Taking Data

Observation starts by running an EXEC file. This file contains commands that control the telescope and the spectrometer and that allows data to be taken. Typical EXEC files that were used to obtain the data shown in this work are:

!Exec for Flats and Arcs
CONFIG TARGET
FLAT
ARC

!Exec to observe target star
CONFIG TARGET
SET OBJECT
BREAK PEAK UP ON OBJECT
STARTGROUP
DO 40 QUADS_DEC95
ENDGROUP

!This is QUADS_DEC95
!Object/Sky pairs; rows 121 & 146
OBJECT
SLIDE SLIT -30.5
SKY
SKY
SLIDE SLIT 0
OBJECT

The commands used in the above EXECs give CGS4/Telescope the following instructions:

• config target loads the CONFIG file `target'.
• flat instructs CGS4 to make a flat field observation.
• arc instructs CGS4 to make an arc observation.
• set object drives CGS4's motors to the positions specified by CONFIG.
• break peak up on object pauses the EXEC and displays an instruction to the observer. In this case `Peak up on object'.
• startgroup starts a loop.
• do 40 quads_dec95 execute `quads_dec95' 40 times.
• endgroup marks the end of the loop.
• object instructs CGS4 to take data and label it as an `object' observation.
• slide slit -30.5 makes the telescope slew 30.5 along the slit
• sky instructs CGS4 to take data and label it as a `sky' observation.
• slide slit 0 makes the telescope slew back to the original position on the slit.

The data acquisition process starts by running the CGS4 dark current monitoring EXEC. This EXEC instructs the spectrometer to take two bias frames and four dark frames. Bias and dark frames are images of the detector without any light shining on it. In the ideal case, the number of counts in each pixel, when no light is shining on the detector array is zero. In practice, each pixel will always have a non-zero count, even if radiation is not arriving at the detector. A bias frame records the 'zero' level of each pixel in the array. Dark frames allows one to infer how the number of counts increase with time. The CGS4 dark current monitoring EXEC takes two 0.12 seconds exposures in the STARE mode (single read-out), labelling them bias, and two 1 second exposures in ND_STARE mode (read non-destructively), one 5 seconds exposure and one 60 seconds exposure in STARE mode, labelling these dark. The array tests are necessary to assess how well the detector array is behaving. Having performed the array tests, the observer can start taking his/her own data.

The first integration must consist of a set of exposures to a uniform light source, carried out in a non-oversampling mode. This is called a Flat Field. It will be used in data reduction to correct for the different sensitivity of each pixel in the detector and for the intensity gradients that result from the instrument's optics. The next frame to be taken is called an Arc. The detector is exposed to the light emitted by an arc lamp containing a known gas. The gas emits only at certain lines, whose wavelengths are known. In general, this frame can be wavelength calibrated, i.e. a wavelength can be assigned to each pixel number along the dispersion direction. This calibration can be used in subsequent frames. If, for a given observing mode, the number of arc lines available in an arc frame is scarce and does not allow one to obtain a good enough wavalength calibration atmospheric lines such as OH emission lines and H₂O can be used to perform the wavelength calibration. At this stage one can start taking data from astronomical objects.