CGS4 Data Types
    
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CGS4
Handbook
A Summary of CGS4 Data Types
CGS4 recognises data types of Object, Sky, Bias, Dark, Flat, Arc . These
correspond to the commands
Object, Sky, Bias, Dark, Flat and Arc
in an EXEC, or can be selected for a single observation. For each command
data taking is carried out as follows: (see the Guides to CGS4 configs
and execs for more detailed information).
-
Object, Sky - the setup for these is identical, except that if Sky
is selected the observation type in the file header is "sky" - and it will
be recognised as a sky frame by the data reduction system. CGS4 takes data
according to the object setup you have defined in define_astronomical_config
when creating the config.
-
Bias - Bias observations can only be taken if the acquisition mode
in your config was set to STARE (they have no meaning otherwise). If this
was the case then setting up for a bias observation means that the data
will be taken with the detector blanked off and the exposure time will
automatically be set to 0.12secs, the exposures per integration will be
set to 30 and the integrations per observation to 3. This gives a good
standard bias. The bias level is extremely stable - normally if you take
a bias at the begining of the night using the array_tests exec, the you
do not need to take any others.
-
Dark - for either STARE or NDSTARE acquisition mode a dark is executed
as follows. The detector is automatically blanked off and data is taken
with the same exposure time, exposures per integration and integrations
per observation as you have setup for your object observations. So darks
have the same exposure time as the object frames they would be subtracted
from. The dark current is low ~0.2 e/s and stable. Generally if you are
observing by nodding along slit for sky subtraction then you do not need
to take darks because the dark is subtracted off when the obj-sky pair
is formed.
-
Flat - A flat can only be taken if the config contains the necessary
information to take flats i.e., if you used the add_flat/arc_information
menu when creating the config. If this was done then the CGS4 motors set
up is the same as for your object observation, and the sampling, lamp and
exposure times are set according to the flat information in your config
file.
-
Arc - An Arc can only be taken if the config contains the necessary
information to take arcs i.e., if you used the add_flat/arc_information
menu when creating the config. If this was done then the CGS4 motors set
up and sampling are the same as for your object observation, and the lamp
and exposure times are set according to the arc information in your CONFIG.
If you have specified an Echelle_arc_cvf_wavelength to get a calibration
line in a different order then that will be set to instead of the object
cvf position. If you are taking low-resolution spectra with a central wavelength
longer than 2.8um, the blocking filter will automatically be set for observation
of 1.4-2.5um lamp lines in second order.
NB: if you attempt to take a flat or an arc with a config
that does not contain the necessary information you will get an error message
reporting an illegal exposure time - because without having defined the
flat/arc information no exposure time has been selected.
Last Modification Date: 1998/09/16 - Last Modification Author: Tom
Kerr, Original Author: Gillian Wright
Tom Kerr (tkerr)
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