number of exposures per visit?

I wrote code this weekend to look at the question of how we should visit a star in the upcoming Terra Hunting Experiment. The current (straw-person) plan is that we will observe each visible star once per night for ten years, with one exposure of a sensibly-chosen exposure time at each visit. Is this a good idea? I was interested in this problem for two reasons. The first is that binning is sinning, with the corollary that bigger bins are worse than finer bins, and a single, long exposure is a very big bin. The second reason is that when there are non-trivial noise sources (like the quasi-periodic variations from p-mode oscillations of the surfaces of Sun-like stars), a few negatively- (or interestingly-) correlated noise draws can be combined in ways that are substantially more informative than by taking the average.

Of course, if you split an exposure (with a standard CCD, say) into sub-exposures, you take on real costs: There is a read time, which is time you aren't integrating, and there is a read noise, that affects each new exposure. So the best strategies are a complicated function of read time, read noise, and the signal-to-noise at which the stellar p-mode oscillations are visible in any realistic data. Related: There are amazingly different and interesting strategies with up-the-ramp detectors that are used in the infrared.

One final comment is that the objective, in my strongly held view, is to optimize the amount of information (about, say, the center-of-mass radial-velocity changes of the target star) per unit wall-clock time. We are paying for wall-clock time; let's get as much as we can out of it.

what is measured with stellar kinematics?

In work on Galaxy dynamics, from stellar kinematics, we measure relative velocities and relative positions, of nearby stars relative to the Sun (or really the Solar System barycenter). These relative positions and velocities are coordinate free, in the sense that they don't imply a rest frame for anything (and indeed, the SS barycenter is not anywhere near the rest-frame position or rest-frame velocity of the Milky Way or Local Group or anything else).

In addition to this, any measurements we make are insensitive to any overall or external acceleration: If the Milky way is in free-fall, accelerating towards some external “great attractor” or anything else, none of these observables are affected in any way by that acceleration. So what is it that stellar kinematics can really be used to measure? I think somehow the answer has to be Galilean covariant (covariant to boosts and translations), but even better it should be generally covariant (in the Newtonian sense, which is well defined, apparently).

I did some research on this subject, and the literature is all about Newton–Cartan theory, but this theory is a Newtonian limit of general relativity. That isn't quite what we care about in stellar kinematics, since in stellar kinematics, we don't get to see any orbits as a function of time (we don't observe geodesics or geodesic deviation). What, exactly do we observe? I think what we observe is something about gradients of accelerations, but I don't know yet. Great project for this summer.

unitary evolution of the Universe

I spent the day with Juna Kollmeier (CITA) talking about epistemology, physical cosmology, and project management (especially academic management). I found myself saying to her the following argument (which I have not seen written down anywhere): Imagine that our Universe is hamiltonian (or lagrangian; it doesn't matter for these purposes). And imagine that our Universe is a simulation being run inside some bigger universe, which is also hamiltonian.

If our Universe is being observed in any sense by any system in that bigger universe, then there ought to be a loss of unitarity in our Universe. That is, there should be a violation of Liouville's theorem, or a violation of key conservation laws, or an information sink. And there is! At black hole horizons, there is an information paradox: Information that goes in never comes back (an evaporating black hole evaporates thermally, or so we think). Thoughts?

SDSS-V AC meeting

Today I chaired the annual Advisory Council (AC) meeting for the SDSS-V project. The AC protects the interests of the partners, who gave money and other resources to the project. We had many presentations from different parts of the project and OMG this project is amazing. I learned a ton and feel very happy that our money is well spent. This activity counts as research because project management is a key part of science.

The AC meeting was followed by touring the observing hardware (I love it; the SDSS Telescope is incredibly important to everything I have done since the late 1990s), followed by actually looking through the 3.5m telescope at Apache Point Observatory.

how to maximize the yield of planets?

There were discussions this week at University of Warwick about the Terra Hunting Experiment strategy and likely detection capability. Various take-homes include that we need to mitigate lots of stellar noise, and that we care deeply about the covariance (as a function of separation in time) of adjacent measurements. I advocated that we split our ten-year survey into two or three surveys, of varying length. In the first, we learn about the stars, and in the last, we go to town on the very most promising targets. There was general agreement that this is a good idea. But now we need a very specific plan for what this means. As my loyal reader knows, in my view, the decisions must be based on repeatable operations, so that we have some hope of learning statistical things about populations in the end.

finding the fiber robots in the SDSS-V focal planes

At the end of the day I met with Conor Sayres (UW) to discuss the problem of measuring the position of focal-plane fiber-carrying robots given images from in-telescope cameras (focal viewing cameras) inside the telescopes that are operating the SDSS-V Project. We have not installed the fiber robots yet, but Sayres has a software mock-up of what the focal viewing camera will see and all its optics. We also discussed some of the issues we will encounter in commissioning and operation of this viewing system.

Later, in the night, I worked on data-driven transformations between focal-plane position (in mm) in the telescope focal plane and position in the focal viewing camera detector plane (in pixels). I followed the precepts and terminology described in this paper on interpolation-like problems. My conclusion (which agrees with Sayres's) is that if these simulations are realistic, the fitting will work well, and we will indeed know pretty precisely what all the fiber robots are doing.

astronomy in film

One of my jobs at NYU is as an advisor to student screenwriters who are writing movies that involve science and technology. I didn't get much research done today, but I had a really interesting and engaging conversation with film-writers Yuan Yuan (NYU) and Sharon Lee (NYU) who are writing a film that involves the Beijing observatory, the LAMOST project, and the Cultural Revolution. I learned a lot in this call!

best RV observing strategies

I really solidly did actual coding today on a real research problem, which I have been working on with Megan Bedell (Flatiron) for a few years now. The context is: extreme precision radial-velocity surveys. The question is: Is there any advantage to taking one observation every night relative to taking K observations every K nights? I succeeded!

I can now show that the correlations induced in adjacent observations by asteroseismic p-modes makes it advantageous to do K observations every K nights. Why? Because you can better infer the center-of-mass motion of the star with multiple, coherently p-mode-shifted observations. The argument is a bit subtle, but it will have implications for Terra Hunting and EXPRES and other projects that are looking for long-period planets.

asteroseismic p-mode noise mitigation

I had a call with part of the HARPS3 team today, the sub-part working on observations of the Sun. Yes, Sun. That got us arguing about asteroseismic modes and me claiming that there are better approaches for ameliorating p-mode noise in extreme precision radial-velocity measurements than setting your exposure times carefully to null the modes. The crew asked me to get specific, so I had a call with Bedell (Flatiron) later in the day to work out what we need to assemble. The issues are about correlated noise: Asteroseismic noise is correlated; those correlations can be exploited for good, or ignored for bad. That's the argument I have to clearly make.

open-fiber proposal

The SDSS-V project uses robot fiber positioners to take millions of short (15-ish minutes per visit) spectra in the visible and infrared. Because of the geometric constraints of the fiber positioners, and the targeting, there will be many, many unusued fibers—meaning, many opportunities to add additional spectroscopic targets! The project issued an internal call for proposals for the open fibers. Today I spent time writing one, which is very simple: It is to fill out the unobserved parts of the ESA Gaia color-magnitude diagram, but targeting stars for spectroscopy that do not already have a nearby star with a spectrum. The word “nearby” implies a resolution (how nearby?). The proposals are due in a few days and we still don't know exactly what our resolution should be! Also, do we treat variable stars differently from non-variable stars? We have work to do!

a survey to support EPRV target selection

In a very low research day, Megan Bedell (Flatiron) and I discussed proposals for inter-disciplinary and inter-group spectroscopic surveys of very bright stars. These would give general information about abundances, binarity, activity, variability, and suitability for further study (by, say, extreme precision radial-velocity projects). She has been thinking about target selection, and we discussed ways to make it very very simple. My position (as my loyal reader knows) is that it is better to be simple and somewhat inefficient than it is to be complex and very efficient. For legacy value, anyway, which is the whole point of a survey like this.

stellar survey

Bedell (Flatiron) and I were on the Terra Hunting Experiment science call where we discussed the idea that the whole extreme precision radial-velocity (EPRV) community might collaborate on doing some big target-selection surveys of relevant bright stars. Different surveys will want to make different choices, but we all want the same kinds of input data to make those choices. So maybe we should just band together and observe the heck out of the possible targets (bright main-sequence stars)? If you are in the community and you want in, send us email!

SDSS-V review, day 1

Today was day one of a review of the SDSS-V Multi-object spectroscopy systems. This is not all of SDSS-V but it is a majority part. It includes the Milky Way Mapper and Black-Hole Mapper projects, two spectrographs (APOGEE and BOSS), two observatories (Apache Point and Las Campanas), and a robotic fiber-positioner system. Plus boatloads of software and operations challenges. I agreed to chair the review, so my job is to lead the writing of a report after we hear two days of detailed presentations on project sub-systems.

One of the reasons I love work like this is that I learn so much. And I love engineering. And indeed a lot of the interesting (to me) discussion today was about engineering requirements, documentation, and project design. These are not things we are traditionally taught as part of astronomy, but they are really important to all of the data we get and use. One of the things we discussed is that our telescopes have fixed focal planes and our spectrographs have fixed capacities, so it is important that the science requirements both flow down from important scientific objectives, and flow down to an achievable, schedulable operation, within budget.

There is too much to say in one blog post! But one thing that came up is fundraising: Why would an institution join the SDSS-V project when they know that we are paragons of open science and that, therefore, we will release all of our data and code publicly as we proceed? My answer is influence: The SDSS family of projects has been very good at adapting to the scientific interests of its members and collaborators, and especially weighting those adaptations in proportion to the amount that people are willing to do work. And the project has spare fibers and spare target-of-opportunity capacity! So you get a lot by buying into this project.

Related to this: This project is going to solve a set of problems in how we do massively multiplexed heterogeneous spectroscopic follow-up in a set of mixed time-domain and static target categories. These problems have not been solved previously!

do we need to include the committee in our model?

Josh Winn (Princeton) and Lily Zhao (Yale) both came in to Flatiron for a couple of days today to work with Megan Bedell (Flatiron), Didier Queloz (Cambridge), and me. So we had a bit of a themed Stars and Exoplanets Meeting today at Flatiron. Winn talked about various ways to measure stellar obliquities (that is, angles between stellar-rotation angular momentum vectors and planetary system angular-momentum vectors). He has some six ways to do it! He talked about statistical differences between vsini measurements for stars with and without transiting systems.

Zhao and Queloz talked about their respective big EPRV programs to find Earth analogs in radial-velocity data. Both projects need to get much more precise measurements, and observe fewer stars (yes fewer) for longer times. That's the direction the field is going, at least where it concerns discovery space. Queloz argued that these are going to be big projects that require patience and commitment, and that it is important for new projects to control facilities, not just to apply for observing time each semester! And that's what he has with the Terra Hunting Experiment, in which Bedell, Winn, and I are also partners.

Related to all that, Zhao talked about how to make an observing program adaptive (to increase efficiency) without making it hard to understand (for statistical inferences at the end). I'm very interested in this problem! And it relates to the Queloz point, because if a time allocation committee is involved every semester, any statistical inferences about what was discovered would have to model not just the exoplanet population but also the behavior of the various TACs!

THE Meeting, day 2

Today at the Terra Hunting Experiment meeting, we got deeply into software and calibration issues. The HARPS family of instruments are designed to be extremely stable in all respects, but also monitored by a Fabry–Perot signal imprinted on the detector during the science exposures. The calibration data are taken such that the data obtain absolute calibration information (accuracy) from arc exposures and relative calibration (precision) from F—P data. There was discussion of various replacements for the arcs, including Uranium–Neon lamps, for which the atlas of lines is not yet good enough, and laser-frequency combs, which are not yet reliable.

Another big point of discussion today was the target selection. We (the Experiment) plan to observe a small number (40-ish) stars for a long time (1000-ish individual exposures for each star). The question of how to choose these targets was interesting and contentious. We want the targets to be good for finding planets! But this connects to brightness, right ascension, stellar activity, asteroseismic mode amplitudes, and many other things, none of which we know in advance. How much work do we need to do in early observing to cut down a parent sample to a solid, small sample? And how much can we figure out from public data already available? By the end of the day there was some consensus that we would probably spend the first month or so of the project doing sample-selection observations.

At the end of the day we discussed data-analysis techniques and tellurics and stellar activity. There are a lot of scientific projects we could be doing that would help with extreme-precision radial-velocity measurements. For instance, Suzanne Aigrain (Oxford) showed a toy model of stellar activity which, if correct at zeroth order, would leave an imprint on a regression of stellar spectrum against measured radial velocity. That's worth looking for. The signal will be very weak, but in a typical spectrum we have tens of thousands of pixels, each of which has signal-to-noise of more than 100. And if a linear regression works, it will deliver a linear subspace-projector that just straight-up improves radial-velocity measurements!

SCIMMA workshop, day 2

I officially don't go to meetings on the weekend! That said, I did go to day 2 of a workshop on multi-messenger astrophysics (and, in particular, the field's computing and information infrastructure needs) at Columbia University today. A lot happened, and there were even some fireworks, because there are definitely disagreements among the physicists, the computer scientists, the information scientists, and the high-performance computing experts about what is important, what is hard, and what is in whose domain! I learned a huge amount today, but here are two highlights:

In its current plan (laid out at the meeting by Mario Juric of UW), the LSST project officially doesn't do any scientific analyses; it is only a data source. In this way it is like ESA Gaia. It is trying to do a lot of social engineering to make sure the community organizes good data-analysis and science efforts around the LSST data outputs and APIs. Famously and importantly, it will produce hundreds of thousands to millions of alerts per night, and a lot of the interest is in how to interact with this firehose, especially in multi-messenger, where important things can happen in the first seconds of an astrophysical event.

During Juric's talk, I realized that in order for us to optimally benefit from LSST, we need to know, in advance, where LSST is pointing. Everyone agreed that this will happen (that is, that this feed will exist), and that (relative to the alerts stream) it is a trivial amount of data. I hope this is true. It's important! Because if you are looking for things that happen on the sky, you learn more if you happen to find one that happens inside the LSST field while LSST is looking at it. So maybe looking under the lamp-post is a good idea!

The LCOGT project was represented by Andy Howell (LCOGT). He talked about what they have learned in operating a heterogeneous, global network of telescopes with diverse science goals. He had various excellent insights. One is that scheduling requires very good specification of objectives and good engineering. Another is that openness is critical, and most break-downs are break-downs of communication. Another is that there are ways to structure things to reward generosity among the players. And so on. He talked about LCOGT but he is clearly thinking forward to a future in which networks become extremely heterogeneous and involve many players who do not necessarily all trust one another. That's an interesting limit!

planets around hot stars

My research highlight for the day was a conversation with Ben Pope (NYU) about projects involving hot stars. We have been kicking around various projects and we realized in the call that they really assemble into a whole research program that is both deep and broad:

There are problems related to finding transiting planets around hot stars, which is maybe getting less attention than it should, in part because there are technical challenges (that I think we know how to overcome). And planets found around hot stars might have very good properties for follow-up observations (like transit spectroscopy, for example, and reflected light), and also good prospects for harboring life! (Okay I said it.)

There are problems related to getting stellar ages: Hot stars have lifetimes and evolutionary changes on the same timescales as we think exoplanetary systems evolve dynamically, so there should be great empirical results available here. And hot stars can have reasonable age determinations from rotation periods and post-main-sequence evolution. And we know how to make those age determinations.

And: The hot-star category includes large classes of time-variable, chemically peculiar stars. We now at Flatiron (thanks to Will Farr and Rodrigo Luger) have excellent technology for modeling spectral surface features and variability. These surface maps have the potential to be extremely interesting from a stellar model perspective.

Add to all this the fact that NASA TESS will deliver outrageous numbers of light curves, and spectroscopic facilities and surveys abound. We have a big, rich research program to execute.

what's permitted for target selection?

Because I have been working with Rix (MPIA) to help the new project SDSS-V make plans to choose spectroscopic targets, and also because of work I have been doing with Bedell (Flatiron) on thinking about planning radial-velocity follow-up observations, I find myself saying certain things over and over again about how we are permitted to choose targets if we want it to be easy (and even more importantly, possible) to use the data in a statistical project that, say, determines the population of stars or planets, or, say, measures the structural properties of the Milky Way disk. Whenever I am saying things over and over again, and I don't have a paper to point to, that suggests we write one. So I started conceiving today a paper about selection functions in general, and what you gain and lose by making them more complicated in various ways. And what is not allowed, ever!

expected future-discounted discovery rate

My tiny bit of research today was on observation scheduling: I read a new paper by Bellm et al about scheduling wide-field imaging observations for ZTF and LSST. It does a good job of talking about the issues but it doesn't meet my (particular, constrained) needs, in part because Bellm et al are (sensibly) scheduling full nights of observations (that is, not going just-in-time with the scheduling), and they have separate optimizations for volume searched and slew overheads. However, it is highly relevant to what I have been doing. It also had lots of great references that I didn't know about! They also make a strong case for optimizing full nights rather than going just-in-time. I agree that this is better, provided that your conditions aren't changing under you. If they are changing under you, you can't really plan ahead. Interesting set of issues, and something that differentiates imaging-survey scheduling from spectroscopic follow-up scheduling.

I also did some work comparing expected information gain to expected discovery rate. One issue with information gain is that if it isn't information gain in this exposure (and it isn't, because we have to look ahead), then it is hard to write down the information gain, because it depends strongly on future decisions (for example, if we decide to stop observing the source entirely!). So I am leaning towards making my first contribution on this subject be about discovery rate.

Expected future-discounted discovery rate, that is.

writing projects

I spent a bit of research time on the weekend on writing projects. In one, I am writing about the algorithmic observing strategies that involve sensible objectives, adaptation to what's known previously, look-ahead to the future, and a discount rate. The idea is that observing decisions should be made algorithmically but also just-in-time. And perhaps simply and interpretably, which is even harder.

In another writing project—which is perhaps not reslly research by my strict rules—I am trying to set down my thoughts about the moderation of the submissions to arXiv. Why? Because this blew up this week and I didn't agree with a lot of the things that people were saying, on all sides. Of course if I really write something good, will the arXiv accept it? I think it will get rejected by moderation!