May 28th, 2008| 01:00 pm | Posted by vlk
The most widely used tool for detecting sources in X-ray images, especially Chandra data, is the wavelet-based wavdetect, which uses the Mexican Hat (MH) wavelet. Now, the MH is not a very popular choice among wavelet aficianados because it does not form an orthonormal basis set (i.e., scale information is not well separated), and does not have compact support (i.e., the function extends to inifinity). So why is it used here?
Continue reading ‘Mexican Hat [EotW]’ »
Tags:
Chandra,
ciao,
convolution,
correlation,
EotW,
Equation,
Equation of the Week,
Fourier Transform,
gaussian,
MexHat,
Mexican Hat,
MH,
multiscale,
wavdetect,
wavelet Category:
Algorithms,
Astro,
Imaging,
Jargon |
1 Comment
May 21st, 2008| 01:00 pm | Posted by vlk
There is a lesson that statisticians, especially of the Bayesian persuasion, have been hammering into our skulls for ages: do not subtract background. Nevertheless, old habits die hard, and old codes die harder. Such is the case with X-ray aperture photometry. Continue reading ‘Background Subtraction [EotW]’ »
Tags:
aperture photometry,
background,
background marginalization,
background subtraction,
celldetect,
Chandra,
ciao,
EotW,
Equation,
error propagation,
ldetect,
local detect,
wavdetect,
X-ray Category:
Algorithms,
Astro,
Jargon |
6 Comments
Oct 21st, 2007| 03:59 pm | Posted by vlk
wavdetect is a wavelet-based source detection algorithm that is in wide use in X-ray data analysis, in particular to find sources in Chandra images. It came out of the Chicago “Beta Site” of the AXAF Science Center (what CXC used to be called before launch). Despite the fancy name, and the complicated mathematics and the devilish details, it is really not much more than a generalization of earlier local cell detect, where a local background is estimated around a putative source and the question is asked, is whatever signal that is being seen in this pixel significantly higher than expected? However, unlike previous methods that used a flux measurement as the criterion for detection (e.g., using signal-to-noise ratios as proxy for significance threshold), it tests the hypothesis that the observed signal can be obtained as a fluctuation from the background. Continue reading ‘The power of wavdetect’ »
Tags:
AXAF,
ChaMP,
Chandra,
ciao,
Power,
source detection,
Type II error,
wavdetect,
wavelet Category:
Algorithms,
Imaging,
X-ray |
1 Comment