SIP and its inverse

One of the many unfortunate things about the TAN-SIP polynomial-distorted WCS convention is that it carries around parameters for forward and reverse directions, even though these can be derived from one another; indeed, an iterated reverse operation can be correct to machine precision, whereas the polynomial description of the inverse of a polynomial distortion can never be correct to machine precision. But anyway, since we are going to use TAN-SIP (dammit), I wrote code today to make the reverse coefficients with as much precision as possible. It works, but it is not yet ready for prime time.


ready to move

Officially sanctioned research (see sidebar) was deprecated today as I readied my office and work for my impending relocation to NYU.


bad galaxy pictures

I chose examples of bad sky determination from among our pictures of RC3 galaxies (a particularly egregious example is NGC 4631) and found the SDSS runs from which they are made to create a sky testbed.


polynomial WCS, SDSS sky

Banged my head against the fitting of polynomial terms in the WCS tweak code. I think I am doing something wrong in the interpret step, where I convert the fitting parameters back into standard WCS parameters.

Made a plan with Blanton to solve the SDSS sky level problem; ie, the determination of the large-scale sky level to very high accuracy (possible because the survey is a drift scan) for finding low surface-brightness galaxies and making pretty pictures. Sky determination is currently the single worst problem with our pretty pictures.


WCS tweak fast

I finished the linearized WCS tweak and it works and it is fast. Now I just have to upgrade idlutils to deal with SIP WCS.


iterated, linearized, WCS tweak

I bit the bullet and figured out how to linearize and fit a reasonable set of WCS parameters, and how to reinterpret the fit output as conventional WCS parameters.

The basic idea is to treat the tangent plane choice as a separate thing from the relationship between x,y in the image and ideal u,v coordinates on the tangent plane. So you fix the tangent point (in the image and on the sky), and then fit for the relationship between x,y and u,v on that fixed tangent plane. Then you re-interpret some parts of the solution as an adjustment to the tangent point on the sky (you don't want to change the tangent point in the image because sometimes that is known a priori). Then you iterate, starting at the updated tangent point.

I started to write the code. The most difficult thing was to figure out how to interpret fit output coefficients in terms of the standard WCS parameters.


MCMC WCS tweak test, library

My very simple, very slow, very limited WCS tweak code based on MCMC (see two days ago) is tested and works. A code that iteratively linearizes the WCS derivatives around the iteratively updated tangent plane projection will be much faster, but takes more thought to do correctly. The MCMC code can be used, I guess, to check the output of the linearized code once I write the latter.

My friend Darcy Duke helped me find papers (on the "lost in space" problem) in the MIT Engineering Library.


anomalous groups

I spent some time working on a new method for finding anomalous groups in the Berlind catalog—anomalous in the properties of the galaxies they contain. Quintero has been assigned the job.


MCMC WCS tweak

I got lazy about linearizing small changes to WCS (especially since images that are near the celestial poles are nightmares), so I coded up (but have yet to test) a WCS tweak code that takes no derivatives at all—a Markov-Chain Monte Carlo code. I am sure there are a million reasons this is a dumb thing to do, but I can do it, so I did. Debugging and testing will be left for tomorrow.



By my own rules (see sidebar), I can't count what I did for most of the day—refereeing a paper for The Astrophysical Journal—as research, but I can count giving extremely detailed comments to Quintero on his nice paper on how the distribution of galaxy properties depends on the distance the galaxies are from the centers of rich clusters. He finds some great results, but (like me), no direct evidence that there is anything exciting happening in the infall regions. I think his most interesting result (though perhaps the hardest to get across) is that he confirms, very clearly, the information theory result from Blanton et al (2005) that morphology-like properties of galaxies are only related to environmental density through their relationships to star-formation-history-like (SFH) properties. The morphology–density relation is actually just a morphology–SFH relation painted onto a more fundamental SFH–environment relation.


astrometry input, prism spectroscopy

I modified the method by which we extract the SDSS x,y lists we input (Sam inputs) into the astrometry engine, to make the cuts not based on specific, calibrated magnitudes, but only on brightness ranking. Sam and I decided that we would go live on the WWW with the current pointing, rotation, and scale capability, as soon as his new students have tweaked it up and have WWW interfaces working. With only small changes (and little analysis), Sam's success rate is up to about 96 percent.

Blanton and I discussed, among many other things, the scientific output of a pilot program of prism spectroscopy as Burles et al have been contemplating. At worst, it will be better than COMBO-17, on much less telescope time. At best it will be much better, and answer qualitatively new questions about galaxy evolution.


group meeting

Group meeting. I learned a lot! Spent the rest of the day on things related to moving back to NYU.


astrometry tweak

I worked on the back end of the automated astrometry system: the part that tweaks up Sam's determination of pointing, rotation, and scale to a high-quality WCS. I keep remembering (and then forgetting) that the general solution to this "tweak" step is not trivial, if you want it to be hands-off but robust to all kinds of stuff you might get.


lost in space

Today I looked into the "lost in space" problem: the determination of spacecraft orientation based on TV camera images of star fields from cameras fixed on the spacecraft body. This bears a lot of resemblance to the automated astrometry problem, although all solutions to the lost in space problem assume perfect knowledge of the "plate scale" of the images, and usually require very large image fields of view. Unfortunately, I can't link to the literature on this (or read it on the internet), because it is all in proprietary engineering publications. People: please stop publishing public research results in proprietary journals! (Although I love the thought that the journal editors think a theoretical paper on spacecraft orientation from 1995 is such valuable intellectual property that they can't let anyone see it without paying $2.95! I guess I will have to go to the engineering library.)


astrometry paper 1

On my week in the wilderness I started paper I in the automated astrometry series. This was inspired by Sam going "alpha" with the SDSS automatic WCS system. It seems to solve about 90 percent of SDSS fields straight out of the box (ie, give it an x,y list of stars in an SDSS image, it gives you back the pointing, rotation, and scale); it remains to be seen how we can tweak it up so it does them all.



I added a figure to the post-starburst environments paper at the insistence of Morad (someone has to keep me honest), and submitted it.

[post script added next day:] I go on vacation for a full week—with no email or phone even—see you all again on August 15.


ready to submit

I got the post-starburst environments paper ready to submit! That was painful. I will submit it late tomorrow, God willing.


environments, meta-astrometry

I got the K+A environments paper closer to being ready for submission.

I discussed with Sam possible projects for his new students working on astrometry projects, including a staged introduction of functionality, from mundane to wondrous, at astrometry.net. In particular, we discussed the value of what I would call a "footprint server" that, for any point on the sky (or every possible image footprint on the sky), returns all the known, public astronomical images whose footprints overlap that point (or image). Although some archive searches come close to this, none of them exactly perform this function, to my knowledge. (I think they fail to do this because the database tables they work off generally know about pointing but generally don't know about imager orientation or geometry.)


astrometry tweak, post-starburst environments

Sam has nearly nailed the blind astrometry for SDSS, so it is time to work on the "tweak" step that takes Sam's output and produces a precise, fine-tuned WCS for the image. I started on this today by drafting code to make an approximate WCS header from Sam's output; this and the original x,y list can be passed to the Fink-Hogg code (created as part of the photometricity monitor robot) to get a precise WCS (or report that no such precise WCS can be found).

In other news, the post-starburst environments paper reaches the end of its three-week waiting period (a period when SDSS collaborators can comment on the paper and request authorship or content changes) this week. I worked a bit on getting that ready to submit.