Large-scale retrospective relative spectro-photometric self-calibration
  in space
  We consider the application of relative self-calibration using overlap
regions to spectroscopic galaxy surveys that use slit-less spectroscopy. This
method is based on that developed for the SDSS by Padmanabhan at al. (2008) in
that we consider jointly fitting and marginalising over calibrator brightness,
rather than treating these as free parameters. However, we separate the
calibration of the detector-to-detector from the full-focal-plane
exposure-to-exposure calibration. To demonstrate how the calibration procedure
will work, we simulate the procedure for a potential implementation of the
spectroscopic component of the wide Euclid survey. We study the change of
coverage and the determination of relative multiplicative errors in flux
measurements for different dithering configurations. We use the new method to
study the case where the flat-field across each exposure or detector is
measured precisely and only exposure-to-exposure or detector-to-detector
variation in the flux error remains. We consider several base dither patterns
and find that they strongly influence the ability to calibrate, using this
methodology. To enable self-calibration, it is important that the survey
strategy connects different observations with at least a minimum amount of
overlap, and we propose an "S"-pattern for dithering that fulfills this
requirement. The final survey strategy adopted by Euclid will have to optimise
for a number of different science goals and requirements. The large-scale
calibration of the spectroscopic galaxy survey is clearly cosmologically
crucial, but is not the only one.
