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<H1><A NAME="SECTION001130000000000000000"> </A><A NAME="7181"> </A>
<A NAME="remove-particle-hits"> </A>
<BR>
Removal of particle hits
</H1>
Cosmic ray<A NAME="7183"> </A> events pose a serious problem in long <TT>CCD</TT> exposures. Their
removal is a rather delicate step, because in high-contrast images
(such as well-exposed echelle spectra) there is always a danger of
damaging the scientific contents of the frame. Particle hits can be
removed from scientific exposures by splitting the exposure and comparing
spectra of the same target obtained under the same instrumental configuration.
Offsets of the target resulting from the positioning of the target on the
entrance slit of the spectrograph and variations of exposure time must be
accounted for. Commands <TT>AVERAGE/IMAGE</TT> and <TT>AVERAGE/WEIGHT</TT> offer
number of options to compare the images and reject particle hits. In
the case of
echelle spectra of sources with very little variation of the spectral
information along the slit, one can also exploit the knowledge provided
by the order definition as to where in the frame the relevant data is located.
<P>
The removal of unwanted spikes, above an otherwise featureless background
such as the inter-order space of echelle spectra,
is done most easily with a median filter. Therefore, in a first step a median
filter is applied to the entire frame. This then enables the true background
to be determined as described in Section `Background Definition' of this
chapter. The subtraction of the background calculated
completes the second step, and, as far as the inter-order space
is concerned, the final result is reached already. The removal of cosmics from
the object spectrum forms the third step
and is restricted to the regions covered by the spectral orders
in the background-corrected, but otherwise raw frame. This step comprises
the following operations performed within a sliding (along and separately
for each spectral order) window of user-specified width:
<UL>
<LI>
For all <I>x</I><SUB><I>i</I></SUB> of the window, normalize the spatial profile to the total
flux in the associated slice (i.e. all <I>y</I><SUB><I>j</I></SUB> of the order considered).
<LI>
Form the `true' spatial profile as the median over the individual profiles
at all <I>x</I><SUB><I>i</I></SUB> in the window.
<LI>
For all pixels (
<!-- MATH: $x_{c}, y_{j}$ -->
<I>x</I><SUB><I>c</I></SUB>, <I>y</I><SUB><I>j</I></SUB>) in the central slice, <I>c</I>, of the window
compare their re-normalised flux, <I>f</I><SUB><I>c</I>,<I>j</I></SUB>, with the properly scaled
contents <I>m</I><SUB><I>j</I></SUB> of
pixel <I>j</I> in the median profile. If the difference exceeds the expected
statistical error of <I>f</I><SUB><I>i</I>,<I>j</I></SUB> (calculated from the number of
photons detected and the readout noise) by a user-specified factor,
replace <I>f</I><SUB><I>c</I>,<I>j</I></SUB> with <I>m</I><SUB><I>j</I></SUB>.
</UL>
If the threshold for substitution by the median is set properly
(<IMG
WIDTH="46" HEIGHT="54" ALIGN="BOTTOM" BORDER="0"
SRC="img287.gif"
ALT="$\sim$">4 <IMG
WIDTH="44" HEIGHT="54" ALIGN="BOTTOM" BORDER="0"
SRC="img288.gif"
ALT="$\sigma$">)
and the spectral information within the spatial
profile does not change (point sources are best), this
procedure does not redistribute the flux or dilute the point spread
function.
<P>
These three steps are the backbone of command <TT>FILTER/ECHELLE</TT>.
The final output frame is the merger of the median-filtered
inter-order domains with the spectral orders after having been
subjected to step three. Note that the background has been subtracted
already. A keyword <TT>BACKGROUND</TT> with contents <TT>SUBTRACTED</TT> is
appended to the frame as a flag to subsequent high-level procedures so
as not to have to go through the very time consuming step of the
background modeling again. Delete that descriptor or change its contents if
re-modeling of the residual background is desired.
<P>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
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