<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"> <!--Converted with LaTeX2HTML 98.1p1 release (March 2nd, 1998) originally by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds * revised and updated by: Marcus Hennecke, Ross Moore, Herb Swan * with significant contributions from: Jens Lippmann, Marek Rouchal, Martin Wilck and others --> <HTML> <HEAD> <TITLE>Definition of objects and sky subtraction</TITLE> <META NAME="description" CONTENT="Definition of objects and sky subtraction"> <META NAME="keywords" CONTENT="vol2"> <META NAME="resource-type" CONTENT="document"> <META NAME="distribution" CONTENT="global"> <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1"> <LINK REL="STYLESHEET" HREF="vol2.css"> <LINK REL="next" HREF="node684.html"> <LINK REL="previous" HREF="node679.html"> <LINK REL="up" HREF="node676.html"> <LINK REL="next" HREF="node684.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html10034" HREF="node684.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html10031" HREF="node676.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html10025" HREF="node682.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html10033" HREF="node1.html"> <IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="icons.gif/contents_motif.gif"></A> <BR> <B> Next:</B> <A NAME="tex2html10035" HREF="node684.html">Extraction of objects</A> <B> Up:</B> <A NAME="tex2html10032" HREF="node676.html">Multi-Object Spectroscopy</A> <B> Previous:</B> <A NAME="tex2html10026" HREF="node682.html">Fitting the dispersion curve</A> <BR> <BR> <!--End of Navigation Panel--> <H1><A NAME="SECTION003640000000000000000"> Definition of objects and sky subtraction</A> </H1> <P> <TT>DEFINE/MOS</TT> helps you to localize your objects and sky regions and by default works automatically. It averages <!-- MATH: $\fbox{{\small \tt XBIN}}$ --> <IMG WIDTH="56" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1221.gif" ALT="\fbox{{\small \tt XBIN}}"> (<B>20</B>) columns around the position <!-- MATH: $\fbox{{\small \tt SCAN\_POS(1)}}$ --> <IMG WIDTH="126" HEIGHT="29" ALIGN="BOTTOM" BORDER="0" SRC="img1222.gif" ALT="\fbox{{\small \tt SCAN\_POS(1)}}"> (<B>0 = center of frame</B>) (in world-coordinates!). In the target frame the program will detect objects above the threshold ( <!-- MATH: $\fbox{{\small \tt THRESH}}$ --> <IMG WIDTH="77" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1223.gif" ALT="\fbox{{\small \tt THRESH}}">, <B>-0.04</B>, see below) relative to the local background within the search window <!-- MATH: $\fbox{{\small \tt WIND}}$ --> <IMG WIDTH="56" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1224.gif" ALT="\fbox{{\small \tt WIND}}"> (<B>5</B>) and fits a gaussian to the spatial profile of any detected object. The threshold may be given in absolute (> 0.0) or relative (< 0.0) numbers. It may be advisable to do at least a rough sky subtraction ahead of this command to facilitate the detection of the objects. In this case you have to use an absolute threshold for the detection of the object spectra afterwards. One may also think about rebinning the object frame to constant wavelength steps because then the search could be done in the same wavelength region for all slitlets. The limits of the objects are defined at the position where the gaussian fit has reached the detection limit <!-- MATH: $\fbox{{\small \tt INT\_LIM}}$ --> <IMG WIDTH="84" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1225.gif" ALT="\fbox{{\small \tt INT\_LIM}}"> (<B>0.001</B>). A safety margin of 3 pixels is taken on both sides of each object where no sky is automatically defined (can be overridden manually later) and the remaining part of the slitlets is taken as sky region. The results are stored in <!-- MATH: $\fbox{{\small \tt WINDOWS}}$ --> <IMG WIDTH="87" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1226.gif" ALT="\fbox{{\small \tt WINDOWS}}">.tbl (<B>window</B>) and can be displayed in the overlay channel display and/or the graphics window. If you are not satisfied with the results you can change the windows interactively. You may also choose the interactive mode from the very beginning with <TT>DEFINE/WIND</TT>. Then no automatic search is performed; instead you enter the the objects and sky regions for each slitlet by keyboard input. By default the sky region is defined as the complete slitlet. <P> The sky fit methods ( <!-- MATH: $\fbox{{\small \tt SKYMET}}$ --> <IMG WIDTH="77" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1227.gif" ALT="\fbox{{\small \tt SKYMET}}">) available for <TT>SKYFIT/MOS</TT> are a simple median along CCD columns within each slitlet (<TT>skymet=median</TT>) and a more appropriate polynomial fit along the columns (<TT>skymet=polynomial</TT>), respectively. These two methods use only rows marked as sky regions in the table <!-- MATH: $\fbox{{\small \tt WINDOWS}}$ --> <IMG WIDTH="87" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1228.gif" ALT="\fbox{{\small \tt WINDOWS}}">.tbl (<B>window</B>) to fit the sky background. With <TT>skymet=nowindows</TT>, however, the table <!-- MATH: $\fbox{{\small \tt WINDOWS}}$ --> <IMG WIDTH="87" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1229.gif" ALT="\fbox{{\small \tt WINDOWS}}">.tbl is ignored and the sky is determined as a simple median over the full slitlet. The limits of the slitlets are taken in this case from <!-- MATH: $\fbox{{\small \tt MOS}}$ --> <IMG WIDTH="45" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1230.gif" ALT="\fbox{{\small \tt MOS}}">.tbl. This mode may be useful for a preliminary sky determination, before the object positions are known. If no sky regions are marked in some slitlet, the input frame is just copied to the sky frame for this slitlet. In this way, after sky subtraction, the slitlet contains only zeros thereby marking that the sky background is unknown for this slitlet. The keyword <!-- MATH: $\fbox{{\small \tt SKYMET}}$ --> <IMG WIDTH="77" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1231.gif" ALT="\fbox{{\small \tt SKYMET}}"> contains the order of the polynom fit or the width of the median filtering, respectively. If a polynom fit is performed the cosmics must be rejected. <TT>SKYFIT/MOS</TT> rejects (but not replaces) pixels that exceed a given limit before the fit is performed. Read out noise, gain and the detection limit (in units of <IMG WIDTH="19" HEIGHT="21" ALIGN="BOTTOM" BORDER="0" SRC="img1232.gif" ALT="$\sigma$">) must be given by keywords <!-- MATH: $\fbox{{\small \tt SKYMET(1),SKYMET(2),REJTHRES}}$ --> <IMG WIDTH="307" HEIGHT="30" ALIGN="BOTTOM" BORDER="0" SRC="img1233.gif" ALT="\fbox{{\small \tt SKYMET(1),SKYMET(2),REJTHRES}}">. <P> <HR> <!--Navigation Panel--> <A NAME="tex2html10034" HREF="node684.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html10031" HREF="node676.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html10025" HREF="node682.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html10033" HREF="node1.html"> <IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="icons.gif/contents_motif.gif"></A> <BR> <B> Next:</B> <A NAME="tex2html10035" HREF="node684.html">Extraction of objects</A> <B> Up:</B> <A NAME="tex2html10032" HREF="node676.html">Multi-Object Spectroscopy</A> <B> Previous:</B> <A NAME="tex2html10026" HREF="node682.html">Fitting the dispersion curve</A> <!--End of Navigation Panel--> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>