CRAFTS for H i Cosmology. I. Data Processing Pipeline and Validation Tests

We present the calibration procedures and validation of source measurement with the data of the Commensal Radio Astronomy FAST Survey for H i intensity mapping by the Five-hundred-meter Aperture Spherical Radio Telescope. Using a 70 hr drift-scan observation with the L-band (1.05–1.45 GHz) 19 beam receiver, we obtain the data covering a 270 deg2 sky area. We employ both the pulsar backend and the spectrum backend to calibrate the spectral time-ordered data (TOD) before projecting them onto HEALPix maps. We produce calibrated TOD with a frequency resolution of 30 kHz and time resolution of 1 s and the map data cube with a frequency resolution of 30 kHz and spatial resolution of 2.95 arcmin2. We examine the pointing errors, noise overflow, radio-frequency interference (RFI) contamination, and their effect on the data quality. The resulting noise level is ∼5.7 mJy for the calibrated TOD and 1.6 mJy for the map, consistent with the theoretical predictions within 5% at RFI-free channels. We also validate the data by principal component analysis and find that the residual map looks thermal noise dominated after removing 30 modes. We identify 447 isolated bright continuum sources in our data matching the NRAO VLA Sky Survey catalog, with relative flux error of 8.3% for TOD and 6.6% for the map level. We also measure the H i emission of 90 galaxies with redshift z < 0.07 and compare them with H i-MaNGA spectra, yielding an overall relative H i integral flux error of 16.7%. These results provide an important first step in assessing the feasibility of conducting cosmological H i detection with CRAFTS.