Planck Pre-Launch Status Papers - APC - Université Paris Diderot ...

J. A. Tauber et al.: Planck pre-launch status: The optical systemwere to: (a) measure the qualitative RF properties of the opticalsystem; and (b) validate the ability of GRASP to predict theflight patterns based on geometrical information. The key resultsof this campaign are: (a) a GRASP model that can be appliedto the geometry of the flight reflectors (referred to as FM for“Flight Model”); and (b) the difference between predicted andmeasured patterns, which provides a quantitative measure of theuncertainty in the modelling based on ground information.The RFQM, including a representative focal plane structureand all the important associated payload elements (e.g. baffle,V-groove) was placed in a CATR and used to measure 4π radiationpatterns of flight-like horns at frequencies between 30 GHzand 320 GHz, including two orthogonal polarisation directions.The surfaces of the reflectors were measured using photogrammetry,and the alignment of the object tested was measured insituin great detail, in both cases using techniques similar tothose later used to align the FM. The radiation patterns of thefeedhorns used were separately measured. The geometry of theGRASP model used for flight predictions was based on all thesemeasurements.Fig. 9. (Top)ThedistributionofWFEsat353GHzfor3000casesofthermo-elastic deformations drawn from the error distributions of allknown cases leading to misalignment (see description in Sect. 5.2).(Bottom)Thedistributionofdefocuserrors(i.e.thedifference along theprincipal axis between the locations of the focal plane formed by thetelescope and that of the feedhorn phase centre). The horizontal axis isdefocus in millimetres. Note that the specific horn used in this analysisis displaced from its optimal location along the focal axis by about0.35 mm; this is a result of the global optical design which necessarilyrequired that some of the horn locations were not optimised.to rule out human error. With this (relatively low) accuracy, thepredicted location was able to reproduce the measured one 6 .6. Radio frequency verificationAlthough the design and verification of the Planck telescopewere based on specified WFE levels, ultimately it is the radiofrequency (RF) performance that matters. Measuring the RF performanceof the optical system at in-flight conditions (low temperature,vacuum) is infeasible due to the dimensions involvedin a measurement setup. A Shack-Hartmann measurement ofthe entire telescope at λ10 µm was initially planned but eventuallydiscarded as too complex and costly. Instead, an RF measurementcampaign based on the qualification model of thePlanck telescope (the “RFQM”) was implemented as a means ofbuilding confidence in the process of estimating the flight predictions.The objectives of this campaign (Forma et al. 2008)6 The accuracy is probably far higher than that specified when takinginto account all the information available in the pattern. A deviationof 0.5 mm between prediction and measurement was found along thedefocus direction, which could not be related to any known systematiceffect, and represents the limit of the uncertainty. This deviation wasalso seen on the RFQM, and is as yet unexplained. As a consequence,it is not safe to claim a superior accuracy in the measurement than thequoted one.6.1. Main beamsThe RFQM main beams 7 were measured with a high samplingdensity. Examples are shown in Fig. 10. Thepredictionmodelfor the main beams is based on physical optics and the onlyinputs to be adjusted are geometrical ones. The comparison ofpredictions and measurements shows differences that increasesignificantly from low to high frequencies. In terms of total integratedpower, the differences vary from a few tenths of onepercent (i.e. within measurement uncertainties) to 6−7 percent.These differences can be partly attributed to measurement errorsand other CATR-induced systematics. However, even at low frequencies,where the errors are very low and systematics wellunder control, differences of ∼3% can be seen in some cases.Therefore the ability to predict the in-flight patterns based ongeometrical information acquired on the ground has been validatedto an accuracy of a few percent in total power, increasingto levels of 5−6percentatthehigherfrequencies.Cross-polar measurements were also performed for each detector(see Fig. 11), but were affected in some cases by significantsystematic effects caused by (a) the weakness of the signals;(b) greater sensitivity to misalignments in the CATR; and(c) both poor cross-polar characteristics and poor knowledge ofthe CATR transmitter horns.6.2. Far side lobesWhen it comes to the computation of far side lobes, pure physicaloptics computations are too time consuming to be practicaland it becomes necessary to use other techniques. The alternativeprovided by GRASP is “Multi-GTD” (Multi-ray GeometricalTheory of Diffraction, GRASP Manual 2008). In addition to thegeometry of the system, it is also necessary to provide GRASPwith the families of rays that it propagates to the far field. Thenumber of families is theoretically infinite and needs to be restricted.The first iteration to this was blind, on the basis ofexperience and a first estimation of the amount of power carriedby each ray family. The second iteration was based onthe RFQM measurements, i.e., wherever there was a significant7 Defined as the patterns within square windows around peak of 2.5,1.0, 0.8, and 0.35 degrees at 30, 70, 100, and 320 GHz respectively. Thewindows are sized to include at least the −40 dB contours.Page 11 of 22