SPIRE Detector Angular Offset and Instrument Mode Mask Calibration Products Edward Polehampton & Marc Ferlet

Transcription

1 Page: 1 f 12 SPIRE Detectr Angular Offset and Instrument Mde Mask Calibratin Prducts Edward Plehamptn & Marc Ferlet 1.1 Reference Dcuments RD1 Calibratin Prducts fr SPIRE Data Prcessing SPIRE-RAL-DOC RD2 SPIRE Instrument User Manual, Issue 1.0 SPIRE-RAL-PRJ RD3 Definitin f a cmbined fcal plane aperture fr the SPIRE SPIRE-RAL-NOT instruments, Issue 1.0, 25 January 2001 RD4 SPIRE Instrument PFM Filters End Item Data Package (EIDP) SPIRE-UCF-DOC RD5 SPIRE Spacecraft-Instrument Alignment Matrix (SIAM) SPIRE-RAL-NOT RD6 SPIRE Pixel identificatin frm ptical mdel Excel spreadsheet, versin Intrductin These calibratin prducts prvide the angular ffsets f the detectrs in each array, and specify an integer mask that can be used t identify which detectrs are chpped ut f the field f view when the BSM is mved. The Instrument Mde Mask prduct is divided int separate tables fr each "Instrument Mde" (ie. POF2, POF3) as the BSM chp thrw will be different in each. This mask is used in the demdulatin and dendding tasks t ensure that the final map given t astrnmers des nt include areas where the inside f the instrument has been cmbined with the sky. This dcument describes the backgrund needed t determine the detectr ffsets and which detectrs t include in the instrument mde mask calibratin prduct fr the Phtmeter, and t explain why a similar prduct is nt required fr the Spectrmeter.

2 Page: 2 f Lcatins f specific detectrs in the field f view The psitins f detectrs in the field f view are determined frm the spreadsheet, SPIRE_Pixel_identificatin_frm_ptical_mdel_v6.xls (RD6) this is an extensin f the SPIRE Spacecraft-Instrument Alignment Matrix (RD5) t cver all detectrs. This spreadsheet gives the psitin f each detectr in mm in the fcal plane in the ESA system (X esa, Y esa, Z esa ), and the directin csine vectrs fr each detectr (A esa, B esa, C esa ). These directin csine vectrs are used in the SIAM t specify the directin csine matrix that transfrms the Herschel centred crdinate system t a system centred at any ne f the SPIRE aperture psitins. The spacecraft X, Y and Z axes are specified as fllws: X esa sunshield Y esa Z esa Several pints that are imprtant t nte when interpretting the spreadsheet: 1. The X, Y, Z crdinate system is fixed with respect t the satellite 2. The ptics f the telescpe cause a flip-rtatin abut the X axis such that a mvement n the instrument plane twards the sunshield (psitive Z) causes a mvement away frm the sunshield n the sky (negative Z). The same effect als ccurs in the Y-directin (psitive mvement in Y n the fcal plane is negative mvement in Y n the sky). SPIRE HIFI PACS +Z PACS SPIRE +Z HIFI +Y sunshield +Y sunshield Instruments as seen n the fcal plane frm M2 Herschel FOV n the sky with instrument psitins The angular psitins f each SPIRE detectr in the crdinate system centred at the Herschel bresight, alng the spacecraft Y and Z axes are given by, θ θ Y Z π = cs 2 π = cs ( B ) esa ( C ) esa

3 Page: 3 f 12 This crrectin is calculated as fllws (example fr just the Z cmpnent): X esa We are interested in the angular psitin in the Z directin i.e. θ Z A esa θ Z V C esa Zesa The directin csine vectr specifies the nrmalised vectr in the directin f that detectr ( V =1). Strictly this applies in 3D, but as the angles are small, we can make an apprximatin that is true in the 2D plane f each cmpnent*, and calculate the angular psitin using, 1 Cesa π 1 = Cesa θ = Z sin r V cs r 2 V In rder t determine the angular ffsets f the detectrs within the SPIRE aperture crdinate system, we need t knw their psitins relative t the SPIRE aperture used, θ θ Y Z SPIRE SPIRE = θ = θ Y det Herschel Z det Herschel θ θ Y aperture Herschel Z aperture Herschel The lcatins f detectrs in the SPIRE S14 aperture crdinate system fr each array are shwn in Figure 1. 1 C esa *Nte: the strict calculatin in 3D wuld be, θ Z = tan Aesa but the difference in angular psitin with the frmula abve is at mst Example t determine the psitin f PSWJ15 relative t aperture S14 Frm the spreadsheet, aperture S14 is centred at PSWE8. On the instrument fcal plane in millimetres: Psitin f PSWE8: Y esa = -2mm, Z esa = -90.6mm Psitin f PSWJ15: Y esa = mm, Z esa = mm Therefre, the relative psitin f PSWJ15 n the instrument plane with respect t PSWE8 is, (-30.45mm, mm) which is in the negative-negative quadrant f the Y-Z plane. On the sky in arcsecnds: Directin csine vectr t PSWE8 = [ , , ] Directin csine vectr t PSWJ15 = [ , , ] Applying the equatins abve gives the relative angular ffset f PSWJ15 with respect t PSWE8 as, (221.5", 108.0") which is in the psitive-psitive quadrant f the Y-Z plane. Z esa PSW lcatin n the sky: aperture S14 centred at PSWE8 PSWJ15 (θ Y-SPIRE, θ Z-SPIRE ) Herschel telescpe bresight SPIRE PSW array X esa Y esa

4 Page: 4 f 12 PSW SSW PMW SLW PLW SLWA1 PLWE9 PLWA1 Figure 1: The lcatin f specific detectrs in the Spire frame centred at aperture S14 (upper plts). The caligned detectrs are clured blue. The 8'x4' bundary is shwn fr the Phtmeter and the 2.6' field fr the Spectrmeter. The plt n the left shws the lcatins f the Phtmeter and Spectrmeter in the frame centred at the Herschel bresight frm HSpt, with the Sun directin marked. All plts shw the psitins n the sky rather than in the instrument plane.

5 Page: 5 f Backgrund t Instrument Mde Mask An Instrument Mde Mask is required t take accunt f the effect f the limiting apertures within the instrument that restrict the field f view n the sky. There are several places in the ptical train that limit the field f view n the detectr arrays the aperture at the Herschel fcal surface, and the reflective surface f mirrr M6 (PM6 fr Phtmeter, and SM6 fr Spectrmeter). Fr the Spectrmeter, there is an additinal dynamic vignetting caused by a mismatch with SM12 which changes with SMEC psitin. This causes the OPD dependent vignetting which shws up as a drp in the baseline f the interfergrams. Instrument entrance aperture Beam steering mirrr Instrument entrance aperture M3 Figure 2: The Phtmeter (left) and Spectrmeter (right) ptical designs shwing the instrument entrance aperture, beam steering mirrr (M4), PM6, SM6 and SM12 (frm the Instrument User Manual). Only the upper half f the spectrmeter ptics are shwn. M3 In summary, the three effects are: Cutff due t entrance aperture Changes with BSM psitin Phtmeter & Spectrmeter Field effect (static) vignetting Fixed with respect t detectr psitins Phtmeter (PM6) & Spectrmeter (SM6) Pupil effect (dynamic) vignetting Changes with SMEC psitin Spectrmeter (SM12) Entrance Aperture As the entrance aperture is befre the BSM in the ptical train, the detectr arrays are mved under this aperture as the BSM mves. Figure 3 shws a phtgraph f the aperture taken in the lab during PFM testing. The specificatin fr the entrance aperture is given in RD3 (Figure 4 and Table 7 f that dcument). The dimensins f the aperture are reprduced here in Figure 4 (frm RD3) and 5 frm (RD4).

6 Page: 6 f 12 Figure 3: Phtgraph f the instrument entrance aperture (during PFM testing). Figure 4: The dimensins f the entrance aperture frm RD3. Figure 5: The dimensins f the entrance aperture frm RD4.

7 Page: 7 f 12 The entrance aperture is lcated at the Herschel fcal surface and s is tilted at 18º in rder t match the fcal surface curvature. As the aperture is nt curved itself, nly the central axis is actually n the fcal surface and this means that the shape shuld be slightly distrted when prjected nt the sky. Hwever, assuming this distrtin is negligible, the size f the aperture n sky can be determine frm the plate scale. Measured data fr the telescpe indicate a flight value fr the fcal length f m. SPIRE is ffset in the telescpe fcal plane (actually curved) and s the effective fcal length is slightly reduced, giving a plate scale f arund 7.28"/mm. This shuld nly vary at % r sub % relative level acrss the SPIRE field f view. The resulting aperture with the psitins f the detectrs is shwn in Figure 6. As the dimensins f the aperture in RD3 and RD4 agree t within 0.5 mm, the (slightly smaller) values frm RD3 have been used here. The (0,0) psitin is set t PSWE8 this crrespnds t pint "X" in Figure 4, which is defined frm Herschel Optical Datum t be x= mm, y=0.00 mm and z= mm in RD4. The separatin between PSWE8 and SSWD4 is ( ", "), determined frm RD5. The reasn that the centre f the Spectrmeter array is ffset in bth Y and Z directins is that the Herschel fcal plane is curved in bth directins, but the aperture is flat and the 18º tilt nly allws fr the curvature in the Z-directin. Figure 6: The lcatin f the Phtmeter and Spectrmeter arrays within the entrance aperture in spacecraft Y-Z crdinates n sky (with PSW E8 as the rigin). Figure 1 gives the mapping between psitin and detectr name fr all arrays Field Effect Vignetting The mirrrs PM6 and SM6 ccur in the ptical train between the BSM and the detectrs. They are versized with respect t the beam centres f individual detectrs, but nt with respect t the full beam sizes, and this means that they cause a static vignetting t the uter detectrs in the array. Fr the Phtmeter, the versizing f the mirrr is crrect fr PSW, but with the larger beam sizes f PLW, there is sme vignetting f the uter detectrs. This has been bserved in the lab data. Fr the spectrmeter, there is vignetting f the uter ring f detectrs. Figure 8 shws the mirrrs PM6 and SM6 with the psitins f the detectr centres marked fr PLW and SSW. The clured dts crrespnd t a pint surce bserved with each detectr i.e. the mirrrs are nt exactly at an image plane, s a pint surce appears extended (by a differing amunt acrss the mirrr). This shws that althugh the beam centres all fall within the bundaries f the mirrr, part f the full beam ftprint can fall ff the edge, particularly fr the spectrmeter (the clured ellipses represent a single pint at beam centre they d nt shw the whle beam size, which will be much bigger).

8 Page: 8 f 12 Figure 8: The psitin f detectr centres n mirrrs PM6 (fr PLW) and SM6 (fr SSW). Nte that each clured ellipse represents a single pint and nt the full beam ftprint. These mirrrs are the rigin f the quted unvignetted fields f view in the descriptins f the instrument. These are usually quted as 8' 4' fr the Phtmeter and a circle f diameter 2.6' fr the Spectrmeter. In actual fact, these are just apprximatins t shw the bundary inside f which all f the flux shuld be recvered. The actual transmissin acrss the arrays has been measured in the lab and is nt symmetrical. Figure 9: Detectrs with nminal bundaries f static vignetting Pupil Effect Vignetting There is a further vignetting fr the Spectrmeter that depends n the ptical path difference. This is due t the fact that the beam changes size depending n the psitin f the SMEC. The mirrr SM12 is lcated between the SMEC and the detectrs, and is nt versized sufficiently t reflect all f the beam at all SMEC psitins. This causes a fall ff in transmissin fr the ff-axis detectrs that is dependent n SMEC psitin. Figure 10 shws the effect n the interfergrams fr SSW, with a fall ff in transmissin twards higher OPD (the central detectr des nt shw this dynamic OPD-dependent vignetting because the full flux is always reflected by the mirrr at all SMEC psitins).

9 Page: 9 f 12 Figure 10: Interfergrams fr all detectrs in the SSW array shwing the dynamic vignetting which increases fr detectrs away frm the axis. 1.5 Phtmeter Chpped Observatins In Phtmeter chpped bservatins, the BSM mves the detectr array ftprint n the sky underneath the entrance aperture. During this chpping, sme detectrs at the edge f the array may end up utside f the bundary f the aperture, and s nly receive radiatin frm the (cld) inside f the instrument. Or frm the alternative pint f view, the BSM mves the sky and entrance aperture ver the detectr array. If the chp thrw is large enugh, the edge f the aperture may be mved int the edge f the array, meaning thse detectrs nly receive radiatin frm the inside f the instrument. The current chp thrws are as fllws: Chp Thrw Jiggle Psitins POF2 63" 7 POF3 126" 64 Fr POF2 bservatins, the jiggle ffsets are relatively small and we can just mask detectrs which have any part f their beam ftprint utside f the aperture. Hwever, fr POF3, the jiggle pattern is larger, and s sme detectrs may be inside fr sme jiggle psitins and utside fr thers. The simplest slutin wuld be t mask all detectrs which have any jiggle psitin utside f the aperture. Figure 11 shws the detectrs chpped utside the field fr POF2 (individual jiggle psitins nt shwn). In Figure 12, all jiggle psitins are pltted (with beam sizes 18", 25", and 36"), shwing the masked detectrs (in blue). In these figures, detectrs that shuld be masked n the right hand side f the aperture are clured in blue. In Figure 12 it is als clear the tw detectrs must als be masked n the left hand side fr PMW and PLW.

10 Page: 10 f 12 PSW PMW PLW Figure 11: Detectr psitins in the aperture fr POF2 with a chp thrw f 63". The left chp is shwn in red, and the right chp in green. Masked detectrs are shwn in blue. PSW PMW PLW Figure 12: Detectr psitins fr a POF3 64 pint jiggle bservatin with a chp thrw f 126". The left chp is shwn in red, and the right chp in green. Masked psitins are shwn in blue.

11 Page: 11 f 12 The final masked detectrs shuld be: POF2 PSWA1, PSWB1, PSWC1, PSWD1, PSWE1, PSWF1, PSWG1, PSWH1, PSWJ1 PMWA12, PMWA13, PMWB12, PMWC13, PMWD12, PMWE13, PMWF12, PMWG13 PLWA1, PLWB1, PLWC1, PLWD1, PLWE1 POF3 PSWA1, PSWA2, PSWA3, PSWB1, PSWB2, PSWB3, PSWC1, PSWC2, PSWC3, PSWD1, PSWD2, PSWD3, PSWE1, PSWE2, PSWE3, PSWF1, PSWF2, PSWF3, PSWG1, PSWG2, PSWG3, PSWH1, PSWH2, PSWH3, PSWJ1, PSWJ2, PSWJ3, PSWB4, PSWD4, PSWF4, PSWH4, PSWA4, PSWC4, PSWE4, PSWG4, PSWJ4 PMWA11, PMWA12, PMWA13, PMWB10, PMWB11, PMWB12, PMWC11, PMWC12, PMWC13, PMWD11, PMWD12, PMWE11, PMWE12, PMWE13, PMWF11, PMWF12, PMWG11, PMWG12, PMWG13, PMWD1 PLWA1, PLWA2, PLWB1, PLWB2, PLWC1, PLWC2, PLWD1, PLWD2, PLWE1, PLWE2, PLWC3, PLWE3, PLWA3 In additin, the fllwing detectrs shuld be masked n the left hand side f the aperture fr POF3 PMWA1, PMWG1 PLWE9, PLWA9 An alternative slutin wuld be t stre the entrance aperture bundary in a calibratin file, and mask specific jiggle psitins in a new mdule in the cde. The advantage f this methd wuld be that nly the actual pintings utside f the aperture wuld be masked, and als it wuld nt matter if the chp thrw r jiggle pattern was changed later. 1.6 Spectrmeter Jiggled Observatins The Spectrmeter des nt have chpped bservatins, but des make jiggled maps: Chp Thrw Jiggle Psitins SOF2 int 0" 4 SOF2 full 0" 16 The jiggle psitins are pltted ver the aperture in Figure 13 (with beam sizes f 16.3" fr SSW and 35" fr SLW). This shws that sme detectrs (r psitins) may need t be masked ut in the spectral cube creatin stage.

12 Page: 12 f 12 SSW SLW Figure 13a: Psitins in a Spectrmeter intermediate sampled Jiggle map ver the aperture. SSW SLW Figure 13b: Psitins in a Spectrmeter fully sampled Jiggle map ver the aperture.