R&D Projects at Lunt Solar Part 1

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It’s probably doesn’t come as any surprise that Lunt Solar Systems LLC is involved in R&D projects outside the field of Solar Filters and Telescopes.
But it is because of our highly technical and specialized skills that we are often asked to develop unique optical elements that fall within our capabilities and my interests.
Some of these R&D programs are confidencial. Some of the ongoing projects are not, and may not only be of interest, but they may encourage readers to suggest other uses for these technologies.

One of our more recent projects has been the developement of a novel solid etalon design.
The intial concept was for the design of an ulta thin, ultra stable, ultra precise, and ultra narrow bandpass micro etalon.

The project’s scope of work consisted of manufacturing a solid etalon that was approximately 0.3mm thick (0.012″) at the Hydrogen-alpha line.
Multiple matched etalons needed to be produced. The desired size was 4mm x 4mm.
The Hydrogn-alpha emission line was chosen due to our knowledge of manufacturing at that specific wavelength. However, Hydrogen-alpha is not the desired wavelength of the finished product.

The results have been extremely succesful.

At Lunt Solar we produced what can essencially be referred to as a wafer etalon.
The solid etalon was manufactured from a 80mm piece of low expansion UV grade material. (not zero expansion).
The “spacer” layer was polished to 0.3mm thick and send out for testing prior to further work. We needed to show verification of the spacer layer’s precision prior to going to the next step.

Results:

The Wafer was first tested for surface flatness. Surface flatness was not the concern given that the etalon would be used in transmission.
It was the Transmitted Wavefront that was specified.

As you can imagine the wafer did suffer from some surface error due to being held in a fixture.

The surface flatness was measured at 1/6th of a wave, some of that coming from astigmatism due to the fixture point.
Power was the major contributing factor to this error due to the method of manufacture.

The Transmitted wavefront results were very encouraging.
The interferometer showed a peak to valley error over 75mm (the aperture of the system) of 1/61th wave at 532nm. The RMS being better than 1/250th wave (the limit of the test system).
This test was over 75mm. The system was not capable of realizing a measurement over 4mm aperture, and the engineer was not able to speculate just what that result would be could it even be measured.

These results were followed by some interesting conversations with the test facility who wanted to know how we had tested the wafer ourselves.
For those that have visited Lunt Solar in Tucson, you will know that I test ALL optics for flatness by eye only.

The wafer was sent out for coating via a low temperature, ion assisted process. The wafer was coated on both surfaces with what I now refer to as the new hybrid high reflector.
Some initial tests needed to be done to assure a “tune” at the desired wavelength.
In this case we tuned slightly low of Hydrogen-alpha in order to utilize the standard method of heat to bring the etalon accurately on band.

The end result was a pellicle wafer of about 80mm diameter. Given the highest degree of precision for the entire etalon, we could now be assured of matching etalons of 4mm x 4mm after the wafer had been diced.

The net result as far as specifications go are as follows:

Bandpass: 0.2 Angstroms
Free Spectral Range: 11 Angstroms
Finesse: 27

A few technology applications immediately come to mind given the controllable FSR and the narrow bandpass. I would certainly like to hear your thoughts on possible applications as well.

I am obviously excited about the prospect of getting this system assembled into the back end of a SCT telescope very soon. The wider acceptance angle of this design vs a conventional higher index spacer material will make the modification to existing SCTs in the market fairly simple and compact.
The fact that the etalon itself is thin and has little mass will allow for rapid temperature change and stabilization.
The use of the proven materials will allow for a mass-produceable 0.2 Angstrom bandpass system with a single etalon. I can hear the calls for a double stack system already…

Moving outside my area of expertise and free thinking for a moment…

The product is of a robust solid design and the CWL of the system can be fixed to a specific wavelength or tuneable.
The thickness of the system lends itself well to the bonding to CCDs. Maybe multiple etalons can be bonded to a single CCD to provide a chemical signature detector?
I have thought about it’s use in applications such as ionized gas/chemical detection for a while.

Used in these applications the signal to background noise ratio would be of huge advantage, eliminating all background noise and providing >90% T at the desired wavelengths.

We provide 0.5A bandpass looking directly at the Sun. Ambient daylight would be of no issue.

Ancillary light sources such as laser or short wavelengths could be used to further excite the desired compounds?
Are there military applications for ionized gas/chemical detection for munitions, gunfire etc?? The addition of signature trace elements to munitions that can be optically traced?

Other applications may include laser line stabilization, optical fiber multiplexing, etc…

This is just one of our current R&D projects. It is thanks to this project and a few others which I will outline in another blog that we are rapidly improving the performance of our core products. Success and failure though our ongoing research projects are already seeing a positve impact on the contrast and resolution of even our most basic systems..

Upcoming products will focus on enhanced performance and the ability to accessorize systems for use as educational tools.

I encourage your feedback and comments and thank you for reading.