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Configuration Parameter Notes

Notes on Configuration parameters

GRATING: Select the grating to use. Only two gratings are in CGS4 at any one time - if in doubt your support scientist or telescope operator can advise which they are. When observing with the Echelle you should normally select ECHELLE_AUTO_ORDER.

WAVELENGTH: Enter the central wavelength for your spectrum in microns.

ORDER: Enter the order to use the grating in. If you have selected ECHELLE_AUTO_ORDER the value here will be ignored.

The CGS4 sensitivity page contains information about the appropriate choice of orders for the gratings as a function of wavelength. Normally echelle_auto_order should be used for echelle observations. However see the echelle notes for details of where this is not appropriate. If in doubt your support scientist can advise you.

CVF_OFFSET_(ECH_ONLY): If necessary enter an offset in microns for setting the CVF when you are using the Echelle. Normally the default of 0.0 should be used. This value is ignored for the 40 and 150 l/mm gratings. The precise calibration of the CVF for Echelle observations can depend on which row you choose to observe, the order and the slit width. Sometimes it is desirable to set the CVF to a slightly different wavelength from the grating to correct for this. Your support scientist will help you check the CVF setting for your wavelength and order. Enter a non-zero value for a CVF offset with caution and only after checking flats taken with NO CVF offset.

POLARISER: The polariser should be NONE for normal spectroscopy. Select PRISM or GRID if you want to do spectropolarimetry.

You cannot do spectropolarimetry with the echelle and prism.

SLIT_WIDTH: Choose a 1-pixel, 2-pixel or 4-pixel wide slit. There is 1 pixel per resolution element and so a two pixel wide slit will give reduced, resolution, but can be useful in poor conditions. The 4-pixel wide slit is not available with the Echelle.

POSITION_ANGLE: Enter the position angle of the slit on the sky in degrees EAST of North. 0 deg is a N-S alignment. Due to a historcal reason, CGS4 will not accept angles greater than about 10 degrees. To request a slit position angle greater than this, you need to specify an angle 180 degrees away. E.g., if you want 45 degrees east of north, you need to specify -135 degrees as the position angle.

OBJECT_SAMPLING: The sampling required x the number of pixels to sample over.

There is 1 resolution element per pixel so the detector is stepped to fully sample. To Nyquist sample your spectra a sampling of 2 should be selected (ie 2 data points per resolution element). Many users prefer to slightly over-sample and choose a sampling of 3. To help eliminate bad pixels from your spectra you can also choose to carry out the sampling over 2 pixels. This means that each data point in your final spectrum has been observed with two neighbouring pixels. When the data reduction interleaves the raw integrations data from a bad pixel is replaced by that from its good neighbour. Most observers therefore chose sampling of 2x2 or 3x2 for this reason.

As an example, if you choose sampling of 2x2 the data in the final spectrum is taken at the following detector positions :

resolution elements     |   |   |   |   |
detector pos1:          1   2   3   4   5  ...256
detector pos2:            1   2   3   4   5
detector pos3:              1   2   3   4   5     
detector pos4:                1   2   3   4   5
Except for at the begining and end, or where there is a bad pixel, each data point in the final array is the average of two measurements. For 2x2 sampling the reduced spectrum has 514 data points on the x-axis.

DATA_ACQUISITION_MODE: Choose ND_STARE, STARE or CHOP.

For exposure times less than 1 STARE is a little more efficient but NDSTARE may aso be used. Use ND_STARE for all other observations that do not require chopping. ND_STARE uses a multiple non-destructive read algorithm to give optimum read noise performance.

SINGLE_EXPOSURE__SECS: Enter the on-chip exposure time in seconds to be used for observations of your source. See the optimum exposure times for a guide to what value to enter here. The minimum exposure time is 0.12secs for the 256x178 array. For the 256x48 array, the minimum time is 0.023 secs, and for the 256x32 array, the minimum time is 0.016 secs.

EXPS/INT_OR_EXPS/CHOPBEAM: Enter the number of exposures to be coadded into an integration (integ) at each array sample position.

For long exposures on faint sources set object_exp_per_integ to 1. For very short exposures on a bright source it is a good idea to choose a value so that the time per integration is a few seconds for best observing efficiency.

CHOP_CYCLES/INTEGRATION This only applies to CHOP mode, and is ignored otherwise. Enter the number of chop cycles you want before nodding the telescope (one chop cycles is an object-sky pair).

FLAT_SAMPLING: choose AS_OBJECT in most cases

For Echelle users it is now possible for the data acquisition to take over-sampled flats with the same sampling factor as your object observations to help remove CVF ripple.

FLAT_LAMP: choose your required lamp. The numbers refer to black-body apertures. There is a table of flats of appropriate values as a function of grating wavelength and order. If you choose off then the calibration unit will not be used - useful for if you want to observe the sky, or the dome etc for a flat.

FLAT_ACQUISITION_MODE: Normally this will be NDSTARE

FLAT_EXPOSURE_SECS: typically 0.12 - 1 sec. See the table of flat exposure times.

FLAT_EXP_PER_INTEG: The number of exposures to be coadded into an integration. typically 50-100.

FLAT_INTEG_PER_OBS: The number of integrations to be taken and returned to the Vax. Typically 1.

ARC_LAMP: Choose your required lamp. There is a table of recommended arcs lamps as a function of wavelength. Plots of sample arc spectra can also be found at that page. If you choose OFF for the lamp then the calibration unit will not be used - useful for if you want to observe sky lines for wavelength calibration.

ECHELLE_ARC_CVF_WAVELENGTH: Defaults to the same wavelength as for your object observation.

Because the wavelength coverage on the array with the Echelle is not very large it is possible to observe at wavelengths where there are no lamp lines which would fall on the array. This allows you to select a different CVF wavelength from the grating, to look at lamp lines in a different order from your source.

If the value shown here is -1 the default CVF setting - ie the same CVF wavelength as for your object will be used. If you have just recalled or created a CONFIG then the default wavelength should appear here.

ARC_ACQUISITION_MODE: Usually NDSTARE

ARC_EXPOSURE_SECS: Typically 0.12 secs with the 40 l/mm grating and long camera.

ARC_EXP_PER_INTEG The number of exposures to be coadded by the into an integration at each detector sampling position. Typically 5.

All ARCS are automatically taken with the same sampling and sampling range as your object observations. Although on-chip exposure times are short, you should add sufficient exposures so that the time per sampling position is at least 0.5 secs, to prevent the background increasing due to the detector translation stage getting warmer.

ARC_INTEG_PER_OBS The number of integrations to be taken and returned to the Vax at each detector sampling position. typically 1.

Contact: Tom Kerr. Updated: Tue Jan 25 11:09:01 HST 2005

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