shows the metal beam that Jeff made, originally to hold a lens or mirror, but here without anything in the holder. The blue thing you see through the holder is Steve's homemade "work of art" aperture (paper, blue tape, and a metal washer), which is in front of the wavefront sensor. The wires going across the holder mark the center of the aperture (my photo wasn't perfectly aligned). In the bottom right photo, the reflective mirror at the other end of the instrument is shown (reflecting some tubing!). There is blue tape on each side of the mirror, holding thin wires to mark the exact same spot in space as the wires in front of Steve's aperture (except translated down the length of the instrument) on the surface of the mirror. 

And it worked! Below is a not-great photo of the screen of the computer, showing what the wavefront sensor was detecting when we had Steve's set-up aligned. The green cross in the center marks the true  center of the image, and we were trying to line up our circular image so its center matched that green cross. Actually, first we just had to get the circular image, then make fine adjustments to center it. We did pretty good! (I think there might have been one more iteration of the adjustments after this photo was taken.)

So, step 2 of the puzzle is solved, at least we think so. :) Tomorrow we will start on step 3, which is adding in the lenses that actually go in the instrument and trying to recreate that light beam path that goes out through a pin hole, through the lenses, off the mirror, back through the lenses, and back through the pinhole. This is probably the hardest task we have to do during this trip, or at least the one with the most uncertainty. But that's what makes puzzles fun, right? If you went in knowing the answer it would be boring. Onward towards discovery! 

Sunrise at LCO this morning. How many telescopes can you spot?

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