Timelines

General description

The timelines, or TOIs for "time-ordered information," are vectors of signal or of pointing or of some other quantity, giving the value of that quantity as a function of time during the mission. For LFI, refer to Planck-2015-A02, and for HFI, to Planck-2015-A07.

The TOIs described here are sampled regularly at the (instrument-dependent) detector sampling frequency and span the full science mission. They thus consist of a large amount of data. This section is concerned with the signal TOIs and their associated pointing TOIs. For selected detectors the DPCs provide a single timeline of cleaned and calibrated signal and three timelines of coordinates, corresponding to the two angular coordinates and one orientation angle.

Obviously these vectors are very long (about 1.38×10¹⁰ samples for HFI, from 2.5×10⁶ to 5.5×10⁶ for LFI) and thus need to be split into multiple files for export. Here the data are split by operational day (OD) as follows:

• HFI has 12 files per OD, one for pointing and one for signal for each channel, covering ODs 91 to 974, all sampled at F_samp = 180.3737 Hz;
• LFI has six files per OD, three each for signal and pointing, since the sampling frequency is channel dependent, and covering ODs 91 to 1543, sampled at F_samp = 32.5079, F_samp = 46.5455, and F_samp = 78.7692 Hz.

The signal timelines are encoded as single-precision real values, but the pointing vectors had to be encoded as double-precision reals to maintain the required accuracy. The result is that the total volume of the full dataset is approximately 30 TB. All files also contain the OBT.

The signal timelines have been cleaned of all known instrumental systematic effects, they have been calibrated in flux, and the solar and Earth motion dipole signals have been removed. But they have not been cleaned of the low-frequency, "1/f" noise that needs to be removed via a destriping tool. The methods consists of removing offsets or "baselines" determined by minimizing the differences in the signals at points on the sky where they intersect. For HFI an offset per ring is determined; for LFI the the baseline is computed every 0.246, 0.988, and 1.000 seconds for the 30, 44, and 70 GHz channels, respectively and maintain the same structure of the signal timelines.

In the case of HFI these offsets are determined using the full mission and all the valid detectors per channel, and they are then used for all the maps produced, i.e., for those maps using any fraction of the mission (year, survey) or any subset of the detectors (single detector, detector set), and it has been shown that using offsets determined from a limited part of the mission and/or a subset of the detectors yields maps that are less consistent than otherwise.

In the case of LFI these offsets are determined using the full mission and all the valid detectors per channel; those values have been used for the production of the full mission period maps. Note that baseline used for shorter period maps are determined on those data periods to avoid noise cross-correlation effects and those are not delivered.

The offsets are delivered separately, as described below.

Furthermore, for HFI and LFI, there are three sets of offsets produced: the primary set using the full rings; and the secondary ones using the first and second half rings, only. The differences between the primary and secondary sets are fairly minor, but they are necessary to include to reconstruct the maps as they were built by the HFI-DPC and LFI-DPC. In particular the offsets are useful for users wishing to build a map of a small area of the sky.

The HFI delivers its offsets in a ROI, or "ring-ordered Information" file. That file contains a table of N_rings = 26766 rows by N_bolometers in which each cell contains a 3-element vector containing the full-ring, first half-ring, and second half-ring offsets. The indices of the signal TOI delimiting the rings are given in a second ROI file with the same N_rings rows containing global parameters.

The LFI delivers its offsets in a TOI format, the structure is exactly the same as that used by the science timelines. The user can simply subtract one-by-one the offset timelines from the science timelines and then generate a map using the result. In the case of the half-ring baselines, a vector has been added in the OBT extension; this vector contains "1" or "2," depending which half ring it should be applied to.

HFI indexing

Each FITS file contains keywords giving the first and last index of the section of the full TOI contained in the file. These indices are those of the internal DPC representation; they begin at 0, corresponding to the switch-on of the instrument, and run to some very large number (about 25 billion for HFI) representing the full mission. Only the science part of the mission is exported, which is about 60% of the total for HFI. Left out are the early mission phase (cool-down and validation phases) and the warm phase of HFI, during which LFI continued to collect data. The HFI keywords also indicate which rings are included in each OD, but note that normally an OD does not begin at the same time (with the same index) as a ring.

Note that for historical reasons, the OD definitions of the two DPCs differ; for LFI they occur at the transition between pointing periods, whereas for HFI they do not. However, this has no significance for the user, since this splitting is somewhat arbitrary anyway, and what counts is the full vector once it is rebuilt in the user's own work space.

HFI timelines at the DPC are indexed from 0 to 25×10⁹, which correspond to instrument switch-on and switch-off, respectively. Of these the indices 1.4×10⁹ to 151.5×10⁹ correspond to the science mission and are exported and delivered. Each file contains a keyword giving the first and last index of the data in that file, and EndIndex(OD)+1 = BeginIndex (OD+1). The ROI "Global" file gives the Begin/End Indices of each Ring, or Pointing Period, and can be used to destripe the signal TOIs with the offsets provided in the "Destriping-Offsets" ROI file.

Production process

HFI processing

The input TOIs are in ADUs, representing the voltage signal at the end of the electronics (see Detection chain for details). The processing applied to remove instrumental effects and to calibrate them is described in detail in the HFI TOI processing section, and in Planck-2015-A07 we give a very brief summary here for convenience. This pipeline performs the following operations.

ADC correction

This corrects for the uneven size of the ADC bins (see ADC correction).

Demodulation

This is performed around a variable level, which is determined from the valid input data (using a validity flag from a previous version of the processing), and the data are converted to engineering units (V) using known conversion coefficients.

Despiking

The redundancy within a ring is used to determine where glitches occur. Once identified, the glitches are fitted with templates. A glitch flag is produced that identifies the strongest part of the glitches, and a timeline of glitch tails. produced from the template fits, is subtracted from the demodulated timeline. Finally, the flagged regions are replaced with data from an average over the pointing period in order to not leave holes in the timeline that would perturb the usage of Fourier methods on them

Dark template removal

The two dark bolometers are demodulated and despiked as above; the resulting timelines are then smoothed and used as an indicator of the overall temperature variations of the bolometer plate. Where the variations are consistent with each other, they are combined and removed from the bolometer signal timelines using appropriate coupling coefficients. The few percent of the data where they are not consistent are flagged as not valid on the timelines.

Conversion to absorbed power

The timeline is converted to watts of absorbed power using the bolometer function. This includes a nonlinearity correction and removal of the 4-K cooler lines (i.e., the electromagnetic interference of the 4-K cooler with the bolometer readout wires, which induces some sharp lines in the signal power spectra at frequencies of the 4-K cooler's fundamental and its multiples, folded by the signal modulations). Fourier coefficients of the relevant lines are determined on a per-ring basis, and then removed from the data. The quality of the removal depends on the bolometer.

Deconvolution by the time transfer function

This is done to correct for the non-instantaneous time response of the bolometers. The function itself is modeled using up to eight time constants, which are adjusted primarily on planet data and also from comparisons of the northward and southward scans of the Galactic Plane. It is then removed using Fourier techniques.

Jump correction

This corrects some rare jumps in the signal baseline (there are on average around 0.3 jumps per bolometer per pointing period). The jumps are detected and characterized on smoothed TOIs, and corrected by adding a constant to part of the signal timeline. The origin of the jumps is not known.

The results of this set of processing steps are a timeline of signal (in absorbed watts) and a "valid data" flag timeline for each of the 50 valid bolometers that are processed, out of the 52 HFI bolometers; these timelines contain the full-sky signal, i.e., including the solar and orbital dipoles, the Zodiacal light, and contributions from the far sidelobes. The dipoles are necessary for the flux calibration and are removed at the mapmaking stage. The remaining two bolometers (143-8 and 545-3) show semi-random jumps in the signal level, typically jumping over 2 to 5 different "pseudo-baseline" levels, a behaviour known as "random telegraphic signal" (RTS), so that these are commonly called the RTS bolometers. Finally, ring-level statistics of different types (mean, median, rms, kurtosis, etc.) are determined on a per-ring basis for all timelines, and a selection based on these statistics is used to discard anomalous rings, which are recorded in a ring-level flag for each valid bolometer timeline (see the Discarded rings section).

Throughout this processing, some planet signals are masked in the timeline in order to avoid ringing effects in the processing. This concerns only Jupiter at 100 and 143 GHz, Jupiter and Saturn at 217 GHz, and Jupiter, Saturn, and Mars at 353, 545, and 857 GHz (thanks to various improvements in the despiking and in the transfer functions it is no longer necessary to flag these three planets in the CMB channels, as was done in the 2013 release). Since they move on the sky, the portion of the sky masked during one survey is observed during another one, and no hole is left in the final map. In parallel, the planet data are processed in a similar way, but with different parameters for the despiking step. These results are processed separately to determine the beam shapes and the focal plane geometry.

Next, the TOIs are calibrated in astrophysical units using the results of the calibration pipeline (see the Map-making and calibration section) and the best estimates of the zero-point offsets (a constant level for each bolometer). These values are given in the RIMO. Also, the solar and Earth-motion dipole signals are computed and removed.

These TOIs are accompanied by several flags that are described below. The most important one is the "Total" flag, which identifies all the samples that were discarded by the DPC mapmaking, as described in HFI TOI processing. This flag includes the portion beyond 72 min for the rings that are longer; this is not used for science analysis because of the slight drift of the satellite pointing direction (spin axis) during these long acquisition periods.

At this point the TOIs still contain the low frequency (1/f) noise, which should be removed before projection onto a map. That cleaning step is called "destriping" or "baseline removal". The HFI-DPC does its destriping at ring-level, meaning that a constant is added to the signal of each ring in order to minimize the difference at the ring crossings, where the signal should be the same for all detectors (with the necessary precautions as described in Map-making and calibration section); should the user want to use the DPC's offsets, they are provided separately (see ROI files below). Maps produced from these TOIs, and after subtraction of the DPC's destriping offsets, are not identical to the maps that are delivered. This is discussed in Section A.2 of Planck-2015-A08.

As indicated above, the brightest planets are masked in the TOI in order to avoid ringing problems. For users specifically wanting to study these planets, we provide separate timelines covering just the planet transits. These timelines also include transits of Uranus and Neptune, which are not masked in the regular TOIs. They are produced with less agressive deglitching options in order to work on the rapidly changing baselines. These are the data used the reconstruction of the focal plane geometry and also for the determination of the scanning beams.

The table below lists the 16 transits, giving for each the begin/end ring number and the begin/end OD in which that ring is found. Note that transit 15 is split into two parts because some tests were done during the transit. The last row is a region processed in the same manner but without a planet transit, which is included for comparison.

Planet transits

Number	Beg Ring	End Ring	Beg OD	End OD	Comment
1	2700	3211	160	176	Jupiter, Mars, Neptune
2	3892	4099	203	212	Uranus
3	4575	4775	233	240	Saturn
4	7900	8150	330	337	Mars
5	8979	9188	367	376	Neptune
6	9550	9750	392	402	Saturn
7	9940	10300	411	425	Jupiter, Uranus
8	14018	14227	537	544	Neptune
9	14820	15190	567	583	Jupiter, Uranus
10	15925	16275	612	624	Saturn
11	20016	20225	734	743	Neptune
12	20900	21650	775	804	Saturn, Uranus
13	21780	22150	808	818	Jupiter
14	24992	25202	916	921	Neptune
15a	25830	25849	937	938	Mars, Uranus
15b	25864	26225	947	957	Mars, Uranus
16	26650	27005	968	974	Jupiter
17	12000	12150	479	483	background

The pointing (see common Detector Pointing section for details) is determined from the AHF produced by the MOC, which gives the direction and orientation of the LOS of a fiducial position in the focal plane at frequencies of 8Hz during stable pointing and 4 Hz during manoeuvres. This is interpolated to the timestamps of data samples, corrected for the wobble and other time-dependent offsets determined from the observed positions of a large number of sources around the sky, and finally converted to the LOS of each detector using the quaternions in the IMO. These pointing data are valid for both the regular and the planet TOIs.

LFI processing

The input TOIs are in ADUs, representing the voltage signal at the output of the electronics (see Radiometer Chain Assembly (RCA) for details). The processing applied to remove instrumental effects and to calibrate them is described in detail in the LFI TOI processing section, and in Planck-2015-A03; here we give a very brief summary for convenience. That pipeline performs the following operations.

ADC correction

Due to ADC nonlinearity under certain condition, this instrumental effect is removed by applying well know templates directly to the diode signal.

Electronics spikes

This is caused by the interaction between the electronics clock and the scientific data lines. The signal is detected in all the LFI radiometer time-domain outputs as a 1-s square wave with a rising edge near 0.5s and a falling edge near 0.75s, synchronous with the onboard time signal. In the frequency domain it appears as a spike signal at multiples of 1Hz. The 44-GHz channels are the only LFI outputs significantly affected by this effect. Consequently the spike signal is removed from the data only in these channels.

Demodulation

Each LFI diode switches at 4096Hz between the sky and the 4-K reference load. The data acquired in this way are dominated by 1/f noise that is highly correlated between the two streams; differencing those streams results in a strong reduction of 1/f noise. The procedure applied is described in LFI TOI processing, taking into account that the gain modulation factor R was computed on timestreams with the Galaxy and point sources masked to avoid strong sky signals.

Diode combination

Two detector diodes provide the output for each LFI receiver channel. To minimize the impact of imperfect isolation of the data stream of each diode, we perform a weighted average of the time-ordered data from the two diodes of each receiver. The procedure applied is detailed in LFI TOI processing; and the weights used are kept fixed for the entire mission.

Scientific calibration

This step calibrates the timelines to physical units K_CMB, fitting the total CMB dipole convolved with the 4π beam representation, without taking into account the signature due to Galactic stray light.

Gain regularization

The calibration constants computed using the model of the dipole signal suffer from large uncertainties when the Planck spacecraft is badly aligned with the dipole axis. To reduce the noise, we apply an adaptive smoothing algorithm, which is also designed to preserve the discontinuities caused by abrupt changes in the working configuration of the radiometers (e.g., sudden temperature changes in the focal plane).

Removal of solar and orbital dipole signal

The combined solar and orbital dipole is convolved with the 4π beam representation of each radiometer and then removed from its timeline.

Removal of Galactic stray light

The light incident on the focal plane without reflecting on the primary mirror (stray light) is a major source of systematic effects, especially when the Galactic plane intersects the direction of the main spillover. This effect is corrected by removing the estimated straylight signal from the timelines. This signal is computed as the convolution of Galaxy simulation with the beam sidelobes (see details in LFI TOI processing).

After these processing steps the timelines are used for the production of the maps.

The pointing (see common Detector Pointing section for details) is determined from the AHF produced by the MOC, which gives the direction and orientation of the LOS of a fiducial position in the focal plane at frequencies of 8Hz during stable pointing and 4 Hz during manoeuvres. This is interpolated to the timestamps of data samples, corrected for the wobble and other time-dependent offsets determined from the observed positions of a large number of sources around the sky, as well as temperature sensors, and finally converted to the LOS of each detector.

File Names

The file names are of the form:

{H,L}FI_TOI_{fff}-{SCI,PTG,TAI,OFF}_R2.nn_ODxxxx.fits

where

• fff denotes the frequency
• SCI denote signal TOIs
• PTG denote pointing TOIs
• TAI denote OBT-MJD correlation TOI
• OFF denote Baseline TOI
• R2.nn is the version, and
• ODxxxx indicates the OD.

Regarding the HFI TOIs for the 100-217 channels, the R2.00, made public in Jan 2015, contained only the unpolarised bolometer timelines, while R2.02, made public in July 2015, contains all bolometers.

The HFI Planet timelines are named:

{H,L}FI_TOI_{fff}-SCI-planets_R2.nn_ODxxxx.fits

and are provided for all 50 valid bolometers, i.e., SWBs and PSBs.

FITS file structure

All FITS files begin with a primary extension containing no data and a minimal header, which is followed by one or more BINTABLE extensions with data, and with a description of the data in the header keywords. In what follows will usually ignore the primary extension, and count only the extensions containing data.

TOI files

The signal FITS files contain N+1 ,'BINTABLE', data extensions, where N is the number of detectors in that frequency channel. The first extension contains the OBT and its flags, called the global flags since they apply to all detectors, which is followed by one extension for each detector, containing the signal and its local flag. The flags columns are written as byte in which each of the 8 bits (max) encodes one flag timeline. The meaning of each bit is given in the header comments and is subject to change. For the global flag they include:

• HFI
  • Unstable pointing: 1= pointing is not stable (e.g., during repointing maneuvers)
  • Dark correlation: 1 = darks are uncorrelated and data are flagged
  • First/Second half ring: which samples are in which half (only covers the stable pointing part of the ring)
  • HCM: in HCM mode (unstable pointing)
  • and more - see extension header for details

• LFI
  • Bit 0, unstable pointing: 1= pointing is not stable
  • Bit 1, time correlation quality: 1= outside specification
  • Bit 2, special observation: 1= special observation like deep scan

And for the local flag they include

• HFI
  • Total Flag: a combination of the various flags that is the one finally used in the map-making (all samples with Total Flag different from zero should not be used)
  • Data not valid: glitched samples
  • Despike Common: (for PSBs only) glitch on current or other of PSB pair
  • StrongSignal: on Galactic Plane
  • Strong Source: on point source
  • and more - see extension header for details

• LFI
  • Bit 0, Data not valid: 1= Science sample not valid
  • Bit 2: Planet crossing: 1= Science Sample containing planet
  • Bit 3: Moving objects: 1= minor Solar System object (not yet used)
  • Bit 4: Gap: 1= this sample was artificially included due to gap in the data

for HFI, the header extension gives more details on the flags and the meaning of 1 / 0.

TOI file data structure

1. EXTNAME = 'OBT' : Data columns
Column Name	Data Type	Units	Description	Comment
OBT	Double	2-16 sec	On-board time
FLAG	Byte	none	the various bit-level flags
Keyword	Data Type	Value	Description	Comment
OD	Int		OD covered (as in filename)
BEGIDX	Int		first sample index of given OD	Only HFI
ENDIDX	Int		last sample index of given OD	Only HFI
BEGRING	Int		first ring in given OD	Only HFI
ENDRING	Int		last ring in given OD	Only HFI
TIMEZERO	String	1958-01-01z00:00	Origin of OBT	Only HFI
TIMEZERO	Float	106744000000000.	Origin of OBT	Only LFI
n. EXTNAME = DETNAME : Data columns
Column Name	Data Type	Units	Description	Comment
SIGNAL	Real*4	Kcmb or MJy/sr	Value of signal	Comment
FLAG	Byte	none	the various bit-level flags
Keyword	Data Type	Value	Description	Comment
UNIT	String		Units of signal	Only HFI, LFI always Kcmb
DESTRIPE	1/0		whether timeline is destriped	Only HFI
OD	Int		OD covered (as in filename)
BEGIDX	Int		first sample index of given OD	Only HFI
ENDIDX	Int		last sample index of given OD	Only HFI
BEGRING	Int		first ring in given OD	Only HFI
ENDRING	Int		last ring in given OD	Only HFI

The HFI Planet files have the same structure, but the local flags contain a single "Data not valid" flag. Also, the first and last OD of each transit is usually not complete, as they contain only the rings that are included in the transit observations.

The pointing files have a similar structure, except that DETNAME extensions contain 3 columns of Real*8 with the phi, theta, and psi Galactic spherical coordinates of each sample in radians, as shown in the table below. There is no local flag for the coordinates.

Pointing TOI file DETNAME extension structure

n. EXTNAME = DETNAME : Data columns
Column Name	Data Type	Units	Description
PHI	Real*8	radian	longitude
THETA	Real*8	radian	colatitude
PSI	Real*8	radian	roll angle

HFI ROI files

The files provided by HFI are

• HFI_ROI_GlobalParams_RelNum_full.fits
• HFI_ROI_DestripeOffsets_RelNum_full.fits

which are described below.

Global parameters

this ROI file contains a single BINTABLE extension with 3 columns that give, for each ring, the index number of the first sample, the ESA pointing ID of the ring, and the ring start time. All three are encoded as Int*8, and the time is given in units of 2^-16 sec from TIMEZERO, 1958-01-01z00:00, as in the TOI files. Rings are numbered from 240 to 27005, as given in the BEGRING and ENDRING keywords

Destriping offsets

this ROI file contains a single BINTABLE extension with 3 columns that give, for each ring, the offsets to subtract from each bolometer signal timeline or vector. There are three offsets available for each bolometer: full-ring, first half-ring, and second half-ring, for building the full-ring or the two half-ring maps, respectively. The offsets must be subtracted from Index(N) to Index(N+1)-1 of the corresponding signal timeline, where N is the ring number, and the indices are given in the Global parameters file. The offsets are given in the same units as the signal vectors, that is K_cmb for the 100-353 GHz channels, and MJy/sr for the 545 and 857 GHz channels. All values are encoded as Real*4. The rings for which the offsets are set to 0.000 are badrings which are rejected in the mapmaking process. These rings are also flagged by the TotalFlag, as are the portions of rings longer than ~72.5 min, where the drift in the satellite's spin axis becomes important.

TAI TOI files

The TAI TOI files contains one extension with two column, the first is the OBT value (exactly the same reported in the SCI TOI), the second the corresponding Modified Julian day. Note that Leap second where not added.

Time TAI TOI file data structure

1. EXTNAME = 'OBT-MJD' : Data columns
Column Name	Data Type	Units	Description	Comment
OBT	Int*8	2-16 sec	On-board time
MJD	Real*8	day	Modified Julian day

LFI OFF TOI files

The OFF (baseline) TOI files adopt the same file structure of the Science TOI files. Note that in case of OFF timelines related to the half-ring, and addition column in included in the OBT extension to define for each sample if it belong to half-ring 1 or half-ring2.

LFI HouseKeeping files

House keeping timelines are:

• LFI_TOI_DAE-FAST-HK_R2.nn_ODxxxx.fits
• LFI_TOI_DAE-SlowConfiguration-HK_R2.nn_ODxxxx.fits
• LFI_TOI_DAE-SlowCurrent-HK_R2.nn_ODxxxx.fits
• LFI_TOI_DAE-SlowPhaseSwitch-HK_R2.nn_ODxxxx.fits
• LFI_TOI_DAE-SlowVoltage-HK_R2.nn_ODxxxx.fits
• LFI_TOI_REBA-HK_R2.nn_ODxxxx.fits
• SCS_TOI_EssentialASW-HK_R2.nn_ODxxxx.fits
• SCS_TOI_NonEssentialASW-HK_R2.nn_ODxxxx.fits

Each file contains two extension, the first is the OBT (value are sampled at 1 or 10 seconds), the latter contain a variable number of column = 2* number of HouseKeeping stored. Each Housekeeping is accompanied by its flag (normally 0 is NOT 0 the value was considerate invalid or Out Of Limit). The Housekeeping name are the once defined in the LFI Instrnument Operation Manual.