<!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>Object definition and sky subtraction</TITLE> <META NAME="description" CONTENT="Object definition 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="node695.html"> <LINK REL="previous" HREF="node689.html"> <LINK REL="up" HREF="node686.html"> <LINK REL="next" HREF="node695.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html10164" HREF="node695.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html10161" HREF="node686.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html10155" HREF="node693.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html10163" 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="tex2html10165" HREF="node695.html">Object extraction</A> <B> Up:</B> <A NAME="tex2html10162" HREF="node686.html">MOS Cookbook - A</A> <B> Previous:</B> <A NAME="tex2html10156" HREF="node693.html">Rebinning</A> <BR> <BR> <!--End of Navigation Panel--> <H2><A NAME="SECTION003674000000000000000"> Object definition and sky subtraction</A> </H2> <P> To define your object's and sky regions use <P> <BR> <BR> Midas ...> <TT>DEFINE/MOS</TT> <P> <BR> <BR> By default it automatically averages <!-- MATH: $\fbox{{\small \tt XBIN}}$ --> <IMG WIDTH="56" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1387.gif" ALT="\fbox{{\small \tt XBIN}}"> (<B>20</B>) columns around the position <!-- MATH: $\fbox{{\small \tt SCAN\_POS}}$ --> <IMG WIDTH="95" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1388.gif" ALT="\fbox{{\small \tt SCAN\_POS}}"> (<B>0=center of frame</B>) of the frame <!-- MATH: $\fbox{{\small \tt OBJ}}$ --> <IMG WIDTH="45" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1389.gif" ALT="\fbox{{\small \tt OBJ}}">and searches for objects in this averaged frame that have an intensity above <!-- MATH: $\fbox{{\small \tt THRESH}}$ --> <IMG WIDTH="77" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1390.gif" ALT="\fbox{{\small \tt THRESH}}"> (<B>-0.04</B>) when compared to a median over <!-- MATH: $\fbox{{\small \tt WIND}}$ --> <IMG WIDTH="56" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1391.gif" ALT="\fbox{{\small \tt WIND}}"> (<B>5</B>) pixels. <!-- MATH: $\fbox{{\small \tt THRESH}}$ --> <IMG WIDTH="77" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1392.gif" ALT="\fbox{{\small \tt THRESH}}"> can be absolute (> 0) or relative to the median intensity (< 0). It searches the slitlets that are defined by <!-- MATH: $\fbox{{\small \tt MOS}}$ --> <IMG WIDTH="45" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1393.gif" ALT="\fbox{{\small \tt MOS}}">.tbl (<B>mos</B>) and stores the results in <!-- MATH: $\fbox{{\small \tt WINDOWS}}$ --> <IMG WIDTH="87" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1394.gif" ALT="\fbox{{\small \tt WINDOWS}}">.tbl (<B>window</B>). The plot option (<B>0</B>) defines whether you get a two-dimensional display of the result, a graphical plot, both, or nothing (default). <P> If an object is detected a gaussian is fitted to its spatial profile and the limits of the object's region are defined at those pixels where the gaussian fit has reached <!-- MATH: $\fbox{{\small \tt INT\_LIM}}$ --> <IMG WIDTH="84" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1395.gif" ALT="\fbox{{\small \tt INT\_LIM}}"> (<B>0.001</B>) of the central intensity. On both sides of each object a safety margin of 3 pixels is established (can be overridden manually later); the remaining slitlet is defined as sky region. It may be advisable to perform a very crude sky subtraction first to get rid of the sky continuum intensity. This can be done with <TT>SKYFIT/MOS</TT> with <TT>skymet=nowindows</TT>. This command determines the median value along the columns over the slitlets. If you choose this way you should use an absolute threshold for object detection. If you want to search for all objects at the same wavelength region you have to rebin your frame first to constant wavelength steps with <TT>REBIN/MOS</TT>. <P> After the automatic definition of objects' and sky regions you are asked if you are satisfied with the results. If you are not (or are not yet sure), you answer 'no' and can now inspect the results more closely in the graphical plot and also change the results manually. You may also start with the interactive definition using <TT>DEFWIN/MOS</TT>. <P> The results are stored in table <!-- MATH: $\fbox{{\small \tt WINDOWS}}$ --> <IMG WIDTH="87" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1396.gif" ALT="\fbox{{\small \tt WINDOWS}}"> (<B>windows</B>): <P><P> <BR> <P> <TABLE CELLPADDING=3 BORDER="1"> <TR><TD ALIGN="LEFT"><TT> :Obj_Slit</TT></TD> <TD ALIGN="LEFT">number of slitlet for object</TD> </TR> <TR><TD ALIGN="LEFT"><TT> :Obj_Strt</TT></TD> <TD ALIGN="LEFT">first row of object</TD> </TR> <TR><TD ALIGN="LEFT"><TT> :Obj_End</TT></TD> <TD ALIGN="LEFT">last row of object</TD> </TR> <TR><TD ALIGN="LEFT"><TT> :Sky_Slit</TT></TD> <TD ALIGN="LEFT">number of slitlet for sky</TD> </TR> <TR><TD ALIGN="LEFT"><TT> :Sky_Strt</TT></TD> <TD ALIGN="LEFT">first row of sky</TD> </TR> <TR><TD ALIGN="LEFT"><TT> :Sky_End</TT></TD> <TD ALIGN="LEFT">last row of sky</TD> </TR> </TABLE> <BR> <BR> <BR> <BR> <P> The command <TT>SKYFIT/MOS</TT> is used to fit the sky. Normally, the sky regions are taken from the table <!-- MATH: $\fbox{{\small \tt WINDOWS}}$ --> <IMG WIDTH="87" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1397.gif" ALT="\fbox{{\small \tt WINDOWS}}">.tbl. Several sky windows may be defined in each slitlets. To fit the sky background you may use the median along the sky regions (<TT>skymet=median</TT>) or a polynomial fit (<TT>skymet=polynomial</TT>). Method polynomial requires rejection of cosmic rays and bad pixels. These pixels are rejected by <TT>SKYFIT/MOS</TT> before fitting the data. Read out noise (in electrons), gain (electrons/adu) and the rejection criterion in units of <IMG WIDTH="19" HEIGHT="21" ALIGN="BOTTOM" BORDER="0" SRC="img1398.gif" ALT="$\sigma$"> must be specified in keywords <!-- MATH: $\fbox{{\small \tt CCDPAR}}$ --> <IMG WIDTH="77" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1399.gif" ALT="\fbox{{\small \tt CCDPAR}}"> and <!-- MATH: $\fbox{{\small \tt REJTHRES}}$ --> <IMG WIDTH="98" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1400.gif" ALT="\fbox{{\small \tt REJTHRES}}"> (<B>3</B>). <P> <BR> <BR> Midas ...> <TT>SKYFIT/MOS ? ? ? ? 3 poly 8,2.3,3</TT> <P> <BR> <BR> does a polynomial fit of 3<IMG WIDTH="22" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1401.gif" ALT="$^{\rm rd}$"> order in the frame <!-- MATH: $\fbox{{\small \tt OBJ}}$ --> <IMG WIDTH="45" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1402.gif" ALT="\fbox{{\small \tt OBJ}}">.bdf, in the regions defined by <!-- MATH: $\fbox{{\small \tt WINDOWS}}$ --> <IMG WIDTH="87" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1403.gif" ALT="\fbox{{\small \tt WINDOWS}}">.tbl (within the slitlet limits listed in <!-- MATH: $\fbox{{\small \tt MOS.tbl}}$ --> <IMG WIDTH="87" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1404.gif" ALT="\fbox{{\small \tt MOS.tbl}}">) and stores the result in <!-- MATH: $\fbox{{\small \tt SKYFRAME}}$ --> <IMG WIDTH="98" HEIGHT="26" ALIGN="BOTTOM" BORDER="0" SRC="img1405.gif" ALT="\fbox{{\small \tt SKYFRAME}}"> (<B>sky</B>).bdf. <P> <HR> <!--Navigation Panel--> <A NAME="tex2html10164" HREF="node695.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html10161" HREF="node686.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html10155" HREF="node693.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html10163" 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="tex2html10165" HREF="node695.html">Object extraction</A> <B> Up:</B> <A NAME="tex2html10162" HREF="node686.html">MOS Cookbook - A</A> <B> Previous:</B> <A NAME="tex2html10156" HREF="node693.html">Rebinning</A> <!--End of Navigation Panel--> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>