Fitting surface brightness profiles

by Jane on December 14, 2009

It’s time for another session of, “Which tool do you use to accomplish a given astro-task, and why that tool?”  The topic:  fitting surface brightness profiles.  Two likely suspects:  the Archangel package, and iraf’s stsdas.analysis.isophote.  OK, go.

{ 6 comments… read them below or add one }

1 Peter Erwin December 15, 2009 at 4:09 am

You mean “generating surface brightness profiles by fitting isophotes in images”, yes? (“Fitting surface brightness profiles” sounds, to me, like modeling surface-brightness profiles with 1-D analytic functions, as in “we fit the surface brightness profile with a Sersic function”, in which case you could be talking about any minimization code.)

OK, enough pedantic nitpicking: isophote, mainly because of inertia (it’s what was and still is widely available, and I’ve learned to work around its idiosyncrasies). I work with people who use the (updated) code from Bender & Moellenhoff (1987); I’m not sure I’ve ever heard of “Archangel.”

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2 Tom Aldcroft December 15, 2009 at 7:29 am

Sherpa is well-suited for fitting 1-D radial profiles and 2-D image data. The threads below describe in detail how to do this. These examples are for X-ray data but the same methods would apply for any image data.

Radial and elliptical profiles of Image Data

Accounting for PSF Effects in 2D Image Fitting

Radial and elliptical profiles of Image Data

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3 Jane December 15, 2009 at 12:48 pm

Peter, yup that’s a tad pedantic. Perhaps I should have written the long-winded: “for a specific example, to measure the asymptotic total magnitude of an inclined disk galaxy”.

The B&M refs are great, thanks. Is what you refer to a version of stsdas.analysis.isophote, or a separate package? Can you provide a URL?

There’s danger, in writing these, “How do you do task X?” posts, of me sounding like an idiot. When asking, “How?”, I’m really asking, “So, I have a cobbled-together solution to problem X. You may, too. Is there a more efficient, more powerful solution out there? What solution should we recommend to a new grad student?”

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4 Peter Erwin December 15, 2009 at 5:29 pm

Jane: the Bender et al. code is not, that I’m aware of, available on the web. If you’re curious, I could ask around as to its availability (I’d ask Ralf Bender himself, but he’s currently out of town). It’s written in Fortran, and is not connected to any IRAF package.

Hmm… there’s another isophote-fitting package called galphot, which I’ve heard of but never used. (It’s originally by Marijn Franx; Franx et al. 1989 is the relevant reference [1]). There is a version of it here, including (it says) both standalone Fortran code and an IRAF version; but it’s not clear whether or not the latter works on anything other than Suns. There’s a version of galphot built into Astro-WISE, apparently, though that’s not any use unless you happen to be at one of the institutes taking part in Astro-WISE.

[1] Most of these fitting-ellipses-to-isophotes code seem to date from the same two or three years; for example, the stsdas.isophote.ellipse algorithm is based on Jedrzejewski (1987).

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5 Eric Peng December 17, 2009 at 1:28 am

For the ACS Virgo Cluster Survey, we used both the IRAF isophote package and the VISTA “elliprof” package. The latter was used for the Tonry et al SBF survey of nearby early-type galaxies. It works well, but unfortunately the documentation is pretty much nil, and has become impossible to find on the internet with Windows Vista and the VISTA telescope out there. If I can find a link, I will post it here.

I’m intrigued about Archangel, which I hadn’t heard of before now. Anyone use it and have an opinion? The high level stuff being in Python seems like a big plus for today’s user (i.e. the beginning student).

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6 C April 17, 2013 at 3:40 am

I can never get Archangel to work. I have tried for years. It hooks into some ancient version of the cfitsio libraries. I bugged Dr. Schombert about it, and he basically told me to rewrite the install script to point to the appropriate places. Yeah, I’m going to go try galphot.

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