The CALET project is a collaboration to process, distribute and analyse ground and space-based imaging data in the Euclid calibration and deep fields.
It builds on the foundation of the TERAPIX project (1997-2017), which was a national service to reduce data from wide-field cameras and in particular data from CFHT‘s MEGACAM camera. CALET aims to provide data releases from three surveys: the UltraVISTA extension survey, the Hawaii-2-0 survey and the Spitzer large area survey (SLS).
The Euclid mission is satellite which aims to measure the cosmological equation state by combining measurements of the baryon acoustic peak (galaxy clustering) and the amount of correlated ellipticities in background galaxies (gravitational lensing). The two principal instruments on Euclid are a visible camera, VIS, and near-infrared imager and spectrograph, NISP. These measurements, like all modern cosmology experiments, are completely dominated by systematic errors. Two of the most pernicious kinds are shape measurement biases (you are not measuring your galaxy shapes correctly) and photometric redshift errors (you are not measuring your galaxy distances correctly). For a detailed explanation of how the error budget in Euclid is handled, you can read about it here.
For this reason, in addition to the 14,000 degree wide Euclid survey, there is also the deep Euclid survey, covering 40 deg (a nice summary of the characteristics of the Euclid wide and deep fields can be found In this talk from Jarle Brinchmann). The deep fields feature VIS images around forty times the exposure time of the Euclid wide. Like the Euclid wide survey, the Euclid deep survey requires ancillary ground-based data, because the Euclid telescope contains only a single broad-band imager (there are no filters in VIS). And the Euclid ground based data must be much deeper than the wide data. The idea behind these much deeper surveys is to make measurements for the main Euclid wide surveys at much higher signal-to-noise so that we in effect “look the answer up the back of the book” in order to understand what all the possible sources of systematic errors are.
The vocation of the CALET projet is process this deep ancillary data and produce photometry and photometric redshift catalogues like we have done in the past for the COSMOS project. However, this ancillary data has fantastic science potential by itself, even without Euclid. That science potential will be explained in the next post.