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James Webb Space Telescope
NIRCam Calibration Sources

Schematic illustration of NIRCam filter wheel assembly showing the location
of the calibration source and the flat-field projector.

Light sources are mounted in NIRCam for self-calibration activities, namely flatfielding, verification of internal alignment, orientation, and plate scale. All light sources are fully redundant in terms of their wiring, lamps, and power sources.

Flat Field Illuminators

The pupil wheels for all channels include a flat field illuminator. The source will be a continuum source warm enough to provide flux in even the shortest NIRCam filter. Internal flat fields can be taken by rotating the desired filter into position after rotating the pupil wheel to the flat field pinhole position and turning on the source. Flat fields taken in this manner provide health checks for NIRCam and supplement the sky flats taken through the entire optical train including the telescope. These internal flats should be very good for measuring high-frequency (pixel-to-pixel) response variations and may be adequate for use over ~200x200 pixel areas as will be required for use in the initial primary mirror segment capture.

Dark Position

The flat field pupil wheel position is also used for dark frames. The flatfield projector on the pupil wheel is opaque to light entering NIRCam from the OTE, and therefore can be used to take darks, with the flat field light turned off. 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). The PAPPA uses the same bench mounted continuum light source as the flat-field projector. 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 comparisons of the two types of image will show whether NIRCam is properly aligned with the OTE pupil, and allow adjustments to be made (if necessary) using the 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.