Second-order contamination correction for NOT grism #4

Grism #4 of the ALFOSC instrument at the Nordoc Optical Telescope suffers from severe second-order contamination. I have developped an empirical method to correct for that. I did several tests which showed that the method works fairly well. Here is an example with the spectra of the SNe 2005cf. The spectrum with grism #4 is compared with the one taken with grism #5, which does not have from order contamination.

Before ...

... and after the correction.