Input Data and Preprocessing

The information involved in a reduction session consists of user data and system tables. User data is a set of echelle images, observed with the same instrument configuration, including a wavelength calibration image ( WLC), a flat field image ( FLAT) and astronomical objects OBJ. Optionally, this set will include standard stars ( STD) to be used in the absolute or relative flux calibration, and dark images ( DARK). Catalogues with comparison lines and absolute fluxes for standard stars are available in the system as MIDAS tables.

Before starting the actual reduction some preprocessing of the data is required to correct for standard detector effects as follows:

• Rotation of input frames.
  After this rotation, the dispersion direction of the echelle orders will be horizontal, with wavelengths increasing from left to right and spectral order numbers decreasing from bottom to top of the image. As always in MIDAS, the origin is the pixel , located in the lower left corner of the image.
• Updating START and STEP descriptors.
  Descriptors START and STEP must be set to 1.,1. for all images processed. Session keyword CCDBIN must be set to the original binning factor along x- and y-axis. Image rotation and descriptors update are performed by the command ROTATE/ECHELLE.
• Cleaning of bad columns.
  First, bad columns -- bad rows after the rotation -- can be removed with the command COMPUTE/ROW. The cleaning of bad columns is required for FLAT images where the variation of the intensity due to these columns can affect the automatic detection of the orders.
• Cleaning of hot pixels.
  Hot pixels can be eliminated by filtering the images. In case the observation has been splitted in several exposures and more than one image is available with the same information, the images can be averaged with the command AVERAGE/WINDOW; this command can, optionally, interpolate pixel values with large deviations from the average value. Removal of hot pixels is required for DARK images and is recommended for OBJ exposures. General methods to clean bidimensional spectra are described in Chapter (Removing Cosmic Ray Hits). The command FILTER/ECHELLE, adapted to echelle spectra is described in Section
• Subtraction of dark current from FLAT, OBJ and STD frames. The dark level is estimated from a series of DARK exposures of short duration which are averaged to reduce the effect of the read--out noise of the CCD and to eliminate hot pixels as described before. If preflashing is necessary, a set of preflashed DARK exposures should be obtained in a similar manner. It is advisable to obtain a set of DARK images with similar exposure times as the object and standard star frames, or to scale the dark level to the observed exposure.
• Checking exposure times in OBJ and STD frames. For images generated by ESO instruments, the exposure time (in seconds) is stored in the descriptor O_TIME(7). If necessary this descriptor can be created as O_TIME/D/1/7 with the command WRITE/DESCRIPTOR.