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<H1><A NAME="SECTION00650000000000000000"> </A>
<A NAME="ccd:naming-convention"> </A>
<BR>
Calibration Frames and Naming Convention
</H1>
The CCD package is based on so-called data sets. A data set contains
a science frame, all its associated raw calibration frames and the master
calibration frames created by combining and processing the raw
calibration frames. Depending on the processing to be done on the science
frame one or more master calibration frames are to be created.
<P>
Basically, the creation of a master calibration frame can be done
in two ways. Either one creates a MIDAS catalogue which contains the
names of all single calibration frames to be combined in to the master
frame, or, in the case of pipe line reduction, one use the Association
Table. Since the first method is straightforward we concentrate on the
use of the Association Table.
<P>
In order to achieve a maximum of efficiency and to interface the
package with the Data Organizer, the naming convention for
master calibration frames is identical to the naming convention for
the naming of the master calibration frames in the latter package. Here
the name of a master calibration frame is a composition of the generic
prefix and all frame numbers of the calibration frames to be used and
therefore selection in the DO context. <I>E.g.</I> the master
calibration frame <TT>susi_12_123_1245</TT> is a combination of
the frames <TT>susi0012</TT>, <TT>susi0123</TT>, and <TT>susi1245</TT>.
After the association process by the DO, the name of the
master calibration frame is stored in a separate column in the
Association Table. The name of this frame is defined as described above.
The names of single calibration frames are however also available
in the Association Table. This obviously offers the possibility of
simply combining all single raw calibration frames in a master one.
To execute this brute force combining the column for the master frame
should contain an asterisk <TT>*</TT>. The name of the master will then
be a composition of the name of the science frame to which the master
calibration frame is associated plus the postfix <TT>_exp</TT>. Here,
<TT>exp</TT> is the exposure type, stored in a MIDAS frame descriptor
(<TT>EXP_TYPE</TT>), and the name of the column with this exposure
type in the Association Table and containing the names of the raw
calibration frames. <I>E.g.</I> the master calibration frame
<TT>susi0100_bias</TT> is created by combining all bias frames
associated to the science frame <TT>susi0100</TT> and stored in the
Association Table.
<P>
Currently, the following exposure types are supported:
<UL>
<LI><TT>bias</TT> - bias frames:
<BR>
These are zero second integration exposures obtained with the
same pre-flash (if any) you have used for your scientific exposures.
The bias frame will correct for the small positive voltage
added to the true signal from the CCD and determines the photometric
zero point of the electronic system.
<LI><TT>dk</TT> - dark current frames:
<BR>
These are long exposures taken with the shutter closed. Dark
emission can be caused by several sources (<I>e.g.</I> overall
background emission, luminescence form source on the CCD) and will
add charge linearly with exposure time.
<LI><TT>ff</TT>, <TT>ff-dome</TT>, <TT>ff-screen</TT> - flat field frames:
<BR>
These are used to remove the pixel-to-pixel variations across
the chip. In some cases dome flats (exposure of an illuminated
screen) or projection flats (exposures of a quartz lamp
illuminating the spectrograph slit) will be sufficient to remove
the chip variations.
<LI><TT>ff-sky</TT> - blank sky exposures:
<BR>
As an alternative to the dome or projector flats
many observers doing direct imaging try exposures of blank
sky field(s). A clear advantage is that the sky field to be obtained
from the blank sky exposures have exactly the colour of the night sky.
However, this method of flat field can only be used in absence of
fringing and low telescope background emission.
</UL>
<P>
Other calibration frames that can be used in the calibration process are:
<UL>
<LI>illumination frames:
<BR>
This calibration frame may be used to correct for the fact that
the flat field calibration frame do not have the same illumination
pattern as the observations of the sky. If this is the case,
applying the flat field correction may cause a gradient in the sky
background.
<LI>fringe frames:
<BR>
It may happen that using a (thinned) CCD a fringe pattern becomes
apparent in the frame. The pattern is caused by interference of
monochromatic light (<I>e.g.</I> night sky lines) falling on the chip
and are not removed by other calibration and correction steps.
To correct one needs to construct needs a really blank sky frame.
</UL>
<P>
<DIV ALIGN="CENTER">
<B>*** WARNING ***</B>
<BR>
</DIV>
In principle, the CCD package allows the use of any name for the
calibration frames. However, to make the reduction of CCD frames as
easy as possible it is recommended to use the above described naming
scheme. It is highly recommended to use it. Using different names may,
under particular circumstances, lead to complications, in particular
in the case of pipe line reduction.
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
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