Calibration Procedures in Spectropolarimetry
In the realm of spectroscopic polarimetry, calibration is an essential aspect to ensure precision and accuracy in observations. This involves calibrating various components such as grating spectropolarimetry, linear polarizations, and waveplate rotators. The calibration data, maintained by SALT, plays a crucial role in the calibration process. Furthermore, the application of calibration involves steps like removing zero points, adjusting for efficiencies, and converting linear to circular polarizations. This systematic approach ensures data integrity in the obtained results. To understand more about the nuances of calibration in spectropolarimetry, explore the detailed calibration procedures discussed in the content.
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Robert Stobie Prime Focus Imaging Spectrograph Linear Spectropolarimetry - Calibration K Nordsieck Dani l: Precision Me: Accuracy RSS polarimetric optics what needs to be calibrated calibration data maintained by SALT calibration is applied in distributed software polsalt known problems in current calibration plans for future Nov 14, 2022 SALT Workshop 1
RSS Spectropolarimetry - overview Camera Collimated Beam Doublet Silica/ CaF2 Filters E Shutter WR 79 900 l/mm 20220923 O Grating/ FP etalons Triplet CaF / NaCl/ CaF CCD Collimator Quadruplet Silica/ CaF / Silica/ CaF RSS Spectropolarimetry: object on-axis at focal plane, though waveplate, dispersed by grating, and split into O and E spectra Linear spectropolarimetry: images taken at 2 halfwave plate rotations 1,2 45 apart. Qraw : hw pos 0,4: 0 ,45 Uraw : hw pos 2,6: 22.5 ,37.5 Triplet Silica/ NaCl/ Silica 2 Beamsplitter 2 2 Waveplates 2 Field Lens Silica/ CaF2 Focal Plane Sraw= [(O1 - O2)/(O1 + O2) - (E1 - E2)/(E1 + E2)] beamsplitter UV Calcite Prism mosaic 150 mm grating Polsalt pipeline: wavecal, extract, rawstokes, calibrate, plot. quarterwave halfwave stack of 3: MgF2/crystal quartz 300 x 100, 60 mm Nov 14, 2022 SALT Workshop 2
Calibration Note: much of this is in the Observer's Guide: http://astronomers.salt.ac.za/wp-content/uploads/sites/71/2022/06/3170AM0013_Polarimetry_Observers_Guide_V1.2.pdf What needs calibrating? here: grating spectropolarimetry: Status On-Axis Done Done Done TBS TBS Calibration Linear Pol Zero-Point Linear Pol Efficiency Linear Pol PA Zero-Point Rho, waveplate rotator Circular Pol Zero-Point Circular Pol Efficiency Linear-Circular Conversion Origin Tel Mirrors. dep Waveplate. dep Full FOV in work in work NA TBS TBS Full Track in work in work NA TBS TBS ?, likely negligible Waveplate ADC, Waveplate TBS TBS TBS Nov 14, 2022 SALT Workshop 3
Calibration Data Maintained by SALT (commissioning data): Linear Pol Zero-Point. Observe known unpolarized standards with several gratings RSSpol_Linear_TelZeropoint_20061030_v01.txt Linear Pol Efficiency, PA. Observe quartz lamp through polaroid with several gratings RSSpol_HW_Calibration_20061030_v04.txt Linear Pol PA Zero-Point. Observe known polarized standard RSSpol_Linear_PAZeropoint.txt: 120 Nov 14, 2022 SALT Workshop 4
Applying the Calibration Sky Telescope N Y Spectrograph Y,r Azimuth Rotator rho In specpolfinalstokes: for each rawstokes, zeropoint rotated from telescope to spectrograph frame and removed after combination of all rawstokes for the observation, polarization divided by HW efficiency PA rotated to spectrograph frame using HW PA data, then to sky using Slit PA and PA zeropoint Fits header will contain: calibration file names slit PA, rotator rho, telescope azimuth Slit PA Nov 14, 2022 SALT Workshop 5
Current calibration precision Precision On-Axis 0.05% 0.2 deg ripple 0.5 deg ripple 0.5 deg ripple 0.1 deg Calibration Linear Pol Zero-Point Linear Pol Efficiency Linear Pol PA Zero-Point Full FOV 1.5% Full Track 0.1%? On-axis: variable small PA "ripple" for high-SN, highly polarized objects Off-axis (long slit): PA ripple + zeropoint change PA ripple due to assembly alignment errors in 6 elements of halfwave plate Variation due to sensitivity of waveplate to change in illumination of pupil over track and FOV (depends slightly on grating) Nov 14, 2022 SALT Workshop 6
Calibration Plans for future (This year) Finish calibration revision to take into account illumination variations over track, FOV to be released in githib as new version of specpolfinalstokes, starting with PG0300, PG0900 gratings (next year) Finish calibration revision for other gratings, new imaging spectropolarimetry mode to be released in github as new polsalt version, incorporating MOS capability, new wavecal and extraction Do circular polarimetry calibration Extend calibration into UV Nov 14, 2022 SALT Workshop 7
Questions? Extra added attractions? Nov 14, 2022 SALT Workshop 8
MOS, Imaging SpectroPolarimetry MOS spectropolarimetry need zeropoint calibration over full FOV Imaging spectropolarimetry dispersed vertically by beamsplitter chromatic splitting slitless. Find highly polarized objects MOS. very lores spectropolarimetry to photon limit MOS support, needs mask design extraction M30 20200822 Nov 14, 2022 SALT Workshop 9
Circular Polarimetry Calibration data needed Circular Pol Zero-Point?, likely negligible Circular Pol Efficiency (Waveplate) Linear-Circular Conversion (ADC, Waveplate) Have started Linear-Circular Conversion by observing known star with high linear, zero circular Have ongoing commissioning program to observe known magnetic stars for Zeeman Effect Nov 14, 2022 SALT Workshop 10
UV Polarimetry UV spectropolarimetry very rare on large telescopes Not currently feasible: arc optics is opaque below 3550 Ang New calibration will be good to 3900 Ang (polaroid limitation) UV calibration differences lamp too weak, use cluster of hot stars observed through UV polaroid, over variety of tracks Have old UV spectropolarimetry (T Pyx, 2011) that is not yet publishable because we do not have a calibration Nov 14, 2022 SALT Workshop 11
