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<!--Table of Child-Links-->
<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></A>

<UL CLASS="ChildLinks">
<LI><A NAME="tex2html624"
  HREF="node8.html#SECTION00810000000000000000">Measuring Apertures</A>
<LI><A NAME="tex2html625"
  HREF="node8.html#SECTION00820000000000000000">Sky Estimation</A>
<UL>
<LI><A NAME="tex2html626"
  HREF="node8.html#SECTION00821000000000000000">Region Growing</A>
</UL>
<BR>
<LI><A NAME="tex2html627"
  HREF="node8.html#SECTION00830000000000000000">Placing the Apertures</A>
<LI><A NAME="tex2html628"
  HREF="node8.html#SECTION00840000000000000000">Finding the Ensemble Photometry Solution</A>
<LI><A NAME="tex2html629"
  HREF="node8.html#SECTION00850000000000000000">Annotations</A>
<LI><A NAME="tex2html630"
  HREF="node8.html#SECTION00860000000000000000">Running Aperture Photometry</A>
<LI><A NAME="tex2html631"
  HREF="node8.html#SECTION00870000000000000000">Creating Recipe Files</A>
<UL>
<LI><A NAME="tex2html632"
  HREF="node8.html#SECTION00871000000000000000">Target Stars</A>
<LI><A NAME="tex2html633"
  HREF="node8.html#SECTION00872000000000000000">Standard Stars</A>
<LI><A NAME="tex2html634"
  HREF="node8.html#SECTION00873000000000000000">Creating the Recipe File</A>
<LI><A NAME="tex2html635"
  HREF="node8.html#SECTION00874000000000000000">Working without an Image Frame</A>
<LI><A NAME="tex2html636"
  HREF="node8.html#SECTION00875000000000000000">Creating Recipies from the Command Line</A>
</UL></UL>
<!--End of Table of Child-Links-->
<HR>

<H1><A NAME="SECTION00800000000000000000"></A><A NAME="ch:aphot"></A>
<BR>
Aperture Photometry
</H1>

<P>
The basic funtion of the aperture photometry routine in <SMALL>GCX </SMALL>
is to measure the flux of a number of stars in the image (which will
be expressed as an <EM>instrumental magnitude</EM>), and estimate
it's expected error. 

<P>
As an additional function, if some of the stars are <EM>standard
  stars</EM> of known magnitude the program will calculate the standard
magnitude of the measured stars using the standard stars as a
reference (ensemble photometry).<A NAME="tex2html30"
  HREF="footnode.html#foot658"><SUP><SPAN CLASS="arabic">7</SPAN>.<SPAN CLASS="arabic">1</SPAN></SUP></A>
<P>

<H1><A NAME="SECTION00810000000000000000">
Measuring Apertures</A>
</H1>

<P>
To measure the flux of a star, we add together the intensity values
from a circular region around the target star (the central aperture), 
and subtract the
estimated background contribution. The background is estimated from
values in a annular region surrounding the star at a distance.

<P>
Normally, we choose the size of the central aperture to be large
enough to include most of the star image. Common ranges are between 
3 and 5 times the FWHM of the star image.<A NAME="tex2html31"
  HREF="footnode.html#foot660"><SUP><SPAN CLASS="arabic">7</SPAN>.<SPAN CLASS="arabic">2</SPAN></SUP></A>
<P>
The annular sky aperture is chosen to be far enough from the star so
that it includes an insignificant amount of flux from it. The default
values of the measurement aperture radiuses are 6 pixels for the
central aperture and 9/13 pixels for the sky aperture. These values
are appropiate for star images between 2.5 and 3.5 pixels FWHM. If one obtains
consistently tighter star images, reducing the central aperture would
help improve the SNR of faint stars. The three radiuses are specified
by options under <EM>Aperture Photometry Options</EM>.

<P>
Assuming there are <SPAN CLASS="MATH"><IMG
 WIDTH="20" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
 SRC="img31.png"
 ALT="$N$"></SPAN> pixels inside the central aperture, the total
flux (star + background) is:
<BR>
<DIV ALIGN="RIGHT" CLASS="mathdisplay">

<!-- MATH
 \begin{equation}
F_T = \sum_i I_i
\end{equation}
 -->
<TABLE WIDTH="100%" ALIGN="CENTER">
<TR VALIGN="MIDDLE"><TD ALIGN="CENTER" NOWRAP><IMG
 WIDTH="82" HEIGHT="48" BORDER="0"
 SRC="img58.png"
 ALT="\begin{displaymath}
F_T = \sum_i I_i
\end{displaymath}"></TD>
<TD CLASS="eqno" WIDTH=10 ALIGN="RIGHT">
(<SPAN CLASS="arabic">7</SPAN>.<SPAN CLASS="arabic">1</SPAN>)</TD></TR>
</TABLE>
<BR CLEAR="ALL"></DIV><P></P>
Where the sumation is taken over the pixels in the central
aperture. If <SPAN CLASS="MATH"><IMG
 WIDTH="18" HEIGHT="20" ALIGN="BOTTOM" BORDER="0"
 SRC="img27.png"
 ALT="$\widetilde{B}$"></SPAN> is the estimated background level, the
star's flux is taken to be:
<BR>
<DIV ALIGN="RIGHT" CLASS="mathdisplay">

<!-- MATH
 \begin{equation}
F = \sum_i I_i - N \widetilde{B}
\end{equation}
 -->
<TABLE WIDTH="100%" ALIGN="CENTER">
<TR VALIGN="MIDDLE"><TD ALIGN="CENTER" NOWRAP><IMG
 WIDTH="127" HEIGHT="48" BORDER="0"
 SRC="img59.png"
 ALT="\begin{displaymath}
F = \sum_i I_i - N \widetilde{B}
\end{displaymath}"></TD>
<TD CLASS="eqno" WIDTH=10 ALIGN="RIGHT">
(<SPAN CLASS="arabic">7</SPAN>.<SPAN CLASS="arabic">2</SPAN>)</TD></TR>
</TABLE>
<BR CLEAR="ALL"></DIV><P></P>
and the star's instrumental magnitude is:
<BR>
<DIV ALIGN="RIGHT" CLASS="mathdisplay">

<!-- MATH
 \begin{equation}
M_I = -2.511886\log\left(\sum_i I_i - N \widetilde{B}\right)
\end{equation}
 -->
<TABLE WIDTH="100%" ALIGN="CENTER">
<TR VALIGN="MIDDLE"><TD ALIGN="CENTER" NOWRAP><IMG
 WIDTH="268" HEIGHT="56" BORDER="0"
 SRC="img60.png"
 ALT="\begin{displaymath}
M_I = -2.511886\log\left(\sum_i I_i - N \widetilde{B}\right)
\end{displaymath}"></TD>
<TD CLASS="eqno" WIDTH=10 ALIGN="RIGHT">
(<SPAN CLASS="arabic">7</SPAN>.<SPAN CLASS="arabic">3</SPAN>)</TD></TR>
</TABLE>
<BR CLEAR="ALL"></DIV><P></P>

<P>
The estimated error of the instrumental magnitude is calculated taking
most known random error sources into account. A detailed description
of the error model and the way the instrumental magnitude error is
calculated can be found in Appendix&nbsp;<A HREF="node10.html#ap:noise">A</A>.

<P>

<H1><A NAME="SECTION00820000000000000000">
Sky Estimation</A>
</H1>

<P>
To calculate the instrumental magnitude above we used an estimate of
the sky background near the star. This value is calculated from the
pixels in the annular ring.

<P>
Given the relatively large size of the sky annulus, it is very
likely that we will find unwanted stars in at least some of the 
annuli. We must therefore use a robust algorithm to obtain the
expected sky value. 

<P>
The program offers a number of algorithms: <EM>average, median,
  mean-median, <SPAN CLASS="MATH"><IMG
 WIDTH="14" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
 SRC="img61.png"
 ALT="$\kappa$"></SPAN>-<SPAN CLASS="MATH"><IMG
 WIDTH="15" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
 SRC="img62.png"
 ALT="$\sigma$"></SPAN> and synthetic mode</EM>. The first four 
are described in Section&nbsp;<A HREF="node7.html#sec:combining">6.5</A>.<A NAME="tex2html32"
  HREF="footnode.html#foot675"><SUP><SPAN CLASS="arabic">7</SPAN>.<SPAN CLASS="arabic">3</SPAN></SUP></A>
<P>
It is generally not recommended to use average, as it is not robust. 
The others, while not having a problem with robustness, will not
produce the best estimate (which is the <EM>mode</EM><A NAME="tex2html33"
  HREF="footnode.html#foot677"><SUP><SPAN CLASS="arabic">7</SPAN>.<SPAN CLASS="arabic">4</SPAN></SUP></A> of the sky annulus pixel values) when the distribution of the sky
 values is skewed. In this case (which arises whenever the sky level is
 relatively low), the synthetic mode is the best algorithm.
The synthetic mode is calculated as follows:

<P>
The histogram of the sky values is created. Then, the histogram is
clipped using a <SPAN CLASS="MATH"><IMG
 WIDTH="14" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
 SRC="img61.png"
 ALT="$\kappa$"></SPAN>-<SPAN CLASS="MATH"><IMG
 WIDTH="15" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
 SRC="img62.png"
 ALT="$\sigma$"></SPAN> algorithm in order to eliminate
the effect of unwanted stars and other defects. The mean and median of
the clipped histogram are computed, and the synthetic mode is defined
as:
<BR>
<DIV ALIGN="RIGHT" CLASS="mathdisplay">

<!-- MATH
 \begin{equation}
\widetilde{B} = {\rm mode} = 3 \cdot {\rm median} - 2\cdot {\rm mean}
\end{equation}
 -->
<TABLE WIDTH="100%" ALIGN="CENTER">
<TR VALIGN="MIDDLE"><TD ALIGN="CENTER" NOWRAP><IMG
 WIDTH="261" HEIGHT="28" BORDER="0"
 SRC="img63.png"
 ALT="\begin{displaymath}
\widetilde{B} = {\rm mode} = 3 \cdot {\rm median} - 2\cdot {\rm mean}
\end{displaymath}"></TD>
<TD CLASS="eqno" WIDTH=10 ALIGN="RIGHT">
(<SPAN CLASS="arabic">7</SPAN>.<SPAN CLASS="arabic">4</SPAN>)</TD></TR>
</TABLE>
<BR CLEAR="ALL"></DIV><P></P>
If the distribution is not skewed, the mean equals the median,
and the mode would be equal to both. 

<P>
The desired sky estimation algorithm is selected by the <EM>Aperture
  Photometry Options/Sky method</EM> option. The rejection band for the
  <SPAN CLASS="MATH"><IMG
 WIDTH="14" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
 SRC="img61.png"
 ALT="$\kappa$"></SPAN>-<SPAN CLASS="MATH"><IMG
 WIDTH="15" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
 SRC="img62.png"
 ALT="$\sigma$"></SPAN> and synthetic mode algorithms is set by 
<EM>Aperture Photometry Options/Sigmas</EM>. 

<P>
Methods that rely on outlier rejection work best when a relatively
large sky aperture is used. 

<P>

<H2><A NAME="SECTION00821000000000000000">
Region Growing</A>
</H2>

<P>
Spurious stars in the sky annulus present a problem. While their peaks
are easily rejected by the clipping algorithm, their ``tails'' are
not, and will affect the estimated sky value. To avoid this, <SMALL>GCX </SMALL>can
grow the area of rejected pixels, by including all pixels within a
given radius of an outlier. This option can be enabled by setting the
<EM>Aperture Photometry Options/Region growing</EM> option to a value
larger than 0. Region growing is only applied when the
<SPAN CLASS="MATH"><IMG
 WIDTH="14" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
 SRC="img61.png"
 ALT="$\kappa$"></SPAN>-<SPAN CLASS="MATH"><IMG
 WIDTH="15" HEIGHT="15" ALIGN="BOTTOM" BORDER="0"
 SRC="img62.png"
 ALT="$\sigma$"></SPAN> and synthetic mode algorithms are used.

<P>

<H1><A NAME="SECTION00830000000000000000">
Placing the Apertures</A>
</H1>

<P>
In <SMALL>GCX </SMALL>all photometry targets are specified using their world coordinates (right ascension, 
declination and epoch). The targets and standards are generally taken
from a particular star file called a <EM>recipe file</EM>. The WCS of the
frame is fitted, then the coordinates of the standards and targets are
transformed to frame coordinates. The resulting positions are used as
initial positions for the measuring apertures.

<P>
If the <EM>Aperture Photometry Options/Center apertures</EM> option is set
the program will try to detect stars in the immediate vicinity of the
initial positions, and center the apertures on the detected stars. The
maximum distance from the initial position to the detected star is
specified by <EM>Aperture Photometry Options/Max centering error</EM>. If
this value is exceeded, the star is marked with the <EM>not found</EM>
flag and the aperture is not moved. Otherwise it is marked with the
<EM>centered</EM> flag.

<P>
If the apertures were centered, the amount by which each star was moved
is indicated by a line extending from the center of the star symbol in
the direction in which the star was moved. The length of the line is
a factor of <EM>Star Display Options/Plot error scale</EM> longer than the 
star's displacement.

<P>

<H1><A NAME="SECTION00840000000000000000">
Finding the Ensemble Photometry Solution</A>
</H1>

<P>
If we have the instrumental magnitudes of the target stars and at
least one standard star, we can calculate the standard magnitude of
our targets<A NAME="tex2html34"
  HREF="footnode.html#foot1243"><SUP><SPAN CLASS="arabic">7</SPAN>.<SPAN CLASS="arabic">5</SPAN></SUP></A> by simply adding the standard magnitude to the difference
in instrumental magnitudes between the target and the standard. This
is the simplest form of differential photometry.

<P>
We can however obtain significant advantages using more than one
standard in the reduction:

<UL>
<LI>The errors of the standard stars will average out. This reduces
  both the contribution of their instrumental magnitude error and that of their
  standard magnitude error. It also reduces the contribution of the
  conformity error that is caused by the stars having different
  colors.
</LI>
<LI>Even more importantly, using several standards in reduction will
  provide valuable information about the quality of the frame. The
  instrumental magnitudes of the standard stars have to follow their
  standard magnitudes within limits set by the expected errors. If
  this doesn't happen we know that there was a problem with the
  frame. If it does happen, we are almost certain that the frame has
  good data.
</LI>
</UL>

<P>
We try to find the best estimate of the <EM>frame zero point</EM>,
i.e.&nbsp;the value which is added to the instrumental magnitudes to
obtain standard magnitudes. If we had no errors, all the standard
stars' instrumental magnitudes would differ from their standard
magnitudes by exactly the zero point value. This of course is never the
case in practice. The differences will be dispersed above and below
the zero point. We call the difference between a standard star's
standard magnitude and the sum of it's instrumental magnitude and the
zero point the star's <EM>residual</EM>. 

<P>
We want to choose the zero point is such a way that the residuals 
are minimised. More specifically, we try to minimize the sum of the
residuals' squares. It is easy to see that the residuals' sum of
squares is minimised if the zero point is chosen so that the average of
the residuals is zero.

<P>
There are two problems with this approach: First, by using many
standards, we have a good chance that a few of the have ``bad''
values. They could be affected by a cosmic ray hit or a speck of dust
that wasn't there when the flat was taken, or the catalog value may be
in error. Or one of the standards may turn out to be variable.
Secondly, if we use both bright and faint standards, the errors of the
brighter ones are known to be lower. We would like the faint stars to
have less influence on the resulting zero point than the bright ones.

<P>
The algorithm used takes care of both these problems. It assigns
weights to each standard star according to it's estimated error, and
iteratively downweights stars that have residuals that are larger than
expected. For a detailed description, see Appendix&nbsp;<A HREF="node11.html#ap:robust">B</A>.

<P>
The algorithm produces it's best estimate of the frame's zero point,
and a ``diagnostic'' value called the <EM>mean error of unit weight</EM>,
usually abbreviated to <EM>meu</EM> or <EM>me1</EM>. The mean error of unit
weight is a number that shows how well the spread of the residuals matches the
estimated errors. It should have a value close to unity. A larger
value shows that we have some error sources we didn't take into
account. A consistently smaller value indicates that our error
estimating parameters are overrated, and the estimated errors are too
large.

<P>
Finally, the standard magnitude of the target stars is calculated by
adding their instrumental magnitude to the estimated zero point. The
error is the quadrature sum of the target's instrumental magnitude
error and the zeropoint error.<A NAME="tex2html35"
  HREF="footnode.html#foot705"><SUP><SPAN CLASS="arabic">7</SPAN>.<SPAN CLASS="arabic">6</SPAN></SUP></A>
<P>

<H1><A NAME="SECTION00850000000000000000"></A><A NAME="sec:annotations"></A>
<BR>
Annotations
</H1>

<P>
The instrumental magnitude obtained is given the name of the filter
the frame was taken with. The filter name is obtained from the <TT>  FILTER</TT> field. If the field is not present, or the <EM>Aperture
  Photometry Options/Force iband</EM> option is set, the filter name is
taken from <EM>Aperture Photometry Options/Instrumental band</EM>.
If any pixel within the central aperture exceeds <EM>Aperture
  Photometry Options/Saturation limit</EM> the star is marked with the
  <EM>bright</EM> flag.

<P>
Relevant information from the fits header and recipe header is carried
on to the observation report. The fields include:
<TABLE CELLPADDING=3>
<TR><TD ALIGN="LEFT">object</TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=255>the name of the target object, taken from the fits
  header or recipe.</TD>
</TR>
<TR><TD ALIGN="LEFT">ra, dec</TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=255>World coordinates of target object.</TD>
</TR>
<TR><TD ALIGN="LEFT">equinox</TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=255>Equinox of world coordinates and star coordintes in
  the report.</TD>
</TR>
<TR><TD ALIGN="LEFT">mjd</TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=255>Modified Julian Date of integration start from <TT>JDATE</TT>
  or <TT>MJD</TT> fits fields.</TD>
</TR>
<TR><TD ALIGN="LEFT">exptime</TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=255>Integration time from the <TT>EXPTIME</TT> field.</TD>
</TR>
<TR><TD ALIGN="LEFT">airmass</TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=255>Frame airmass, from either the <TT>AIRMASS</TT> field or
  calculated from the geographical coordinates and time.</TD>
</TR>
<TR><TD ALIGN="LEFT">aperture</TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=255>Telescope aperture from the <TT>APERT</TT> field.</TD>
</TR>
<TR><TD ALIGN="LEFT">telescope</TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=255>Telescope name from the <TT>TELESCOP</TT> field.</TD>
</TR>
<TR><TD ALIGN="LEFT">filter</TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=255>Filter used from the <TT>FILTER</TT> field or as set by the
  user.</TD>
</TR>
<TR><TD ALIGN="LEFT">latitude</TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=255>Location of the observing site from the <TT>LAT-OBS</TT>
  field.</TD>
</TR>
<TR><TD ALIGN="LEFT">longitude</TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=255>Location of the observing site from the <TT>LONG-OBS</TT>
  field.</TD>
</TR>
<TR><TD ALIGN="LEFT">altitude</TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=255>Altitude of the observing site from the <TT>ALT-OBS</TT>
  field.</TD>
</TR>
<TR><TD ALIGN="LEFT">observer</TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=255>Name of observer from the <TT>OBSERVER</TT> field.</TD>
</TR>
<TR><TD ALIGN="LEFT">sequence</TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=255>A string describing where the sequence in the recipe
  originated, from the <TT>sequence</TT> field of the recipe.</TD>
</TR>
</TABLE>

<P>

<H1><A NAME="SECTION00860000000000000000">
Running Aperture Photometry</A>
</H1>

<P>
To run the aperture photometry routine on a frame, load the frame into
gcx (<EM>File/Open Fits</EM>), then load a recipe file or another star
file that contains standard and target stars.<A NAME="tex2html36"
  HREF="footnode.html#foot1245"><SUP><SPAN CLASS="arabic">7</SPAN>.<SPAN CLASS="arabic">7</SPAN></SUP></A>
<P>
Then fit the frame's WCS using <EM>Wcs/Auto Wcs</EM> and finally run the
aperture photometry routine with <EM>Processing/Aperture Photometry
  to File</EM>. A report file will be created, that lists all the standard and target
stars with their instrumental and standard magnitudes, general
information about the frame and fit information. More details about
the report format can be found in Appendix&nbsp;<A HREF="node12.html#ap:format">C</A>.

<P>
When reducing a large number of frames, it is more convenient to
invoke <SMALL>GCX </SMALL>from the command line, perhaps from a script. To reduce
frame <TT>frame.fits</TT> using the recipe file <TT>vs.rcp</TT> and
append the format at the end of the <TT>rep.out</TT> file, we can use:
<BLOCKQUOTE>
<TT>gcx -P vs.rcp -o rep.out frame.fits</TT>
</BLOCKQUOTE>
In addition, if we have a master dark frame <TT>mdark.fits</TT> and a
master flat frame <TT>mflat.fits</TT>, we can combine CCD reduction for
the frame with the aperture photometry, like this:<A NAME="tex2html37"
  HREF="footnode.html#foot741"><SUP><SPAN CLASS="arabic">7</SPAN>.<SPAN CLASS="arabic">8</SPAN></SUP></A><BLOCKQUOTE>
<TT>gcx -d mdark -f mflat -P vs.rcp -o rep.out frame.fits</TT>
</BLOCKQUOTE>
Aperture photometry reports from several frames can be combined by
simply concatenating the files together. The combined file can be used
for further refining the data reduction with the multi-frame reduction
routine (Chapter&nbsp;<A HREF="node9.html#ch:multiframe">8</A>). 

<P>
Selected information from the (combined) report file can be set out in
a tabular format using the report converter function of <SMALL>GCX.</SMALL> The
format of the table is specified in the <EM>File and Device
  Options/Report converter output format</EM> option. Possible values for
the format are described in Appendix&nbsp;<A HREF="node14.html#ap:repconv">E</A> and the on-line
help. After setting the format,<A NAME="tex2html38"
  HREF="footnode.html#foot1246"><SUP><SPAN CLASS="arabic">7</SPAN>.<SPAN CLASS="arabic">9</SPAN></SUP></A> invoke the report converter using:
<BLOCKQUOTE>
<TT>gcx -T rep.out -o rep.txt</TT>
</BLOCKQUOTE>
Which will convert the report file <TT>rep.out</TT> to a table 
named <TT>rep.txt</TT>.

<P>

<H1><A NAME="SECTION00870000000000000000"></A><A NAME="sec:recipe"></A>
<BR>
Creating Recipe Files
</H1>

<P>
Having a recipe file is central to running aperture photometry in <SMALL>GCX</SMALL>. Fortunately, creating one is relatively straightforward. 

<P>
Let's
create a recipe file for the <TT>uori-v-001.fits.gz</TT>&nbsp; frame, which is included in
the <SMALL>GCX </SMALL>distribution. Open the frame and match the WCS (using
<EM>Wcs/Auto Wcs</EM>). The WCS matching command leaves the GSC field
  stars and the detected stars visible. 

<P>

<H2><A NAME="SECTION00871000000000000000">
Target Stars</A>
</H2>

<P>
First, we add our target: 
select <EM>Stars/Add from Catalog</EM>, and enter it's name at the
prompt (uori). An object symbol will appear on the screen (around the
bright star near the center, which is U Orionis). Select it, and bring
up the star editing dialog using <EM>Stars/Edit</EM> or right-click on
the star and select <EM>Edit Star</EM> from the pop-up menu. Change the
star's type to ``AP Target'' and click <EM>Ok</EM>. The symbol on the
image should change to a big cross, indicating the star is a target.

<P>
If we don't have GCVS installed, we can identify the star from a star
chart and edit a field star or even a detected star and make it the
target. We normally want to change the star's name to something
descriptive and check that the coordinates are correct.

<P>
If there is no star at the desired position (which can happen if we
prepare a recipe for a very faint variable), just edit any star,
change the coordinates to the desired ones and the type to target.
A recipe can have any number of targets; more can be added in
the same way.

<P>

<H2><A NAME="SECTION00872000000000000000">
Standard Stars</A>
</H2>

<P>
Now we need some standard stars. If we have a chart we want to use as
the base of the recipe, we can create it on-screen similarly with the
target (by editing field stars). The difference is that the standards
are marked as ``Standard Star'', and we need to enter their standard
magnitudes. Several magnitudes can be entered in the ``Standard
magnitudes'' field of the edit dialog. A magnitude is given as:
<BLOCKQUOTE>
<TT><SPAN CLASS="MATH"><IMG
 WIDTH="18" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
 SRC="img64.png"
 ALT="$&lt;$"></SPAN>band<SPAN CLASS="MATH"><IMG
 WIDTH="18" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
 SRC="img65.png"
 ALT="$&gt;$"></SPAN>(source)=<SPAN CLASS="MATH"><IMG
 WIDTH="18" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
 SRC="img64.png"
 ALT="$&lt;$"></SPAN>magnitude<SPAN CLASS="MATH"><IMG
 WIDTH="18" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
 SRC="img65.png"
 ALT="$&gt;$"></SPAN>/<SPAN CLASS="MATH"><IMG
 WIDTH="18" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
 SRC="img64.png"
 ALT="$&lt;$"></SPAN>error<SPAN CLASS="MATH"><IMG
 WIDTH="18" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
 SRC="img65.png"
 ALT="$&gt;$"></SPAN></TT>
</BLOCKQUOTE>
where <SPAN CLASS="MATH"><IMG
 WIDTH="18" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
 SRC="img64.png"
 ALT="$&lt;$"></SPAN><EM>band</EM><SPAN CLASS="MATH"><IMG
 WIDTH="18" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
 SRC="img65.png"
 ALT="$&gt;$"></SPAN> is the name of the standard band,<A NAME="tex2html39"
  HREF="footnode.html#foot763"><SUP><SPAN CLASS="arabic">7</SPAN>.<SPAN CLASS="arabic">10</SPAN></SUP></A> <EM>(source)</EM> is an optional
field describing where the magnitude value originated, <SPAN CLASS="MATH"><IMG
 WIDTH="18" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
 SRC="img64.png"
 ALT="$&lt;$"></SPAN><EM>  magnitude</EM><SPAN CLASS="MATH"><IMG
 WIDTH="18" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
 SRC="img65.png"
 ALT="$&gt;$"></SPAN> and <SPAN CLASS="MATH"><IMG
 WIDTH="18" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
 SRC="img64.png"
 ALT="$&lt;$"></SPAN><EM>error</EM><SPAN CLASS="MATH"><IMG
 WIDTH="18" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
 SRC="img65.png"
 ALT="$&gt;$"></SPAN> are the star's magnitude and
error, respectively. The error field is optional, and it's absence
means that we don't know the error of the magnitude. A few examples of
magnitude entries are:

<P>

<P></P>
<DIV ALIGN="CENTER">
<TABLE CELLPADDING=3>
<TR><TD ALIGN="LEFT"><TT>v(aavso)=12.5</TT></TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=184>A typical example for a value taken from a paper
  aavso chart; the error is unknown.</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT>v=12.53/0.05 ic=11.2/0.03</TT></TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=184>A star for which we know the
magnitudes in two bands.</TD>
</TR>
<TR><TD ALIGN="LEFT"><TT>b=13.2 v=12.7/0.1 r=12.2</TT></TD>
<TD ALIGN="LEFT" VALIGN="TOP" WIDTH=184>We know three magnitudes, but only one error.</TD>
</TR>
</TABLE>
</DIV>

<P>
Another way to get standard stars is to use the tycho catalog. Remove
all field stars, then select <EM>File/Load Field Stars/From Tycho2
  Catalog</EM>. 
The Tycho stars will show up as field stars. All we have to do is to 
mark the ones that we want to use as standards. 

<P>

<H2><A NAME="SECTION00873000000000000000">
Creating the Recipe File</A>
</H2>

<P>
Now we can finally create the recipe file. Select
<EM>File/Create Recipe</EM> and enter a recipe name (or press the
``...'' button and select a file name). Then select which stars
we want to include in the recipe file. We will most certainly want the
standard and target stars, but we may include objects and field stars 
to be used for WCS matching if we envision using the recipe on a
machine that doesn't have catalogs installed.

<P>
To verify our newly created recipe, remove all stars
(<EM>Stars/Remove All</EM>) and load the file we just
created (using <EM>File/Load Recipe</EM>). Run the photometry routine
(<EM>Processing/Aperture Photometry to File</EM>) and check the output.

<P>

<H2><A NAME="SECTION00874000000000000000">
Working without an Image Frame</A>
</H2>

<P>
In the above examples, we have used a frame of the field as a
backgound on top of which we loaded the stars. This is not
required. If we select <EM>Stars/Add from catalog</EM> without having a
frame loaded, the program will create a blank frame with the size set
by <EM>File and Device Options/New frame width</EM> and <EM>height</EM>, and
set it's WCS with the center of the frame pointing at the selected
object, and the scale as set by <EM>File and Device Options/New frame
  scale</EM>. 

<P>

<H2><A NAME="SECTION00875000000000000000">
Creating Recipies from the Command Line</A>
</H2>

<P>
If we want to create many recipies at a time, it can be more
convenient to use the command line. To create a recipe from the Tycho2
catalog, use:
<BLOCKQUOTE>
<TT>gcx -make-tycho-rcp 20 -j uori -o uori.rcp</TT>
</BLOCKQUOTE>
This will create a recipe using Tycho stars situated within a 20
minutes radius from U Orionis and save the result to <TT>uori.rcp</TT>.

<P>
If we have a sequence file in a format supported by <SMALL>GCX </SMALL>, such as the
``<TT>.dat</TT>'' files made available by Arne Henden at:
<BLOCKQUOTE>
<TT>ftp://ftp.nofs.navy.mil/pub/outgoing/aah/sequence</TT>
</BLOCKQUOTE>
it can be converted to a recipe file using the following command:
<BLOCKQUOTE>
<TT>gcx -import henden &lt;uori.dat -mag-limit 15 |<SPAN CLASS="MATH"><IMG
 WIDTH="13" HEIGHT="35" ALIGN="MIDDLE" BORDER="0"
 SRC="img66.png"
 ALT="$\backslash$"></SPAN>
<BR>
gcx -p - -set-target uori &gt;uori.rcp</TT>
</BLOCKQUOTE>
The first part of the command reads the <TT>uori.dat</TT> file and
converts it to a <SMALL>GCX </SMALL>star file, keeping only stars brighter that the
<SPAN CLASS="MATH"><IMG
 WIDTH="35" HEIGHT="18" ALIGN="BOTTOM" BORDER="0"
 SRC="img67.png"
 ALT="$15^{\rm th}$"></SPAN> magnitude, and writes the star file to the standard
output. The second parts of the command reads the the file from the
standard input, adds ``uori'' as a target, and writes the resulting
rcp file to uori.rcp.

<P>

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