What are the “insect antennae” structures in front of the astronaut’s visor? They appear to be attached to the MMU, but do not appear in the MMU User Guide, static displays or Weightless Environment Training Facility photos. https://ntrs.nasa.gov/api/citations/19790008382/downloads/19790008382.pdf
Found a 2nd source so removed "partial answer notation" (still looking for details on what the lights actually indicated).
The "antennae" are cue light extenders.
(From Aviation Week and Space Technology, January 23, 1984 - the article is here but paywalled)
This schematic shows the cue lights located at the base of the "antennae"
The cue lights are stated to
allow a visible indication of thruster commands and isolation valve operation
The fiber-optic extenders are referenced in the FMEA portion of the document below, sadly without further explication.
The crewmembers couldn't easily see the lights without the additional devices aka antennae.
I'm satisfied with this answer except I want to know exactly what triggered the lights, and I haven't found that yet. Will edit if I find more info.
Reference: Independent Orbiter Assessment of the MMU
AFAIK, and it is hard to find a lot of info atm, they are for the propellant gauges. They supply information.
Thruster Lights While flying the Manned Maneu- vering Unit, the astronaut keeps track of propellant with two gauges located on either side of the helmet's face plate. An astronaut needs to know how much propellant is left in the unit, because when the propellant is gone, the astronaut is no longer able to maneuver. The astronaut needs to keep enough fuel in reserve to return safely to the Shuttle. Generally, this means an astronaut can use up half the fuel to maneuver away from the Shuttle and keep the remaining half for the return.
The displays on the module are a 12-digit LED display, a built-in test equipment indicator and an analog suit pressure gauge.
The display and control module is connected to the hard upper torso and to the PLSS by both internal and external hookups. A multiple-function connector links the display module to the service and cooling umbilical, thus enabling the use of the display module controls during suit checkout inside the airlock station.
The display module interacts with a microprocessor in the PLSS that contains a program that enables the crew member to cycle the display through a series of systems checks and thereby determine the condition of a variety of components. The microprocessor monitors oxygen pressure and calculates the time remaining at the crew member's present use rate. It signals an alarm at high oxygen use in the primary oxygen tanks. It also monitors water pressure and temperature in the cooling garment. The carbon dioxide level is monitored and an alarm is signaled when it reaches high concentrations in the suit. The microprocessor monitors the power consumed and signals at high current-drain rates and also when an estimated 30 minutes of battery power is left. All the warnings are displayed on the LED display.
The display module also has a fiber-optic cable that is used when the MMU is connected to the EMU. The fiber-optic cable connects the display unit to the MMU. A fiber-optic cable is more reliable and more covenient and safer to use than an electrical connector for extravehicular applications. The MMU is mounted on the back of the portable life support system. When the MMU is connected, the display module also provides a cycled readout of propellant pressures, temperatures, and battery condition (in the MMU) and an audible thruster cue. The C/W system warns of low propellant, low battery, and failed components.