by Mandeep Gill
In Part I of this 2-part series on what astronomers do while observing, we looked at the why and wherefore for taking the wide-field data for the cosmological side of things, and here in Part 2 we continue with first discussing the other mode that most people think of when they imagine what astronomers do at a telescope: searching for specific interesting objects.
The “funner stuff”: hunting for specific objects
While these periods of object hunting total in the end to less time that we are operating during DES operations at the telescope, they do keep our attention and alertness quite focused while they are happening. These hunts can be for anything from Planet 9 in the outer Solar System, to asteroids that may be potentially hazardous to the Earth, to a visitor from deep space zooming by the Sun, to some counterpart of events seen in other types of observatories like ultra-high energy cosmic ray telescope arrays, gravitational wave detectors or neutrino observatories. Most would generally agree that it is more exciting to be part of a team searching for these types of events when we’re actually at the telescope, just in case we happen upon something in that very moment.
However, even finding a new object sometimes take more than one image; for example, to find Planet 9, at least a couple of nights of observations are required to see the planet in motion against the non-moving stellar background.
A typical day, and night, of observing
So this is all prologue to say that mostly in my 11 nights of observing, we were taking data in the wide-field survey mode, where a typical day might look like this: at 4pm local Chilean time we have have a ‘ops’ (operations) videocon meeting with operations manager Tom Diehl and others at Fermi National Laboratory (the central hub for DES) and folks involved in data management, supernova data collection etc., at other places in the US to review how the previous night’s observations went, and also go over the current night’s plan.
We then do a couple of setup steps to prepare for the night, like accepting the data assessment of all the images from the previous night, if we got word from the Data Management crew at the ops meeting that the quality was good enough (i.e., clear and stable enough atmosphere) to do so. We do this so that that area of the sky will not need to be re-imaged later.
Then we generally head back down the hill to the dorm cafeteria for dinner. (Sometimes together, or independently, as there are normally two to three DES observers per night—three for all my shifts this run.)
We next head back up from the dorm around 8pm at this time of year (since we were nearing the start of the Chilean summer, it was getting dark by about 9pm).
Once back in the control room, we initially do a couple of preliminary setup steps at the start of the night: making sure the camera focal plane and electronics are ready to take exposures (by “flushing the buffers,” etc.) and then taking twilight standard star fields so we will know the precise brightness of whatever objects we'll be looking at that night (which we call photometry).
From there, the system is mostly automated, so that we turn on the software (called the Observing Tactician or “ObsTac”) and in theory just let it run all night. ObsTac knows which fields in the 5000-square-degree footprint have already been observed, and also the last time the SNe fields were observed (which were discussed in Part I), so that it can make sure to go back and get an observation of those fields once a week.
That how it goes if all runs smoothly—but Blanco is an old telescope with various little persnickety aspects, and a good fraction of the time, things don't go smoothly all night. Several times, we did have problems with the software. Thankfully, there’s always at least one Telescope Operator or “telop” in the control room with us. A telop is the person who knows the scope and knows how to diagnose and fix things when they go wrong. Generally a telop has been on the job much longer than any of the observers have been on DES or coming down to CTIO—sometimes for more than a decade. So the telops have seen the various situations come up, and usually know much better what to poke, prod, or restart, to get things to work right.
While the telop monitors their systems, the DES team monitors the data quality, observing sequence, etc. During this run, we had problems a couple of nights with the network which caused data transfer issues, during which we couldn’t take data. This causes major stress for everyone while we hover over telops to see if there is anything that can be done to get the system going again. This is because we want the data to be flowing well and into the can, with the clearest and most stable skies possible—even if we on the DES side have little we can do directly about it when some problems come up.
One of the things that observers have particular responsibility for is to carry out the observation plan. That means starting on time, seeing that the observations are taken in the order that is expected, and making sure that the last scheduled observations of the night finish during early twilight, but before the Sun rises. This is because the CCDs (charge-coupled devices) that take the images are so sensitive that even a little bit of time with the Sun shining on the detector plane may potentially damage them. Indeed, these are extremely red-sensitive CCDs that are the best in the world in the wavelength range they cover, which is what allows us to do deep images of the hundreds of millions of galaxies we need to do, to make the exquisite cosmological and astrophysical measurements we have been able to make so far. Thankfully, the protection is automated and built-in as there are protective photodiodes installed outside of the telescope scaffolding so they are sensitive to scattered light as well as some photodiodes on the focal plane itself (where the CCDs sit) that prevent exposure to excess light.
As one example of just how sensitive the camera is, the tenth brightest star in the sky, Achernar, which is only visible from the Southern Hemisphere because it's so low in the sky (technically, at a very southern declination) is actually in the DES footprint, and came up in one of our images. It was so bright in the exposure that several of the 62 CCDs were totally saturated, and ghost images from internal reflections off various surfaces and lenses in the scope showed up across the entire exposure. While this made for a dazzlingly beautiful image to look at, very little of that image will be useful for actual science images.
Our efforts in hunting mode
Now we turn our attention back to the second hunting-for-objects mode discussed above, where we follow up on alerts from other types of detectors around the world earlier in that day.
During my time there we did have one of those events, which we technically call a Target of Opportunity or ToO, as a result of an IceCube high-energy neutrino event that was seen way down in Antarctica. When this happens early enough in the day, the team makes a decision at the 4pm meeting for whether or not to interrupt the normal preplanned automated ObsTac sequence for that night and insert the commands (written in a language called JSON) to slew the telescope and look in the direction that IceCube (in this case) has indicated, to see if we can associate any particular optical emission with it. This is precisely what was done in the case of the gravitational wave event back in August 2017, which led to the observation of the binary neutron star collision in optical light. So far, no specific object has been associated with that ToO, but I know that KIPAC alumnus Keith Bechtol was sending ToO requests to Blanco for some weeks afterwards in case anything might show up once the whole sequence of images was analyzed. As a result of these searches, at the minimum, an "upper limits" paper will be published discussing that the brightness of the optical counterpart had to be below a certain value or else we would have seen it in DES images.
DES observers: Staying alert and on top of it all
So this is the primary work: monitoring the data quality and software operations during the night to see that all is coming in okay, inserting the ToO scripts at specific requested times during the night, and optimizing as we can (sometimes manually) things like slew times (by minimizing them), the amount of air we’re looking through (also minimizing that), etc. We further optimize observations as we can, e.g. usually trying to point the telescope as straight up as possible so that we can minimize the amount of atmosphere it's looking through (either manually, or more implicitly, through code written into ObsTac), because the more air we look through, the more blurred the images. (This is by the way why astronomers like to launch telescopes into space, but that costs 10 to 100 times what building a similar-aperture telescope on the ground costs, which really cuts down on the number of space telescopes the astronomical community can afford to launch!)
End of the night—or early in the morn!
At the end of the night, which was around 6am when we were down there, when the Sun begins to rise, we reverse the process we started at the beginning of the night: we take some standard star fields for photometry, then turn off the high voltage to the camera, write the end-of-night report, and head down the hill to the dorm.
A planet-hunter amongst us
One thing—there was an actual planet-hunter with us! Scott Sheppard, an astronomer I met a year ago when he was planet-hunting at the Blanco after the evidence of Planet 9 came out. This is because Carnegie Institutes buy several nights per year of time on the scope. Now, chances are that Planet 9 is out there (based on indirect orbital analyses of other outer solar system objects) but it is not a done deal. There are other possibilities for why their orbits are skewed that way, so the planet-hunters will just keep looking. Scott didn't find it during the time he was up at the Blanco during his half of the night (we were able to chat with him as he looked, and see him swapping between screens as he searched for a moving planet) but he has been on teams which have found several moons around Jupiter, Saturn, Uranus, and even Neptune (the links will go to example moons found by Sheppard, et al.), along with plenty of other bodies in the solar system. This was pretty fascinating to me, and long talks with Scott about the dynamics of the Solar System (Saturn's rings, possibilities of life on the moons Enceladus, Europa, and Titan, Trojan asteroids around each planet, interstellar object Oumuamua, etc.) really brought back the powerful excitement I felt about our own little neighborhood Solar System back when young.
And closing that loop felt quite fitting, indeed.
Finally, I close with a few photos of my co-observers, and some friends we connected with during out time at CTIO:
Truly, there is something wonderful about being at the telescope taking data for the whole collaboration one is a part of. For those few nights that any of us go observing, right there, right then, we are in charge of the data-taking for all several hundred of our collaborators (as well as for the entire public, when we release the data to them, as we just did for the first time). Indeed, after watching countless talks with results from the analyzing the data, and looking at computer screens where we poke and prod it to extract out the answers to our questions, this is the very first time for many of the astronomy graduate students and postdocs that they are, in fact, sitting at the controls of the telescope, taking the images of the night sky that all of those analyses will ultimately derive from.
If one has not observed before, I daresay that this is the point at which on feels that kinship going back to Hubble, Vera Rubin, the Herschels, Galileo, and the Greeks and Arabs before that—with all the astronomers who have come before, and indeed, with all humanity that has looked up and wondered: what’s up there, what’s out there, how did we get here, where are we going.
Further, it’s while sitting at those terminals that one feels a true sense of responsibility to the other collaboration members, as well as that camaraderie and "esprit de corps" when part of a larger team that is all pulling in the same direction: to tease out Nature’s secrets, and push forward the frontiers of human knowledge just a little bit (and sometimes, if Nature smiles upon us, a lot!).
And that is a beautiful, wonderful, amazing, and magical adventure that one can only feel fortunate and profoundly grateful to be a part of, in this short human life of ours.
Lori White on figures and co-editing, Tom Diehl for edits, especially in the conclusion, and Josh Frieman (DES Spokesperson) for useful comments.