Best*. Photometry. Ever. III: Prototyping Holographic Diffusers

In the first installment of this series, Penn State Research Associate Ming Zhao presented “the problem” of precise NIR photometry.

In the previous installment, we made a detour into the not-so-distant past and revealed how fate had intervened to give Suvrath Mahadevan the idea to install holographic diffusers on a spectrograph that needed to be blurrier.  Survath recommended we try the same thing for photometry.  

In this installment, Ming and I describe the efforts of many people to prove the diffuser could actually do what we needed.

We immediately looked into this possibility with Suvrath’s consultation, and searched for places that make diffusers and contacted all of them. Unfortunately, none of them made diffusers that fit our specifications, but luckily, some of them do have development capability.  After a few months of searching and back and forth communications, we were referred to a vendor that sent us some sample diffusers to further investigate the feasibility of Suvrath’s idea for photometry. 

We tested the samples on sky with Davey Lab’s rooftop telescope with the kind help of Prof. David Burrows and Lea Hagen, a dedicated and hard working graduate student who was in charge of the telescope back then.  We inserted the diffusers into the optical path and imaged some stars, but the results were disappointing — we did not get the nicely diffused images we had hoped for.

We tested the samples’ performance in Suvrath’s lab with the help of Sam Halverson, a bright graduate student works on instrumentation in Suvrath’s group.  Those tests were critical, as they allowed us to learn that the diffuser will only work when the beam sizes of the stars are much larger than those of the diffusing sub-structures in the diffuser (i.e., collimated beams that fill the diffuser are the best).  

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Test images of laser light through the prototype diffuser (left) and without the diffuser (right) taken in Suvrath Mahadevan’s lab at Penn State.  The diffuser successfully spreads the light out over many pixels, just like we want.

We started thinking about diffusers for the MINERVA array, but found it would be hard there because the filter wheels are not in a collimated beam, and the result would be a lot like our rooftop tests.  But the lab tests showed us this would actually work great (in principle) on the Wide-field Infrared Camera (WIRC) at Palomar (where Ming does his secondary eclipse work), since the beam there is collimated at the filter wheel, then re-imaged onto the detector.  So Ming discussed this with his colleague Prof. Heather Knutson at Caltech.  She agreed that we should put a diffuser at the Palomar 200-in, and none of what happened next would have happened without a big push from her. 

We still had to convince Caltech to put one of these things into WIRC, and we still weren’t sure that it would actually work on sky.  Fortunately, there was a Palomar Science Meeting coming up (thanks for advocating me to go, Jason — MZ).  Heather managed to get Ming on the schedule to give a long talk about the great secondary eclipse work they had been doing together.  Ming ended his talk with the results of our diffuser tests and argued that Palomar would be much more efficient at this work with a diffuser installed (no more praying for perfectly bad seeing!).  Heather followed later up with Palomar administrators and convinced them that this was the right way to proceed. 

Then, over the next several months, Heather led the effort to communicate with the vendor, spent large funds on the development and purchase of the diffuser, and coordinated with Caltech’s staff on the implementation. We worked closely with her to ensure the quality and the specs of the newly developed diffuser met our requirements, since the opening angle we needed was at the manufacturing limit.  She also coordinated with Eugene Serabyn’s team at JPL to test the final product at their Palomar testbed (more on this next time). Finally, by the end of last year, a diffuser was delivered and installed on one of WIRC’s filter wheels, and everything was ready for on-sky test, thanks to all the teamwork and efforts of the Palomar staff and everyone involved.  

But of course, it couldn’t be that easy…  Next time:  first results on-sky

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