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<!DOCTYPE HTML>
<html lang="en">
<head>
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<title>Munipack ‒ Photometric Corrections Tutorial</title>
</head>
<body>
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<section>
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* update commands (synchro with actual dataset)
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<h1>Photometric Corrections</h1>
<p class="abstract">
Description for photometric pre-processing of images.
How to create of averaged biases, darks or flat-field frames
and how to apply ones on scientific images.
</p>
<h2>Introduction</h2>
<p>
The preprocessing is image is absolutely
necessary for any correct photometry work. The application
corrects unwanted fluency of devices as zero offset, dark
current or light responsibility.
</p>
<p>
More detailed description of the corrections is included
in pages:
<a href="man_bias.html">Averaged bias frame</a>,
<a href="man_dark.html">Averaged dark frame</a>,
<a href="man_flat.html">Averaged flat-field frame</a> and
<a href="man_phcorr.html">Photometric corrections</a>.
</p>
<p>
As an excellent introduction for preprocessing, the textbook
<a href="http://www.cambridge.org/gb/knowledge/isbn/item2713446/">
To Measure the Sky</a> by F. R. Chromey would be recommended.
</p>
<h2>Sample Data</h2>
<p>
A sample data are available as
<a href="https://integral.physics.muni.cz/ftp/munipack/munipack-data-blazar.tar.gz">munipack-data-blazar.tar.gz</a>.
Use commands
</p>
<pre>
$ cd /tmp/
$ tar zxf munipack-data-blazar.tar.gz
</pre>
<p>
to unpack it to a desired directory.
We will assume that the sample data are unpacked to <samp>/tmp</samp> directory
as <samp>/tmp/munipack-data-blazar</samp>.
</p>
<p>
The sample dataset includes an observation of this blazar together
with correction frames (flat-fields and dark-frames).
</p>
<div class="table">
<table>
<caption>An overview</caption>
<tr><th>Description</th><th>Filemask</th><th>Exposure</th></tr>
<tr><td>scientific images</td><td style="text-align:right;"><samp>0716_[1-9]R.fits</samp></td><td style="text-align:right;">120 sec</td></tr>
<tr><td>dark-frames of scientific images</td><td style="text-align:right;"><samp>d120_[1-7].fits</samp></td><td style="text-align:right;">120 sec</td></tr>
<tr><td>flat-fields</td><td style="text-align:right;"><samp>f10_[1-9]R.fits</samp></td><td style="text-align:right;">10 sec</td></tr>
<tr><td>dark-frames of flat-fields</td><td style="text-align:right;"><samp>d10_[1-9].fits</samp></td><td style="text-align:right;">10 sec</td></tr>
</table>
</div>
<h2>Working directory</h2>
<p>
As a first important step, we will create a working directory. For example,
create directory <samp>/tmp/munipack-data-blazar</samp> and switch to by the command:
</p>
<pre>
$ mkdir /tmp/work
</pre>
<p>
The name does not matter. It is highly recommended to use
a new empty directory to prevent any lost of data
(especially of original images!).
</p>
<h2>Preparation</h2>
<p>
Some functionality of preprocessing tools requires correct
setting of FITS header keywords. The keywords are preset,
but sometimes local conventions are different. Therefore there
are available some <a href="man_env.html">environment variables</a>
for individual tuning.
</p>
<p>
It is recommended to set the variables by the way in your
bash shell as the initial step (modify by your needs):
</p>
<pre>
$ FITS_KEY_FILTER='FILTER'
$ FITS_KEY_EXPTIME='EXPTIME'
$ FITS_KEY_TEMPERATURE='TEMPERAT'
$ FITS_KEY_DATEOBS='DATE-OBS'
$ MUNIPACK_TEMPERATURE_TOLERANCE=1 # Celsius degree
$ MUNIPACK_EXPTIME_TOLERANCE=1e-6 # seconds
$ export FITS_KEY_FILTER FITS_KEY_EXPTIME FITS_KEY_TEMPERATURE FITS_KEY_DATEOBS
$ export MUNIPACK_TEMPERATURE_TOLERANCE MUNIPACK_EXPTIME_TOLERANCE
</pre>
<p>
To avoid need of repeated typing of the keywords, add these commands
to your ~/.bashrc profile.
</p>
<h2>Average Of Dark Frames</h2>
<p>
To create an average dark frame for scientific exposures, run the command:
</p>
<pre>
$ cd /tmp/work
$ munipack dark -o d120.fits /tmp/munipack-data-blazar/d120_*.fits
$ ls d120.fits
d120.fits
</pre>
<p>
Munipack is invoking the module <samp>dark</samp> intended
to average of images specified as the last argument.
The asterisk matches all images begins with <samp>d120_</samp>
together and ending with the suffix <samp>.fits</samp>.
The processed mean is stored as <samp>d120.fits</samp>.
</p>
<p class="indent">
Averaging uses a robust mean method by default.
It reduces of fluency of cosmic-rays and similar single-frame defects on
final products. On other side, it requires much more computer resources
than simple averaging by the arithmetical mean (switch <samp>-a</samp>)
</p>
<p class="indent">
Note that, this step can be omitted (a single dark frame can
be used only), but one is preferred from a statistical point
of view. The result image is frequently called as master-dark.
</p>
<figure>
<img class="figure" src="d120_1.png" alt="d10_1.png" title="A dark image">
<figcaption>
A randomly selected dark image.
</figcaption>
</figure>
<figure>
<img class="figure" src="d120.png" alt="d120.png" title="Mean of dark-frames">
<figcaption>
Mean of dark-frames (master-dark).
</figcaption>
</figure>
<h2>Average Of Flat-Fields</h2>
<p>
Because flat-fields are light frames similar to scientific frames,
ones needs similar preprocessing. Especially, we must correct
its for dark frames.
</p>
<p>
So, the first step is preparation of the dark with 10 s of exposure
times for flats
</p>
<pre>
$ munipack dark -o d10.fits /tmp/munipack-data-blazar/d10_*.fits
</pre>
<p>
The exposure times must be exactly the same for both darks and flats.
</p>
<p>
With this dark we can easy create the averaged flat as:
</p>
<pre>
$ munipack flat -o f_R.fits -dark d10.fits /tmp/munipack-data-blazar/f10_*R.fits
</pre>
<p>
The internally corrected
flat-fields are scaled by its mean intensity and its dispersion to
a unified output level. A robust mean is made on the uniform scaled flats
and an output flat is stored as <samp>f_R.fits</samp>.
</p>
<p class="indent">
The key feature of <samp>flat</samp> is the scaling and a robust mean of single
flats. The procedure is pretty effective for short series of the twilight
sky's exposures when brightness rapidly decrease. Also, a long over-night series
of non-identical fields (like many blazar fields) will produce excellent
results.
</p>
<figure>
<img class="figure" src="f10_1.png" alt="f10_1.png" title="A flat-field image">
<figcaption>
A randomly selected flat-field image.
</figcaption>
</figure>
<figure>
<img class="figure" src="autoflat.png" alt="autoflat.png" title="A scaled robust mean of flats">
<figcaption>
A scaled robust mean of flat-fields (master-flat).
</figcaption>
</figure>
<h2>Dark And Flat-Field Corrections</h2>
<p>
Original scientific images can be corrected for dark-frames by running:
</p>
<pre>
$ munipack phcorr -t . -dark d120.fits /tmp/munipack-data-blazar/0716_*R.fits
</pre>
<p>
<samp>phcorr</samp> action subtracts, the previously created mean-dark
<samp>d120.fits</samp> given as a first non-optional argument,
from every scientific exposures of 0716+71 and newly created images
will be stored in
the current working directory (given by option -t . (tee and dot!)
with image names identical to original ones.
</p>
<figure>
<img class="figure" src="0716_original.png" alt="0716_original.png" title="An original image">
<figcaption>
A randomly selected scientific exposure of blazar 0716+714.
</figcaption>
</figure>
<figure>
<img class="figure" src="0716_dark.png" alt="0716_dark.png" title="An original image with the dark-frame subtracted">
<figcaption>
A randomly selected scientific exposure of blazar 0716+714 with the d120 dark-frame subtracted.
</figcaption>
</figure>
<p>
By analogy of dark correction, scientific images (subtracted for dark)
can be corrected for flats by
</p>
<pre>
$ munipack phcorr -t . -flat f_R.fits /tmp/munipack-data-blazar/0716_*R.fits
</pre>
<p>
We can see that the current directory images are used.
</p>
<figure>
<img class="figure" src="0716_final.png" alt="0716_final.png" title="A fully corrected scientific exposure">
<figcaption>
A randomly selected, fully corrected (dark and flat), image of blazar 0716+714.
</figcaption>
</figure>
<p>
For convenience, both correction can be appplied together
</p>
<pre>
$ munipack phcorr -t . -flat f_R.fits -dark d120.fits /tmp/munipack-data-blazar/0716_*R.fits
</pre>
<h2>See Also</h2>
<p>
Manuals:
<a href="man_bias.html">Bias</a>,
<a href="man_dark.html">Dark</a>,
<a href="man_flat.html">Flat-field</a>,
<a href="man_phcorr.html">Photometric corrections</a>.
</p>
</section>
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