With the calibration light sources disabled, the flat field pupil wheel position is a blank position used for dark frames. This position on the pupil wheel is opaque to light entering NIRCam from the OTE, and therefore can be used to take darks. The similar pupil alignment pinhole projector assembly (PAPPA) discussed in the next section contains both outward-facing pinholes and inward-facing pinholes. This could allow light from the OTE to leak through to the FPAs and it is therefore inappropriate to use it as a dark/opaque position.
Pupil Alignment Pinhole Projector Assembly (PAPPA)
The pupil wheels include a pupil alignment pinhole projector assembly similar to the flat-field illuminator (i.e. a cavity with pinholes), but the same calibration light source has been disabled. However, the SW PAPPA has on both sides (inward and outward) a pattern of 18 pinholes associated with the primary mirror segments. The PAPPA will be used in conjunction with the pupil imaging lens (PIL) located in the short wavelength channel. Images taken through the PAPPA and the PIL will accurately map the internal NIRCam pupil onto the SW FPA. Images of a bright star taken using the PIL will map the OTE pupil, and this will aid in aligning NIRCam with the OTE pupil. Adjustments to NIRCam alignment are made (if necessary) using the Focus Adjust Mechanism (FAM). The LW PAPPA will only be used during ground-test, and does not have inward-facing pinholes because there is no PIL in that channel that could allow them to be imaged onto the FPA.
Image Quality & Alignment LEDs
The mechanical coronagraphic mounts at the telescope focal surface have LED point source emitters that can be used to check that image quality and alignment in NIRCam has not changed. These LEDs emit in narrow bandwidths, and so must be used with specific filters in the filter wheel and either of the two coronagraphic pupils. There are separate and redundant LEDs for the short- and long-wavelength channels, but those in the long-wavelength channels have been disabled.