Subject: echomop

From: Ignacio Negueruela <[email-masked]>
Date: Tue, 4 May 1999 17:15:47 +0200 (MET DST)

Dear Norman,

 I hope you're still responsible for ECHOMOP.
 
 I am finding a weird problem when processing spectra taken with different
instruments - I have until now found it with UES data, FOCES data (a
fibre-fed spectrograph) and BFOSC data a multiinstrument using a
combination of two grisms.

 The problem is a sort of ripple superposed on the continuum - as a matter
of fact, it's not a propoer sort-of-sinusoidal wave, but looks more like
some missing counts every so many wavelength units. I am tempted to
attribute this to sky subtraction, since in most cases there shouldn't
really be any sky to subtract from the image (the seeing being bad enough
for the star to occupy the whole slit or fibre-hole). My idea is that,
given the relatively curved shape of the orders, ECHOMOP leaves out pixels
that belong to the object (or counts them as sky). I cannot be sured
becuase I really don't know how ECHOMOP determines the extent of the order
at each point. I have experimented quite a lot with ECH_SPATIAL, changing
the parameters for individual orders, but I cannot find a correlation
between, say,  the number of sky pixels and the quality of the resulting
spectrum. Also, I cannot extend indefinitely the size of the object, due
to order overlapping.

 Are you conscious of this problem happening to someone else and do you
have any recipe to treat it?

 Thanks,

   Ignacio.

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Ignacio Negueruela - ESA External Research Fellow    Phone +39 06 40796012
SAX Science Data Center, Agenzia Spaziale Italiana   Fax   +39 06 40796291
c/o Telespazio, via Corcolle 19            email: [email-masked]
I00131 Roma, Italy                  WWW:sax.sdc.asi.it/~ignacio/index.html