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A&A 520, A12 (2010)Table 1. Summary of optical requirements for each spectral band.Fig. 1. Drawing of an HFI detection assembly chain. The back-to-backhorn (front and back horns) couples the incoming radiation from thetelescope to a detector horn which then couples the radiation to the bolometricdetector. Filters are located in between the two horn assembliesin order to define the spectral band. A lens refocusses the radiation fromthe back horn to the detector horn.Band Target Spillover Edge tapercentral freq. resolution (%) (dB)100 GHz 9.2 arcmin 1 (0.5) −25 (−30) at 25 deg143 GHz 7.1 arcmin 0.7 (0.5) −28 (−30) at 27 deg217 GHz 5 arcmin 0.5 (0.3) −30 (−32) at 26 deg353 GHz 5 arcmin 0.5 (0.3) −30 (−32) at 26 deg545 GHz 5 arcmin 0.5 (0.3) −30 (−32) at 26 deg857 GHz 5 arcmin 0.5 (0.3) −30 (−32) at 27 degNotes. Numbers in parentheses refer to the goal we were aiming at.We will focus on the HFI optical design performances.Section 2 describes the optical concept of the focal plane unit(FPU) based on experience gained from previous instruments.Section 3 reviews the optical requirements based on the scientificgoals. Sections 4 and 5 describe the design of the FPU andthe solutions adopted to reach the specificationsdefinedbytherequirements. In Sect. 6 we set out our best prediction of thepre-launch HFI optical performances deduced from the variouscalibration campaigns. Section 7 is then dedicated to the beamsimulations performed for the single and multi-mode channelsassuming an ideal telescope 2 .2. Focal plane unit conceptThe Planck-HFI detection system is based on highly sensitivebolometers. A specific detection assembly configuration was developedfor Planck-HFI drawing upon the heritage from previousCMB experiments.The detectors are feedhorn coupled in order to meet therequirements on beam shape definition and straylight control.Notwithstanding recent progress inantennacoupled bolometerperformances, when Planck-HFI was designed, the only choicewas to use corrugated feedhorns. Moreover, it has been shown(Maffei et al. 2008) thatlocatingquasi-opticalcomponentsinfront of the horn aperture will impact its beam characteristics.When cryogenically cooled detectors are used (such as bolometers),ground based and balloon borne experiments need to havequasi-optical components such as a dewar window and interferencefilters (Ade et al. 2006) infrontofthecoldoptics.Thisinevitablyresults in main beam distortion and an overall increasein sidelobe levels. Because HFI is in space, it is possible toavoid the use of quasi-optical components in front of the thehorn. In order to do so and taking thermo-mechanical constraintsinto consideration, a triple horn configuration has been adopted,where the filters are located between a back-to-back horn andthe detector horn. In this position, the filters will have a smallerimpact on the beam shape.This configuration was first used as a 90 GHz radiometerprototype (Church et al. 1996), and subsequently in the experimentBOOMERanG (de Bernardis et al. 2000). It was then optimisedfor Planck-HFI and operated in the Archeops experiment(Benoit et al. 2002), the balloon borne version of HFI.The thermo-mechanical purpose of this triple horn configuration,forming the detection assembly (or pixel − Fig. 1), ispresented in detail in a joint paper (Ade et al. 2010). Here we explainhow the optical optimisation has been performed and alsocompare the theoretical modelling with the measured results.2 In the context of this paper, “ideal telescope” must be understood as atelescope model with design alignments and smooth mirror theoreticalsurfaces.3. Optical requirementsThe scientific goals of Planck-HFI (Tauber et al. 2010b) dictatethe instrumental specifications such as the sensitivity, the frequencycoverage or the spatial resolution. Taking into accountthe constraints of a space mission, these specifications are translatedinto a set of optical requirements that are listed below.3.1. Spatial resolutionThe size of the Planck primary mirror results from a trade-offbetween the desired resolution and the size and weight limitswhich can be flown on-board a medium size space mission. Thediffraction limit dictates that for frequencies above 300 GHz, aresolution of a few arcminutes can be reached. However, calculations(Planck community 2005) haveshownthatpointsourcecontamination would be too high to extract useful informationat high multipoles in the CMB power spectrum. Also, in orderto be compliant with a correct sampling of the sky (due to dataacquisition rate and speed of rotation of the satellite), a maximumresolution of 5 arcmin has been set (Table 1). To do so,two techniques can be used: either under-illuminating the telescope,resulting in a smaller effective aperture diameter, or alternativelymaking use of multi-mode optics. We have chosento slightly under-illuminate the telescope for the 353 GHz bandand to use multi-mode channels (Murphy et al 2001)forthetwohighest frequency spectral bands (545 GHz and 857 GHz). Thelatter technique has the advantage of increasing the sensitivityof the detection assembly, each mode bringing its contributionto the power detected, but has the drawback of resulting in abeam which is more complicated to model and less predictablethan single-mode channels.We will describe in this paper the general principles of theoptical optimisation, valid for all the HFI channels, and deferamoredetaileddescriptionanddiscussiononthedesignofthemulti-mode channels to a specific paper to follow later (2010).3.2. Spillover, straylight and sidelobe rejectionSince the signal from the CMB anisotropies is weak, it is crucialto reduce unwanted signals to a minimum. These parasitic signalswill come not only from the instrument self-emission surroundingthe focal plane, as well as from potential bright objects(such as the Earth, the Moon or bright stars for example).The off-axis emission of these bright objects within the spectralbands of observation can reach the detector through the antennafar-sidelobes, through multiple scattering on the baffles and instrumentor through the part of the horn beam looking directly atthe sky.The fraction of the horn beam coupling to the telescopewill create the antenna main beam through which the CMB andPage 2 of 15
B. Maffei et al.: Planck pre-launch status: HFI beam expectations from the optical optimisation of the focal planeforeground signals will be observed. The remaining part of thehorn beam couples either to the sky via sidelobes or to tothe instrument through absorption ormultiplereflections.Thespillover can be defined as the overall radiative power that doesnot intercept the telescope reflectors, thus directly reaching thedetector antennas. This will result in an unwanted signal not directlyoriginating from the source of interest. It is therefore animportant parameter in assessing straylight control.The reduction of the spillover and the maximisation of thepower concentrated in the horn main beam is of great importance:not only the horn beam sidelobes have to be reduced, butthe main beam has to be as close to a Gaussian profile as possible,as a Gaussian beam does not change its shape or developsidelobes as it propagates (at least for single-mode case).In order to be consistent with the science requirements, thespillover has to be maintained to within one percent. A moredetailed requirement per band is given in Table 1. Anequivalentparameter defining the telescope illumination and the potentialstraylight contamination is called edge taper. Assuming aGaussian illumination of the secondary (M2) and primary (M1)mirrors by the horns, the edge taper defines the value of the illumination(in dB) at the edges of the mirrors. The further off-axisthe horn is with respect to the axis of the mirror, the less symmetricalthe edge taper value is for such a feedhorn position. Anaverage edge taper requirement corresponding to the spilloverrequirement is given in Table 1 for each spectral band.Thus, with a fixed telescope diameter, we need to make atrade-off between maximum resolution and spillover reduction.3.3. Optical efficiencyAssuming that we are observing the CMB radiation as a sourceand that the field of view of the detection assemblies is filled withthis radiation (extended source), the power absorbed by each detectorfrom the CMB source is:∫P = AΩɛ(ν)B (ν, T CMB ) dν (1)∆ν∆ν is the spectral bandwidth of the channel and B(ν, T CMB )isthebrightness distribution of the emission of the CMB according toPlanck’s law.The throughput, AΩ,isgivenbyn(λ) · λ 2 ,wheren(λ) = 1forasingle-modechannel(A being the effective aperture area of thefront horn, Ω the horn beam solid angle and λ the wavelength).In the case of the multi-mode channels n(λ) isadjustedtoproducethe required Ω to match the telescope aperture. This will beaddressed further in a future paper (2010).In this equation, ɛ(ν), the optical efficiency, is the only parameterremaining which can affect the amount of power receivedby the detector from the source. This will then directlyimpact the instrumental sensitivity.The optical efficiency takes into account several parametersassociated with the performance of each component forming thecold optics. It includes the detector efficiency η which is set bythe detector design and manufacture. One of the major tasks wasto optimise the optical efficiency in order to reach a requirementof ɛ = 0.25 when averaged over the spectral bandwidth and withagoalofɛ = 0.3.3.4. Focal surface curvatureThe chosen off-axis Gregorian telescope configuration is designedto accommodate the relatively large size of the focal planeFig. 2. Curvature of Planck focal surface from Thales study resultingfrom simulation of a perfect telescope. Each cross represents the theoreticallocation of the phase center of a front horn.formed by both instruments given the optical and mechanicalconstraints. The optimisation performed by Thales Alenia Spaceto reduce the averaged cross-polarisation (Dragone-Mitsugushicriteria) and minimise the straylight pickup as well as theaberration effects for each pixel resulted in an aplanatic configurationconsisting of two ellipsoidal reflectors giving a curvedfocal surface with an off-centre apex (Fargant et al. 2000). Thisshape has a pronounced slope across the focal plane and with differentorthogonal curvatures. This curved focal surface (Fig. 2)is computed assuming an ideal telescope. Each individual fronthorn has been designed such that its phase centre is located onthis theoretical focal surface (on each cross of Fig. 2) andorientedin such a way that its main beam aims at the centre of theprimary mirror.Additionally many of the single-mode detection assembliesare sensitive to polarisation (Table 7). The detectors (PSB, seeSect. 5.2) are oriented so that the telescope depolarisation effectis corrected.The real mirror surfaces, misalignments (within tolerances)and thermal effects will lead to perturbations in the focal surfaceshape, resulting in a de-focusing of the feedhorns (Tauber et al.2010a).3.5. Shadowing constraints on designBecause the focal surface is not planar, potential shadowingof one of the horn beam by an adjacent front horn could occur.This is a possibility not only between horns of the sameinstrument, but also between LFI and HFI. Studies on horn mutualcoupling and shadowing within Planck focal plane instrumentsby Thales (internal report) and then by the LFI team(D’Arcangelo et al. 2005) haveshownthatclearingeachhornbeam by +/−45 degrees respectively to its boresight axis isenough to make any shadowing effects on a given horn mainbeam pattern unnoticeable (within 1% error of the measurementsystem used).To meet the required overall sensitivity HFI needs to haveeight detection assemblies for each single-mode band dedicatedto CMB measurements (100, 143, 217 and 353 GHz). In orderto fit this number of assemblies within the focal plane, tworows of pixels for the first three spectral bands are needed. ThisPage 3 of 15
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ABSTRACTThe European Space Agency
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