In my view, all opera performances should be recorded for archival purposes. Especially if you’ve already got microphones involved. In case an involved party is thinking that someone might take an archival recording and sell it, there is literally no profit to be made selling a professional opera recording much less an amateur opera recording. There should be no concern for lost revenue by allowing an archival recording to be made. The purpose of an archival audio recording is to document what a performance sounded like as a guide for future productions.  I’ve been recording archival audio during Fort Worth Opera productions for the past 10+ years and no one has ever asked to buy a copy. On the other hand, I have been asked by the conductors at times to share with them a recording of a recent performance so they can hear if their tempos are working or not. Archival audio recordings can be an important part of a promoting a new work and making sure it can be accurately recreated later. How?

At a minimum there are inexpensive stand alone pocket size audio recorders that can be used for audio capture. If you want a better quality recording, you can use separate microphones and separate recording devices. If you have the means, you can record multitrack audio right of the production audio console. Most of the time there are floor mics to pick up the singing and pit mics to help the singers hear the music. Those same microphones can be used for an archival recording.