Healpix Rings LFI

LFI ring objects are constructed from calibrated and destriped timelines (computed using the entire data set). The objects are thus already cleaned of correlated noise as far as possible. The dipole and far sidelobe contribution are removed. Flagged TOI samples are discarded, including the pointing manoeuvre. The objects are not corrected for bandpass or for beam effects.

The objects are constructed as follows. For each sample, the theta, phi pointing angles in Galactic coordinates are converted into a Healpix pixel number at resolution Nside=1024 in nested pixeling scheme. The psi angle is discretized by dividing the range [0,2pi[ uniformly into Npsi=4096 bins. The first bin covers the angle [0,2pi[/Npsi. This corresponds to an angular resolution of 3.5' in theta, phi, and 5.3' in psi. The combination of pixel number and psi index defines a LFI ring bin.

The TOI samples that fall into same bin are coadded. Along with the coadded signal, the ring objects store the number of samples into the bin. Only elements with non-zero hit count are stored.

Coadding a TOI into a ring object at resolution Nside=1024 typically compresses it by a factor of 20. The typical size of a 4D map file is 8 GB for each horn. The full LFI data set takes 90 GB of disk space.

The main differences with respect to HFI rings is that LFI rings store the coadded signal instead of the average, and that the distribution of psi angles is preserved.

FITSIO file format

The ring objects are stored as FITSIO files with two binary tables.

One ring object holds the data for one horn. Because the M pointing is used for both radiometers, and flagging is symmetrical, pixel number, psi index and hit count columns are identical and only need to be stored once.

The first data extension (HDU 2) has 5 columns:

1. pixel: Healpix pixel number (int32)
2. ipsi: psi index (int32)
3. weight: Number of hits per radiometer (float32)
4. signal1: Coadded signal for radiometer M (float32)
5. signal2: Coadded signal for radiometer S (float32)

Pointing periods are appended one after another. Within a pointing period, data is arranged first in the order of increasing pixel number, and within the same pixel number, in the order of increasing psi. The signal column is in the units of Kelvin. Though usually an integer, the number of hits is stored as float to allow for non-uniform weighting.

Another data extension (HDU 3) contains the pointing period information that is needed for the interpretation of data of HDU 2. HDU 3 contains five columns, one row per pointing period.

1. pointingID: Pointing period identifier (int32)
2. sample_offset: Starting point (zero-based offset) of the pointing period in HDU 2 (int64)
3. nsamples_PID: Number of rows belonging to the current pointing period (int32)
4. start_SCET: Start time of the pointing period (float64), in SCET time units
5. end_SCET: End time of the pointing period (float64), in SCET time units

Resolution parameters are stored in header keywords Nside and Npsi. Other keywords specify the Healpix pixeling scheme Ordering=NESTED, and the direction of polarization sensitivity in radians (psipol1, psipol2, for M and S radiometers, respectively. An estimate of the white noise stDev per sample in TOI domain is stored in keywords sigma1 and sigma2. White noise per 4D element will be this divided by sqrt(number of hits).