In this work the intensity calibration of the CDS Grazing Incidence Spectrometer is discussed, and plasma diagnostic techniques have been used to check the pre-flight relative intensity calibration of the four GIS detectors and for determining the second-to-first order relative intensity calibration of the GIS 3 and GIS 4 detectors.
A preliminary study has been performed in order to determine the effect of the ghosting problem on some observed emission lines, and to correct for it, whenever possible. This allowed the selection of a number of lines (Table 1) which have proved not to be significantly affected by the ghosting problem.
These lines have been used for checking the relative intensity calibration between the four detectors of the GIS instrument. A general agreement is found between the present results and the pre-flight intensity calibration by Bromage et al. (1996) listed in Table 5. Therefore the use of the pre-flight calibration values is recommended for intensity calibration of GIS spectra. No evidence was found for a wavelength dependent variation of sensitivity along the detectors.
For the first time, a second order sensitivity calibration has been determined. Correction factors have been obtained which may be applied to the first order sensitivities of GIS 3 and GIS 4, where nearly all the second order lines appear.
The accuracy of the present check of the GIS sensitivity is mainly limited by instrumental problems, such as fixed patterning, ghosting and anomalous line profiles, and by the uncertanties of the theoretical data used for the analysis. Most of the GIS lines which have proved to be useful for the present work agree among themselves within 30%. The higher limit is mainly due to weaker lines. We consider this higher limit as the accuracy of the sensitivity calibration of the GIS instrument determined in the present work.
The problems of ghosting, fixed patterning and instrumental line profiles have been discussed, and several methods for smoothing the fixed patterning were compared. A 3-point boxcar smoothing was found to be adequate.
Lines in Table 1 are recommended for further diagnostic studies of active region and quiet Sun spectra observed with GIS. However, it is important to be aware that the spectra analysed had GIS Setup IDs of 41 (quiet Sun) and GSET_ID=47 (active region), and therefore the ghosting analysis that allowed the selection of usable lines can in principle only be applied to other observations having the same GSET_ID. The position and amount of ghosting is not expected to vary for a given GSET_ID. What varies is the degree of ghosting, which can change drastically depending on the target source.
Any application of the presented results, in terms of ghost analysis, to other observations taken with a different GSET_ID should be treated with great caution. Also, any long term gain depression could modify the relative efficiencies of the GIS detectors, and therefore any application of the pre-flight calibration to observations taken before or after September 1996 should be checked. However, current indications are that detector sensitivities are stable with time.
There is much work still being done, both with the atomic physics and the GIS detector calibrations. These include software for refining and semi-automating the ghost detection and correction, second order calibrations for long term gain depression, as well as work with the other valid GSET_IDs.
G. Del Zanna work has been supported by a scholarship from the University of Central Lancashire. G. Del Zanna and B.J.I. Bromage are grateful for the use of PPARC Starlink computing facilities. We are grateful to all the SOHO and in particular CDS team members who have made this solar mission such a success. SOHO is a ESA and NASA joint mission. We are grateful to Dr. J.S. Kaastra for useful comments on the original manuscript.
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