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<h1>Mosaics</h1>
<p class="abstract">
How to montage multiple exposure to a single frame covering
large area of heavens.
</p>
<h2>Whirlpool Galaxy</h2>
<p>
<a href="http://en.wikipedia.org/wiki/Whirlpool_Galaxy">Whirlpool galaxy (M51)</a>
is a pretty photogenic object showing nice example of interacting (merging) of galaxies.
The primary, grand design galaxy, has a spiral arm deformed by interaction
with the small second one with a bar and without any arms.
Whirlpool galaxy is so large that fills full field of view of many
telescopes. Therefore we must apply mosaics technique to acquire
peripheral parts of Whirlpool.
</p>
<p>
A scene of the galaxy interaction covers unusually large part
of the sky. Night photographers are in doubt on which point
theirs equipment must be centred.
A large telescope can capture fainter objects (details)
but a small field of view permits portraying just only part of an
object. A small telescope has larger field of view, but detect
only brighter parts. Mosaics offers amazing possibility
to use of large telescope and also cover of larger part of sky.
On the contrary, the way is more time consuming and requires
advanced processing methods.
</p>
<p>
There, images of large part of neighbourhood of Whirlpool
galaxy taken on Vyškov's Observatory (17°01'20.6" E, 49°17'01.4" N)
by 0.4 m Newtonian telescope and Moravian Instrument's G2 camera via
a filter of RGB by Jaroslava Kocková and Filip Hroch
are served as an example. The data has poor quality, the observation
was having interrupted by rapidly changing clouds and some technical
problems with both CCD camera and telescope was solved. Please,
consider the images as an example for mosaic construction
(not a perfect imaging).
</p>
<p>
<a href="ftp://integral.physics.muni.cz/pub/munipack/munipack-data-m51.tar.gz">Download</a> data for mosaics of Whirlpool galaxy and unpack it:
</p>
<pre>
$ wget ftp://integral.physics.muni.cz/pub/munipack/munipack-data-m51.tar.gz
$ tar zxf munipack-data-m51.tar.gz
$ cd munipack-data-m51/
</pre>
<p>
To construct a mosaics, please follow commands in this tutorial.
</p>
<h2>Photometric Precorrections</h2>
<p>
The first step is photometric precorrections of images.
</p>
<p>Construct of mean dark frame for Whirlpool (for darks
with 120s exposure time):
</p>
<pre>
$ munipack dark -o dark120.fits dark_005?.fits dark_008?.fits
</pre>
<p>
Tip. Inspect FITS header of dark120.fits. One will have zero temperatures
because input images has its chip temperatures pointed by CCD-TEMP keyword.
Munipack expects the keyword as TEMPERATURE. To get correct values,
set an appropriate <a href="man_env.html">environment variable</a>:
</p>
<pre>
$ export FITS_KEY_TEMPERATURE="CCD-TEMP"
</pre>
<figure>
<img src="m51_dark.png" alt="m51_dark.png" title="Averadge of dark frames">
<figcaption>
Average of dark frames for object
</figcaption>
</figure>
<p>
Analogically, the mean dark frame for flat-field frames will be
constructed as
</p>
<pre>
$ munipack dark -o dark10.fits dark_003?.fits dark_004?.fits
</pre>
<p>
Flat-field frames can be corrected for dark as
</p>
<pre>
$ munipack phcorr -dark dark10.fits flat_Green_*.fits
</pre>
<p>
And the mean flat-field can be prepared as
</p>
<pre>
$ munipack flat -o flat_Green.fits flat_Green_*.fits
</pre>
<figure>
<img src="m51_flat.png" alt="m51_flat.png" title="Averadge of flat-field frames">
<figcaption>
Average of flat-field frames for object
</figcaption>
</figure>
<p>
All frames of Whirlpool can be corrected for both dark and flat-field frames
by the single command:
</p>
<pre>
$ munipack phcorr -dark dark120.fits -flat flat_Green.fits m51_Green_*.fits
</pre>
<figure>
<img src="m51_Green_0053.png" alt="m51_Green_0053.png" title="A corrected image">
<figcaption>
An image with all photometric precorrections applied.
</figcaption>
</figure>
<p>
The corrected images has the same name as the original which has been
backup-ed to ones endings with ~. It is highly recommended
visually compare images before and after precorrections.
</p>
<h2>Astrometry and Photometry Calibration</h2>
<p>
The first delicate point of our analysis is detection of stars
on images. The detection is mostly on base of presumption that
all stars are Gaussian-like peaks with an identical profile. The profile
has the same width at half of maximum (FWHM) which an image
of a point source imagined via our (optical) telescope (apparatus).
The FWHM is in pixels and is in range 2-10 for optical non-space
telescopes. The value can be easy estimated and checked with
Photometry tool in xmunipack:
</p>
<figure>
<img src="m51_photometry_tool.png" alt="m51_photometry_tool.png" title="Photometry Tool">
<figcaption>
Photometry Tool in action
</figcaption>
</figure>
<p>
With help of the tool, we can found that right parameters for
out images are: FWHM=6 and threshold is 10 sigma above background level.
With the information, all stars on all frames will be found and
aperture photometry will be easy:
</p>
<pre>
$ munipack find -f 6 -th 10 m51_Green_*.fits
$ munipack aphot m51_Green_*.fits
</pre>
<p>
An arbitrary image can show detected stars (see pic)
</p>
<figure>
<img src="m51_aphot.png" alt="m51_aphot.png" title="Detected Objects">
<figcaption>
Detected objects
</figcaption>
</figure>
<p>
The perfect arrange of images is prepared with help of precise astrometry.
To prepare astrometry, we should list a part an astrometry catalogue. The
UCAC5 is an ideal for our purposes. We are selected cone
about centre of Whirlpool galaxy in radius 0.2° because our
fields are spread over the area.
</p>
<pre>
$ munipack cone -c UCAC5 -r 0.2 202.47 +47.2
</pre>
<p>
and run the astrometry calibration
</p>
<pre>
$ munipack astrometry m51_Green_00*.fits
</pre>
<p>
Note use of --maxmatch option. In this case, when we need large
part of sky, we also need more than default count of stars.
</p>
<p>
The astrometry will take a while. The warnings like
</p>
<pre>
Mutual match for files `m51_Green_0062.fits' and `cone.fits' failed.
</pre>
<p>
can be safety ignored. The visual inspection give our understand why
the match failed.
</p>
<h2>Final Mosaic</h2>
<p>
The last step is composition of images to a mosaic. Try
</p>
<pre>
$ munipack kombine --rcen 202.47 --dcen +47.2 --width 1000 --height 1000 \
> --adjust --expomask m51_Green_00??.fits
</pre>
<p>
You can play with the values of centre of projection (--rcen, --dcen)
and also with size of output image.
</p>
<p>
Kombine trying to fine adjust of background level of individual
exposures with --adjust option.
</p>
<p>
Photon (count) rates are normalises on the same exposure time for parts
of images with different exposure time using of --expomask when an exposure
mask is created during merging.
</p>
<figure>
<img src="m51_mosaic.png" alt="m51_mosaic.png" title="Mosaic of Whirlpool galaxy">
<figcaption>
Mosaic of Whirlpool galaxy (non-calibrated frames). The logistic function
and some fine tuning of parameters has been applied.
</figcaption>
</figure>
<p>
The image is not perfect, specially due to clouds. On other side, we can
observe how the image was merged from single exposures (omitting --expomask).
</p>
<p>
Some CCD device problem are appeared: the residuals for hot column at
left part on coordinates (93,221-3) copied from flat-fields and the
dark rows at centre of the image. There is no way hot to correct it
by a standard way (the image must be patched).
</p>
<h2>Perfecting Mosaics by Photometric Calibration</h2>
<p>
The final image is affected by clouds which visible obscured our
object and reduces amount of collected light.
We should perfect of final image by using
of photometrically calibrated images.
</p>
<p>
The photometry calibration can be used to derive
different attenuation of light on different frames due to observing
conditions (clouds).
The calibration is not absolute and we not need know photometry
system and filter (moreover our identification of the filter as 'Green'
is probably incorrect).
</p>
<pre>
$ for A in m51_Green_00??.fits; do
munipack fits --update-key FILTER="'V',Replacement of Green" $A;
munipack phcal --photsys-ref Johnson --area 0.3 -c cone.fits \
--col-ra RAJ2000 --col-dec DEJ2000 -f V --col-mag Gmag $A
done
$ munipack kombine --rcen 202.47 --dcen +47.2 --width 1000 --height 1000 \
--adjust --expomask m51_Green_00??_cal.fits
</pre>
<figure>
<img src="m51_cal_mosaic.png" alt="m51_cal_mosaic.png" title="Mosaic of Whirlpool galaxy">
<figcaption>
Mosaic of Whirlpool galaxy (calibrated frames)
</figcaption>
</figure>
<h2>Tips</h2>
<p>
Some recommendations for creating of photogenic frames:
</p>
<ul>
<li>Take images under excellent atmospheric conditions: suppress fluency
of clouds, urban lights and similar polluting factors.
Avoid observations near horizon and where changes of background
can by expected.</li>
<li>Use stable equipment (temperature regulated camera) and the best
flat-fields.</li>
<li>Cover all the area by similar exposures.</li>
<li>Try large overlaps (ideally per half of field of view)</li>
<li>Select the best exposures only.</li>
</ul>
<h2>See Also</h2>
<p>
Manuals:
<a href="man_kombine.html">Kombine</a>,
<a href="man_astrometry.html">Astrometry</a>,
<a href="man_aphot.html">Aperture Photometry</a>,
<a href="man_phcorr.html">Photometric corrections</a>.<br>
A bash script summarising of this tutorial: <a href="mosaic.sh">mosaic.sh</a>.
</p>
<p>
<a href="http://montage.ipac.caltech.edu/">Montage</a>
is much more advanced tool.
</p>
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