PACS Spectroscopy performance and Doc ID: PICCKLTN041...

PACS ICCPACS Spectroscopy performance and

calibration

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PICCKLTN0412.331. May 20111 of 16

Prepared by Bart Vandenbussche

With inputs by Joris Blommaert

Alessandra Contursi

Helmut Feuchtgruber

Christophe Jean

Albrecht Poglitsch

Pierre Royer

Roland Vavrek

Approved by

Authorised by

PACS Spectroscopy performance and calibration

PICC-KL-TN-041


calibration


PICCKLTN0412.331. May 20112 of 16

Distribution List

Recipients Affiliation Nr. of Copies

Document Change Record

Issue Date Description

1.0 10. Mar. 2010 First version limited to flux calibration accuracies and gross correction factors to the ground flux calibration

1.1 11. Mar. 2010 Incorporated comments + added a few clarifications

2.0 6. Apr. 2011 Updated flux calibration error budget; included beam efficiencies

2.1 25. May. 2011 Included 2nd pass ghosts; formatting for public release

2.2 26. May. 2011 Included unchopped spectroscopy uncertainties, beam size as a function of wavelength

2.3 31. May. 2011 Included Wavelength calibration section

Table of Contents1 Introduction...................................................................................................................................3

2 Flux calibration..............................................................................................................................3

3 PACS spectrometer beam efficiency............................................................................................5

4 Spectral leakage and ghosts........................................................................................................9

5 Wavelength calibration...............................................................................................................13


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1 IntroductionThis document provides details on the calibration accuracy and the necessary information to optimally interpret PACS spectroscopy observations.

New issues of this document will be released with new versions of the pipeline and new versions of the calibration files in the Herschel interactive data analysis system.

The present version (2.1) of this document is applicable for PACS results obtained with pipeline and hipe software release versions 6.0 and 7.0 and PACS calibration files version 12 and later.

Processing software version can be verified in the FITS header (CREATOR keyword) or in hipe under help > about hipe

The version of the set of calibration files used for the processing can be found in the FITS header (calTreeVersion keyword) or in hipe with the command print (getCalTree())

2 Flux calibration2.1 Absolute flux scaling

PACS calibration files version 12 and later provide a signalflux density conversion based on inorbit measurements. Therefore the user should not apply the groundtoflight correction factors (1.1 and 1.3) anymore.

2.2 Flux calibration accuracies

The PACS spectrometer flux calibration accuracy is limited by detector response drifts and slight pointing offsets. These limit both the absolute flux accuracy and relative accuracy within a band. Corrections for both effects are under study by the PACS ICC and will be provided to the user in forthcoming hipe software versions. Awaiting these corrections, the following accuracies shall be assumed when interpreting PACS spectroscopy data:

2.2.1 Absolute flux calibration accuracy

This accuracy applies for single line fluxes or continuum flux densities at a given wavelength in any spaxel. Beware of the correction needed for flux falling out of the pixel for point sources (section 3.3 ).

The absolute flux calibration accuracy was determined from observations of ~30 absolute flux sky calibration sources (fiducial stars, asteroids, planets). The table below shows the RMS of (PACS observation / predicted model flux) and peaktopeak scatter around the expected flux densities. This is the absolute flux calibration accuracy to assume for a single PACS spectroscopy observation.

Spectral Band RMS Peak-Peak accuracy

B2A (50-70 micrometer) 11% +/- 30%

B3A (50-70 micrometer) 11% +/- 30%

B2B (70-100 micrometer) 12% +/- 30%

R1 (100-220 micrometer) 12% +/- 30%


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2.2.2 Relative flux accuracy within a band and detection limit for broad features

Broad spectral features (a few micrometer) and continuum shape difference can be introduced by transient effects and pointing offsets. Corrections for these effects are under study. In the mean time, such features should not be interpreted blindly. Note that due to the origin of these effect, they will be seen differently in every observation, so dividing two PACS spectra will not eliminate these instrumental effects.

The table below summarises the resulting accuracy to assume when comparing relative line fluxes within a spectral band. When comparing line fluxes across spectral bands, the absolute flux accuracies in section 2.2.1 apply.

This is also the current limit on detection of broad spectral features (solid state features, dust continuum shape).

These numbers apply to the wavelength regions not affected by spectral leakage (see section 4 )

Spectral Band Broad spectral feature detection limit

relative line flux accuracy within a band

B2A (50-70 micrometer) 10%


B2B (70-100 micrometer) 10%

R1 (100-220 micrometer) 20%

2.2.3 Relative flux accuracy between spaxels

The PACS spectroscopy pipeline in version 6.0 uses a fixed nominal response value for every pixel. These values are based on flux calibration standard measurements of sources placed on the central spaxel. The absolute calibration of the surrounding spaxels is tied to the central spaxel via a flatfield determined on the telescope background. These flatfields reproduce very well. When we divide the 'master flatfield' by the individual flatfields measured, we see a standard deviation of 23%. The table below gives the relative accuracy (peakpeak) when comparing line or continuum fluxes in different spaxels. The main uncertainty in interpreting line flux differences in different spaxels is the knowledge about the source structure and the beam probed by the different spaxels.

Spectral Band Relative line & continuum flux accuracy between spaxels



B2B (70-100 micrometer) 10%

R1 (100-220 micrometer) 10%


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2.3 Flux calibration accuracy of unchopped spectroscopy modes

The absolute flux calibration of the PACS spectrometer is based on observations of flux calibration standards using chopped spectroscopy modes. There are hints of systematic differences in the response scaling between chopped and unchopped mode due to response transients within the chopping pattern. These are well within the flux calibration uncertainties listed in section 2.2 .

Background subtraction for unchopped spectroscopy measurements is done by subtracting the telescope background spectrum measured at an offposition. Repeated offposition background spectrum measurements show a ~4% peaktopeak reproducibility in total absolute flux (telescope + source) and a ~1% inband shape error for the longest scans we have observed so far. In observations of low continuum sources this will dominate the continuum flux accuracy. For a 20 Jy source, having a ~200 Jy telescope background these uncertainties translate to ~40% continuum uncertainty and ~10% inband continuum shape error.

3 PACS spectrometer beam efficiency3.1 Detector sampling of the PSF

The PACS spectrometer spaxels sample a part of the PSF delivered by the Herschel telescope. The telescope PSF becomes larger with wavelength, and shows substantial departure from a gaussian profile due to the telescope wavefront errors, mainly caused by the threepoint mount of the telescope dish. At different wavelengths, different fractions of the PSF structure are seen by the different spaxels. This is illustrated in Figure Error: Reference source not found.

Given the pointing accuracy of the spacecraft, this means that the fraction of the PSF falling onto the central spaxel can vary substantially. This is presently the main limitation of the flux calibration.

The full wavelengthdependent characterisation of the PSF is ongoing at the PACS ICC. In this section we provide some crucial firstorder information to interpret flux values seen in a PACS spaxel.

Figure 1: Figure 1: PACS spectrometer detector positions overlaid on the telescope PSF at 75um (left) and 150um (right). Color scaling of the PSF is chosen to enhance the lobes and wings of the psf.


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3.2 Measured beam efficiencies

A full characterisation of the spectrometer beam efficiencies is ongoing. The beam efficiencies have been measured via raster maps on Neptune at a few selected wavelengths. The Neptune visibility window in spring 2011 has been used to complete these measurements at different wavelengths in every spectral band. Together with the detailed telescope PSF models this will allow us to provide reliable model beam efficiencies well sampled across the PACS spectral coverage.

The measured beam maps measured in every spaxel will be made available on the HSC PACS web page.

In figure 2 we show the measured beam efficiencies at 62, 75, 125 and 150um. Figure 3 shows the gaussian width of the measured beams as a function of wavelength.. The pixel size dominates the width of the beam efficiency up to 150um.

Figure 2: PACS spectrometer beam efficiency as measured from oversampled raster maps on Neptune. From top left, to bottom right: 62um, 75um, 125um, 150um. The contours indicate 10%, 50% and 90% of the peak response.


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Figure 3: Width of the PACS spectrometer beams as a function of wavelength. We show the FWHM in two directions of the assymetric 2D gaussian fit (blue squares, red diamonds) and the mean of the two (yellow triangles). Note that the beams are not gaussian, these numbers are a rough indication of the beam size only,.


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3.3 Recovering full beam line fluxes and flux densities for point sources

The PACS spaxels have a projected size of 9.4”x9.4” on the sky. The fraction of the Herschel+PACS PSF seen in one spaxel varies with wavelength. In order to recover full beam line fluxes or flux densities for point sources, a wavelength dependent correction factor needs to be applied to the line fluxes or flux densities as measured in the central spaxel only.

The wavelength dependent fraction between the point source flux seen in the central spaxel and the full beam flux has been modelled and scaled to actual measurements of the PACS spectrometer PSF maps measured on Neptune. Version 2 of the spectrometer.pointsourceloss calibration file lists these inorbit correction factors. (PcalSpectrometer_PointSourceLoss_FM_v2.fits, available in the hipe as calTree.spectrometer.pointSourceLoss.). The correction curve is depicted in Figure 4 and tabulated in appendix This correction curve assumes a perfect centering of the point source in the central spaxel.


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4 Spectral leakage and ghosts4.1 First-pass spectral leakage seen in all spaxels

The order selection filters of the PACS spectrometer have a steep but not perfectly vertical transmission profile a the cutoff wavelengths of the spectral bands. The PACS spectra near the band borders of Bands R1, B3A and B2B are affected by higher or lower order wavelengths leaking into the spectra. Interpretation of spectral features (unresolved or continuum fluxes) in the spectral leakage regions should be avoided without consulting a PACS expert. Figures 6 5 show the wavelength regions affected. Band B2A is not affected by this spectral leakage.


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4.2 Second-pass ghost seen in some non-central spaxels

A second pass in the optics of the PACS spectrometer can cause a ghost image on some spaxels. Figure 8 shows the spaxels where a second pass ghost might appear, and the location of the corresponding spaxels where the originating, real emission is located. The ghost appears shifted in wavelength.

If a source in one of the originating spaxels shows a strong spectral line, typically an atomic fine structure line, a weak, broadened line can be seen at an offset wavelength in the corresponding spaxel affected by 2nd pass ghosts. The peak flux of this line is typically ~5% of the line peak of the originating line. The integrated line flux can be up to ~14% of the integrated line flux of the originating line. An example is shown in figure 12. The wavelength offset between the originating line and the ghost line depends on the spectral order of the band, and varies with wavelength. A few examples for the strongest fine structure lines in the PACS wavelength range are given in table 1. Appendix B. tabulates ghost wavelength against originating wavelength for all PACS bands affected.

Before interpreting broad spectral lines in spaxels potentially affected by the 2nd pass ghosts, observers should use these tables and, if available, the spectra observed in the corresponding ghost source spaxel, for the presence of a strong line at the originating wavelength.

Point source observations, well centered on the central spaxel, are not affected. The central column of the IFU is not contaminated by 2nd pass ghosts.

Figure 8: Location of the spaxels where a second pass ghost might appear. In black are the module numbers (this is the numbering in the PACS frames product), in white the row and column numbers in the PACS cube products. The arrows indicate the spaxel where the originating, real emission is located.


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Figure 9: Example of the second pass spectral leak: the strong (real) line emission at 145.5 (OI) and 157.7 (CII) in module 13 leak into module 10, where they are seen as broadened spectral lines around 108 and 122 micron.

Table 1: Example 2nd pass ghost wavelengths corresponding to prominent fine structure lines in the PACS wavelength range

Band Originating line wavelength ghost wavelengthB3A O I 63.2 54B2B O III 88.4 72R1 O I 145.5 108R1 CII 157.7 122R1 N II 205.3 178


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5 Wavelength calibration5.1 Wavelenght calibration accuracy

The PACS wavelength calibration relates the diffraction grating position to the wavelength seen in every detector pixel. This calibration was derived from laboratory measurements of a water vapour absorption cell, and further refined inflight based on fine structure lines in planetary nebulae. For ideal extended sources the required accuracy of better than 20% of a spectral resolution element is met throughout all bands. While at band borders, due to leakage effects and lower S/ N, the RMS calibration accuracy is closer to 20%, values even better than 10% are obtained in band centres.

For point sources the wavelength calibration may be dominated by pointing accuracy.

5.2 Wavelength shifts with source position

The 5x5 PACS spaxels of the integral field unit are imaged onto a 1Dslit. The 1Dslit image is then dispersed by the diffraction grating. As with any diffraction grating spectrometer, moving the source center with respect to the slit center in the dispersion direction will result in a slight shift of the wavelength seen in a detector pixel. Therefore, lines observed in a point source with PACS will appear slightly shifted in wavelengths if the source is not perfectly centered on the spaxel. This is illustrated in figure 10. Since the PSF peak is resolved in the slit, a distinct skew can be seen as well when the source is near or beyond the spaxel edge. These features should be taken into account before interpreting wavelength shifts or line profiles in terms of velocity. Pointing offsets along the dispersion direction do not result in a wavelength shift. The sign (red or blushift) and magnitude of the shift for different wavelenths is shown in figures 11and 12.

Figure 10: When a point source is offset in dispersion direction with respect to the center of a spaxel, the observed lines will show a distinct shift in wavelength and a skew.


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Figure 11: If a point source is offset in dispersion direction, wavelengths will be shifted to the red when moving 'up', e.g. from module 12 to module 17 in the PACS IFU. When the source is offset 'down', the wavelengths will be blueshifted.

Figure 12: The wavelength shift, seen when a point source is not centered perfectly, depends on the observed wavelength and band, and the pointing offset. The black dashed line shows the wavelength shift seen for a point source offset by 1.5". The blue, green and red dashed line shows the wavelength shift for a pointing offset of 2". The solid red, green and blue line show the wavelength shift seen when the source is centered on the spaxel edge.


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A.Point source correction factorsFraction of a point source full beam flux seen in one PACS spaxel. Values are based on PcalSpectrometer_PointSourceLoss_FM_v2.fits. Wavelengths are in micrometer.

Wavelength Fraction Wavelength Fraction Wavelength Fraction

50 0.75 107 0.66 164 0.49

51 0.75 108 0.66 165 0.48

52 0.74 109 0.65 166 0.48

53 0.73 110 0.65 167 0.48

54 0.73 111 0.65 168 0.48

55 0.73 112 0.64 169 0.47

56 0.72 113 0.64 170 0.47

57 0.72 114 0.64 171 0.47

58 0.71 115 0.64 172 0.46

59 0.71 116 0.63 173 0.46

60 0.71 117 0.63 174 0.46

61 0.71 118 0.63 175 0.46

62 0.71 119 0.62 176 0.45

63 0.7 120 0.62 177 0.45

64 0.7 121 0.62 178 0.45

65 0.7 122 0.62 179 0.44

66 0.7 123 0.61 180 0.44

67 0.7 124 0.61 181 0.44

68 0.7 125 0.61 182 0.43

69 0.7 126 0.6 183 0.43

70 0.7 127 0.6 184 0.43

71 0.7 128 0.6 185 0.43

72 0.7 129 0.59 186 0.42

73 0.7 130 0.59 187 0.42

74 0.7 131 0.59 188 0.42

75 0.7 132 0.59 189 0.41

76 0.7 133 0.58 190 0.41

77 0.7 134 0.58 191 0.41

78 0.7 135 0.58 192 0.41

79 0.7 136 0.57 193 0.4

80 0.7 137 0.57 194 0.4

81 0.7 138 0.57 195 0.4

82 0.7 139 0.56 196 0.4

83 0.69 140 0.56 197 0.39

84 0.69 141 0.56 198 0.39

85 0.69 142 0.55 199 0.39

86 0.69 143 0.55 200 0.38

87 0.69 144 0.55 201 0.38

88 0.69 145 0.55 202 0.38

89 0.69 146 0.54 203 0.38

90 0.69 147 0.54 204 0.37

91 0.69 148 0.54 205 0.37

92 0.69 149 0.53 206 0.37

93 0.68 150 0.53 207 0.37

94 0.68 151 0.53 208 0.36

95 0.68 152 0.52 209 0.36

96 0.68 153 0.52 210 0.36

97 0.68 154 0.52 211 0.36

98 0.68 155 0.52 212 0.35

99 0.68 156 0.51 213 0.35

100 0.67 157 0.51 214 0.35

101 0.67 158 0.51 215 0.35

102 0.67 159 0.5 216 0.34

103 0.67 160 0.5 217 0.34

104 0.66 161 0.5 218 0.34

105 0.66 162 0.49 219 0.34

106 0.66 163 0.49 220 0.33


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B.Second-pass ghosts - wavelength offsets In the table below we list for each band the correspondence between wavelength at which a second pass ghost is seen, and the wavelength of the originating emission. See section 4.2 .

Band R1 Band 2B Band B3Aghost wavelength originating wavelength ghost wavelength originating wavelength ghost wavelength originating wavelength102 140.2 68 84.6 50 59.9103 141.1 69 85.5 51 60.7104 141.9 70 86.3 52 61.5105 142.8 71 87.2 53 62.4106 143.6 72 88.0 54 63.2107 144.5 73 88.9 55 64.0108 145.4 74 89.7 56 64.8109 146.2 75 90.6 57 65.6110 147.1 76 91.4 58 66.4111 147.9 77 92.3 59 67.2112 148.8 78 93.1 60 68.1113 149.6 79 94.0 61 68.9114 150.5 80 94.8 62 69.7115 151.3 81 95.7 63 70.5116 152.2 82 96.5 64 71.3117 153.0 83 97.4 65 72.1118 153.9 84 98.2 66 73.0119 154.8 85 99.1 67 73.8120 155.6 86 99.9121 156.5 87 100.8122 157.3 88 101.7123 158.2 89 102.5124 159.0125 159.9126 160.7127 161.6128 162.4129 163.3130 164.1131 165.0132 165.9133 166.7134 167.6135 168.4136 169.3137 170.1138 171.0139 171.8140 172.7141 173.5142 174.4143 175.3144 176.1145 177.0146 177.8147 178.7148 179.5149 180.4150 181.2151 182.1152 182.9153 183.8154 184.7155 185.5156 186.4157 187.2158 188.1159 188.9160 189.8161 190.6162 191.5163 192.3164 193.2165 194.0166 194.9167 195.8168 196.6169 197.5170 198.3171 199.2172 200.0173 200.9174 201.7175 202.6176 203.4177 204.3178 205.2179 206.0180 206.9

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