One of the ways Astronomers tell what stars are made of is by spectrometry. The light emitted from every element or compound in nature is slightly different. By studying the light frequencies arriving from a star, astronomers can tell what it is made of.
Furthermore, - if somewhere on the way to Earth, the light happens to travel through a gas cloud, - the gasses in the cloud will filter out certain frequencies and add their own. This allows astronomers to tell what the gas cloud is made of.
There is a lot of interest about global warming caused by atmospheric pollution at the moment and the same techniques are being used on Earth to tell what gases are in the atmosphere and what effects they are having. To measure the effect of those gasses in the laboratory, scientists use gas Analysers, and Oldham optical make the optics for some of them.
The gas analyser described here works in the same way as light from a star travelling through a gas cloud and being partially absorbed. They can't fit a star into the laboratory, but they can use artificial light sources such as lasers, deutrinium lamps and even standard tungsten filament bulbs depending on the frequency of light needed to carry out the tests.
An
example schematic of a gas analyser is adjacent. Here a
laser is shown as the light source and is set up to shine a beam of light through a chamber containing the gas
being analysed. A detector measures the light after it has passed through the
gas.
As usual nowadays, a computer is involved and this processes the readings and converts them into something a human being can understand.
There may be a reference sample taken from the same laser beam via a beam-splitter before it passes through the gas. If so, a comparison between the two is done inside the computer, - but for a lot of work done with gas analysers in the laboratory, we are told this extra complication is unnecessary.
For a lot of gasses, it needs the light to travel through a lot of gas before a significant amount of light is absorbed. That's no problem in space as a gas cloud could easily be a light-year across, so that is plenty of gas even if it's at very low pressure.
Funnily enough, - it's not possible to fit a light-year into a laboratory!
Laboratory testing cannot use massive distances like light-years and although it can use greater concentrations of gas than in interstellar space, it often requires path lengths into the hundreds of metres. The best path length to use is dependant on the gas itself and how much light it absorbs and what frequency of light is being used. So what is really needed for a good gas analyser is a chamber of variable length anywhere between a couple of metres and hundreds of metres depending on the gas under test and the light source being used.
Since it's not even practical to fit a chamber that is hundreds of metres long in the average laboratory, - this is where some clever optics have to come in.
And even if the laboratory had the room for a long tube that was hundreds of metres long - it would need a very large gas sample to fill it!
However, - If you stick a mirror at one end of a reasonably sized chamber, it reflects the beam back down the chamber and doubles the path length. The mirror can be mounted at a slight angle to the beam so it is deflected slightly to the side making room to fit the detector alongside the laser.
You can probably think of a few other arrangements where you can bounce the laser beam a few times around the chamber, - but we guess most of your ideas thought up at short notice would not allow the path length to be altered easily? The arrangement would probably all have to be taken apart and set up again for a different path length?
However
other people have had longer to think about the problem. Two early thinkers in
the field were J U White and D R Herriott, and the diagram opposite was taken from a
later paper by two gentlemen who followed on from them called S M Chernin and E G Barskaya, published in 1991.
This uses three circular mirrors together with one large and one small rectangular mirror, all having identical spherical radii of curvature. The arrangement achieves a large number of reflections and the numbers of reflections can be quickly varied by adjusting the angle of the mirrors.
In the design adjacent, the laser beam enters by one hole, reflects a large number of times off the mirrors and finally exists at a hole alongside the entrance hole. The number of reflections are controlled by the angle of the circular mirrors on their horizontal and vertical axis. Simply altering the angles alters the path length.
So using this method, a chamber that is physically about two Metres long can apparently be any reasonable selection of path lengths up into hundreds of metres long.
Oldham Optical has supplied a set of mirrors, (Seen opposite
before being aluminised.) for the "Highly
Instrumented Reactor for Atmospheric Chemistry", (HIRAC), which is a
"leading edge" Gas Analyser that is part of the
facilities of the School of Chemistry at Leeds University. The researchers
report the optics as working well.
You might think these optics would be easy to make because they are so small? - but they do have their own special problems.
If we are making a 20" F/5 Newtonian primary mirror for a customer, the important thing is the surface quality, - usually 1/4λ or better. It is not critical if the focal length departs from the nominal 100" a small amount. If say there is an error of say 1" on the focal length, the telescope focuser usually has plenty of tolerance to cope, so the dimension is not often critical.
So tolerance on focal length for a standard Newtonian mirror is typically only a bit better than 1%.
But when a series of mirrors are built into an optical system such as the gas analyser, the radius of curvature becomes highly critical. All the mirrors in the system must be manufactured to exactly the same radius of curvature, - else the laser beam disperses and is lost after a small number of reflections. Achieving the same focal length on a batch of mirrors is a lot more difficult than just manufacturing one in isolation, - but Oldham Optical can do it!
For this set of mirrors, - all had to have the same focal length within a tolerance of 0.06%.
Here is a photo of the HIRAC chamber in the School of
Chemistry at Leeds University. As you can see, there is a lot more to it than
just five funny little mirrors.
The stainless steel chamber can duplicate atmospheric conditions at any altitude/pressure and over a wide temperature range. It can be filled with a gas sample and left "to cook" for some time while chemical reactions take place.
It has various test gear connected to it ranging from proprietary spectrometer and trace gas analysers to a locally developed unit that they are very proud of that detects hydroxyl radicals called FAGE. (Fluorescent Assay by Gas Expansion)
(Don't ask us what it does, - we needed spell-check just to spell Fluorescent!, - but its apparently a very handy thing to have if you want to measure pollution in the atmosphere.)
Anyway, - the chamber and test gear allow Leeds University to carry out experiments mirroring what happens in the "real atmosphere".
The set of mirrors supplied by Oldham Optical are part of the test gear in the chamber. They are set 1785mm apart and are often set to give 70 reflections before the beam exits. This gives a path length of about 135 Metres.
There is a trade-off between the losses due to reflection on the mirrors and the amount of light absorbed by the gas. The value of 70 reflections "is about right" for a lot of the testing Leeds are doing at the moment, but they have used it with 128 reflections and think they could push it a bit further if required.
The picture adjacent gives you a better idea of the size and scale of the beast. It is a view in through one of the side access ports and you can see where it is on the chamber by referring to the picture above.
The gentleman crouched inside on the foam rubber cushion is Andrew Goddard. He is one of the researchers involved with the project and he is seen installing the rectangular field mirror.
(Looks more like he is having a "passport sized photograph" taken!)
And Finally, - there is a spectacular picture of the blue lighting in the chamber. Some people think it's something to do with "The Fly".
Well!
- It's not quite as exotic as that. The chamber needs to simulate the light from
the sun because it causes chemical reactions to take place in
the atmosphere. In practice it's the light at the blue end of the spectrum that
causes most of the chemical changes so the lamps appear blue because they do
not need to provide light at the red end of the spectrum.
There are eight quartz tubes, each containing three actinic UV lamps.
If you want a super-fast sun-tan, - please apply direct to Leeds University?
And if you are a company thinking of building your own Gas Analyser, - you now know one firm who you can approach to supply the optics!
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