[boojum]

"The opening angle is about 60 [degrees]."

"Reason: rise in the frequency response of the microphone axis"

The pic shows the mics parallel. I will find the PDF and attach it in a mo'.

The "textbook" I am speaking of is the "textbook" plaement of a pair of mics above the conductor/director and aimed into the chorale/orchestra. It is common to the books on mic placement as a standard approach. It works well almost always and is not a "smoke and mirrors" setup; it is plain vanilla. How's that for mixing metaphors? ;o)

OK, found it and attached it. It is on page 30, 10.2.2. I am going by the pic. My German is over 50 years old now. If it were not for Google I would not have the translations above. ;o)