Difference: SpirePhotometerBeamProfileAnalysis2 (3 vs. 4)

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Analysis Details of SPIRE Photmeter Beam Profiles

( Bernhard Schulz, October 2014)
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Initial Beam Profile

The beam profile was reconstructed from four fine scan maps with obsids: 1342186522, 1342186523, 1342186524, 1342186525, and two observations of the same sky region 1342255134 and 1342255135 three years later. The Neptune observations followed the small proper motion of the object. The scan pattern had, unlike standard scan map observations, very narrowly spaced scan legs. The maps scanning in spacecraft-y direction consisted of 150 scan legs separated by 3.9", the orthogonally scanning maps comprised of 210 scans at the same separation. There were actually two sets of each map, each starting in opposite directions to test for dependencies on scan direction. The second set of scans in Y-direction actually started mid-way between the previous +Y scan legs (2" offset), leading to an overall finer spacing of legs along the Z-coordinate. The +Z and -Z directions started with almost perfect overlap.Neptune moved about 6.1" between the mid-points of the first and last observation. The observations were special calibration observations that yielded a number of calibration results, for example the positions of all detector pixels, relative flatfields, and the ratio between integral and peak based flatfields, the so-called extended source gains.
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 Table 1: Observation IDs, times and positions of all six maps.

The last two "shadow" observations were made after Neptune had left the region 3 years later at about the same time of year to ensure the same observing geometry w.r.t the Zodiacal cloud. These were performed in a non-moving reference frame with the same narrow scan pattern as the Neptune observations. Each scan direction was executed only once. The duration of each map was equivalent to the corresponding ones on Neptune, i.e. spending a total of half the observing time that was used on Neptune.

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Figure 1: Depictions of the combined scan patterns of Neptune observations (left) and Shadow observations (right) overlaid over the PSW maps at standard 6" pixel size.
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  • For all readouts that had been deselected previously, indicated by bit #5, reset master bit and bit #5 (only Neptune observations).
  • Run Naive Mapper on destriped level 1 data twice, once with pixel size 1" and once with standard pixel size.
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 Figure 2: The reconstructed raw maps for array PSW at 1" pixel size and standard 6" pixel size. The standard pixel size maps offer higher signal to noise in exchange for poorer spatial resolution.

Background Subtraction

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Although Neptune and shadow maps appear very consistent in its background features at first sight, a direct subtraction of the maps (see Fig. 3 left) shows a number of artifacts hinting at a small astrometric shift between both maps and a photometric difference. This is particularly obvious when using the maps at standard pixel size benefitting from higher S/N. The shadow map over-subtracts the sources by about 10%. Scaling down the shadow map by 10% gives a better removal, however the best background subtraction was achieved empirically by Gaussian smoothing of the shadow maps with sigmas = [0.10, 0.03, 0.10] for PSW, PMW PLW respectively and shifting the PSW and PMW by 1.8" in the spacecraft X-direction. The smoothing of the shadow map effectively accounts for the smearing of the background galaxies in the Neptune images that were made while the telescope tracked the proper motion of the planet.
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Although Neptune and shadow maps appear very consistent in its background features at first sight, a direct subtraction of the maps (see Fig. 3 left) shows a number of artifacts hinting at a small astrometric shift between both maps and a photometric difference. This is particularly obvious when using the maps at standard pixel size benefitting from higher S/N. The shadow map over-subtracts the sources by about 10%. Scaling down the shadow map by 10% gives a better removal, however the best background subtraction was achieved empirically by Gaussian smoothing of the shadow maps with sigmas = [0.10", 0.03", 0.10"] for PSW, PMW PLW respectively and shifting the PSW and PMW by 1.8" in the spacecraft X-direction. The smoothing of the shadow map effectively accounts for the smearing of the background galaxies in the Neptune images that were observed while the map center tracked the proper motion of the planet.
 
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Figure 3: The .
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Figure 3: Difference maps of Neptune and shadow maps for PSW without any correction (left) and with a small shift by 1.8" in spacecraft X-direction and Gaussian smoothing of the shadow map with sigma=0.1" before the subtraction (right).
 
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With the advent of the shadow maps, previous efforts involving detection and subtraction of point sources, and fitting of a warped background plane, became unnecessary.
 
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(bis hier!)
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Looking for "Triton Contamination"

 
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Figure 4: Overplot of Triton positions during the observation over PSW isophotes of the Neptune map as thick black trace. The levels are in Jy and the small tick marks on the side are arc seconds.
 
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Editing done up to here

 
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Background fit and removal

The background was not flat at the level 0.1e-5 from the peak, most likely due to Zodiacal Light background variation, however galactic Cirrus could also be the reason. A two dimensional polynomial was fitted and removed. The removed background can be examined in the following files:

0x5000241aL_PLW_pmcorr_1arcsec_bgfit.fits
0x5000241aL_PSW_pmcorr_1arcsec_bgfit.fits
0x5000241aL_PMW_pmcorr_1arcsec_bgfit.fits

 
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Overlaying the ecliptic coordinate system over the background fit (contours) shows that a an interpretation by a Zodiacal light gradient is at least qualitatively consistent.
 
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Using IRSKY to obtain a rough estimate through extrapolation of its Zodiacal model to SPIRE wavelengths at the position of the observation and one degree towards the ecliptic pole, the following gradients were obtained.
 
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Gradient over 0.4 deg 0.085 0.108 0.252
  PSW PMW PLW
IRSKY Model Result
Difference over 1 degree [MJy/sr] 0.050 0.020 0.011
Gradient in [mJy/beam/deg] 0.51 0.20 0.11
Fitted Background Map Result
The gradients are somewhat similar although the crude extrapolation probably introduces large errors.

Background source removal

A large number of extragalactic sources contaminated the maps. These were removed in HIPE using Sussextractor.

Array Before Removal After Removal Source Positions
PLW
PSW
PMW
 

Solid Angles

After background tilt subtraction and background source removal the signal was averaged over concentric annuli around the source and this radialized beam profile was plotted against radius (Plot 1). To ensure symmetry, e.g. flatness of the background, the same was also plotted for three sectors (Plot 2 and 3). From the radialized beam profile a new background was determined and subtracted based on the necessity to have only positive flux within the errors (Plot 4).
 
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