Contamination from Adjacent FP Orders and Strong Spectral Lines

The grating profile shape measured by each mini-scan was distorted if more than one FP order was transmitted or if there was a sudden change in source continuum level (e.g. due to a strong spectral line). To find mini-scans where the contamination from adjacent FP orders was minimised, the dataset was searched for scans where a single FP order was within 1 FWHM of the grating response maximum. Figure 3.9 shows the limits used and the maximum distortion to the profile shape that was permitted by this condition. This resulted in a change to the shape of less than 7.6% of the maximum response for grating second order (SW1-SW5) and less than 10% of the maximum response for grating first order (LW1-LW5). This criterion was equivalent to a varying limit on the FP free spectral range with distance from the centre of the grating profile. The free spectral range was allowed to be smaller when the FP was scanning near the grating profile maximum than when it was scanning in the wings. This maximised the amount of data that could be used and meant that the order contamination was similar across the whole profile.

Mini-scans containing strong absorption lines would have distorted the shape of the final profile. This was particularly true for wide lines that crossed more than one mini-scan. Any mini-scans that contained the lines shown in Table 3.3 were rejected from the analysis.

Table 3.33.3: Mini-scans that contained these lines were discarded from the analysis.

Line	Wavelength ($\mu$m)
OH	52.25, 53.35
OH	79.12, 79.18
OH	84.42, 84.60
OH	119.23, 119.44
$[$OI$]$	63.184
$[$OI$]$	145.53
$[$OIII$]$	88.36
CH	149.1, 149.4
$[$NIII$]$	57.33
H$_{2}$O	179.5