def _kt_kt_prime_factor(airmass):
<EOL>return <NUM_LIT> * np.exp(-<NUM_LIT> / (<NUM_LIT> + <NUM_LIT> / airmass)) + <NUM_LIT:0.1><EOL>
Calculate the conversion factor between kt and kt prime.
Function is useful because DIRINT and GTI-DIRINT both use this.