Difference between revisions of "Effective Beams"
(→Comparison of the images of compact sources observed by Planck with FEBeCoP products)
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<gallery widths=heights=perrow=caption="Comparison images of compact sources and effective beams, PSFs">
Revision as of 16:36, 18 June 2013
- 1 Product description
- 1.1 Comparison of the images of compact sources observed by Planck with FEBeCoP products
- 1.2 Histograms of the effective beam parameters
- 1.3 Sky variation of effective beams solid angle and ellipticity of the best-fit Gaussian
- 1.4 Statistics of the effective beams computed using FEBeCoP
- 2 Production process
- 3 Inputs
- 4 Related products
- 5 File Names
- 6 Retrieval of effective beam information from the PLA interface
- 7 Meta data
- 8 References
The effective beam is the average of all scanning beams pointing at a certain direction within a given pixel of the sky map for a given scan strategy. It takes into account the coupling between azimuthal asymmetry of the beam and the uneven distribution of scanning angles across the sky. It captures the complete information about the difference between the true and observed image of the sky. They are, by definition, the objects whose convolution with the true CMB sky produce the observed sky map.
The full algebra involving the effective beams for temperature and polarisation was presented in [Mitra, Rocha, Gorski et al.] #mitra2010, and a discussion of its application to Planck data is given in the appropriate LFI #planck2013-p02d and HFI #planck2013-p03c papers. Relevant details of the processing steps are given in the Effective Beams section of this document.
Comparison of the images of compact sources observed by Planck with FEBeCoP products
We show here a comparison of the FEBeCoP derived effective beams, and associated point spread functions, PSF (the transpose of the beam matrix), to the actual images of a few compact sources observed by Planck, for all LFI and HFI frequency channels, as an example. We show below a few panels of source images organized as follows:
- Row #1- DX9 images of four ERCSC objects with their galactic (l,b) coordinates shown under the color bar
- Row #2- linear scale FEBeCoP PSFs computed using input scanning beams, Grasp Beams, GB, for LFI and B-Spline beams,BS, Mars12 apodized for the CMB channels and the BS Mars12 for the sub-mm channels, for HFI (see section Inputs below).
- Row #3- log scale of #2; PSF iso-contours shown in solid line, elliptical Gaussian fit iso-contours shown in broken line
Histograms of the effective beam parameters
Here we present histograms of the three fit parameters - beam FWHM, ellipticity, and orientation with respect to the local meridian and of the beam solid angle. The shy is sampled (pretty sparsely) at 3072 directions which were chosen as HEALpix nside=16 pixel centers for HFI and at 768 directions which were chosen as HEALpix nside=8 pixel centers for LFI to uniformly sample the sky.
Where beam solid angle is estimated according to the definition:(effbeam)/max(effbeam) i.e.,
Sky variation of effective beams solid angle and ellipticity of the best-fit Gaussian
- The discontinuities at the Healpix domain edges in the maps are a visual artifact due to the interplay of the discretized effective beam and the Healpix pixel grid.
Statistics of the effective beams computed using FEBeCoP
We tabulate the simple statistics of FWHM, ellipticity (e), orientation ( ) and beam solid angle, ( ), for a sample of 3072 and 768 directions on the sky for HFI and LFI data respectively. Statistics shown in the Table are derived from the histograms shown above.
- The derived beam parameters are representative of the DPC NSIDE 1024 and 2048 healpix maps (they include the pixel window function).
- The reported FWHM_eff are derived from the beam solid angles, under a Gaussian approximation. These are best used for flux determination while the the Gaussian fits to the effective beam maps are more suited for source identification.
|frequency||mean(fwhm) [arcmin]||sd(fwhm) [arcmin]||mean(e)||sd(e)||mean() [degree]||sd() [degree]||mean() [arcmin ]||sd() [arcmin ]||FWHM_eff [arcmin]|
Beam solid angles for the PCCS
- - is the mean beam solid angle of the effective beam, where beam solid angle is estimated according to the definition: (effbeam)/max(effbeam), i.e. as an integral over the full extent of the effective beam, i.e. .
- is the beam solid angle estimated up to a radius equal to one and up to a radius equal to twice the .
|Band||[arcmin ]||spatial variation [arcmin]||[arcmin ]||spatial variation-1 [arcmin]||[arcmin ]||spatial variation-2 [arcmin]|
FEBeCoP, or Fast Effective Beam Convolution in Pixel space [Mitra, Rocha, Gorski et al.], is an approach to representing and computing effective beams (including both intrinsic beam shapes and the effects of scanning) that comprises the following steps:
- identify the individual detectors' instantaneous optical response function (presently we use elliptical Gaussian fits of Planck beams from observations of planets; eventually, an arbitrary mathematical representation of the beam can be used on input)
- follow exactly the Planck scanning, and project the intrinsic beam on the sky at each actual sampling position
- project instantaneous beams onto the pixelized map over a small region (typically <2.5 FWHM diameter)
- add up all beams that cross the same pixel and its vicinity over the observing period of interest
- create a data object of all beams pointed at all N'_pix_' directions of pixels in the map at a resolution at which this precomputation was executed (dimension N'_pix_' x a few hundred)
- use the resulting beam object for very fast convolution of all sky signals with the effective optical response of the observing mission
Computation of the effective beams at each pixel for every detector is a challenging task for high resolution experiments. FEBeCoP is an efficient algorithm and implementation which enabled us to compute the pixel based effective beams using moderate computational resources. The algorithm used different mathematical and computational techniques to bring down the computation cost to a practical level, whereby several estimations of the effective beams were possible for all Planck detectors for different scanbeam models and different lengths of datasets.
Pixel Ordered Detector Angles (PODA)
The main challenge in computing the effective beams is to go through the trillion samples, which gets severely limited by I/O. In the first stage, for a given dataset, ordered lists of pointing angles for each pixels---the Pixel Ordered Detector Angles (PODA) are made. This is an one-time process for each dataset. We used computers with large memory and used tedious memory management bookkeeping to make this step efficient.
One important reason for separating the two steps is that they use different schemes of parallel computing. The PODA part requires parallelisation over time-order-data samples, while the effBeam part requires distribution of pixels among different computers.
The computation of the effective beams has been performed at the NERSC Supercomputing Center. The table below shows the computation cost for FEBeCoP processing of the nominal mission.
|PODA/Detector Computation time (CPU hrs)||85||100||250||500||500||500||500||500||500|
|PODA/Detector Computation time (wall clock hrs)||7||10||20||20||20||20||20||20||20|
|Beam/Channel Computation time (CPU hrs)||900||2000||2300||2800||3800||3200||3000||900||1100|
|Beam/Channel Computation time (wall clock hrs)||0.5||0.8||1||1.5||2||1.2||1||0.5||0.5|
|Convolution Computation time (CPU hr)||1||1.2||1.3||3.6||4.8||4.0||4.1||4.1||3.7|
|Convolution Computation time (wall clock sec)||1||1||1||4||4||4||4||4||4|
|Effective Beam Size (GB)||173||123||28||187||182||146||132||139||124|
The computation cost, especially for PODA and Convolution, is heavily limited by the I/O capacity of the disc and so it depends on the overall usage of the cluster done by other users.
In order to fix the convention of presentation of the scanning and effective beams, we show the classic view of the Planck focal plane as seen by the incoming CMB photon. The scan direction is marked, and the toward the center of the focal plane is at the 85 deg angle w.r.t spin axis pointing upward in the picture.
The Focal Plane DataBase (FPDB)
The FPDB contains information on each detector, e.g., the orientation of the polarisation axis, different weight factors, (see the instrument RIMOs):
The scanning strategy
The scanbeam modeled for each detector through the observation of planets. Which was assumed to be constant over the whole mission, though FEBeCoP could be used for a few sets of scanbeams too.
- LFI: GRASP scanning beam - the scanning beams used are based on Radio Frequency Tuned Model (RFTM) smeared to simulate the in-flight optical response.
- HFI: B-Spline, BS based on 2 observations of Mars.
(see the instrument RIMOs).
Beam cutoff radii
N times geometric mean of FWHM of all detectors in a channel, where N
|channel||Cutoff Radii in units of fwhm||fwhm of full beam extent|
|30 - 44 - 70||2.5|
|sub-mm||4||18.327635(545GHz) ; 17.093706(857GHz)|
Map resolution for the derived beam data object
- LFI frequency channels for
- HFI frequency channels for
Monte Carlo simulations
FEBeCoP software enables fast, full-sky convolutions of the sky signals with the Effective beams in pixel domain. Hence, a large number of Monte Carlo simulations of the sky signal maps map convolved with realistically rendered, spatially varying, asymmetric Planck beams can be easily generated. We performed the following steps:
- generate the effective beams with FEBeCoP for all frequencies for dDX9 data and Nominal Mission
- generate 100 realizations of maps from a fiducial CMB power spectrum
- convolve each one of these maps with the effective beams using FEBeCoP
- estimate the average of the Power Spectrum of each convolved realization, C'_\ell_'^out^'}, and 1 sigma errors
As FEBeCoP enables fast convolutions of the input signal sky with the effective beam, thousands of simulations are generated. These Monte Carlo simulations of the signal (might it be CMB or a foreground (e.g. dust)) sky along with LevelS+Madam noise simulations were used widely for the analysis of Planck data. A suite of simulations were rendered during the mission tagged as Full Focalplane simulations, FFP#. For example FFP6
Beam Window Functions
The Transfer Function or the Beam Window Functionrelates the true angular power spectra with the observed angular power spectra :
The window functions are estimated by performing Monte-Carlo simulations. We generate several random realisations of the CMB sky starting from a given fiducial , convolve the maps with the pre-computed effective beams, compute the convolved power spectra , divide by the power spectra of the unconvolved map and average over their ratio. Thus, the estimated window function
Beam window functions are provided in the RIMO.
Beam Window functions, Wl, for Planck mission
The effective beams are stored as unformatted files in directories with the frequency channel's name, e.g., 100GHz, each subdirectory contains N unformatted files with names beams_###.unf, a beam_index.fits and a beams_run.log. For 100GHz and 143GHz: N=160, for 30, 44, 70 217 and 353GHz: N=128; for 545GHz: N=40; and 857GHz: N=32.
Retrieval of effective beam information from the PLA interface
In order to retrieve the effective beam information, the user should first launch the Java interface from this page: http://www.sciops.esa.int/index.php?project=planck&page=Planck_Legacy_Archive
One should click on "Sky maps" and then open the "Effective beams" area. There is the possibility to either retrieve one beam nearest to the input source (name or coordinates), or to retrieve a set of beams in a grid defined by the Nside and the size of the region around a source (name or coordinates). The resolution of this grid is defined by the Nside parameter. The size of the region is defined by the "Radius of ROI" parameter.
Once the user proceeds with querying the beams, the PLA software retrieves the appropriate set of effective beams from the database and delivers it in a FITS file which can be directly downloaded.
The data format of the effective beams is unformatted.
Cosmic Microwave background
(Planck) Low Frequency Instrument
(Planck) High Frequency Instrument
Early Release Compact Source Catalog
Data Processing Center
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.
Planck Legacy Archive
Ring-Ordered Information (DMC group/object)
Flexible Image Transfer Specification