Definition of Useful Non-Prime Data

The data recorded on the 9 non-prime detectors had a grating transmission ranging from zero to maximum and this determined the signal to noise ratio of the final flux. The reliability of the flux level was also affected if more than one FP order were included within the grating transmission range. To give a quantitative evaluation of good mini-scans, `useful' data were defined. A mini-scan was defined as having good signal to noise if it had at least one data point located above 90% of the maximum grating transmission. Figure 1.11 shows two mini-scans that meet this criterion and one that does not. Assuming that the grating response was approximately Gaussian (see Section 1.4.1), the 90% level occurred at a distance from the profile centre of,

$$\lambda_{{\rm r}}^{2}=-\ln(0.9)\left(\frac{\Delta\lambda_{\mathrm{g}}}{2\sqrt{2 \ln 2}}\right)^{2}$$ (A.13)

where $\Delta\lambda_{\mathrm{g}}$ is the FWHM of the grating response profile. This gives a cut-off distance of $\lambda_{{\rm r}}=0.195$ FWHM from grating response function centre. Most of the prime data meet this limit as they were designed to cover the peak in grating transmission.

Figure 1.111.11: The grating response function shape (solid line) is shown with the limits used to define `useful' data. The red and green mini-scans are classed as useful whereas the blue mini-scan is not.

Well order sorted data were defined as having only one FP order within 1 FWHM from grating response function centre. Any orders occurring outside of this range did not contribute significantly to the flux as the grating transmission had fallen to $\sim6$% of its maximum value. Mini-scans recorded on non-prime detectors that met the above two conditions were comparable with data recorded on prime detectors. However, the first condition was much more important in defining the usefulness of non-prime data because a correction for FP order contamination was developed and is described in Section 3.6.