<!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>Combining Dark Frames</TITLE> <META NAME="description" CONTENT="Combining Dark Frames"> <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="node47.html"> <LINK REL="previous" HREF="node45.html"> <LINK REL="up" HREF="node42.html"> <LINK REL="next" HREF="node47.html"> </HEAD> <BODY > <!--Navigation Panel--> <A NAME="tex2html2021" HREF="node47.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html2018" HREF="node42.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html2012" HREF="node45.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html2020" 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="tex2html2022" HREF="node47.html">Combining Flat Fields</A> <B> Up:</B> <A NAME="tex2html2019" HREF="node42.html">Preparing Your Calibration Frames</A> <B> Previous:</B> <A NAME="tex2html2013" HREF="node45.html">Combining Bias Frames</A> <BR> <BR> <!--End of Navigation Panel--> <H2><A NAME="SECTION00684000000000000000"> </A> <A NAME="ccd:combining-dark-frames"> </A> <BR> Combining Dark Frames </H2> Similarly to the bias combination one can obtain an average dark current frame. However, one should consider whether the dark correction should really be applied: one needs a reasonable number of dark frames in order not to degrade the S/N, and obviously this takes telescope time. Because the dark level depends on the exposure time, weighting the input dark frames should be considered. Another possibility would be simply to take the average dark value and to scale that number with the exposure time. Filter type is not important. Table <A HREF="node46.html#ccd:dark-keywords">3.5</A> shows the default setting for combining dark frames. <P> <BR> <DIV ALIGN="CENTER"><A NAME="1848"> </A> <TABLE> <CAPTION><STRONG>Table 3.5:</STRONG> Keywords for combining dark calibration frames</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>Content </B></TH> <TH ALIGN="LEFT"><B>Default </B></TH> <TH ALIGN="LEFT"><B>Description </B></TH> </TR> <TR><TD ALIGN="LEFT"><TT> CCD_IN</TT></TD> <TD ALIGN="LEFT">input name</TD> <TD ALIGN="LEFT">?</TD> <TD ALIGN="LEFT">table of input frames</TD> </TR> <TR><TD ALIGN="LEFT"><TT> DK_TYP</TT></TD> <TD ALIGN="LEFT">descriptor value</TD> <TD ALIGN="LEFT">*</TD> <TD ALIGN="LEFT">exposure type to check, if any</TD> </TR> <TR><TD ALIGN="LEFT"><TT> DK_SIG</TT></TD> <TD ALIGN="LEFT">yes|no</TD> <TD ALIGN="LEFT">no</TD> <TD ALIGN="LEFT">create sigma image</TD> </TR> <TR><TD ALIGN="LEFT"><TT> DK_MET</TT></TD> <TD ALIGN="LEFT">comb. method</TD> <TD ALIGN="LEFT">avsigcl</TD> <TD ALIGN="LEFT">type of combining operation</TD> </TR> <TR><TD ALIGN="LEFT"><TT> DK_DEL</TT></TD> <TD ALIGN="LEFT">yes|no</TD> <TD ALIGN="LEFT">no</TD> <TD ALIGN="LEFT">delete cal. frames afterwards</TD> </TR> <TR><TD ALIGN="LEFT"><TT> DK_STA</TT></TD> <TD ALIGN="LEFT">mean|median|mode</TD> <TD ALIGN="LEFT">mode</TD> <TD ALIGN="LEFT">correct. by Mode/Median/Mean</TD> </TR> <TR><TD ALIGN="LEFT"><TT> DK_EXP</TT></TD> <TD ALIGN="LEFT">yes|no</TD> <TD ALIGN="LEFT">yes</TD> <TD ALIGN="LEFT">scale by exposure time</TD> </TR> <TR><TD ALIGN="LEFT"><TT> DK_SCA</TT></TD> <TD ALIGN="LEFT">yes|no</TD> <TD ALIGN="LEFT">no</TD> <TD ALIGN="LEFT">scale by MMM</TD> </TR> <TR><TD ALIGN="LEFT"><TT> DK_OFF</TT></TD> <TD ALIGN="LEFT">yes|no</TD> <TD ALIGN="LEFT">no</TD> <TD ALIGN="LEFT">add offset obtained from MMM</TD> </TR> <TR><TD ALIGN="LEFT"><TT> DK_WEI</TT></TD> <TD ALIGN="LEFT">yes|no</TD> <TD ALIGN="LEFT">yes</TD> <TD ALIGN="LEFT">use a weighted average</TD> </TR> <TR><TD ALIGN="LEFT"><TT> DK_SEC</TT></TD> <TD ALIGN="LEFT">area</TD> <TD ALIGN="LEFT">[<,<:>,>]</TD> <TD ALIGN="LEFT">area for computing MMM</TD> </TR> <TR><TD ALIGN="LEFT"><TT> DK_RAN</TT></TD> <TD ALIGN="LEFT">real,real</TD> <TD ALIGN="LEFT">-99999,99999</TD> <TD ALIGN="LEFT">valid pixel range</TD> </TR> <TR><TD ALIGN="LEFT"><TT> DK_CLP</TT></TD> <TD ALIGN="LEFT">real</TD> <TD ALIGN="LEFT">3.,3.</TD> <TD ALIGN="LEFT">low/high sigma clipping factor</TD> </TR> <TR><TD ALIGN="LEFT"><TT> DK_NUL</TT></TD> <TD ALIGN="LEFT">real</TD> <TD ALIGN="LEFT"><TT> NULL(2)</TT></TD> <TD ALIGN="LEFT">value for rejections</TD> </TR> </TABLE></DIV> <A NAME="ccd:dark-keywords"> </A> </TD></TR> </TABLE> </DIV> <BR> <P> If the bias is stable enough to allow taking averages, one might argue that it is not really needed. In that case, provided a good linearity of the CCD, one could do with the subtraction of an average dark frame. If the bias is unstable, one should be careful with simply combining bias frames. In that case the better solution might be an average of two bias frames, taken just before and after each flat or sky exposure. <P> <HR> <!--Navigation Panel--> <A NAME="tex2html2021" HREF="node47.html"> <IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="icons.gif/next_motif.gif"></A> <A NAME="tex2html2018" HREF="node42.html"> <IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="icons.gif/up_motif.gif"></A> <A NAME="tex2html2012" HREF="node45.html"> <IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="icons.gif/previous_motif.gif"></A> <A NAME="tex2html2020" 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="tex2html2022" HREF="node47.html">Combining Flat Fields</A> <B> Up:</B> <A NAME="tex2html2019" HREF="node42.html">Preparing Your Calibration Frames</A> <B> Previous:</B> <A NAME="tex2html2013" HREF="node45.html">Combining Bias Frames</A> <!--End of Navigation Panel--> <ADDRESS> <I>Petra Nass</I> <BR><I>1999-06-15</I> </ADDRESS> </BODY> </HTML>
