Blue Collar Scientist

The final video from the Junk Bond Observatory trip is up at YouTube.

If you sent in a question, and haven’t heard it answered yet, it will be covered here. Enjoy!

This big hype up to yesterdays huge NASA announcement doesn’t appear to be justified to me. The announcement doesn’t strike me as that big a deal.

Basically, the big NASA story that we’ve been waiting breathlessly for is that astronomers have found a supernova remnant created by a star that went supernova during the US’ Civil War. This is significant because of two facts:

1. Prior to this, the most recent supernova we’ve known of in the Milky Way is over 400 years old.
2. The statistics of stellar populations tells us that there should be seeing about three supernova in the Milky Way each century - meaning we have roughly a 12 supernova deficit (well, 11 after yesterday’s announcement) since that 400 year old remnant.

The thing is, we understood already why we don’t see these things go off - even though they should all be as bright as the brightest planets at the distances within our own galaxy. Basically, the hypothesis is that lots of these supernova aren’t visible due to extinction - obscuration by the huge loads of galactic dust that get in the way of our seeing them.

So we’ve understood that supernova must be going off fairly frequently for a long time now. But we’ve also understood why we can’t easily see them. So now they find a remnant that helps confirm that (a) they are going off like we thought, and (b) we can’t easily see them for precisely the reasons that we thought.

Yeah, it’s a cool discovery. I’m fine with that. But it is cool because it shows the predictive power of the scientific method. We saw a discrepancy in the data. We made some discoveries that might explain the discrepancy. Then, at length, we confirmed that yep, that data does indeed explain the discrepancies.

But as far as the object itself, I’m completely unable to get worked up about it. Maybe it’s because it isn’t in the solar system. Maybe I’m just still a little blown up from my trip.

Whatever. If you want to learn about it, you apparently will get no opportunity from my apathy-riddled brain; instead, the Bad Astronomer has excellent coverage.

My previous two videos about Junk Bond Observatory have inspired some Q&A, so I’ve posted part one (of two) of some answers to the questions that have been posted to the blog as comments, and/or e-mailed to me. This is a bit dry, I know, but it is what I do for a living, and I don’t know how to make it interesting unless you are really obsessed with high performance telescopes that aren’t being controlled by grad-student-ware¹. Anyway, here it is:

If you want to keep up with my future videos, you can subscribe to my YouTube channel.

The next two videos coming up will be part two of the Q&A, and a video of a short presentation I gave at a religious school.

1. This is a slightly derogatory term referring to kludged-together software, generally written in some deplorably out of date language or development environment, with many bad architectural choices made, by the graduate students tasked with writing it in return for peanuts and water from the drinking fountain down the hall. In astronomy, grad-student-ware (cf. shareware, freeware) seems to be responsible for more observatory downtime and observational overhead than any other source. [↩]

I’ve finally completed the video about data reduction at Junk Bond Observatory. In this video, we answer the question of how Junk Bond Observatory got its name, and provide a demonstration of what our images look like and how we turn them into scientific results. The demonstration by Dave is followed by a higher-resolution screen recording to show you the details, so stick with it to the end if you are interested in seeing the images up close. (And yes, I know this isn’t the best way - I’ve been learning whole new continents of my computing system doing this, and I am learning, but if I wait to post until I’ve learned everything, I’ll never get around to posting this stuff.)

In the next video, I’ll have answers to the rest of the questions that people asked after the first video was posted, and hopefully it won’t take me so long to get it posted. What can I say - I’m on a working trip. Work comes first, and even though the formal purpose for the visit was fulfilled sometime yesterday, I’ve still been quite busy taking care of loose ends and addressing things that I felt needed to be dealt with before I left.

Tomorrow will be a semi-travel day for me; I’ll be leaving JBO around noon Arizona time and heading up to Tucson, with a long stop to visit some people in between. Then Wednesday is a travel day. Blogging will continue to be light, sporadic, and possibly sub-standard until I’m home and have had a chance to sleep properly. But I’m hoping to get a few things posted from the hotel in Tucson, so stay tuned.

One of the things I emphasize to my students is that a lot of the better research going on today is interdisciplinary, in which scientists from completely different fields collaborate to study a phenomenon and the scientific results are improved from the participation of folks that have different knowledge and different backgrounds.

Yesterday I spent a little time in my own cross-disciplinary scientific world. I wasn’t really contributing anything, I was soaking up the awesome coolness that is Tom Kaye.

Tom’s a sort of modern gentleman-scientist, of the sort that nearly went extinct shortly after Darwin’s time when the cost of doing scientific research began to require funding that was not available even to the very rich. I’ve known Tom by reputation for years; back in 2000, he had a telescope set up at a friend’s observatory where he made the first amateur astronomer detection of an exoplanet using the radial velocity method. He’s also the guy that got hold of Norm Oberle’s 1-meter mirror blank; I knew Norm back when I lived in Ohio, have seen the blank, and knew that someone had bought it, but never knew who until I went to dinner with Tom a few nights ago.

Tom’s neck-deep in astronomy, but he’s also a paleontologist, and he’s specifically looking at a possible connection between gamma-ray bursters, the K-T boundary extinction, and the Chicxulub impactor. To support this research, Tom has a bunch of fossils, K-T boundary samples, microscopes, and atomic composition analysis equipment.

And when I say microscopes, I mean microscopes. He’s got everything from a simple stereo microscope, to a couple of the nicest compound binocular microscopes ever made, and even two electron microscopes. We slapped a spider leg into one of the electron microscopes and took a look at it in all its hairy, spikey glory. Really cool stuff - I’ve never had a chance to play with a microscope before.

Tom was kind enough to donate a bunch of hadrosaur teeth to me for use in my educational programs, along with a sauropod stomach stone and some 35 million year old fossilized poop. And we’re going to work at the beginning of next school year on putting together some brief educational videos for use in the classroom, and maybe even set him up so that he can visit my classes through webcam to talk about his research.

It’s amazing the people you meet in my line of work.

I have previously posted on the International Year of Astronomy - and I had kind of a mixed reaction to IYA overall.

Having stumbled upon the IYA website back at the beginning of January, my main reaction to it was that at the time, it was pretty heavy on bureaucracy, and pretty light on things that I thought would actually help astronomy EPO. I didn’t just seagull¹ the IYA - I offered constructive criticism.

Today I learn that Pamela Gay, one of the really truly helpful people in the world of astronomy education, has been hired into the IYA’s bureaucratic apparatus.

Let me say as clearly as possible: This is a good thing. A very good thing. This single move gives IYA more credibility in my mind than anything I’ve heard so far, including things the two people associated with it left as comments on my previous post.²

Pamela recalls some painful experiences she’s had in the past turning FITs images into play-nice JPEGs and other “standard” pc/mac image formats. I’ve had exactly the same problems. And she reports that NASA/ESA/ESO have come out with FITsLiberator as a Universal Binary (and Win compatibility too) - which is relevant to me, since I’m now doing most of my work of a new Macbook. This is indeed a Good Thing, and something that will help advance IYA’s stated goals, although it isn’t clear exactly what IYA had to do with the FITsLiberator update.

Pamela mentioned the IYA’s Portal to the Universe project (without linking, for shame), and I got my hopes up, thinking that maybe someone had taken to heart the plea from my last post on the topic (I’ve edited it in a few places to bring it up to date):

As I write, I have spent 65 hours 135 hours preparing an EPO presentation for adults on the history of astronomy that gets presented at CCSO next week³. If I’m lucky, I’ll get to give it at the Eagle River Nature Center sometime in the future - this is planned as a two-use program. There’s an outside chance it will get recycled in a few years and I’ll get a bit more mileage out of it.

The program starts with ancient conceptions of the structure of the universe and covers every really major astronomical discovery or theory since then. By “really major” I mean things as significant as heliocentrism, the cepheid period-luminosity relationship, the expansion of the universe, the cosmic microwave background, and the like. My presentation has 70 slides 95 slides, and all but two of them are images or animations. What has taken the most time has been (a) finding the illustrations and (b) getting the permissions to use them. (I know I can claim fair use - but sometimes I need permissions because the venue hosting the talk demands that I have them, regardless of the law; other times, I need permissions because it would be fair use to use the image in my talk, but not on the web or in the newspaper in promotion or coverage for the talk, etc, etc.)

What I really could use is a public domain or Creative Commons licensed image and animation library. Something that is captioned by experts so that I know exactly what I’m seeing in the image. For example, I don’t have a good animation of stellar parallax - there should be movies out there put together from FITS images that I can use. The movies exist, in varying quality, but getting permission to use a high quality, useful animation isn’t happening. There’s no animation that illustrates the origin of the cosmic background raditation that I can find. Even though I can picture how to illustrate it, I’m no artist. It is hard to find an illustration of the Keplerian thought transition from “orbs” to “orbits” - a fairly important advance in thinking about planets not as things affixed to spheres with rotate, carrying the planet along with it, but as things which are out in space attached to nothing and revolving in orbits.

Unfortunately, Portal to the Universe is not the image/animation library I had hoped. Portal to the Universe looks like it is going to be a super-cool, mega-feed-aggregator for everything good about astronomy. That’s great. I’m not against that. I just want more.

Pamela, IYA, please:

What I really could use is a public domain or Creative Commons licensed image and animation library, captioned by experts.

I’m not trying to be histrionic by using the big type. IYA obviously has some money to spend on improving astronomy outreach, and they’ve obviously got some political clout. Please start using that leverage to ask researchers to release significant images under Creative Commons or GPDL or some other free license.

I’m not alone. I’ve been associated with astronomy clubs for most of my life, and in each club, there are always five or ten people who are going out to talk to a school class or the Girl Scouts or similar groups once or twice a year - especially if they have kids of their own in the system. In the last five or ten years, as multimedia projectors have become common, the standards for presentations have risen sharply. As a consequence, these people are spending more time in PowerPoint or Keynote preparing their shows, and they are running up against the same problems I am. They ask: Should I just use the image and not tell anyone?

Usually you can do so, and do so legally. But what you can’t do is use it, and then (legally) give it to your pal in the astronomy club when he’s going to a different classroom to talk on the same subject. You can’t post the presentation file on the astronomy club website for everyone to use, for fear someone will object to the content. Even if we, personally, as individuals are willing to take the risk, people with fiduciary responsibility in the club (quite rightly) won’t allow it, due to that same risk.

IYA, if you can make a start of such a library, I, and people like me - and there are lots of us doing this independently in the trenches - will benefit by:

• Being able to share our presentation files without the fear of getting sued by some university bureaucrat protecting their “rights” to some image or other. I’m not saying it has happened, but I have heard stories about threats, and we’ve all heard about RIAA ruining peoples lives with lawsuits directed at the wrong defendant, demanding outrageous damages, etc. Give us a legitimate way to use the images and share presentations and dramatically cut the amount of preparation we have to do - that will advance what you are all about.
• Being able to do better education and outreach outside of our specialties. I have hundreds of thousands of images of asteroids taken with ground-based telescopes, because that is what I research - asteroids. I do not have even a single image of cosmic background radiation anisotropy that I know for sure I’m permitted to use. The good news? Since asteroids hit planets, and killed the dinosaurs, I’m often asked to talk about asteroids, not CMB anisotropy, so in those cases I have lots of cool images for those talks. The bad news? I’m also often asked to talk about the big bang, because that’s a pretty fundamental astronomical issue, and it is one that schoolteachers don’t always grasp enough to teach it well. (The captioning is important here - news release captioning is often mangled by institutional PR writers with no knowledge of the subject, and I often find that the information that goes with such images is a bit less helpful than it could be. I’m no cosmologist, but I’m not stupid either, and I’d benefit from an expert perspective on the kinds of issues we see streaming by in the feeds that basically just reproduce astronomy press releases. The proof of this are the large number of papers I get from arXiv when my interest has been spurred by a press release - I understand most of them.)
• One-stop shopping. I could probably cut my preparation time by more than half, because (a) I wouldn’t have to go google-surfing for images I can steal, and (b) I wouldn’t have to beg for permissions from dozens of different people, and keep track of when I’m getting referred to someone else for the permissions instead, when I’ve got permission, when I’ve been denied permission, etc. If I can cut prep time in half, I can (a) do three more EPO activities in the saved time, or (b) take some time off and not go crazy for doing so many EPO activities.

Institutions would benefit by preserving their copyright if they chose a creative commons - attrib - no derivative - no commercial license. (Oh, and by the way - I put my money where my mouth is. This blog is CC licensed. I’m also a magazine writer, and my writing has earned me some not insignificant money. If I can do it, I’m sure some astronomers can too.)

Thank you, IYA, for listening to my thoughts.

PS -

Last time I posted about IYA, people with the IYA asked me to call them on the telephone. Don’t take this personally, folks - but I am busy. I’m in the schools two or three times a week doing astronomy and physics education, and I’m not a teacher. I’m out in front of the general public six to eight times a year doing the same thing. I have a blog. I have a job. I’m in the inconvenient time zone of UT -9 (-8 for DST). And I tend to prefer my solitude anyway. But it isn’t that I don’t want to talk to you, its just that anytime I might call you I have five other things that are more important to do. If you want to talk to me, leave a comment and ask me to send my phone number - I’ll send it. It is easier to be interrupted than interrupt myself.

1. Seagull: to swoop in, crap all over everything, and fly away. [↩]
2. I didn’t allow one of the comments through, because of the number of phone numbers it contained. This blog attracts some readers that you just don’t want to give your phone number to. [↩]
3. It got presented, it was well received, it was great! [↩]

For years, astronomers have tried to catch a supernova in the earliest stages of its explosion. They’ve gone to the extremes of building highly automated telescopes to search hundreds of galaxies each night for new supernova, often revisiting the same galaxy several times per night just so that if one did happen to be going off, they could catch it several times early in the process. Researchers in the highly competitive field have even collaborated with each other to insure they are getting most efficient coverage possible.

Catching a supernova early on in its detonation is of interest to astronomers for two reasons. The first reason is that certain supernova are useful standard candles for calibrating intergalactic distances. The better the measurement of supernova’s brightness over time, the better the distance measurement will be. So there’s an incentive to observe a supernova regularly over as much of its lifetime as possible. The other reason is that the very earliest stages of the explosion should tell us interesting things about how supernovae explode. We’ve got the broad strokes of supernova formation pretty well figured out, but watching a supernova in the early stages will allow us to figure out the details.

Astronomers studying supernovae have had some success with their strategies. They’ve caught quite a few supernovae on their rise to brightness, but they’ve never really achieved their holy grail - observing a supernova from the earliest stages of its detonation.

Back on January 9, that all changed. On that day, I got an e-mail from the ATEL service reporting that the Swift satellite had discovered a bright X-ray transient while it was observing a two-week old supernova in the galaxy NGC 2770:

Follow-up observations of SN2007uy with the Swift/XRT reveal a new transient source about 95 arcsec away from the SN position at RA=09:09:30.7, Dec=+33:08:19 (J2000). This position coincides with the outskirts of the host galaxy of SN2007uy (NGC 2770).

This sort of thing is fairly routine. I get ATEL messages, and alerts from other services, primarily because some of the astronomers who run my software use those services to report and find targets of opportunity. It is nice to know what my clients and colleagues are up to - makes me feel like I’m still in the loop, even though I’m not doing the day-to-day thing at an observatory anymore.

After the initial report, several other astronomers chimed in, some with negative observations, and the early thinking was that the object was a flare from a soft gamma-ray repeater, or a weak X-ray flash - two classes of phenomena that are pretty interesting in their own right, but nothing that would keep me up nights.

But within a few hours, an optical counterpart had been discovered by J. Deng and Y. Zhu at the National Astronomical Observatory of China:

We took a 60s unfiltered CCD image and a 600s R-band image of NGC 2770 on 2008 Jan 10 18:19 UT and 18:22 UT with the BFOSC [Beijing Faint Object Spectrograph and Camera] instrument of the NAOC 2.16m telescope at the Xinglong Observatory, China. An optical source was clearly seen in both images which was conincident in position with the X-ray transient reported in GCN 7159. It was roughly 2 mag fainter than SN 2007uy in the same galaxy….. We suspect that this source is the SN counterpart of the X-ray transient (a XRF). Spectroscopic observations are in progress despite the cloudy weather condition.

Just fifteen hours later, Stefano Valenti reported:

Following the report of J. Deng and Y. Zhu (GCN7160) that the transient was roughly 2 magnitudes fainter than SN2007uy on Jan. 10.76, in R band, it seems that the luminosity of the transient is rising rapidly, supporting the very fortunate possibility that Swift has observed, in real time, the explosion of a massive star.

This was exciting - electrifying - because in the past, nothing like this had ever been done. For forty years, astronomers have theorized that when a supernova goes off, you should first get a burst of neutrinos, then some gravitational waves, and then some X-rays, all within seconds or minutes of each other. A couple days after that, you’ll start to get the visible light, so the best that all the astronomers using optical telescopes to hunt for supernova could possibly accomplish was to discover something that had exploded a day or two before.

It looked pretty good for this being the first discovery of a supernova at the very moment of the explosion. One of the things that astronomers were looking for was something called “shock breakout radiation” - a big burst of x-rays and ultraviolet light that results from the explosion’s shock wave emerging through the stellar surface. Shock breakout radiation had not, to the best of my knowledge, ever been observed before. Then several astronomers reported on ATEL that:

the optical transient was clearly detected in the U, V, and B bands of the UVOT. The source was marginally seen in the UVW1 image. The very rough B and V magnitude is 18.8 and 18.4, respectively. The Swift observations indicate that the optical transient becomes brighter in the past 24 hours. We note that the optical transient was not seen in the UVOT observations taken on 2008 Jan 9.

The “U” band referred to is ultraviolet light, so this was a good sign - they could not see the supernova at nearly the moment of detonation, but they could see it, and measured the brightness of the UV radiation from it, two days later. That’s almost exactly what theory predicted they should be able to do!

Over the next few days, a flood of messages about this newly-discovered object came down the pike, and after a while, the excitement died down. Almost a month ago, the observers submitted a paper to Nature, and the preprint appeared on the arXiv server. Everyone has been very responsible about not discussing it due to the embargo while the paper undergoes peer review, but now Science News has published a story, and given my small readership, I don’t see the point of being quiet about it any more.

The data suggests that this was a Type Ibc supernova discovered at the moment of detonation, that the progenitor was a Wolf-Rayet star, that shock breakout radiation was observed, that radio, optical, and long-lasting x-ray counterparts were observed, that fast ejecta was observed, and that all the observed phenomena fit very nicely into a model of how supernova explode.

Furthermore, the observations are inconsistent with one hypothesis about how supernova generate their radiation. The “relativistic outflow” model proposes that a lot of supernova emission is the result of particles that are accelerated to a substantial fraction of the speed of light, something that did not happen in this case.

And finally, it looks as though a wide-angle X-ray instrument similar to the one the Swift astronomers were using would allow scientists to detect several hundred core-collapse (Type Ib, Ic, and Type II) supernovae each year at the instant of explosion. This would not only allow the model to be tested with a lot of different supernovae, but scientists using neutrino and gravitational wave detectors might benefit from having some constraints on the position and time of the events these sensors are designed to detect.

The bottom line is, these results are very much what science is all about. Some of the hypotheses about supernovae made predictions that didn’t stand up to the observations of SN2008D very well at all, but others fit the observations just about perfectly. It opened up new lines of inquiry and research, and suggested some things that astronomers could do to do even better science in the future. And last but not least, it showed where we still have some things to learn about supernova that aren’t fully explained yet.

It is a good time to be an astronomer!

Martin Rundkvist of Aardvarchaeology writes about the battle between scientism and antiscientism in what some of my colleagues have described, a little less than generously I think, as a “soft science” - archaeology. Martin, who I met at TAM 5.5, writes about the necessity of interpreting data, even in the “hard” physical sciences like astronomy. One part in particular brought back memories (emphasis mine).

Could it be that the anti-scientism archaeologists believed that their work was fundamentally different from natural science because it involved interpretation? Well, in fact, yes. They tended to hint that natural scientists just read their conclusions off of their source material using fancy instruments, and that this would never work with cultural source material. The truth, as anybody who’s ever done real scientific research knows, is that all data must be interpreted in order to be understood and generate knowledge. Hundreds of gigabytes of observational data on quasars from a radio telescope is not astronomical knowledge. It is the necessary raw material of such knowledge. And the first interpretation of such a dataset that is published will not be accepted as knowledge until it has been thoroughly discussed and perhaps repeatedly (though ultimately unsuccessfully) challenged.

Let me tell you, Martin may be more right than he knows.

Replace “hundreds of gigabytes” with “several gigabytes,” “radio telescope” with “optical telescope,” and “quasars” with “asteroids” and you’ll have the position I was in back in 1999. I was a nobody, working at a small, privately owned observatory in southern Arizona. A commercial 0.4 meter Schmidt-Cassegrain was in use at the observatory, popularly referred to as the “POS,” the “telescope that caught on fire,” and “that damn telescope.” It was all we had while a 0.8 meter was constructed and installed, and it suffered from 30% downtime and a huge suite of mechanical and control system idiosyncracies.

My job was writing software to automate this telescope, and its successor, the 0.8 meter. Very early in the game I adopted Bob Denny’s ACP and PinPoint software, because these tools solved a lot of very difficult - or at least tedious to code - problems for me. It took care of things like using COM ports to send commands to telescope firmware, and analysis of images. However, an efficient high-level control system was not, at the time, included, so that is substantially what I set about to write.

The telescope was a major problem. It wouldn’t point accurately. Most telescopes that have problems pointing accurately like this run a layer of software that models the pointing errors. Up to a couple dozen elements go into the model, and when you need the telescope to point somewhere, you tell the model where, and it invisibly calculates the corrections to be made and passes them onto the telescope. In this way, your somewhat balky, misbehaving telescope is supposed to work well. But this telescope wouldn’t point inaccurately in a consistent way. Of the couple dozen coefficients that went into the error model, several of them were stochastic. We needed a 60 percent improvement in pointing, but we could only get about ten percent or so. The solution to this was to take an image after every telescope movement, compare the stars in the picture with a list of stars and their positions from a star catalog, and figure out where we were “really” pointed and make an empirical correction - a small, second slew - afterwards. It added several seconds onto every observation.

The optical mounting was substandard, and allowed the telescope mirror to flop around by several millimeters depending on where in the sky the telescope was pointed. This led to more pointing errors, and to football-shaped stars in our images. The telescope was made of steel, and as the air temperature dropped as the night went on, the telescope shrank. That threw the telescope out of focus.

I jury-rigged a huge bolt to the optical chassis to keep the mirror from flopping around and installed an ingenious focuser that had a temperature sensor and focus-loss model built into firmware that was installed on a BASIC Stamp (remember those?). I wrote all kinds of little routines into the control system to check and see if one of more than two dozen of this telescope’s failure modes had occurred, and recover from it if so. Even so the telescope more often than not failed in some new and novel way halfway through the night, bringing observing to a halt.

The whole thing frankly sucked. The only good thing about it was that we were learning what could go wrong with the 0.8 meter. We benefited a lot from that knowledge.

In the meantime, we were hearing a lot - mostly from the Minor Planet Circulars, where asteroid discoveries are announced - about an outfit called LINEAR. They had a budget in the millions, and were using expensive and classified Air Force technology. They had at first one, then two, telescopes that were designed from the ground up for automated observing of Earth satellites, which they had adapted to hunting for asteroids. They were using a huge, wide-angle CCD camera which we could not have bought even if we could have afforded it (they were classified technology). They had a bunch of experienced software developers to contrast with the single inexperienced one at my observatory (me). And they were sweeping up dozens of new asteroids a night. We didn’t really want to compete with these guys, we just wanted to have an efficient, automated observing capability.

After months of development, we ran the telescope for a full night in completely unattended mode. Then we did it again. And again. It gradually dawned on us that we had a robot on our hands.

We decided to conduct a little stunt. As a way of pointing out to our colleagues that we were automated, we decided to spend a night observing as many known asteroids as we possibly could. We set up a target list of about 400 asteroids, and at the appointed time we unleashed the telescope. We watched it observe the first half-dozen or so, and then went and watched a movie, because the only thing more boring than operating a telescope in person to observe an asteroid is watching a computer do the same thing. Nowadays, people would say “big deal.” At the time, something like this hadn’t been done before for a budget under millions - and we’d had a budget of thousands.

To make useful observations of asteroids, you need to take more than one picture. In our case, we took three images, each separated by about 20 minutes. That’s about 1,200 images by the end of the night.

The following day we had a almost two gigabytes of images sitting on our hard drive. Now this was the late 90’s, and I think the biggest hard drive we had was a two gig drive. I vaguely remember archiving all this data the next day to four or five CD-ROMs.

But it’s only data. Once you have data, you have to reduce it, and that’s what Martin is talking about.

Pictures of asteroids taken in visible light are not very useful for anything except determining the asteroid’s position. The pictures are taken with a fancy digital camera - with cooling modules and extreme sensitivity and so on, but basically just a plain old digital camera - and therefore the picture is made up of pixels. Each pixel covers a certain amount of sky. In our case, each pixel was about 2.8 arcseconds on each side¹.

So you want to measure the position of the asteroid, but your camera’s blocky pixels make doing this precisely difficult. This is where centroiding comes in. The idea is that an image of a star - or an asteroid, which looks a lot like a star, only it moves - is actually a smeared-out, blurry disk, with a brighter center and gradually fading edges. A star image like this will typically have a diameter of four or five pixels. Now the brightest part of the star image is where the star “is,” but just by looking you can’t narrow that down very well because of all this smeared-out light and the ‘bigness’ of the pixels.

It turns out that you can model the star image, and calculate from the model where the brightest part of the star image really is, and you can do it to a resolution much finer than that of your sensor. Most astronomers use a model called the point spread function , but there are other choices as well. By using this model, we could take our 2.8 arcsecond images and measure asteroid positions to about 0.3 arcseconds. Pretty slick.

Turns out, that’s the first layer of interpretation that the investigator imposes on the data set. What method to use to centroid the star and asteroid images can influence both the accuracy and precision of the positional measurements.

The next step is to take the list of positions from our 1,200 images and send them off to Gareth Williams, of the Smithsonian Astrophysical Observatory at Harvard. Gareth would take our positions and use them to calculate orbits. How? Would he compute a Vaisala orbit? Well, if the asteroid was a new discovery, he probably would - but otherwise not. If it were a known asteroid, he would add the observations to an existing list of prior observations and compute a much more precise orbit that makes fewer assumptions. Would he just generate some Keplerian elements as a result of this? Yeah, probably - unless the asteroid was “interesting,” for example if it were going to pass close to Earth or some other planet in a way most asteroids never do. If that happened, would he include Newtonian influences? Certainly. Would he deal in relativistic effects? Probably not, but maybe. Would N-body problems come up? Maybe.

How to deal with all this imposes another layer of interpretation.

The end result of all this work is an idea of where the asteroid is, and where it is going to be. An orbit can be visualized in 3-D as though it were a garden hose. The asteroid is a grain of sand somewhere in the hose - not sure exactly where, but definitely within the hose, and at a certain position along the hose’s length. If the asteroid is not well observed, it might be best to visualize the hose as a big fat fire department hose - the uncertainty is bigger. But if it is a well known asteroid, it might be a pebble in aquarium tubing. This lack of exact knowledge about the asteroid’s orbit is known as ‘orbital uncertainty,’ and it arises from measurement errors back when you reduce the data from your images. Only a few asteroids’ orbits are known so well that the orbital uncertainty is always less than their diameters. But almost everything has an orbital uncertainty low enough that we know for certain that it can’t possibly hit something for the foreseeable future (which is often a hundred years or more distant).

The bottom line is that Martin is right - data from the physical sciences is very heavily interpreted indeed. Even in the ultimate automated observing system, in which the telescope automatically generates data and the software automatically reduces it, the methods of interpreting the data would be imposed by the programmer at design-time. There’s no free lunch.

What is really cool about the physical sciences, though, is that despite all of this interpretation, you can have some reasonable level of certainty about your results. There is simply no other plausible explanation for the phenomenon we call “asteroids” than to believe that there are big chunks of rock (etc) orbiting the sun in very specific, well-measured paths, and that these bodies respond to well-defined physical laws.

Today, the only asteroid work we do is the occasional interesting near-earth object and the occasional discovery of a new main-belt asteroid. But the same system observes variable stars, exoplanet transits, active galactic nuclei, and a bunch of other interesting things.

1. That will seem grossly big to astronomers, but this telescope was optically terrible, and our seeing was also gross. We had a pretty good match between our PSF and our sampling. [↩]

On this day in 1930, amateur astronomer Clyde Tombaugh discovered the ninth planet of the solar system the Kuiper Belt Object the Trans-Neptunian Object the Scattered Disk Object the dwarf planet Pluto. Tombaugh was employed by Lowell Observatory at the time.

Tombaugh discovered the planet¹ in photographs on this day 78 years ago by comparing photographs taken at different times using an instrument called a blink comparator. The planet appeared to hop against the background stars in this instrument. The photos that Tombaugh evaluated had been taken almost a month previously.

Prints of Pluto’s discovery plates. The starfield is dramatically cropped; the plates covered a large area of sky and consequently there were many more stars to evaluate in the original. Click to enlarge.

After reporting the discovery to Lowell Observatory director Vesto Slipher, additional confirming photographs were taken and measured. The discovery was eventually announced on March 13. After conducting a search of archives, it was learned that Pluto had been photographed at least five times prior to its discovery, with the earliest observation, in 1915, being found in 1976.

A faithful-color image of Pluto’s surface constructed from photometric data from partial eclipses of Pluto by its moon Charon. Image courtesy Eliot Young (SwRI) et al., NASA. Click to enlarge.

Pluto’s largest moon, Charon, was discovered in 1978 - right at the beginning of the time that I began following astronomy news as a kid. Pluto has two other moons, Nix and Hydra, discovered in 2005.

Today, the New Horizons mission is headed for a Pluto flyby in 2015.

So lift a glass for Pluto Day!

1. I’m confused. “Dwarf” is an adjective in this usage, right? So a “dwarf planet” is still a planet, right? Except IAU says it isn’t a planet. Nothing like being unambiguous in your terminology…. [↩]