astrometric standard catalog, clustocentrics

I (finally) finished Sam's GALEX-optimized star catalog based on the USNO-B1.0 astrometric standard catalog. It is limited in the B band, and all-sky. Unfortunately, the Galactic Plane is a big issue. I decided that for the next iteration, I need to make a catalog that is uniform in angular density. This means putting a healpix grid on the sphere, and keeping N stars in each healpix pixel. This would be hard work if Finkbeiner hadn't already put healpix into idlutils.

I also went over more of Quintero's paper on clustocentric distance dependencies for SDSS galaxies relative to Berlind's clusters. Quintero's results have a pretty sharp feature (break in dependency slope) at 1 virial radius, which implies that the Berlind mass estimates can't be all that bad.


stupid binary files

In my limited research time today, Sam and I tried to figure out what is incompatible about our binary files. Our research was not illuminating.


small-scale clustering

[Sorry, no posts for a few days because of big—and very, very good—events in my personal life; email for details.]

Today, Morad and I spent a few minutes going over his paper on the LRG–LRG merger rate and small-scale clustering. It is very close now.


quads, dark energy, galaxy simulations, axis ratios

Sam and I discussed ways to prioritize quads, both at index time and at test time in the automated astrometry project. I started to make him a list of astrometric standards that is tailored (somewhat) to GALEX data.

Sheldon led a very nice—and lively—group meeting. He talked about the Dark Energy Survey, and, more importantly, dark energy.

Fabio Governato gave an impressive talk about making galaxies in N-body simulations, with approximate treatments of star formation and stellar-driven feedback. Among other things, he showed that the thick disk is likely to have been thickened by a merger, and the age of its stars is likely to be indicative of the cosmic time of that last major merger. I discouraged him from trying to match morphological type fractions in his simulations, since no two people agree on any galaxy's morphological type!

Ari Maller, Blanton, and I discussed the dependencies of observed galaxy properties on axis ratio.


editing, soft matter

I worked through parts of Quintero's clustocentric distance paper with Quintero. I discussed statistical estimation issues in a paper by Mark Brodwin (JPL) with Blanton and Sheldon. I discussed Willman's paper on Willman 1 stellar populations with Willman. I started to read the latest draft of Masjedi's paper.

Willem Kegel (Utrecht) gave a colloquium in which he showed conditions on colloid particle interactions that lead to the creation of a "cluster" phase. He made a cute analogy with nuclear matter.


blind astrometry parameters, fast K corrections, edge-on galaxies

I worked out the complete list of free parameters (or equivalent) for the blind astrometry system. There are about 10, which is too many; it would be nice if we could remove some or optimize over them. There are some short-term prospects for getting the list down to of order 5, which would be much more manageable. I also worked on outputting better stellar catalogs for Sam's use.

In related news, we decided to try to solve some GALEX images, which may be non-trivial given the large wavelength difference between GALEX (UV) and USNO (visual). Sam is very optimistic, I am guardedly optimistic, Schiminovich thinks we are very unlikely to succeed!

Morad and I discussed, and Morad went some way towards implementing, a K correction method for his next project that will be super-fast. Because the project (the cross-correlation of spectroscopic SDSS LRGs with essentially all imaging sources) involves doing between 107 and 108 K corrections, it needs to be fast. Fortunately, it does not have to be supremely precise, so our approximations are likely to be allowed.

Anil Seth visited from UW today. He showed us some very nice results on resolved stellar populations and vertical structure in nearby, edge-on, low-mass disk galaxies. Apparently the Milky Way has a thinner disk than most of these neighbors (interesting), but it might be comparing apples to oranges, given that the MW is so much more massive.


PSF modeling

Nikhil Padmanabhan and Doug Finkbeiner came to NYU today to talk about astrometry and large data management (for SDSS and Pan-STARRS). We ended up spending most of our time on variable PSF fitting; Fink has a new method that is simultaneously simple, fast, and capable of catching subtle and detailed effects. One of the issues for a future psf.net is the automatic selection of stars apart from galaxies. That's not trivial.


ordering the stars

Sam and I realized that we might be able to speed up astrometry.net by making more use of the flux-ranking of stars. If, at index time, we build quads not out of random stars but out of the brightest set that meet our angular requirements, and if, at test time, we start with the brightest quad in the image and cascade to fainter quads, then we are more likely to make matches sooner. It was a great discussion, but we put many more items onto our to-do list than we took off!


Willman I tidal features, non-bug, Farhi

Beth showed beautiful plots today of Willman 1's tidal features. She, Blanton, and I discussed issues relating to making isopleths of the angular density of stars (it requires smoothing). Of course, there appears to be substructure in the outer parts of the system, but is that substructure (a) real, and, if so, (b) physically signficant?

Sheldon, unintentionally, managed to convince me for a whole hour that I had a serious (and publicly humiliating) bug in my SDSS astrometry (or at least routines that use and display it). Fortunately, there was no bug!

Ed Farhi gave a nice colloquium on quantum computing. His group has had some significant success in demonstrating the theoretical power of quantum computing. I still don't fully understand what aspects of the power come from superposition, and what aspects come from the fact that it is analog rather than digital.


all SDSS fields

I fixed many little bugs in the code that makes Sam's list of x,y lists. I can't believe (actually, I can believe) that such a simple task can take so much time and effort!

Sheldon and I discussed what it would take to make an interactive, google-maps-like interface to the SDSS imaging data.


spectroscopic target selection

Among other things, Morad, Blanton, and I discussed how one selects sources for spectroscopy with a spectrograph that has physical constraints on how close any two sources can be and get spectra. The desideratum is that we be able to extract nonetheless the correlation function on all scales, including those smaller than that corresponding to the mechanical constraint (as Morad has done for SDSS). We discussed two schemes, both of which involve random sparse-sampling of the sources in collision groups, ie, groups of objects that are connected by friends-of-friends with a linking length equal to the mechanical constraint.


IRAS atlas

I was out sick today, but I did manage to shepherd the creation of a huge number of SDSS images of IRAS-selected sources, after a request (and then helpful information) from Nurur Rahman. Here's one:


directions on the sky

I wrote code to overlay arrows on pretty pictures, pointing from one object to another. Here's an obvious realization I had about it: If object positions are given relative to us (eg, in RA,Dec,distance), then their radial distances from us do not come into the directions of the arrows at all. To test my code, I made the arrow pointing from Willman 1 to the UMa dwarf (Willman 2). It is shown below (on a weird coordinate system because I was stress-testing my code).


the whole enchilada

I started to assemble the x,y positions of every compact source in every SDSS field in order for Sam to run every one of them through the blind astrometry system. The plan is to simultaneously run them all, investigate faliures, and modify and expand the index.

I also worked on the To-do lists for the astrometry.net project and what is now the PRIMUS project (formerly prism spectroscopy).


GALEX, SDSS, and Spitzer

Today was all talk all the time. We had David Schiminovich and Ben Johnson (no, not any of these Ben Johnsons) down from Columbia to discuss Ben's recent results on the inter-comparison of GALEX, SDSS, and Spitzer photometry and spectroscopy of normal galaxies. Ben finds that he can separate dust and age very clearly (though they do not separate clearly in optical bands), and that there is great promise for understanding galaxy spectral energy distributions from the far UV all the way through to the far IR. At the same time, he confirms a result of ours that there is diversity among galaxies in the mid-IR at low luminsoities. This motivates me to get cracking on my K corrections for Spitzer, a product that I am (famously) contractually obliged to deliver. We discussed how to proceed on that, especially the desideratum (?) that the spectra we fit not violate energy conservation!



Many discussions today, of Willman I and possible evidence of mass segregation and tidal stripping; of Blanton and Roweis's new k-correct paper (draft); and of Ronin Wu's project of re-identifying the membership of Virgo from SDSS data.


WCS tweak

Found a major bug in the WCS tweak code; fixed it, and I now have something that goes from the output of Sam's quad indexer to a passable TAN-SIP WCS header very quickly. Tested it on Sam's first 100 fields and it works on every single one (that makes it through the quad indexer with a match). There is a cryptic QA file here.


functional astrometry routines!

I now have functional routines that (a) find the best reverse polynomial distortion terms given forward polynomial distortion terms, and (b) invert any x,y to RA,Dec transformation by Newton's method.


astrometry, Willman 1

Caught up with Sam (by phone now) on the astrometry.net to-do list. I looked up the IDs of the SDSS fields that failed to match in the current system, and there is nothing obviously strange about them, but Morad is going to check them.

Caught up with Beth on the status of the Willman I observations. We also discussed the coincidence that Willman I and UMa are so close on the sky.