  Some ideas for onboard processing and photometry with an astrometry satellite
are presented, especially designed for GaiaNIR which may be launched about 2045
as a successor of Gaia. - Increased sensitivity, reduced image overlap, and
simpler PSF calibration in GaiaNIR will result if the proposed initial
processing of data from the detectors is implemented, because the across-scan
smearing will become insignificant. - Filter photometry is required for high
angular resolution as needed for astrometric and astrophysical reasons.
Low-dispersion spectra are questioned because they fail at high star density.
This will be a much greater problem with GaiaNIR than it is with Gaia because
of the larger number of stars expected. It was the aim to collect in this note
all arguments about GaiaNIR photometry which can be stated with words only, in
correspondence with readers. The remaining work to be done for the definition
of photometric equipment on the satellite requires further quantitative
assessments and comparison of various options. Finally, 1) an advantage of
filters is that the photometric observations can also be used for astrometry,
2) the XP spectra in Gaia will give very good astrophysical data for about 400
million single stars with G <~ 18.5 mag, but filters would have been better for
all fainter and for all multiple stars, and 3) it is presently not clear which
advantages for astrophysics low-dispersion spectra in the NIR might have over
filters.
