Date: December 13, 2009
Title: Infrared Instrumentation and the Large Binocular Telescope
Podcaster: Richard Drumm
Organization: Drumm Digital Design
http://theastronomybum.blogspot.com/
Description: Richard Drumm talks with Dr. Mike Skrutskie of the University of Virginia Astronomy Department about his construction of the LMIRcam mid-infrared camera which will be part of the University of Arizona’s LBTI/NIC the Large Binocular Telescope Interferometer/Nulling Imaging Camera, on the LBT on Mount Graham. Dr. Skrutskie will also tell us about the TripleSpec trio of near-infrared spectrographs his laboratory is constructing, one of which is to be installed at the Apache Point Observatory.
Bio: Richard Drumm is President of the Charlottesville Astronomical Society and President of 3D – Drumm Digital Design, a video production company with clients such as Kodak, Xerox and GlaxoSmithKline Pharmaceuticals. He was an observer with the UVa Parallax Program at McCormick Observatory in 1981 & 1982. He has found that his greatest passion in life is public outreach astronomy and he pursues it at every opportunity.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by 3D, Drumm Digital Design of Charlottesville, Virginia. 3D is a full service production company that has astronomy as its passion. Visit http://theastronomybum.blogspot.com/ if you want to contact Richard Drumm of 3D.
Transcript:
Infrared Instrumentation and the Large Binocular Telescope
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[RD]
And thank you once again, to George Hrab for that wonderful musical introduction. His podcast is called the Geologic Podcast. It has nothing to do with geology and everything to do with Geo and Logic! Get it? Anyway…
Hello, I’m Richard Drumm, the President of the Charlottesville Astronomical Society, and I’m also known as Richard Drumm The Astronomy Bum on the internet. Well, if Bill Nye can be the Science Guy, I guess I can be Richard Drumm The Astronomy Bum…
Well, a few days ago I went to the University of Virginia Astronomy Department and I talked to Dr…
[MS]
Mike Skrutskie
[RD]
And I asked him a few questions about infrared astronomy and the infrared instruments that his team designs and builds.
And so, yeah, tell me about LMIRcam, is it already functional on LBT?
[MS] 0:51
Oh no, it’s, it’s just coming together right now, so… What LMIRcam is is an imager, something that takes pictures, that works between 2 and 5 microns in the middle of the infrared part of the spectrum. And it’s meant to go at what we call the combined focus of the Large Binocular Telescope. So the Large Binocular Telescope has two 8.4 meter mirrors on a single mount, just like a pair of binoculars but with monstrous lenses or in this case mirrors.
And ideally what you would like to do is take the light from both of those mirrors and combine it coherently so that you’ve line everything up to a fraction of the width of a human hair so that the light combines as if you were working with a single aperture. And when you do that you get the resolution as if you had one giant telescope 22 and I think it’s 22.8 meters in diameter.
And that gives you exquisitely fine spatial resolution. You can see about 1/30 of an arcsecond. So compared to typical ground-based observations where you’re thinking in arcseconds, an arcsecond or two, or a fraction of an arcsecond.
Here you’re getting down to 1/30 of an arcsecond, so you put those 2 capabilities together, seeing in the mid infrared and then also being able to resolve detail at the level of a thirtieth of an arcsecond, and that gives you access to extrasolar planets directly. That is, if you have Jupiters that have recently formed, where “recently” is say in the last billion years or so, in orbit around stars within 10 to 50 parsecs, so 30 to 150 light years of the Sun. They are self-luminous at these wavelengths.
Jupiter’s self-luminous at this wavelength as well, but it’s cooled down to the point that it’s not emitting a whole lot of flux between 3 and 5 microns. But these younger Jupiters are quite luminous at those wavelengths and we hope to be able to create direct images and ultimately take spectra with this instrument – it’s also a spectrograph – between 3 and 5 microns. And so maybe even provide the first direct images of extrasolar planets, depending on how you define them, because there are people who already claimed to have made those images right now.
[RD] 2:53
Right, with Fomalhaut.
[MS]
Exactly. That’s one of the candidates, there’s a couple of others out there as well.
[RD] 2:59
What sort of criteria do you have to have when you come to design and build a new instrument?
[MS]
Yeah, no, in the infrared it’s interesting because we all take for granted for example that glass is transparent, because you can see through it with your eyes. But you don’t have infrared eyes and you can’t easily asses the transmissivity of glass. It turns out that you know, ordinary window glass or ordinary telescope lens glass BK7 if you’re, you know, an aficionado of specific optical materials, is transparent out to about 2 microns in the infrared and can be used in some infrared optical systems.
But it turns out that to design at least refractive infrared systems that you need to use specialty glasses, really crystalline materials. So I could rattle off a list but they’re, there are things like barium fluoride, calcium fluoride. Silicon is an interesting lens material, as is zinc selenide, indium antimonide is often sometimes for very long wavelengths used as an optical material. Just take a couple elements from the periodic table and put them together and see what their transmissive properties are, some of them are quite agreeable.
Actually very interesting, in the early history of infrared astronomy, in fact one of the pioneers was a fellow by the name of Samuel Pierpont Langley who was also a pioneer of flight, and almost beat the Wright brothers. He’s the guy who flew his airplane into the Potomac River trying to be the first one to demonstrate powered flight.
In any case he was a pioneer of infrared astronomy and used salt, sodium chloride as his optical material, both for prisms and for lenses because it also behaves relatively well at infrared wavelengths. As long as you don’t get it wet. Water is our enemy in the infrared. Water is one of the primary culprits in making the atmosphere opaque. And since salt absorbs water, as you know if you have a salt shaker at home in the summer it gets clogged up, because it picks up the water from the Earth’s atmosphere. And so, guess what, if water gets into your salt it becomes equally opaque and is not a terribly useful material.
One sidelight of this and really exciting is that, you mentioned earlier, we’re working on a project called APOgee at the moment, which is called the Apache Point Observatory Galactic Evolution Experiment, another contorted acronym, but actually quite right on target. The idea is to build an infrared spectrograph that works at very high spectral resolution in the infrared band that’s centered around 1.6 microns, and feed it with 300 optical fibers, so you can take the spectrum of 300 stars at once.
You do that to get the chemical fingerprints of the stars to see what their elemental relative abundances are. That’s sort of a fingerprint of where those stars came from as the Milky Way was being built up. You know, we hope to look at 100,000 stars and disentangle the assembly history of the Milky Way Galaxy and the best of all scenarios. But what is interesting from the instrumentation perspective is this is a large instrument.
The lenses, it’s actually a refractive camera in this instrument, the lenses are almost a half a meter in diameter, and there are 6 lenses in the optical train and half of them are made of silicon. Elemental silicon, crystalline silicon. So they begin as a single crystal of silicon drawn from a bath of molten silicon until you get this boule, this big chunk of silicon that’s about a half a meter in diameter and they slice off lens material and polish it to make these incredible silicon lenses. And those are just in production for us right now and we’ll be very excited to see them integrated in the camera for that instrument.
[RD] 6:35
I’ll wager those are rather expensive, too!
[MS]
They’re not cheap, yes. You don’t want to drop one, let’s say that.
[RD] 6:42
But the rest of the instruments, I suspect, use mirrors.
[MS]
As often as we can. Mirrors are great because they reflect all wavelengths equally. One of the horrible properties of lenses, of course, is chromatic aberration that, that the refractive index changes depending on the wavelength at which you’re observing.
So you look in an optical telescope, a typical refractor will produce blue halos around the star, which is out of focus blue light, because the light is coming to focus at a different place. So in optical telescopes you can commonly compensate for that by mixing glass types with different refractive behaviors. So there’s crown glass and flint glass in a classical telescope where the chromatic aberration effects cancel. We do precisely that with refractive infrared optics, that we have multiple refractive materials, and so you can mix essentially quartz glass, silicon glass with something like barium fluoride and come up with an equally corrected design.
On the other hand, as you just mentioned, you simply use mirrors which don’t have this chromatic aberration problem because it’s all geometry, they all focus wavelengths of light in the same place. And ideal systems are made with mirrors, except that mirrors are less forgiving than lenses to some extent. That lenses because of the properties of refraction don’t need to be quite as precise as mirrors do, so sometimes you have very widefield instruments where the mirrors can be quite complex.
So you mentioned the LMIRcam instrument that we’re working on right now. That has some fascinating mirrors in it, in order to control the distortion, the aberration of those mirrors, they have to have essentially 2 radii of curvature built into these. So you couldn’t polish the way you do ordinary glass, they’re called biconic mirrors, because they have 2 separate conics built into the surface of these mirrors, and the only way to do that is actually with a mechanical cutting. With a machine that figures the mirror mechanically, and then there’s a post polish to a fine finish.
And there are 2 biconic mirrors in the system, which work in tandem, then, to produce an almost perfect image over a wide field for this Large Binocular Telescope. So, it’s not all done with mirrors, but sometimes mirrors are your friend. Other times you try to work with the refractive materials, and this is sort of the art of the design is to come up, usually, with the most compact and inexpensive configuration that takes advantage of, you know, the best materials you have available.
[RD] 8:59
And the other instrument is TripleSpec.
[MS]
Right!
[RD] I love that name! It puts me to mind of a certain liqueur. So you’ve got that at Apache Point and Palomar also.
[MS]
Right! And this is really exciting and I should drag John Wilson from across the hall in here because he’s the fellow who really made all that happen. This was one of the first instruments maybe ever, but certainly that I’ve been involved in that was built by a consortium at some level.
So, this all began as an instrument for Palomar and was led by Cornell University. It turned out that John Wilson I mentioned a second ago was a graduate of Cornell and was working on this instrument in its early stages. He came here to work and the opportunity arose for us to do some design work for TripleSpec, so we started to get involved, just participating in the development of this instrument.
And then as it turned out we said “Gee, we’ve got the resources, maybe we could build a copy.” and we know, you know a couple places that would be desperate for this capability. There are still a limited number of infrared spectrographs in the world, and observatories will beat a path to your door to get one at some level, so we said, “Let’s make the investment, take the leap of faith, and just go ahead and build, you know, another copy.”
At the same time CalTech became interested in getting a copy for the Keck. And so suddenly
[RD] 10:11
You make 3!
[MS]
… there were 3! And there were 3 different instrumentation groups, one at CalTech, one at Cornell, and one here, interested in that common goal, and we all worked together on the instrument design to build 3 nearly identical systems. In the end there are slight differences to adapt them to their telescopes.
The first one completed was Cornell’s copy, and that was deployed to Palomar Observatory about a year or a couple years ago now, and ours followed quick on that one’s heels, in fact we let them make all the mistakes and took advantage of the fact that they did all the learning on our dime in some sense and ours came together a little bit more smoothly, but…
[RD] 10:48
There’s an old saying: “You can tell the pioneers, they’re the ones with arrows in their backs!”
{MS]
Well that’s right! We owe a tremendous debt to the folks at Cornell for making all that work so smoothly. There are some really brilliant people on every corner of this triangle that put this instrument together. But in any case you know, this instrument for us came together about a year and a half ago and was deployed to Apache Point Observatory, which is a very interesting facility because it’s run almost entirely remotely by astronomers involved in the consortium there.
And so the idea is to have a telescope in which instruments are rapidly interchangeable. There’s a Nasmyth port looking into the side of the telescope with a big flange on it, and essentially you can roll up an instrument, close a couple of the clamps, and in 5 minutes or so have a brand new instrument on the telescope. And now TripleSpec is there with an infrared imager, with an optical CCD camera, with an optical spectrograph, all working in tandem on any given night an astronomer either uses one or more of those instruments.
But it was fabulous to see this instrument deployed there and now be able to literally – I’m sitting at my desk right now, I could log into the instrument and start taking data and it’s made to give you a feel when you’re using these widgets on your screen as if you’re sitting right at the telescope. So you can drag stars right down the spectrographic slit and guide on that star and watch the data come in, and feel like you’re sitting in New Mexico instead of here and work quite efficiently, which saves a lot of time and trouble in terms of travel and the like. And on cloudy nights, you know, it’s a little bit more relaxed instead of sitting and wringing your hands in the control room of the telescope, you know, you can take a nap and say “I’ll get up in an hour and see how things are going.” and if it’s really socked in you just give up and go to bed, and it’s your own bed in your own house, so it’s a very nice configuration.
[RD] 12:36
There’s a pizza place just down the road and when…
[MS]
Exactly!
[RD]
All the comforts of home… because it IS home I guess!
Thank you so much, Dr. Skrutskie!
Well maybe we’ll have another podcast in 2010!
[MS]
Well, it’s been my pleasure talking to you and I’d be happy to do it again in a year!
End of podcast:
365 Days of Astronomy
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