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\title{Automated Identification of Spectra in Schmidt Plates}
\titlemark{Automated Identification of Spectra in Schmidt Plates}

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\author{Raquel D\'iaz, J.\ Guichard, O.\ Cardona, L.\ Altamirano}
\affil{Instituto Nacional de Astrof\'isica \'Optica y
Electr\'onica}

\contact{Raquel D�az}
\email{raqueld@inaoep.mx}

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\paindex{Diaz@D\'iaz, R.} 
\aindex{Guichard, J.} 
\aindex{Cardona, O.}
\aindex{Altamirano, L.}  

\authormark{D\'iaz et al. }


\keywords{automated identification, plates: Schmidt}

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We present the first results of a system for automatic analysis of
astronomical images from digitized plates. This system will
automatically identify spectral lines in low resolution spectra
employing image processing and analysis. We propose to carrying
out an identification of spectral lines with minimum human
intervention, considering factors such as type of emulsion,
objective prism, telescope characteristics, etc. Our pursuing goal
is to obtain totally automatic software for the identification of
the spectra and its lines (emission /absorption). In this work we
present the first results of the automatic segmentation and the
calculated spectra in 1-D.
\end{abstract}

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\section{Introduction}
INAOE has a great collection of astronomical plates taken with the
Tonantzintla Schmidt Camera between 1941 to 1995, there are
altogether 15686 photographic plates of three different types,
4484 are spectral type, 8432 are direct type and 2540 are direct
of three images type (Tepanecatl, et. al. 2002). There are also
different types of emulsions (mainly the Series 103a of Kodak
Spectroscopy Plates). This work is focused in the spectral plates
only. The Tonantzintla Schmidt Camera has the following features:
Focal length 231.4 cm, Focal relation 3.2, Plate scale 95
arcsec/mm, Correcting plate 66.4 cm. Mirror 76.20 cm. The
objective prism 3.96 degrees with dispersion of 1533 \AA/mm
between $H_\beta$ and $H_\gamma$; 954 \AA/mm between $H_\gamma$
and $H_\delta$; and 626 \AA/mm between $H_\delta$ and
$H_\epsilon$ (Haro, 1956). The plates have a size of 8x8 inches
and they cover a 5x5 degree square.

Our objective is to develop an algorithm of  optimal and robust
segmentation of stellar spectra on spectral plate images with the
purpose of identifying its spectral lines in an automatic form.
People inside and outside of INAOE are interested in analyzing
this information in an automatic form. For such a task we have the
works  by Borra et. al. 1998, and Zamorano et. al. 1990 as a
basis. In figure 1 we show a flow chart with the processes
necessary to obtain our goal.

\section{Algorithm and Plate Digitization}
For plate digitization an  EPSON Expression 1680 Professional
Firewire scanner is used which is able to scan a full photographic
plate in a few minutes, it has a resolution of 1600 dpi which
correspond to a pixel size of 20 microns using the transparency
(positive) mode. The dynamic range is 8 bit (gray scale of 256
values, spanning the range 0 (dark) to 256 (white)). We get images
of 12600 x 12600 pixels with a size of about 150 MB.  Each
spectrum has approximate dimensions of 350 pixels in length and  8
pixels in width.


\begin{figure}[h]
\begin{center}
\includegraphics[width=9.0cm]{P7-7_1.eps}
\caption{Processes for the automatic identification of spectral
lines.}
\end{center}
\end{figure}

\subsection{Background Identification and Characteristic Curve}
After the digitization we have  images containing both sky,
spectra and spurious objets. The aim of this step is to rawly
identify the spectra and sky regions. We use a segmentation
process for this goal and later  extract the properties of the
resulting regions (Awcock, 1996; Lira, 2002).

The next step  is to obtain the mean sky level or background
identification from the image. This value is calculated by summing
all the pixels forming the region considered  as sky and divided
by the total number.  Taking the sky level as a constant gray
value, the program creates a new image with this value,
subtracting it from the original and creating an image without the
sky contributions but with all the spectral objects, including the
spurious ones.


To obtain the characteristic curve is a complicated task because
we do not have sensitometric spots in our photographic plates.
This implies that we cannot construct a characteristic curve
taking  as a base the densities and the noise associated with them
as is normally done. We considered an alternative way of obtaining
this curve directly from the same plate using all it properties,
because the  characteristic emulsions such as class, exposition
time and development conditions can be different among plates. In
consequence, we obtained as a result of this process a
look-up-table (lut) for each image and modified the image using
this table. We are also analyzing the idea of using stellar
spectra to construct a characteristic curve for each image
(Stienon, 1972). This technique basically involves the use of
spectra of several stars whose monochromatic magnitude differences
are known.


\subsection{Selection and Shape Analysis of the Spectra}

At this point we have a linearized image without sky
contributions, then we applied filters for contrast improvement
and image smoothing   with the purpose of emphasizing the spectra
on the plate and  eliminating the noise. After this a thresholding
segmentation is made, obtaining regions corresponding to each
spectrum. Characteristics  such as position (X,Y), area, diameter,
length of the contour, direction, ellipticity  and anisometry are
extracted. A second process of selection must be made using the
calculated characteristics to eliminate spurious objects (arrows
and/or annotations on the plate) that were not eliminated with the
previous criteria. The objects truncated by the edges of the plate
are eliminated too (Borra 1987 and 1988). Hartmann�s formula was
used in order to know the  spectra length for each type of
emulsion (Sawyer 1948).

Once the spectra that we are interest in are identified, the first
step for their extraction  in 1-D is to find their exact area and
center, with the purpose of considering only lines near to the
spectral center and avoiding lines very near the edges. We repeat
the process for the columns too. This analysis is made in the same
way that IRAF does in the routine of APALL. The average
intensities are stored in a text file which is drawn later to show
the spectra in 1-D.The graphics at this point show only values of
intensity for each point without wavelength calibration.


\section{Results}
We presented the first results of the automatic segmentation and
the calculated spectra in 1-D. The results obtained are comparable
with those in the 1-D spectra using IRAF. The selection of objects
is made with greater precision and in a shorter time than by
visual inspection. The processing time for a whole plate is about
10 min. The results are show in figures 2  and 3. The first tests
were done with plates that have a 103a-O emulsion, it has a rank
of effective sensitivity from  250 to  550 nm, and it allows us to
make an identification of lines with greater clarity in the blue
region of the electromagnetic spectrum. More information about
this is available in http://www.inaoep.mx/$\sim$raqueld/


\begin{figure}[h]
\begin{center}
\includegraphics[width=6cm]{P7-7_2.eps}
\caption{Segmentation of an image by different methods. a) Objects
selected with our method. b) Objects selected with SExtractor.
Note the automatic elimination of unwanted regions (small spectra
and observation notes). }
\end{center}
\end{figure}


\begin{figure}[h]
\begin{center}
\includegraphics[width=9cm]{P7-7_3.eps}
\caption{First results 1-D spectrums. a) Objects analyzed without
any process. b) Objects analyzed with our method. c) Section of
the digitized plate.}
\end{center}
\end{figure}

\section{Future Work}
Currently it is possible to identify the spectral lines by visual
inspection  in 1-D spectra;  we are working  in the process of
automatic identification. We plan to produce a process that
automatically identifies lines in spectra (emission /absorption).
The algorithm should be applied to plates with a different
exposition time and different  class emulsion in order to work
with all the spectral plates of  INAOE`s collection.





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\begin{references}
\reference Awcock, G.W., et al., 1996, Aplied Image Processing, ,
Ed. McGrawHill, Inc.
\reference Borra, E.F., et al., 1987, PASP, 99, 535B
\reference Borra, E.F., et al., 1988, PASP, 100, 1276
\reference Eastman Kodak Spectroscopy, 1973, P-315
\reference Haro, G. 1956,Bol.Obs. Tonantzintla Tacubaya 14, 8
\reference Lira, 2002, Introducci\'on al tratamiento digital de
im\'agenes, F.C.E. 
\reference Sawyer, R.A, 1948, Experimental Spectroscopy, Prentice -Hall, Inc, p.58
\reference Stienon, F.M., 1972, A.A.S.P.B., 5, 17
\reference Tepanecatl, S., et al., 2002, Internal Report INAOE,
No.227
 \reference Zamorano J., et al.,1990, Ap\& SS, 170, 353
\end{references}

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