Calibration Frames and Naming Convention

  The CCD package is based on so-called data sets. A data set contains a science frame and all its associated calibration frames. In order to archive a maximum of efficiency the CCD package uses a simple and straightforward naming convention: the names of the data sets are identical to the names of the science frames. All relevant data sets are assumed to be contained in the association table, generated by the MIDAS Data Organizer.

By combining the calibration frames of a particular exposure type ( e.g. bias) the user can create a master calibration frame. In the CCD context master calibration frame must have a exposure type postfix describing the exposure type of the frame. The extension is obligatory. The first part can be any legal character string. Below follows a listing of calibration frames that currently can be handling in the CCD package. Between brackets is indicated which exposure extension for the master calibration frame is used.

• bias frames ( _bs):
  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.
• dark current frames ( _dk):
  These are long exposures taken with the shutter closed. Dark emission can be caused by several sources ( e.g. overall background emission, luminescence form source on the CCD) and will add charge linearly with exposure time.
• flat field frames ( _ff):
  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.
• blank sky exposures ( _sk):
  As a alternative to the dome of projector flats many observers doing direct imaging try exposures of the blank sky field(s). 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.

Other calibration frames that can be used in the calibration process are:

• illumination frames ( _il):
  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 gradient in the sky background.
• fringe frames ( _fr):
  It may happen that using a (thinned) a fringe pattern becomes apparent in frame. The pattern is caused by interference of monochromatic light ( e.g. 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.

As described above, the CCD package allows the use of any name for the calibration frames as long as the extension is present. However, to made the reduction of CCD frames as automatic as possible a naming scheme for the master calibration frames has been built-in that. It is based on the association of calibration frames with science frames as stored in the association table. In this scheme, the name of a master calibration frame is, except for the exposure type extension, identical to the name of the data set (science frame). Using this convention the system is able to create the master calibration frame from the calibration frames in the association table and associated with that data set.

As a example of the naming convention used in the CCD package, assume a science frame with the name susi010.bdf. The association table contains 5 bias frames associated with this science frame: susi021.bdf to susi025.bdf. Following the default naming scheme the COMBINE/CCD will use these five bias frames to create the master bias frame. The name for this frame will be susi010_bs.bdf, indicating that it is a bias frame constructed from the bias frames associated with data set susi010.

*** WARNING ***

It is highly recommended to use the naming convention as explained above. Using different names is dangerous and may lead to wrong results. In the automatic reduction procedure, that will be discussed below, the system uses this standard naming convention. When using the basic CCD commands, in particular the commands COMBINE/CCD and MKCAL/CCD other names can be chosen.