MagAO for the Layperson #1

For all of the non-astronomers out there (hi dad!) who have been patiently wading through our blog because they love us, here is a very brief crash course on AO. We thought that this might help you to understand why what we’re doing here at MagAO is so exciting.

The first important concept is that, in an ideal world, the resolution of telescope images is directly proportional to the wavelength of light you’re looking at and inversely proportional to the diameter of your telescope. In other words, the formula for resolution is:

resolution = wavelength / telescope diameter

Small resolutions are the best (we generally call this “high resolution” when we’re talking about cameras or TVs), so you can improve by either (a) going to a bigger telescope (increase diameter = divide by a larger number = smaller resolution) or (b) going to shorter wavelength light.

The first and even second and third generation AO systems all operate on infrared light, which has relatively long wavelengths.  We are the first 8m-class telescope system to push to shorter wavelength visible light (hence the name of our blog, VIS-AO). If there is a resolution advantage to shorter wavelength light  (visible over infrared), why did/does anyone bother with infrared AO?

The reasoning comes down to another formula, this time the formula for the atmospheric coherence length (also known as the Fried parameter). The official definition of this quantity is “the area over which an rms wavefront aberration is less than 1 radian”, which is a little opaque so let’s break it down. RMS is “root-mean-square” and is essentially an average. “Wavefront aberration” is the amount that a wave’s position deviates from perfection (perfectly flat, which is why we’re always talking about “flat wavefronts”). The atmosphere is the culprit here, bending our wavefronts from a perfectly flat shape.

One way to think about the Fried parameter is as the size over which you can expect the atmosphere to “behave” the same. The Fried parameter is proportional to wavelength to the 6/5 power. This means that at infrared (long) wavelengths, the Earth’s atmosphere is coherent over bigger patches. So if you want to correct at visible (short) wavelengths, you have to correct on smaller spatial scales. Adaptive secondary mirrors (ASMs), which are physically large compared to the tertiary mirrors used in other AO systems, can fit enough actuators on the back to correct at the necessary spatial scales for VisAO. In short, VisAO is hard, and we are heroic individuals!

The great irony, no matter what wavelengths you’re using for your AO correction, is that light from astronomical objects spends the vast majority of it’s journey to Earth with perfectly flat wavefronts. It’s only in the last 300 miles, the part where it’s traveling through the Earth’s atmosphere, that it becomes distorted. 300 miles seems like quite a distance on the surface (hah!), but it’s actually only a teeny tiny fraction of the entire journey made by light from an astronomical object.

In fact, let’s quantify that. If we’re looking at the very nearest star, Alpha Centauri, its light will have taken about 4.5 years to reach us. At a speed of 186,000mi/sec, that’s:

4.5yr x 186,000mi/sec x 3600sec/hr x 24hr/day x 365days/yr = 26 trillion (26,000,000,000,000) miles

to get to us. But the Earth’s atmosphere, which does all of the wavefront bending, is only 300 miles thick, so the portion of the light’s journey that is spent in the atmosphere is only 300/26 trillion, or 0.00000000001 (=0.000000001%).

That means that 99.999999999% of the journey was complete before the light found itself in need of our AO services, and that was for the very nearest star. If we’re looking at more distant objects, that fraction only increases!

This, of course, is why people put telescopes in space. If you don’t bother with those last 300 miles, then you don’t have to correct your wavefronts at all.

So why do we put telescopes on the ground? Well, there are lots of reasons. My two favorites are (1) you get a lot more bang for your buck on the ground and (2) you can upgrade the technology on a ground-based telescope whenever you want, so it will never become obsolete.

Hope that helps clarify what we’re up to a bit. Thanks for sticking with us!

Beautiful Sunsets are So Overrated

As an Astronomer, it feels traitorous to appreciate this.
Because clouds and telescopes are sworn enemies.

Yet the folks over at Baade powered through all night last night, and with a practically full moon to boot!

And it won’t stop us tonight either! Rest assured that our PI had the foresight to keep the CRO on today, so MagAO commissioning continues unabated.

Look how short that post was!

MagAO Commissioning Day 15: The Return of the Crow

Greetings MagAO blog followers! Tonight, I’m giving Jared and Katie a well-deserved night off from the blogosphere. Don’t worry – they’re still hard at work on the system, and continuing to collect pithy quotes and hilarious anecdotes for your entertainment.

I’ll start with a recap of yesterday, since I neglected to send the “good pictures” to Jared before he posted the blog entry. As you may recall, we had the first real starlight pass through our system last night! Here are a couple of pictures of the action.

Even though they look disappointed, those are actually triumphant smirks on the faces of MagAO PI Laird and AO guru Alfio.
Jared contemplating real starlight on his VisAO camera
First light night at Clay. Thanks Yuri for the awesome picture!
This one is actually from several days ago. Shh.

After we successfully closed the loop on-sky and the day crew left for some much needed rest, the night crew continued to push the system. As we ramped up to 50 modes, we were thwarted repeatedly by a ring of actuators where very high forces were building up and breaking our control loop. While we were mulling on that, we managed to do some important science calibrations. Specifically, we figured out which way was up.  No really.

With all of the rotations and reflections of a complex optical system, “up” is non-trivial, so you have to do some testing to figure out how north, south, east and west on the sky map to your camera output. We also figured out the “plate scale” of our instruments, which just means that we looked at a binary star system of known separation and calculated how big each pixel on our detectors is when it’s mapped onto the sky. We will have to do this more precisely in a few days, but were relieved to get the answer we were expecting. No unit conversion errors here!

Before going to bed, we also took some on-sky science data for the science team to practice with. More on this later.

This is a bird. Jared made me take a picture of it because he believes that this is the culprit who has been pooping on the optics. I'm pretty sure that this is just a random bird, but I indulged him anyway. I insert it here to distract the reader from the long text of my blog entry, so that my advisor won't comment derisively on my liberal arts education tomorrow.

Today the day crew put the CRO back on the telescope to continue their calibration work. They quickly discovered that our actuator ring problem was due to the method used to “slave” the inner rings of unilluminated actuators. Now the system is running well at 400 modes without breaking, and the Italians have gone to bed to rest their brains in preparation for a ramp up to more modes tomorrow. The night crew is hard at work continuing their calibrations.

It’s not a MagAO blog post without a wildlife reference, so I should mention that on my hike up the mountain to the telescope this afternoon, I encountered Vizzy and a friend hanging out under the eaves of the ASB roof. Now, when I first came to Chile as an undergrad 10 years ago, I was told of these mythical creatures known as “vischachas” that were part kangaroo, part bunny, part squirrel. Memories of my dad having me look for jackelopes on ski lifts when I was a kid left me (appropriately) skeptical, so I was surprised to come face-to-face with two real live vischachas today. I watched them for several minutes to make sure that they were not stuffed and part of an elaborate prank. I have yet to see them hop like a kangaroo, so stay tuned for future investigations into the vischacha phenomenon. In the meantime, I hope that the following picture is more convincing than your typical sighting of Nessy or Big Foot.

I snuck up on Vizzy from the backside of the ASB and waited until I saw him move his whiskers.
Katie, Alan, Phil, Jared and Ya-Lin posing with the moon on the Magellan balcony.
Clay and Baade at sunset. I don't have a wide angle lens, so this is the best I could do to give you the whole picture.

Now on to my alterior motive for posting today. I’d like to take this opportunity to answer the question, put to me directly by both my husband and my father after reading the blog, “So… it sounds like things are going great over there…. but why are YOU there again?”

It’s a fair question. What ARE all the rest of us doing? You’ll find evidence of our existence in the footnotes  of previous blog posts  (“so and so arrived today”) and in the backgrounds of pictures, but what are we actually doing?

No we weren’t eaten by Cart-Zilla before we could get any real work done! No, we’re not twiddling our thumbs, sleeping all day, watching movies, or leaving our husbands to fend for themselves for weeks on end for no good reason at all.  We’re here for science.

I asked the newest member of the MagAO team to demonstrate:

Ya-Lin doing SCIENCE!

 

There are a lot of practical problems to be solved before we start collecting science data, and that’s what the rest of us have been working on. We’re making sure that all of the file information that we need is written correctly and automatically when we take data, that we know how to tell the telescope where to point, that we have lots of options for good scientifically-interesting things to point at, that we understand how our instruments are performing and, perhaps most importantly, that we know how to process the data coming out of the system so that we can actually do science when the time comes. For example, a typical VisAO data set might contain 10,000 images. Processing such a large number of images at once requires some careful planning, so we’ve been busy testing different methods to try and speed up the process. So that’s what the rest of us quiet M-star types are doing here while the O and B engineering superstars are burning brightly on the blog.

Warning: I’m about to get deep here. Perhaps it will even make you forget my corny spectral type joke.

In conclusion, I’d like to take a moment to appreciate the complex mechanical, software and optical engineering that goes into a system like MagAO.  Even after being involved in this project for 4 years, I’m not sure that I fully appreciated the complexity until I got here. The MagAO system has to control many components to micron precision all while the telescope is tracking, the mirror is deforming, and control programs are running on four or more separate computers. All of these pieces have to speak coherently to one another, and do so seamlessly so that we observers can do our science.  Everyone knows that science is sexy, but as I’m discovering, engineering can be sexy too!

Therefore, I leave you to contemplate Cart-Zilla, an engineering marvel.

Laird describes this contraption as the result of "Engineers Gone Wild". Presumably there was less toplessness involved than in other iterations of the "Gone Wild" franchise.