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We've all seen those brilliant photographs of Splendid Nebulae,
Astoundingly bright Galaxies and those patches of stars called
Clusters, all these are a result of the magnificent telescopes we've
built, most people just happen to see the telescope as an eye-piece
attached device with which one can gaze at the stars, while this may
be true partly, a telescope's applicability can be truly manifested
only when it can provide photographs (in terms of light) and
information about the stars and other inter-stellar wonders, this is
made possible by a device known as CCD (Charge coupled device), this
being used in digital cameras and other equipment are the most
efficient ideal light detectors today, the telescope receives light
from the sky and this is made to incident on the CCD which produces
images according to the amount of light intensity received and it's
position, the images thus obtained form the raw data for any
observational research and is transmitted to a computer in the form of
images, the following excerpt tells how the raw data is processed.
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Excerpt from:
The Telescope - A blue
print
A dissertation
By
Amoghavarsha JS
M. P. Birla Institute of fundamental research
Bangalore - India
Data
reduction systems:
The images that are transmitted to the analysis computer are of the
format .FITS (Flexible Image Transfer System), this image needs to be
reduced by the application of error corrections and other parameters
before it can be used for observational research, this is done using
special reduction packages and one such widely used package is IRAAF
(Image Reduction and Analysis Facility), this is distributed by NOAO
(National Optical Astronomy Observatories) operated by the Association
of Universities for Research in Astronomy Inc. under contract with the
National Science Foundation.
IRAAF is a software that works on the UNIX platform, since UNIX is
vendor independent and also almost all research software from earlier
times were developed on UNIX based systems at Bell labs, it became the
default OS for the astronomical community.
Advantages of IRAAF over other small software developers:
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Small astronomy departments or groups can have a full-blown data
analysis package, even when they would not have the resources to
write one on their own. |
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More complicated algorithms can be made available. There are a lot
of rather straight0forward reduction problems that most of us would
not be afraid to tackle. |
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The
reduction process would look the same to all astronomers. This is of
particular interest to national observatories and other institutions
which host large number of visiting observers, as it means that
those observers will already know how to reduce data using software
which can be available online. |
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After the observing runs, visiting observers will be able to do any
remaining data reduction at their home institutions. This again
pertains to a national observatory situation. |
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The
algorithm used in the data reduction will be standardized. This
will mean that what you see, for example, a reduced spectrum in the
literature, you will be able to guess what has been done to it and
therefore have a better basis to judge which features are real and
which are not. |
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The
package would set standards for file formats. Even if you go ahead
write your own package, such standards help immensely in
transferring data to another astronomer, the .FITS format itself
being a live example to this. |
Data reduction with IRAAF:
These are some of the CCD reduction works done by IRAAF, these
procedures or reductions are general, irrespective of the kind of
research these reductions have to be applied before using the image
for further analysis.
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Trimming of raw data
It is a procedure where the pixels at
the edges or the region of least concern are trimmed off, for
example in spectroscopic work, only 1 pixel width along the Y-axis
is required for observation, the other pixels provide extra glare
that may hamper the observation and hence those parts are trimmed
off.
Syntax: epar ccdproc trimsec [x:1024, y:400]
/*where x and y are number of pixels to be trimmed off on x and y
axis respectively.*/
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Zero correction
The CCD though kept at immensely cold
conditions of the order of -200 degrees Celsius, still have some
thermal excitations called dark currents, though they are of very
small orders, these dark currents hamper the observation, hence
correction for this has to be applied.
Syntax: epar zerocombine
/*Here we have to specify the file which consists of the values of
the dark current readings taken when the CCD is not exposed to
external light*/
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Flat correction (bias correction)
The CCD has sometime imperfections at
certain points or pixels, these have to be corrected for light
source otherwise they may lead to mistakes by indicating light
source, which is just an imperfection in the CCD, therefore a flat
correction is applied by first exposing the CCD to a bright patch of
light which illuminates all pixels of the CCD equally and then
checking for imperfections on the CCD and then checking for
imperfections on the CCD and then applying the required correction.
Syntax: epar flatcombine
/*Here also we have to specify the file which consists of the values
of the CCD exposed with a bright white wall or a CCD with all pixels
exposed equal amounts of light*/ |
-signing off
amogh |
