Calibration Frames and Naming Convention

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.

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.

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. E.g. the master calibration frame susi_12_123_1245 is a combination of the frames susi0012, susi0123, and susi1245. 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 *. 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 _exp. Here, exp is the exposure type, stored in a MIDAS frame descriptor (EXP_TYPE), and the name of the column with this exposure type in the Association Table and containing the names of the raw calibration frames. E.g. the master calibration frame susi0100_bias is created by combining all bias frames associated to the science frame susi0100 and stored in the Association Table.

Currently, the following exposure types are supported:

• bias - bias frames:
  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.
• dk - dark current frames:
  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.
• ff, ff-dome, ff-screen - flat field frames:
  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.
• ff-sky - blank sky exposures:
  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.

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

• illumination frames:
  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.
• fringe frames:
  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 (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.

*** WARNING ***

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.