<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Glossary — C-Munipack 2.0.8 documentation</title>
<link rel="stylesheet" href="_static/default.css" type="text/css" />
<link rel="stylesheet" href="_static/pygments.css" type="text/css" />
<script type="text/javascript">
var DOCUMENTATION_OPTIONS = {
URL_ROOT: '',
VERSION: '2.0.8',
COLLAPSE_INDEX: false,
FILE_SUFFIX: '.html',
HAS_SOURCE: true
};
</script>
<script type="text/javascript" src="_static/jquery.js"></script>
<script type="text/javascript" src="_static/underscore.js"></script>
<script type="text/javascript" src="_static/doctools.js"></script>
<link rel="top" title="C-Munipack 2.0.8 documentation" href="index.html" />
<link rel="next" title="History" href="history.html" />
<link rel="prev" title="timebat (command)" href="timebat_command.html" />
</head>
<body>
<div class="related">
<h3>Navigation</h3>
<ul>
<li class="right" style="margin-right: 10px">
<a href="genindex.html" title="General Index"
accesskey="I">index</a></li>
<li class="right" >
<a href="history.html" title="History"
accesskey="N">next</a> |</li>
<li class="right" >
<a href="timebat_command.html" title="timebat (command)"
accesskey="P">previous</a> |</li>
<li><a href="index.html">C-Munipack 2.0.8 documentation</a> »</li>
</ul>
</div>
<div class="document">
<div class="documentwrapper">
<div class="bodywrapper">
<div class="body">
<div class="section" id="glossary">
<h1>Glossary<a class="headerlink" href="#glossary" title="Permalink to this headline">ΒΆ</a></h1>
<dl class="glossary docutils">
<dt id="term-air-mass-coefficient">Air mass coefficient</dt>
<dd>The airmass coefficient characterizes the attenuation of the light when it
travels through the atmosphere. Because the atmosphere is not equally transparent
for all wavelengths, this effect becomes important when you want to compute the
absolute magnitudes for measured objects. The coefficient equals to 1.0 when the
object is in the zenith by definition and it is greater than one for altitudes
between the zenith and the horizon. It is not defined for objects at or below the
horizon.</dd>
<dt id="term-altitude">Altitude</dt>
<dd>Altitude is one of the coordinates of the horizontal coordinate system. It is
used for computation of the airmass coefficient. To compute the object’s altitude
at a time of observation, the date and time of observation, object’s equatorial
coordinates and observer’s geographical coordinates must be known.</dd>
<dt id="term-aperture-photometry">Aperture photometry</dt>
<dd><p class="first">The aperture photometry is a method for measurement of brightness of a star
on a CCD frame. After image calibration, the frame consists of two components -
sky background level and a signal from a star. To measure star’s brightness, we
have to separate these two components.</p>
<p class="last">The algorithm of aperture photometry was developed from a method used for
a observations made by means of a photomultiplier. Unlike this, the aperture photometry
for digital images carried out by means of a CCD camera uses the digital image
processing algorithms. The algorithm uses two apertures - an annular shaped one to
measure the background level in vicinity of a star and a circular one to measure
the total flux from a star. The background is subtracted from a star’s signal.
The brightness of a star is defined as a ratio of its signal to a fixed reference value.
Such value, expressed in magnitudes, is called instrumental absolute brightness.</p>
</dd>
<dt id="term-bias-frame">Bias frame</dt>
<dd><p class="first">A bias frame is an image obtained from the CCD camera when all
light is blocked and the exposure is indefinitely short (in ideal case).
It represents the constant bias level, that is preset in the camera’s
electronics that reads out the image data. For ideal CCD camera, the
level should be constant for pixels and the bias could be represented
by a single value. In practice, there are small differences between
pixels.</p>
<p>In the standard calibration scheme, the bias is included in
the dark frame and thus it is subtracted during the dark-frame correction.
In the advanced calibration scheme, the bias must be subtracted
from a scientific frame before the dark-frame calibration is applied.</p>
<p class="last">The bias frame is used in the bias-frame correction phase of the
reduction process. To reduce noise, a master bias frame is often used
instead of a raw bias frame.</p>
</dd>
<dt id="term-bias-frame-correction">Bias-frame correction</dt>
<dd><p class="first">The bias-frame correction subtracts the bias component from a CCD frame.
This step is a part of the reduction process of CCD frames. It is used only in
the advanced calibration scheme.</p>
<p class="last">To apply the correction to a set of frames, a bias frame is required.</p>
</dd>
<dt id="term-catalog-file">Catalog file</dt>
<dd><p class="first">A catalog file is a file that consists of a list of stars (their positions
and magnitudes). Unlike a photometry file, it can contain identification of variable,
comparison and check stars.</p>
<p class="last">A catalog file can be made by means of the Muniwin user interface and it is
used as a reference file in the matching process. In this case, the software restores
the selection of stars from a file and an user don’t need to select the stars again.
It is useful if you repeatedly observe the same field.</p>
</dd>
<dt id="term-dark-frame">Dark frame</dt>
<dd><p class="first">A dark frame is an image obtained from the CCD camera when all
light is blocked. The image represents the thermal current accumulated
in a CCD device during an exposure. The mean value of the thermal current
is proportional to the CCD temperature and exposure duration. For an ideal
CCD camera, the thermal current would be the same for all pixels and
it could be represented by a single value. In practice, some of the CCD
elements are much more sensitive to the thermal current than the rest.
These pixels, which appear as a bright dots on a dark frame, are called
hot pixels. Depending on the calibration scheme, the dark frame may also
contain the bias level.</p>
<p class="last">The dark frame is used in the dark-frame correction phase of the
reduction process. To reduce noise, a master dark frame is often used
instead of a raw dark frame.</p>
</dd>
<dt id="term-dark-frame-correction">Dark-frame correction</dt>
<dd>The dark-frame correction subtracts the thermal component from a CCD frame.
This step is a part of the reduction process of CCD frames. In the standard
calibration scheme, this bias components is subtracted as well, because the bias
is included in the dark frame.
To apply the correction to a set of frames, a dark frame is required.</dd>
<dt id="term-flat-frame">Flat frame</dt>
<dd><p class="first">A flat frame is an image obtained from the CCD camera when a telescope
is pointed towards a luminous area of uniformly distributed intensity across the
field of view. The image represents the spatial distribution of the sensitivity
not only of the CCD elements but whole device. It records the response of the
entire optical system - the telescope, filters, camera’s window, cover glass
and CCD chip itself - to a uniform source of light. Most significant effects that
affect a flat frame are vignetting and sensitivity variations of CCD elements.</p>
<p class="last">The flat frame is used in the flat-frame correction phase of the
reduction process. To reduce noise, a master flat frame is often used
instead of a raw flat frame.</p>
</dd>
<dt id="term-flat-frame-correction">Flat-frame correction</dt>
<dd><p class="first">The flat-frame correction equalizes sensitivity variations of the whole
optical system (telescope + filter + camera). This step is a part of the reduction
process of CCD frames.</p>
<p class="last">To apply the correction to a set of frames, a flat frame is required.</p>
</dd>
<dt id="term-flexible-image-transport-system"><span id="term-fits"></span>Flexible Image Transport System<br />FITS</dt>
<dd><p class="first">The Flexible Image Transport System is a file format designed specifically
to store scientific images and data. Since its first publication in 1981 in has
become the most frequently used file format for storing and manipulating of images
that were carried out by means of the astronomy CCD cameras.</p>
<p class="last">In the C-Munipack project, the FITS format accepts the FITS images on
its input and it uses the format to store the results of operation that produce
a CCD image as an output.</p>
</dd>
<dt id="term-full-width-at-half-maximum"><span id="term-fwhm"></span>Full Width at Half Maximum<br />FWHM</dt>
<dd>Full Width at Half Maximum is used in the C-Munipack software to express
a diameter or width of a star-like objects. The width is measured in the middle
between background level and the maximum value.</dd>
<dt id="term-heliocentric-correction">Heliocentric correction</dt>
<dd><p class="first">The heliocentric correction fixes the variations of the distance between the
observer (Earth) and observed object which occur because the Earth orbits around the
Sun. This effect is not negligible especially in observation of eclipsing variable
stars or exoplanet transitions, when you need to measure the time interval between
two events, that occurred few weeks or months apart. The heliocentric correction
transforms the times in such a way that they are related to a virtual fixed point
placed in one focus of the Earth’s orbit (close to the Sun).</p>
<p class="last">The heliocentric correction is optionally performed during construction of
a light curve. Object’s equatorial coordinates are required.</p>
</dd>
<dt id="term-heliocentric-julian-date"><span id="term-hjd"></span><span id="term-jd-hel"></span>Heliocentric Julian date<br />HJD<br />JD(hel.)</dt>
<dd>The heliocentric Julian date is a time of an event (a particular
observation, time of minimum) with the heliocentric correction applied.</dd>
<dt id="term-julian-date"><span id="term-jd"></span>Julian date<br />JD</dt>
<dd><p class="first">The Julian date (JD) is the interval of time in days and fractions of a day
since January 1, 4713 BC Greenwich noon. It is widely used and recommended by International
Astronomical Union to express the time of an event, for example a particular observation,
minimum of a variable star, etc.</p>
<p class="last">You can use the Muniwin program to convert the Gregorian calendar date to
Julian date and back.</p>
</dd>
<dt id="term-light-curve">Light curve</dt>
<dd><p class="first">A light curve is a graph of brightness of a star, as a function of time. The
light curve is one of the products of the C-Munipack software. It is used especially
in the observation of variable stars. The brightness of a star is expressed in
magnitudes, usually as a difference between two objects - a variable star and a
comparison star. One or more check stars are used in addition. Absolute intensities
are used sometimes, another post-processing step outside the C-Munipack software
is required in this case.</p>
<p class="last">Depending on a type of the light curve, a user must choose a variable star,
a comparison star and optionally one of more check stars. Object’s equatorial
coordinates or observer’s geographical coordinates may be required, too.</p>
</dd>
<dt id="term-master-bias-frame">Master bias frame</dt>
<dd><p class="first">A master bias frame is a combined CCD frame made from a set of bias
frames. By means of the robust mean algorithm, the noise of the resulting
image is reduced.</p>
<p class="last">The master bias frame is used in the bias-frame correction phase of the
reduction process.</p>
</dd>
<dt id="term-master-dark-frame">Master dark frame</dt>
<dd><p class="first">A master dark frame is a combined CCD frame made from a set of dark
frames. By means of the robust mean algorithm, the noise of the resulting
image is reduced.</p>
<p class="last">The master dark frame is used in the dark-frame correction phase of the
reduction process.</p>
</dd>
<dt id="term-master-flat-frame">Master flat frame</dt>
<dd><p class="first">A master flat frame is a combined CCD frame made from a set of flat
frames. By means of the robust mean algorithm, the noise of the resulting
image is reduced.</p>
<p class="last">The master flat frame is used in the flat-frame correction phase of the
reduction process.</p>
</dd>
<dt id="term-matching">Matching</dt>
<dd><p class="first">The matching is the last step in the standard CCD image reduction scheme.
It process a set of photometry files of a same view field and finds a relation
between individual stars on the frames. For example, the star on position (100, 100)
on frame 1 is the same one as star on position (120, 95) on frame 2 etc. Each
star is then assigned a unique identifier, which is the same for the same star
on all frame in the project.</p>
<p class="last">To perform the matching, one frame must be given as a reference frame.
The reference frame is usually one frame from a set of input frames, but another
frame may be used - see catalog files.</p>
</dd>
<dt id="term-photometry">Photometry</dt>
<dd><p class="first">The photometry is a process performed during the CCD image reduction. It
takes a calibrated CCD image and detects stars on it. For each star, the brightness
is determined. The results are saved to a file, which is called the photometry
file.</p>
<p class="last">Current version of the software uses the aperture photometry method to
measure brightness of an object.</p>
</dd>
<dt id="term-reduction-of-ccd-frames">Reduction of CCD frames</dt>
<dd><p class="first">The reduction of CCD frames is a process which takes raw frames carried out
by means of a CCD camera and produces a set of photometry files, one file for
each input frame. The photometry file consists of a set of stars. For each star,
the derived brightness is stored. An unique identifier assigned to each star
is used to identify a particular star between frames.</p>
<p class="last">Depending on a reduction scheme, the reduction consists of the following
three steps: calibration (bias-frame correction, dark-frame correction, flat-frame
correction and time correction), photometry and matching.</p>
</dd>
<dt id="term-reference-file">Reference file</dt>
<dd><p class="first">A reference file is a photometry file which is used in the matching process.
It is either a photometry file for one of source frames or a catalog file of
the same field.</p>
<p class="last">The matching algorithm takes two photometry files at a time and find
corresponding stars on them. To match more than two scientific frames, one frame from
the set is chosen as a reference file and all source frames are matched one by one
against the reference.</p>
</dd>
<dt id="term-scalable-dark-frame">Scalable dark frame</dt>
<dd><p class="first">A scalable dark frame is a special case of the dark frame. It is
used in the advanced calibration scheme that allows to use a dark frame
of exposure duration different from scientific images for the dark-frame
correction.</p>
<p class="last">The scalable dark frame can be made from a dark frame by subtracting
a bias frame. Thus, the scalable dark frame contains only components that
are linearly dependent on exposure duration.</p>
</dd>
<dt id="term-time-correction">Time correction</dt>
<dd>The time correction fixes the time of observation of source frames. This step
of the reduction process is optional and depends on particular situation. For example,
you can use the correction to fix the bias of the PC’s system clock in regards to
the UTC.</dd>
<dt id="term-time-of-observation">Time of observation</dt>
<dd>In the C-Munipack software, the time of a particular CCD frame is always related
to a center of the exposure.</dd>
<dt id="term-track-curve">Track curve</dt>
<dd>A track curve is a graph of spatial offset of CCD frames, as a function
of time. Such curve is used to monitor the stability and precision of telescope
mount and its clock drive.</dd>
</dl>
</div>
</div>
</div>
</div>
<div class="sphinxsidebar">
<div class="sphinxsidebarwrapper">
<h4>Previous topic</h4>
<p class="topless"><a href="timebat_command.html"
title="previous chapter">timebat (command)</a></p>
<h4>Next topic</h4>
<p class="topless"><a href="history.html"
title="next chapter">History</a></p>
<div id="searchbox" style="display: none">
<h3>Quick search</h3>
<form class="search" action="search.html" method="get">
<input type="text" name="q" />
<input type="submit" value="Go" />
<input type="hidden" name="check_keywords" value="yes" />
<input type="hidden" name="area" value="default" />
</form>
<p class="searchtip" style="font-size: 90%">
Enter search terms or a module, class or function name.
</p>
</div>
<script type="text/javascript">$('#searchbox').show(0);</script>
</div>
</div>
<div class="clearer"></div>
</div>
<div class="related">
<h3>Navigation</h3>
<ul>
<li class="right" style="margin-right: 10px">
<a href="genindex.html" title="General Index"
>index</a></li>
<li class="right" >
<a href="history.html" title="History"
>next</a> |</li>
<li class="right" >
<a href="timebat_command.html" title="timebat (command)"
>previous</a> |</li>
<li><a href="index.html">C-Munipack 2.0.8 documentation</a> »</li>
</ul>
</div>
<div class="footer">
© Copyright 2012, David Motl.
Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 1.1.3.
</div>
</body>
</html>
