next up previous contents
Next: Echelle Reduction Up: No Title Previous: CCD-Commands Summary


The reduction of CES spectra is, especially for point sources, very straightforward, and the general instructions given in Chapter 6 for the standard reduction of spectra are fully adequate. The following merely adds a few instrument specific details and hints.

Except where noted otherwise, the reduction of data obtained with the Short or the Long Camera is exactly the same.

As far as the data reduction goes, the main difference between Reticon and CCD is the data format. Since the Reticon is a one-dimensional array, no attention has to (can) be paid to the extraction of the object spectra. The data acquisition system used with the Reticon permits multiple exposures to be combined into a pseudo two-dimensional spectrum. For operations on such data, all commands which work on image rows are useful, e.g., COMPUTE/ROW, AVERAGE/ROW or also EXTRACT/IMAGE.

Blemishes, etc.: 

Background determination: 
For Reticon data, separate observations are required. On CCD spectra, the various background components (bias, scattered light, ghosts, sky, etc.) can usually be estimated from the signal on either side of the object spectrum.

Unless you are sure that features in the background spectra are significant, only subtract the mean value as number or a strongly smoothed background spectrum in order not to add noise to your object spectra.

`Internal' FF lamp
is perfect (apart from vignetting, see above) for the Reticon and usually fully adequate for CCD spectra over most of the wavelength range accessible in the blue pass of the CES. In the red, the phases of the fringes in flatfield and object spectra may be so different that division by such a `flatfield' only makes things much worse.

Dome flats: 
Observers have reported that the position of fringes may depend slightly on telescope position.

Bright stars
without disturbing spectral features and if observed sufficiently close to the target in both position and time, may be used for three purposes:

High spatial frequencies: 
For this application, the spectrum of the flatfield star must have been trailed so that its well exposed part (along the spatial axis) fully covers the relevant positions of object spectra. Division of the object spectrum by the standard spectrum (both assumed bias corrected) will also take care of low spatial frequencies and, perhaps, telluric features as described below.

Low spatial frequencies: 
Command NORMALIZE/SPECTRUM can be used to obtain an approximation to the continuum of the comparison star. Division of this curve into the extracted target spectra will be useful in correcting for vignetting problems and other residual curvatures (echelle ripple) of the flatfielded spectra.

Telluric lines
can, with some luck, be removed by dividing the extracted object spectrum by a suitably normalised extracted flatfield star spectrum. Wherever possible, this should be done prior to wavelength calibration.

Wavelength calibration: 
If you have to be worried about artifacts introduced by the non-linear rebinning, try to be innovative and do not rebin your data at all! If this is not practical, the following details should be considered:

Flux calibration
is not possible for CES spectra unless you managed to observe a standard star with flux data that is extremely well sampled in wavelength.

=01 =11 =1995

next up previous contents
Next: Echelle Reduction Up: No Title Previous: CCD-Commands Summary
Petra Nass