statistics of stellar quadrangles

I worked out and wrote up a short note on the statistics of stellar quadrangles. As one might expect, their number on the sky goes as the angular size allowed for the largest star–star separation to the sixth power, and as the total density of stars on the sky to the fourth power.


almost no astrometry

I did almost no real research today, but I did investigate some scalings of the current astrometry setup with angular scale of the indexed stellar quadrangles.


blind astrometry

After a short vacation in Maine I came back refreshed to work on blind astrometry with Sam and Morad. Sam successfully solved the first-ever SDSS field totally blind, but it was slow because he encountered statistics of false matches that we had not expected. We re-worked the numbers for the millionth time and everything checks out okay. We discussed numerous ways to make everything faster, and we may have settled on a faster scheme, but one that ties the angular scale of the indexed stellar quadrangles to the number density (on the sky) of the magnitude-limited catalog used to construct them. We also tasked Sam and his (future) students with coding work.


massive spectroscopy

I discussed with Burles target selection for the prism-spectroscopy project of Burles, Coil, and Eisenstein. Evidently he is leaning towards the COSMOS survey field, with deep, one-band HST/ACS imaging. We discussed object detection that could take place in the raw frames; ie, without stacking the images. We also discussed slit placement and observing strategy. If fringing can be represented always as additive functions in a restricted range of spatial scales on the detector, it might be possible to estimate it on every source, and then the observing strategy called nod-and-shuffle (which restricts the slit lengths to be long and increases the noise) might not be necessary.


luminosity–environment, drizzle

Quintero, Blanton, and I worked on the dependence of the luminosity distribution on clustocentric distance.

Blanton and I discussed the nascent replacement for STScI drizzle.


HST archive and data

With the help of Karen Levay at STScI (because of previously mentioned StarView bugs), Phil and I got a list of every single Northern Galactic Cap ACS and WFPC2 exposure that was longer than 60 s of exposure time and within 3 arcmin of any SDSS LRG target (observed spectroscopically or not). This search returns 5795 public-domain exposures by 1090 LRGs; we expect about 500 of these to actually be in the images. We start the SFTP on Friday. There is lots of science to do in the HST–SDSS overlap.

Phil and I followed this with some arts and crafts: Tweaking color schemes on images of possible HST-discovered lens systems, and bug reporting and bug fixing in the ACS WCS tools.


HST archive, catching up

Phil Marshall and I worked on putting a huge query into the HST Archive, and hit many snags, most of which derive from the fact that HST is not expecting people to query for 42,000 exposures in one shot. I can't imagine why not. No, I am not kidding: aren't we in the world of big data? Anyway, we failed, but the super-helpful people at STScI are on the case and I think we will be able to do a huge query by the end of the week. Interestingly, the STScIers want us to do a huge query to put their system through its paces.

Blanton and I caught up on a lot, as did Eisenstein and I (by phone); Blanton has some extremely good results on the evolution of galaxies as observed and predicted, and nice work reducing the dimensionality of galaxy spectral variations, conditioned on realistic (read: possible) star-formation histories. Eisenstein (with Burles and Alison Coil) has a great project to take massive numbers of redshifts using a prism rather than a grism or grating.


paper done

I helped Morad finish up and post his paper on the very small-scale clustering and merger rate of LRGs to the SDSS collaboration publications archive. It can be submitted after the three-week waiting period (it saves lives, it really does). Other than that, I only did NSF-proposal-related non-research.



SDSS astrometry

Morad and I helped put Sam on the brink of the first real-world test of blind astrometry on the SDSS data. Other than that I worked on a grant proposal, which does not qualify as research.


Willman 1, geometry

I made some ugly pictures of Willman 1, Beth's odd (and tiny) Galactic globular cluster, based on our KPNO Mosaic imaging data. I am not posting the pictures here because they aren't yet ready for prime-time. Morad helped me diagnose some problems with our pixel masking and now the pictures are re-making.

Sam and I had yet another conversation about geometry!


astrometry hypotheses, environments literature

Sam and I discussed some of the steps involved in the "resolve" part of automated astrometry, where we have to decide among many computer-generated hypotheses about where each field is pointing and its rotation and scale. We think that a single coincidence among quads will be sufficient, but there are issues for finding even that coincidence (what's a coincidence?).

I got comments on the post-starburst environment paper from Tucker, Knaizev, Goto, Humboldt, and Morad. These comments were great; in particular they pointed out that I have completely ignored an important and relevant literature, much of it emanating from SDSS itself. I should hang my head in shame, but in fact I am happy that everyone is doing my work for me.

Preprints: You can either read them or you can write them —Penny Sackett


drizzle, astrometry

talk, talk, talk!

I talked to Schlegel at length about the conceptual problems with drizzle, and reiterated my concerns that (a) it does not optimize a scalar objective function; ie, the image it produces is not the "best" in any conceivable sense; and (b) philosophically, it treats the telescope as a photon bucket, not a device that measures the intensity. This latter problem is subtle, but if you treat the detector as a flux collector, you have to worry about "flux conservation" and "gaps in the detector" etc, but when you think of it as device that measures the intensity field, you just have to determine each pixel's "beam" and then note that each measurement pixel constrains the beam-convolved intensity map. Of course, drizzle exists, and my ideas are vapor-ware, or maybe bar-ware, since they are just polemics shouted in bars.

I talked to Sam and Morad about the current status of the astrometry project, and we heard that the indexing job we started on Friday finished in an hour (which is good) and we checked some numbers relating to methodologies for determining, at test time, which index matches are correct, and which are flukes.


comments, indexing

I discussed projects and drafts with Morad, Quintero, and Blanton.

Sam and I worked through the geometry—and symmetries and ambiguities—of the star quadrangles used for indexing in the current iteration of the blind astrometry project. We checked some code Sam wrote and started an indexing run on the USNO-B subset Morad made us.


WCS bug, virtual observing

Burles found a bug in my ACS WCS code. It was ugly. I fixed it. Then I wept tears of shame.

I discussed Lockman's polemic with Schechter (who is credited with pestering) and Blanton. I think most of Lockman's general, philosophical points are correct, but his conclusions are incorrect: The fact that you need to know the folklore to be a good astronomer is a reason not to maintain the folklore system, but to change it. Of course change requires good people and money. His argument that everyone should get cozy with the raw data is a good one, but it is on a slippery slope: He insists that you must know the raw data (I agree), but he doesn't insist that you build your own correlator. Sounds like an observer (as opposed to an experimentalist) talking.


first draft

I sent out a first draft of the K+A environments paper to the co-authors.


discussions, atlas

I briefly discussed the environment- and luminosity-dependences of the merging rate with Morad, and high-density spectroscopy with Burles.

I reminded myself of the details of our future "Gunn Atlas" built from the SDSS data on low-redshift galaxies (it's a great idea), for my next attempt at NSF funding. Unfortunately, NSF funding proposals do not count as "research" for the purposes of this diary, so I am going to have to shut up about this now.



cluster infall

Three papers I see cited frequently in the context of supporting the view that galaxies are "transformed" from star-forming late types into dead early types as they fall into rich clusters are Poggianti et al (1999), Balogh et al (2000), and Kodama & Bower (2001). The odd thing is that all three of these papers actually conclude that any transformation that occurs must be occuring on very long timescales (longer than 1 Gyr), because they in fact find no direct evidence of the infall at all! The only evidence that there is transformation with infall is that the galaxies inside the clusters look different from those outside.

It may sound crazy, but the data are consistent with a picture in which the morphological and star-formation properties of galaxies are in fact set at a relatively early time, before infall, and the relationship with environment comes from the fact that the galaxies that find themselves inside clusters come from a special (ie, not representative) set of environments and initial conditions.

Today I worked on the problem of putting this argument, or a softened version of it, into the post-starburst environments paper.


introduction, astrometry angular scale

I cranked out an unintelligible, unreferenced, but complete introduction to the post-starburst environments paper. This brings the current version up to what I call a "zeroth" draft—ie, all there but not yet in a form that is ready for the co-authors to read, let alone the public.

Sam and I spent all afternoon playing with SDSS run 745, camcol 3, field 101, looking at its astrometry, the number of USNO-B1.0 stars in the field, and the probability that the field will contain an astrometric index quadruple in our current plan for automated astrometry. It looks like our index ought to make quads with a maximum separation of between 2 and 7 arcmin to have good coverage and good precision when applied to the SDSS data. We realized that if the USNO-B1.0 and all data we ever see is of high precision, the best plan would be simply to index quads at the smallest possible angular scale; the only reason not to do this is that there is positional jitter in both the USNO catalog and any input image, which makes matching more difficult at the smallest scales. Of course at larger scales, camera distortions enter, so it still might be best always to be looking at the smallest available quads.