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<H2><A NAME="SECTION001111000000000000000">
Input Data and Preprocessing</A>
</H2>
The information involved in a reduction session consists of user data
and system tables. User data is a set of echelle images, observed with
the same instrument configuration, including a wavelength calibration
image (<TT>WLC</TT>), a flat field image (<TT>FLAT</TT>) and astronomical
objects <TT>OBJ</TT>. Optionally, this set will include standard stars
(<TT>STD</TT>) to be used in the absolute or relative flux calibration,
and dark images (<TT>DARK</TT>). Catalogues with comparison lines and
absolute fluxes for standard stars are available in the system as <TT>MIDAS</TT> tables.
<P>
Before starting the actual reduction some preprocessing of the data is
required to correct for standard detector effects as follows:
<P>
<UL>
<LI>Rotation of input frames. <BR>
After this rotation,
the dispersion direction of the echelle orders will be horizontal,
with wavelengths increasing from left to right and spectral
order numbers decreasing from bottom to top of the image.
As always in <TT>MIDAS</TT>, the origin is the pixel (1,1),
located in the lower left corner of the image.
<LI>Updating <TT>START</TT> and <TT>STEP</TT> descriptors. <BR>
Descriptors <TT>START</TT> and <TT>STEP</TT> must be set to 1.,1. for
all images processed. Session keyword <TT>CCDBIN</TT> must be set
to the original binning factor along <I>x</I>- and <I>y</I>-axis. Image
rotation and descriptors update are performed by the command
<TT>ROTATE/ECHELLE</TT>.
<LI>Cleaning of bad columns. <BR>
First, bad columns - bad rows after the
rotation - can be removed with the command <TT>COMPUTE/ROW</TT>.
The cleaning of bad columns is required for <TT>FLAT</TT> images where
the variation of the intensity due to these columns can affect
the automatic detection of the orders.
<LI>Cleaning of hot pixels. <BR>
Hot pixels can be eliminated by filtering the images. In
case the observation has been splitted in several exposures
and more than one image is available with the same information,
the images can be averaged with the command <TT>AVERAGE/WINDOW</TT>; this
command can, optionally, interpolate pixel values with large deviations
from the average value. Removal of hot pixels is required for
<TT>DARK</TT> images and is recommended for <TT>OBJ</TT> exposures.
General methods to clean bidimensional spectra are described in
Chapter <A HREF="node676.html#spectral-data">L</A> (Removing Cosmic Ray Hits). The
command <TT>FILTER/ECHELLE</TT>, adapted to echelle spectra is described
in Section <A HREF="node151.html#remove-particle-hits">7.3</A>
<LI>Subtraction of dark current from <TT>FLAT</TT>, <TT>OBJ</TT> and <TT>STD</TT>
frames.
The dark level is estimated from a series of <TT>DARK</TT> exposures
of short duration which are averaged to reduce the effect of the
read-out
noise of the <TT>CCD</TT> and to eliminate hot pixels as described before.
If preflashing is necessary, a set of preflashed
<TT>DARK</TT> exposures should be obtained in a similar manner. It is
advisable to obtain a set of <TT>DARK</TT> images with similar exposure
times as the object and standard star frames, or to scale the dark
level to the observed exposure.
<LI>Checking exposure times in <TT>OBJ</TT> and <TT>STD</TT> frames. For images
generated by ESO instruments, the exposure time (in seconds) is stored in the
descriptor <TT>O_TIME(7)</TT>. If necessary this descriptor can be
created as <TT>O_TIME/D/1/7</TT> with the command <TT>WRITE/DESCRIPTOR</TT>.
</UL>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
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