The method used to correct for the atmospheric transmission and instrumental response is the same discussed in Section 3.2. The standard stars used (listed in Table 3.4) have spectral type A and therefore show Pa Beta and Br Gamma in absorption. As with the standard stars observed during UT9410, these features were removed by fitting a spline to the data region where the line is present.
In contrast with the UT9410 data, where the instrumental response was a straight line, the instrumental response in the UT9512 data is clearly well approximated by polynomial of a higher degree. An example of this is shown in the top panel of Figure 3.11 for the Br Gamma window.
Figure 3.11: Top panel - The solid
line is the Br Gamma observed spectrum of GL380, a K7 V star. The dashed
line is the result of a polynomial fit to the continuum of GL380's
spectrum and it represents the instrumental profile at this
wavelength. Bottom panel - Observed spectrum of the standard star
BS1251, showing Br Gamma in absorption. Also plotted is the same
spectrum after removing Br Gamma.
Shown in this figure are the spectrum of the K7 V star GL380 (solid line) and the general form of the instrumental response (dashed line mainly reflecting the CVF transmission profile) obtained by fitting the continuum of GL380 with a low order polynomial. Note that GL380's spectrum does not have any very wide emission or absorption line capable of distorting the instrumental response by much.
Before using a spline fit to remove the hydrogen absorption line from the standard star's spectrum, the spectrum was divided by the instrumental response obtained from GL380's spectrum. This makes the spectrum of the standard star flat, and now the same procedure used to remove the absorption feature from the standard stars observed in UT9410 can be implemented. After removing the absorption feature, the spectrum has its original shape restored by multiplying it by the instrumental response used before. As an example, the result of removing Br Gamma from the spectrum of BS1251 is shown in the bottom panel of Figure 3.11.
As before, the spectra of the standard stars, with the hydrogen absorption lines removed, were used to correct for the atmospheric transmission and instrumental response.
