<!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>Overscan correction and trimming</TITLE> <META NAME="description" CONTENT="Overscan correction and trimming"> <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="node54.html"> <LINK REL="previous" HREF="node52.html"> <LINK REL="up" HREF="node52.html"> <LINK REL="next" HREF="node54.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html2100" HREF="node54.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html2097" HREF="node52.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html2091" HREF="node52.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html2099" 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="tex2html2101" HREF="node54.html">Bias, Dark and Flat</A> <B> Up:</B> <A NAME="tex2html2098" HREF="node52.html">Running REDUCE/CCD</A> <B> Previous:</B> <A NAME="tex2html2092" HREF="node52.html">Running REDUCE/CCD</A> <BR> <BR> <!--End of Navigation Panel--> <H3><A NAME="SECTION00692100000000000000"> </A> <A NAME="overscan-and-trimming"> </A> <BR> Overscan correction and trimming </H3> If the keyword <TT>SC_SCAN</TT> is set the first action will be to determine the bias offset. For that you can use the overscan region on the chip. This possibility is certainly helpful in case you did not obtain separate bias calibration frames or in case you want to determine the bias offset for each science frame individually. <P> At initialization, keywords are created to contain the overscan and image areas: <TT>OV_SEC</TT> and <TT>IM_SEC</TT>, respectively. However they are not filled yet. You can find these numbers using a number of standard MIDAS core commands to display one or more frames, or to plot a number of lines or columns. Then use the command <TT>SET/CCD</TT> to fill in or to adjust the keywords. <P> The basic command that does the overscan fitting is called <TT>OVERSCAN/CCD</TT>. It needs at least two input parameters: the input and output frame. A third parameter, the overscan area, may be added. If absent it will be taken from the CCD keyword <TT>OV_SEC</TT>. To determine the bias offset a number of CCD keywords, all starting with `<TT>OV_</TT>' will be read, and hence have to be filled correctly. They are displayed in the Table <A HREF="node53.html#ccd:overscan-keywords">3.8</A> <BR> <DIV ALIGN="CENTER"><A NAME="1956"> </A> <TABLE> <CAPTION><STRONG>Table 3.8:</STRONG> CCD keywords for overscan fitting</CAPTION> <TR><TD> <DIV ALIGN="CENTER"> <TABLE CELLPADDING=3 BORDER="1" ALIGN="CENTER"> <TR><TH ALIGN="LEFT"><B>Keyword </B></TH> <TH ALIGN="LEFT"><B>Options </B></TH> <TH ALIGN="LEFT"><B>Default </B></TH> <TH ALIGN="LEFT"><B>Description </B></TH> </TR> <TR><TD ALIGN="LEFT"><TT>OV_SEC </TT></TD> <TD ALIGN="LEFT">coord.</TD> <TD ALIGN="LEFT">?</TD> <TD ALIGN="LEFT">area to be used for fitting</TD> </TR> <TR><TD ALIGN="LEFT"><TT>OV_IMODE </TT></TD> <TD ALIGN="LEFT">yes|no</TD> <TD ALIGN="LEFT">yes</TD> <TD ALIGN="LEFT">interactive fitting?</TD> </TR> <TR><TD ALIGN="LEFT"><TT>OV_FUNCT </TT></TD> <TD ALIGN="LEFT">lin|pol</TD> <TD ALIGN="LEFT">lin</TD> <TD ALIGN="LEFT">type of function to fit</TD> </TR> <TR><TD ALIGN="LEFT"><TT>OV_ORDER </TT></TD> <TD ALIGN="LEFT">integer</TD> <TD ALIGN="LEFT">3</TD> <TD ALIGN="LEFT">order of fit (polyn. fit only)</TD> </TR> <TR><TD ALIGN="LEFT"><TT>OV_AVER </TT></TD> <TD ALIGN="LEFT">integer</TD> <TD ALIGN="LEFT">1</TD> <TD ALIGN="LEFT">number of points to combine</TD> </TR> <TR><TD ALIGN="LEFT"><TT>OV_ITER </TT></TD> <TD ALIGN="LEFT">integer</TD> <TD ALIGN="LEFT">1</TD> <TD ALIGN="LEFT">max. number of iterations</TD> </TR> <TR><TD ALIGN="LEFT"><TT>OV_REJEC </TT></TD> <TD ALIGN="LEFT">real</TD> <TD ALIGN="LEFT">3</TD> <TD ALIGN="LEFT">low/high rejection factor</TD> </TR> </TABLE></DIV> <A NAME="ccd:overscan-keywords"> </A> </TD></TR> </TABLE> </DIV> <BR> <P> Depending on the readout direction of the CCD (keyword <TT>DIRECT</TT>) the command will average the data in the overscan region, and fit these values as a function of line-number (<I>i.e.</I> average in the `x' direction within the overscan region, and fit these as function of `y'). The fit will be subtracted from each column (row) in your frame. In case of a zero order fit the correction is a simple constant. In the other case it will be one dimensional array. <P> After an interactive fit has been completed the command will ask you if you want to fit the next frame interactively as well, or if you want to apply the overscan correction to all subsequent frames. In the latter case the overscan correction will be stored as a separate calibration frame or, in case a zero order fit has been used, a single constant. <P> After the overscan correction is determined, the frames can be trimmed to keep only those parts of the frames that contain valid data. Obviously, this action will speed up the data processing of the subsequent reduction steps. The relevant keyword is <TT>SC_TRIM</TT> (default value <TT>yes</TT>) and <TT>IM_SEC</TT>, containing the data section of the frame. <P> <HR> <!--Navigation Panel--> <A NAME="tex2html2100" HREF="node54.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html2097" HREF="node52.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html2091" HREF="node52.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html2099" 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="tex2html2101" HREF="node54.html">Bias, Dark and Flat</A> <B> Up:</B> <A NAME="tex2html2098" HREF="node52.html">Running REDUCE/CCD</A> <B> Previous:</B> <A NAME="tex2html2092" HREF="node52.html">Running REDUCE/CCD</A> <!--End of Navigation Panel--> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>
