Survey scanning and performance
Contents
Scanning strategy[edit]
Planck spins at 1 rpm about the symmetry axis of the spacecraft. The boresight of the telescope is 85◦ from the spin axis. As Planck spins, the instrument beams cover nearly great circles in the sky. The spin axis remains fixed (except for a small drift due to Solar radiation pressure) for between 39 and 65 spins (corresponding to the dwell times given above), depending on which part of its orbit Planck is in. To high accuracy, any one beam covers precisely the same sky between 39 and 65 times. The set of observations made during a period of fixed spin axis pointing is often referred to as a “ring”. This redundancy plays a key role in the analysis of the data, and is an important feature of the scan strategy.
As the Earth and Planck orbit the Sun, the nearly-great circles that are observed rotate about the ecliptic poles. The amplitude of the spin-axis cycloid is chosen so that all beams of both instruments cover the entire sky in one year. In effect, Planck tilts to cover first one Ecliptic pole, then tilts the other way to cover the other pole six months later. If the spin axis stayed exactly on the ecliptic plane, the telescope boresight were perpendicular to the spin axis, the Earth were in a precisely circular orbit, and Planck had only one detector with a beam aligned precisely with the telescope boresight, that beam would cover the full sky in six months. In the next six months, it would cover the same sky, but with the opposite sense of rotation on a given great circle. However, since the spin axis is steered in a cycloid, the telescope is 85◦ to the spin axis, the focal plane is several degrees wide, and the Earth’s orbit is slightly elliptical, the symmetry of the scanning is (slightly) broken. Thus the Planck beams scan the entire sky exactly twice in one year, but scan only 93% of the sky in six months.
Routine operations[edit]
Planck's orbit during routine operations traced a Lissajous path around the L2 point:
Routine operations started on 12 August 2009. For convenience, we call an approximately six month period one “survey”, and use that term as an inexact shorthand for one coverage of the sky. Nine numbered “Surveys” are defined precisely in the Table below, which also shows the fraction of the sky covered by all frequencies.
The fourth Survey was shortened somewhat so that the slightly different scanning strategy adopted for Surveys 5–8 (see below) could be started before the Crab nebula, an important polarization calibration source, was observed. The coverage of the fifth Survey is smaller than the others because several weeks of integration time were dedicated to “deep rings” (defined below) covering sources of special importance.
The spin axis was commanded to follow the cycloidal path across the sky in step-wise displacements of 2′ (Fig. 4). To maintain a steady advance of the projected position of the spin axis along the ecliptic plane, the time interval between two manoeuvres varied between 2360 s and 3904 s. The fraction of time used by the manoeuvres themselves (typical duration of five minutes) varied between 6% and 12%, depending on the phase of the cycloid. The data taken during manoeuvres are not used in the analysis. Over the nominal mission, the total reduction of scientific data due to manoeuvres was 9.2%.
The path followed by the Planck spin axis was defined with respect to the Ecliptic plane. It corresponded to a motion in longitude which maintained an anti-Sun pointing (about one degree per day), to which was added a cycloidal motion (precession) of the spin axis around the anti-Sun position (fiducial point). The cycloidal path was defined by the functions:
$ λ = θ sin((-1)^n ω (t-t_0)+φ) $ (eq. 1),
$β = -θ cos((-1)^n ω(t-t_0) + φ) $ (eq. 2),
where λ is the angular distance from the fiducial point in Ecliptic longitude, β the angular distance from the fiducial point in Ecliptic latitude, θ the spin axis precession amplitude, ω the pulsation of the precession, φ its phase, n the parameter which controls the motion direction of the precession, t is the time, and $t_0$ is the first time during the Planck survey at which the fiducial point crosses the Ecliptic longitude =0 line.
The following precession parameters were chosen for the scanning strategy of Planck:
- Amplitude θ = 7.5deg. (this value is the lowest possible which allows the entire sky to be covered with all detectors).
- n = 1 (anti-clockwise motion as seen from the Sun).
- Pulsation ω = 2π / 6 months (a faster precession was considered, and has interesting advantages, but was not possible given the need to keep a 7.5 deg. amplitude in order to cover the whole sky).
- The phase of the precession was decided according to a set of criteria (in order of importance): (a) respecting the operational constraints; (b) allowing the largest possible angle between two scans on the Crab Nebula (a polarization calibrator); (c) avoiding null dipole amplitude for the whole mission; (d) optimizing the position of the planets in the beginning of the survey and with respect to the feasibility of their recovery; (e) placing the two “deep fields” where Galactic foregrounds are minimum; (f) allowing a reasonable survey margin (i.e. the time allowed to recover lost pointings if a problem occurs). The phase selected was 340 deg. for Surveys 1 to 4, and 250 deg. for Surveys 5 to 8. The cycloid phase was shifted by 90◦ for Surveys 5–8 to optimize the range of polarization angles on key sources in the combination of Surveys 1–8, thereby helping in the treatment of systematic effects and improving polarization calibration.
The scanning strategy for the second year of Routine Operations (i.e. Surveys 3 and 4) was exactly the same as for the first year, except that all pointings were shifted by 1 arcmin along the cross-scanning direction, in order to provide finer sky sampling for the highest frequency detectors when combining two years of observations.
Routine operations were significantly modified twice more:
- The sorption cooler switchover from the nominal to the redundant unit took place on 11 August 2010, leading to an interruption of acquisition of useful scientific data for about two days (one for the operation itself, and one for re-tuning of the cooling chain).
- The satellite’s rotation speed was increased to 1.4 rpm between 8 and 16 December 2011 for observations of Mars, to measure possible systematic effects on the scientific data linked to the spin rate.
Data were acquired in the normal way during the above two periods, but were not used in the generation of products.
The scientific lifetime of the HFI bolometers ended on 13 January 2012 when the supply of 3He needed to cool them to 0.1K ran out. LFI continued to operate and acquire scientific data until 3 October 2013. On 4 October 2013 Planck ended its routine operations, after executing its last observation of the Crab Nebula, and started its decommissioning activities. On 19 October 2013, the sorption cooler and the two instruments (LFI and HFI) were switched off. The final command to the Planck satellite was sent on 23 October 2013, marking the end of operations.
Add something regarding the final orbit.
Operations were extremely smooth throughout the mission. The total observation time lost due to a few anomalies is about 5 days, spread over the 15.5 months of the nominal mission.
The figure below shows the actual path of the spin axis of Planck during its entire lifetime.
Sky coverage[edit]
As an example, the maps below exhibits the progress of sky coverage during the first months of survey (black colour = unobserved areas).
- HFI 353 GHz integration time after N months of survey. The units here are seconds per square degree, normalized to one detector.
After 1 month
After 2 months
After 3 months
After 4 months
After 5 months
After 6 months
After 7 months
After 8 months
After 9 months
After 10 months
After 11 months
After 12 months
A dynamic visualization of sky coverage by the Planck scanning strategy can be found here and here.
As an additional example, the following figures illustrate the distribution of integration time over the sky for the nominal and “0.1-K” (i.e., until the 3He ran out, see Table) missions, for the 353 GHz frequency channel.
[[File:Integration_time353GHz_nominalmission.pdf|thumb|center|500px|
The total integration time after the end of HFI operations (January 2012) is shown in the following maps. Units are seconds per square degree, normalized to one detector, for all maps. The projection is full-sky Mollweide centred on the Galactic centre.
- Integration time after the end of HFI operations (January 2012). The units here are seconds per square degree, normalized to one detector.
HFI 100 GHz 1,4
HFI 100 GHz 2,3
HFI 143 GHz 1-4
HFI 143 GHz 5-8
HFI 217 GHz 1-4
HFI 217 GHz 5-8
HFI 353 GHz
HFI 545 GHz / 857 GHz
LFI-25 (44 GHz)
LFI 30 GHz
Special Observations[edit]
During routine scanning, the Planck instruments naturally observe objects of special interest for calibration. These include Mars, Jupiter, Saturn, Uranus, Neptune, and the Crab nebula. Different types of observations of these objects were performed:
- Normal scans on solar system objects and the Crab nebula. The complete list of observing dates for these objects can be found in Planck Collaboration (2013).
- “Deep rings”. These special scans are performed on observations of Jupiter and the Crab nebula from January 2012 onward. They comprise deeply and finely sampled (step size 0.′5) observations with the spin axis along the Ecliptic plane, lasting typically two to three weeks. Since the Crab is crucial for calibration of both instruments, the average longitudinal speed of the pointing steps was increased before scanning the Crab, to improve operational margins and ease recovery in case of problems.
- “Drift scans”. These special observations are performed on Mars, making use of its proper motion. They allow finely-sampled measurements of the beams, particularly for HFI. The rarity of Mars observations during the mission gives them high priority.
Below is a list of these observations, with the deep rings (Jupiter and the Crab) and drift scans (Mars only) in boldface.
- Crab: 16-22 September 2009
- Mars: 17-29 October 2009
- Jupiter: 25 October – 1 November 2009
- Neptune: 1-8 November 2009
- Uranus: 6-16 December 2009
- Saturn: 2-8 January 2010
- Crab: 6-12 March 2010
- Mars: 9-18 April 2010
- Neptune: 15-23 May 2010
- Saturn: 13-22 June 2010
- Uranus: 27 June – 5 July 2010
- Jupiter: 1-9 July 2010
- Crab: 15-21 September 2010
- Neptune: 3-11 November 2010
- Jupiter: 9-18 December 2010
- Uranus: 12-21 December 2010
- Saturn: 15-22 January 2011
- Crab: 7-12 March 2011
- Neptune: 18-26 May 2011
- Saturn: 30 June - 9 July 2011
- Uranus: 3-11 July 2011
- Jupiter: 9-15 August 2011
- Crab: 10-18 September 2011 (performed with the 250 deg. phase cycloidal strategy)
- Crab: 20-23 September 2011 (performed with the 340 deg. phase cycloidal strategy)
- Mars: 8-16 December 2011 ("drift scan", Mars allowed to move through the Planck focal plane via its proper motion)
- Mars: 17-26 December 2011 (normal scanning)
- Jupiter: 8-30 January 2012 (deep ring)
- Crab: 26 February - 16 March 2012 (deep ring)
- Jupiter: 1-14 September 2012 (deep ring)
- Crab: 14 September - 1 October 2012 (deep ring)
- Neptune: 19-27 November 2012
- Uranus: 24-30 December 2012
- Saturn: 30 January - 5 February 2013
- Jupiter: February 2013 (deep ring)
- Crab: Feb. 28th - March 15th, 2013 (deep ring)
- Neptune: June 4th - June 12th, 2013
- Uranus: July 9-16th, 2013
- Saturn: July 20-26th, 2013
- Crab: Sept. 10th - Oct. 3rd, 2013 (deep ring)
Note that on Sept. 14-16th, 2013, LFI also performed a special dipole calibration observation.
Unplanned events[edit]
The main unplanned events included the following.
- Very minor deviations from the scanning law included occasional (on the average about once every two months) under-performance of the 1-N thrusters used for regular manoeuvres, which implied that the corresponding pointings were not at the intended locations. These deviations had typical amplitudes of 30 arcsec, and had no significant impact on the coverage map.
- The thruster heaters were unintentionally turned off between 31 August and 16 September 2009 (the so-called “catbed” event).
- An operator error in the upload of the on-board command timeline led to an interruption of the normal sequence of manoeuvres and therefore to Planck pointing to the same location on the sky for a period of 29 h between 20 and 21 November 2009 (“the day Planck stood still”). Observations of the nominal scanning pattern resumed on 22 November, and on 23 November a recovery operation was applied to survey the previously missed area. During the recovery period the duration of pointing was decreased to allow the nominal law to be caught up with. As a side effect, the RF transmitter was left on for longer than 24 h, which had a significant thermal impact on the warm part of the satellite.
- As planned, the RF transmitter was initially turned on and off every day in synchrony with the daily visibility window, in order to reduce potential interference by the transmitter on the scientific data. The induced daily temperature variation had a measurable effect throughout the satellite. An important effect was on the temperature of the 4He-JT cooler compressors, which caused variations of the levels of the interference lines that they induce on the bolometer data (Planck HFI Core Team 2011a). Therefore the RF transmitter was left permanently on starting from 25 January 2010 (257 days after launch), which made a noticeable improvement on the daily temperature variations.
- During the coverage period, the operational star tracker switched autonomously to the redundant unit on two occasions (11 January 2010 and 26 February 2010); the nominal star tracker was restored a short period later (3.37 and 12.75 h, respectively) by manual power-cycling. Although the science data taken during this period have normal quality, they have not been used, because the redundant star tracker’s performance is not fully characterized.
For more information, see [1]. For a complete list of operational events, including contigencies, see the POSH (Planck operational state history).
References[edit]
- ↑ Planck early results. I. The Planck mission, Planck Collaboration I, A&A, 536, A1, (2011).
revolutions per minute
(Planck) High Frequency Instrument
(Planck) Low Frequency Instrument
Operation Day definition is geometric visibility driven as it runs from the start of a DTCP (satellite Acquisition Of Signal) to the start of the next DTCP. Given the different ground stations and spacecraft will takes which station for how long, the OD duration varies but it is basically once a day.
