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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<!--AstroStatistics Home page-->
<title>CASt: Chandra Star Cluster dataset</title>
<!-- InstanceEndEditable -->
<meta http-equiv="Content-Type" content="text/html;">
<meta name="keywords"
content="Center for Astrostatistics, CASt,
Statcodes, SCMA, VoStat">
<link href="assets/cast.css" rel="stylesheet" type="text/css">
</head>
<body style="background-color: rgb(255, 248, 229);" leftmargin="0"
topmargin="0">
<!--body style="background-color: #FFF8E5"-->
<p><img src="assets/HmPg_Ban.gif" alt="Astrostatistics Image" usemap="#Map"
href="http://www.stat.psu.edu" title=""
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</p>
<!-- InstanceBeginEditable name="head" -->
<p> <!--center--><a href="index.html">Home</a></p>
<p><img src="http://astrostatistics.psu.edu/rainbow.gif" border="0"
height="4" vspace="2" width="700"><!-- InstanceBeginEditable name="PageTitle" -->
<!-- InstanceEndEditable --><!-- InstanceBeginEditable name="body" --></p>
<table border="0" cellspacing="0" width="700">
<tbody>
<tr>
<td style="vertical-align: top;"> </td>
<td>
<h2 style="text-align: center;">Chandra star cluster</h2>
<h3>The CASt datasets</h3>
<a href="Chandra_img1.dat" target="ImageView">Chandra_img1.dat</a><br>
<a href="Chandra_img2.dat" target="ImageView">Chandra_img2.dat</a><br>
<a href="Chandra_psf.dat" target="ImageView">Chandra_psf.dat</a><br>
<br>
<p><span style="font-weight: bold;">Astronomical background
</span></p>
<p>Some astronomical images are greyscale with (quasi)Gaussian
statistics. This is typical of visible light and radio
images. However,instruments observing at high energy X-ray and
gamma-ray wavelengths typically detect individual photons, producing
images showing individual photons with Poisson statistics. The
original datasets are often lists of photon events, and most of the
pixels in the resulting images have zero or one photon. Many of
the 1-photon pixels are due to a non-astronomical background, often due
to cosmic rays hitting the detector. Source detection must thus
be based on Poisson statistics and, if the background is low, sources
might be detected with only a handful of photons.
</p>
<p>An additional problem in astronomical image analysis is that
the optics are imperfect. This is particularly true at high
energies where focusing optics are difficult or impossible to
construct. The optical performance of a telescope-detector system
is often summarized by a "point spread function" (PSF), the
hypothetical appearance of an unresolved astronomical source with
infinite signal and no background. If the observed field has
closely-spaced multiple point sources , or structured diffuse emission,
then image deconvolution using the known PSF is performed to recover an
improved view of the true sky distribution of light. <br><br>
</p>
<p><span style="font-weight: bold;">Dataset</span></p>
<p>NASA's Chandra X-ray Observatory, launched in 1999, has the
best-focussing mirror and its Advanced CCD Imaging Spectrometer (ACIS)
is a very low-noise detector. Chandra thus detects extremely
faint X-ray sources and can image complex X-ray structures in great
detail. However, Chandra presents several statistical challenges
for faint source detection and image deconvolution.<br>
</p>
<p>The datasets provide here are off-axis portions of a Chandra
image of the crowded star cluster NGC 6618 which illuminates the bright
HII region Messier 17. This cluster is extremely young and is
studied to help understand the processes of star formation and early
stellar evolution. These figures are obtained from a forthcoming
paper by P. Broos et al. (2006). We show three images here:
</p>
<ol>
<li>The entire ACIS field which is about
17'x17'
in extent (about half the size of the full Moon). The shaded area
shows the portion of the image discussed here. This is the
M17-North region with a cluster of protostars embedded deep in the host
molecular cloud. <br>
<img alt="Chandra M17 image" src="Chandra_M17img.jpg"
style="width: 522px; height: 469px;"></li>
<li>The result of the source detection the
polygons show the PSFs of the detected sources; note that the PSFs of
the Chandra mirror are large and distorted far off-axis. The
source detection is based on a wavelet decomposition of the image
(Freeman et al., ApJ Suppl 138, 185 2002) and based on procedures
implemented in the ACIS Extract and CIAO software packages described in
Townsley et al. (ApJ 593, 874 2003). <img
alt="Chandra M17 sources" src="Chandra_M17src.jpg"
style="width: 512px; height: 487px;"><br>
</li>
<li>An expanded view of the subimage
regions considered here. The pink boxes show the 128x128 pixel
regions with 1"/pixel (1" = 1 arcsecond). Chandra_img1.dat
gives the upper left region and Chandra_img2.dat gives the lower right
region. The PSF is given in the Chandra_PSF.dat file
at this location with the same 1"/pixel resolution. It is given
as a 17x17 pixel array but can be padded
with zeros as needed. <br>
<img alt="Chandra M17 subimages" src="Chandra_M17subimg.jpg"
style="width: 590px; height: 588px;"> </li>
</ol>
<p><span style="font-weight: bold;">Statistical exercises<br>
</span>
<ul>
<li>Recover the sources found using ACIS Extract</li>
<li>Estimate the counts in each source, with confidence
intervals</li>
<li>Deconvolve Chandra_img1.dat with the PSF to reconstruct a
higher resolution image of the sky.<br>
</li>
</ul>
<br>
<span
style="font-style: italic;">This dataset was generously
provided by Patrick Broos (Penn State University).</span><br>
</td>
</tr>
</tbody>
</table>
<br>
<img src="http://astrostatistics.psu.edu/rainbow.gif" border="0"
height="4" vspace="2" width="700"><br>
<table style="width: 700px; text-align: left;" border="0"
cellpadding="2" cellspacing="2">
<tbody>
<tr>
<td style="text-align: center; vertical-align: top;"><a
href="http://www.nsf.gov/"><img src="assets/nsflogo.gif" alt="NSF"
style="border: 0px solid ; width: 65px; height: 65px;" align="middle"></a><a
href="http://www.stat.psu.edu/"><img src="assets/statistics.jpg"
alt="Department of Statistics"
style="border: 0px solid ; width: 68px; height: 58px;" align="middle"></a><a
href="http://www.science.psu.edu/"><img alt="Eberly College of Science"
src="assets/eberlycol.gif"
style="border: 0px solid ; height: 58px; width: 363px;" align="middle"></a><a
href="http://www.astro.psu.edu/"><img src="assets/astronomy.jpg"
alt="Department of Astronomy and Astrophysics"
style="border: 0px solid ; width: 68px; height: 58px;" align="middle"></a></td>
</tr>
</tbody>
</table>
<br>
<div style="text-align: left;"> </div>
</body>
</html>