2010

Pupil Alignment Measuring Technique and Alignment Reference for Instruments or Optical Systems

This technique can be used in any instrumentation that requires measurement of pupil alignment, such as optical instruments and cameras.

A technique was created to measure the pupil alignment of instruments in situ by measuring calibrated pupil alignment references (PARs) in instruments. The PAR can also be measured using an alignment telescope or an imaging system. PAR allows the verification of the science instrument (SI) pupil alignment at the integrated science instrument module (ISIM) level of assembly at ambient and cryogenic operating temperature. This will allow verification of the ISIM+SI alignment, and provide feedback to realign the SI if necessary.

This innovation consists of a 10-mm reflective patch on the +V1 face of a filter or closed position of the SI pupil wheel. The PAR will have a centered alignment crosshair and a minimum of two concentric circular fiducials representing a reference for the SI pupil alignment. The fiducials need not be exactly centered to the nominal SI pupil position, but their alignment relative to the nominal pupil position must be known to 0.2-percent of the pupil diameter. A clocking reference point should also be included in one quadrant to provide a reference.

The SI teams will reference their pupil alignment to the PAR during their instrument alignment, and measure the PAR in the +V1 horizontal and +V1 down orientation at ambient temperature relative to the nominal V Coordinate system. The teams must demonstrate by test and analysis that the SI internal pupil alignment (from the kinematic feet up) is within the 0.5-percent placement allocation in 0-G. In addition, the SI team must demonstrate by test and analysis that the pupil alignment is within the 1-percent placement allocation in 1-G to a knowledge tolerance of 0.5 percent.

For ISIM, the PAR will be used at ambient temperature to verify that the SI has been installed to within allocated tolerances, and that its alignment does not shift due to vibration and other environmental test exposures. Ambient temperature measurements are performed using a PAR ISIM reference fixture to place alignment telescopes along the nominal chief center ray of each SI. The alignment telescopes will measure the offset of each SI PAR from nominal, and verify that the ISIM+SI alignment is within tolerance at ambient temperature. This also allows a non-invasive means of checking SI alignment to ISIM (without removing the ISIM enclosure) after shipping to observatory testing.

During the ISIM level verification, the OSIM will be aligned to ISIM and the optical telescope element (OTE) SIMulator (OSIM) pupil reference fiducials will be projected onto the SI PARs, and the pupil alignment will be mapped. A Global Nominal Pupil (GNP) position, optimizing all of the SIs, will be determined, and used to align the ISIM to the OTE to minimize common path pupil alignment error. The pupil alignment measurement will also verify that the ISIM+SI pupil alignment is within allocated tolerances for all SIs in the +V1 down orientation. Therefore, it is crucial that the SI pupil alignments are known in both orientations for each SI. The final opportunity to discover and correct ISIM+SI pupil alignment errors at cryogenic operating temperature is during ISIM level testing, so it is crucial that a standardized reference (SI PAR) be available. These references will be measured relative to Pupil Imaging Modes for NIRCam and MIRI to verify that the alignment has not changed downstream of the Pupil reference due to shifts of optics, and is the only way to deterministically demonstrate an unvignetted field at the observatory level of assembly.

This work was done by John G. Hagopian of Goddard Space Flight Center. For further information, contact the Goddard Innovative Partnerships Office at (301) 286-5810. GSC-15783-1

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