SPIRE Design Description - Research Services

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Draft SPIRE Design Description Document Observatory Functions: Combinations of Spacecraft and Instrument Functions and Instrument Data Configurations which, with the appropriate input parameters, allow any Observation to be carried out. There are two categories of Observatory Functions: Photometer Observatory Functions (POFs) and Spectrometer Observatory Functions (SOFs). 5.3 Photometer Observatory Functions Table 5-1 - Photometer Observatory Functions Observation Observatory Function Name Comments Point source POF1 Chop without jiggling Accurate pointing and source position Photometry POF2 Seven-point jiggle map Inaccurate pointing or source position Jiggle POF3 n-point jiggle map Field mapping Mapping POF4 n-point jiggle map with raster Extended field mapping Scan POF5 Scan map (no chopping) Large-area mapping Mapping POF6 Scan map with chopping Large area mapping (with 1/f noise) Peak-up POF7 Photometer peak-up Determination of pointing offsets Calibrate POF8 Photometer calibrate Responsivity tracking Engineering Modes POF9 Special engineering/ commissioning modes 5.3.1 Point Source Photometry (POF1 and POF2) For point source photometry, pixel-pixel chopping is used as described in Section 2.2.1.3. The required chop throw is 4Fλ at 500 µm, corresponding to a 10-mm motion of the beam on the array and to a chop angle on the sky of θchop = 126 arcsec. Simple pixel-pixel chopping (POF1) requires pointing accuracy sufficiently good that the loss of signal due to the pointing error is acceptable. The signal loss factors for the photometer beams are shown in Figure 5-5a. 20 16 12 8 4 (a) (b) 250 µm 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Pointing offset (arcsec.) Figure 5-5 - (a) Signal loss vs. pointing error for the three SPIRE photometer bands; (b) beam positions for 7-point map with angular offset θ. 118 350 µm 500 µm θ
Draft SPIRE Design Description Document The required Absolute Position Error (APE) for Herschel (3.7 arc seconds) corresponds to 11%, 6%, and 3% signal loss at 250, 350, and 500 µm respectively. The corresponding figures for the goal APE value (1.5") are 2%, 1%, and 0.5%. For most observations, 11% is not acceptable, but 2% is. Therefore, the required APE is not good enough to allow accurate photometry at 250 µm without peaking up, but the goal is sufficient to allow this. In order to avoid calibration errors due to satellite pointing errors or inaccurate knowledge of the source coordinates, it is envisaged that many point source observations will be carried out by making a 7-point map (POF2). The BSM is used to do to do small map 7-point hexagonal jiggle map with spacing θ arcsec., as shown in Figure 5-5b. A suitable value for θ is ~ 6 arc seconds: this spacing is 1/3 of the beam at 250 µm (so consistent with full sampling), and is almost twice the APE. From such a 7-point map, the total flux of the source can be computed, and the source position can be recovered with an accuracy that depends on the S/N. The central position is made to coincide with one of sets of three overlapping detectors to allow simultaneous optimised observations in the three bands. The chop throw can be set at any desired value within the available range. A value of 126” improves the overall efficiency by allowing the source to be observed all the time in all bands. 5.3.2 Jiggle Mapping (POF3 and POF4) Jiggle-map mode is for mapping objects or regions that are extended with respect to the SPIRE beam but smaller than a few arcminutes in size. The BSM is used to make an n-point jiggle map while chopping with a throw greater than the size of the object to be mapped. The maximum throw is 4 arcminutes (± 2 arcminutes) as illustrated below. 4 x 8 arcminute field of view of the SPIRE array determined by the SPIRE optics Central 4 x 4 arcminute portion BSM at rest position Figure 5-6 Field mapping with maximum chop throw of 4 arcminutes. The detectors in the central square 2 x 2 arcminute portion of a photometer array are deflected by ± 2 arcminutes by the BSM, so that they are alternately chopped from one side of the available field of view to the other. For full sampling at all wavelengths, n = 64. Considering the simplified case of square-packed horns, the step size must be 0.5λ/D = 9 arcseconds at 250 µm and the number of steps must accommodate the need to cover the distance between two beams at 500 µm: 2λ/D = 72 arc seconds. Eight steps in each orthogonal direction are thus required. The geometry of the juggle pattern is hexagonal for the hexagonally packed feedhorns, but the number of steps required is still 64. To allow flexibility in the use of this mode, permitted values of n shall be 16, 32 and 64. The jiggle positions shall be defined by angles ∆θY and ∆θZ in the Y and Z directions (with respect to the BSM rest position). The sequence in which the jiggle positions are visited is 119 BSM at offset = -2 arcmin BSM at offset = +2 arcmin
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