CES

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.

Cameras: 

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

Detectors: 

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.: 

 

• In many echelle orders a prominent ghost appears which, with some bad luck, may sit right on top of the object spectrum. It cannot be `flatfielded away' or otherwise corrected.
• The first 10-15% (in wavelength) of most spectra suffer from vignetting which divison by a flatfield may even enhance rather than remove. The most successful way of correcting it is with a flatfield standard star (see item flatfielding below).
• Reticon data suffer from a periodic ripple which flatfielding does not always remove adequately. For the `normal' 4- or 8-pixel ripple, the command FILTER/RIPPLE can be tried. However, occasionally, also very weird periods occur which can be identified in a Fourier transform (see the various FFT/ commands). Spikes in the real part of the Fourier transform can be removed with the interactive command MODIFY/GCURSOR before the data is transformed back to the original pixel space. Alternatively, FILTER/RIPPLE can be used once the period is known. Note that these corrections have to be done prior to any rebinning (wavelength calibration in particular)!
• The dome flatfield lamp has an emission line at 670.7 nm, apparently due to lithium. Other deviations from a pure continuum are not known, but you may wish to watch for them (this advice applies also to the `internal' FF lamp).
• Some of the strongest absorption lines appear not only in stellar but occasionally also in flatfield exposures!

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.

Flatfielding: 

 

`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:

• For high precision, always flatfield your arc spectra.
• Almost the only relevant comparison source is a thorium lamp. Do not use the argon lines; the lamp contains argon only to start the gas discharge process.
• By far the best laboratory wavelengths are those by Palmer and Engleman (1983). Their list is availabe as MIDAS table TH in directory MID_ARC. Copy this table to your MID_WORK, use SELECT/TAB on column :WAVE and COPY/TT to reduce the size of the table to the range in wavelength of your spectra, then delete the first copy to recover disk space.
• The information given in the descriptor O_COM about the wavelength (i.e., `CRVALX') of the central pixel (i.e., `CRPIXX') and the mean channel width (i.e., `CDELTX') usually is extremely reliable and therefore very useful in the interactive part of identifying the comparison lines.
• Select the threshold in the line searching step so that about 10-25 lines are detected. (As a rule of thumb, it is for normally exposed arc spectra both necessary and sufficient to use all lines which in column :INTENSITY of table TH are listed with a laboratory strength of about 3 units or more.) Only the GAUSSIAN option in command SEARCH/LINE will give useful line positions.
• Identify (IDENTIFY/LINE) about five lines interactively; they should be well distributed over the wavelength range you are interested in.
• Always start with a parabola for the approximation of the dispersion curve (CALIBRATE/WAVE), never user polynomials with degree > 3.
• For normally exposed arc spectra, the automatic identification should identify most of the lines which in table TH are listed (column :INTENSITY) with a laboratory strength of 3 units or more (but, of course, reject blends).
• With the Long Camera, the rms scatter of the computed wavelengths about a fitted second-order polynomial would usually be 1-2 10^-4 nm (if not better). In spectra taken with the Short Camera, the corresponding value may be about two times higher.
• Rebin your spectra to a step in wavelength which is at least two times smaller than the detector pixel width.
• For the rebinning of such very high resolution spectra it is important that the descriptors START and STEP and the relevant variables of programs are of double precision (often applies also to the subsequent analysis of the calibrated spectra). If in doubt or in order to check possible problems, suppress the leading two digits from the laboratory wavelengths (e.g., COMPUTE/TABLE) and later re-introduce them in descriptor START of the calibrated spectra.

.

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