Some Basic Definitions

Preliminary reduction of array data is common to both spectroscopy and photometry and techniques describing the removal of instrumental effects are noted elsewhere (Daly 1987). Before starting to use Portable--CGS4DR, however, you should be familiar with the reduction of spectroscopic data (Puxley 1988). It would also be advantageous to be familiar with Figaro data reduction (Bailey 1990, Bailey 1991). Portable--CGS4DR software knows about the following observation types:

BIAS

--- A short blank observation.

DARK

--- Blank observation with the same on-chip exposure time as another observation.

FLAT

--- Observation of black-body source in calibration unit, the sky or the illuminated dome.

OBJECT

--- Observation of an astronomical object.

SKY

--- Observation to subtract from an OBJECT frame to perform first order sky subtraction.

ARC

--- Uncalibrated observation of the arc lamp or OH-lines from the sky.

CALIBR

--- An ARC observation converted into a wavelength calibrated frame.

STANDARD

--- A standard star observation divided by a model black-body.

In a common data reduction sequence the following operations would be carried out:

1. Apply a bad pixel mask.
2. Subtract a BIAS frame.
3. Linearise the signal.
4. Subtract a DARK frame.
5. Divide by a FLAT field.
6. Interlace integrations taken at different detector positions.
7. Calibrate x-axis into wavelength.
8. Co-add observations and subtract SKY frames.
9. Divide by a STANDARD source.
10. Extract up to 3 spectra nodded along the slit.

These steps are configurable to allow for a wide variety of astronomical configurations. When observations are reduced they are recorded in an index file of the form CGS4'137 yymmdd.INDEX, where yyddmm is the UT date on which the observations were made, in the directory $cgs4_index/. The system will try to replace the date string with an appropriate value whenever possible based upon universal time. The contents of the index file may also be manipulated.