Project Documentation Document RPT-0046 Draft A3 Polarimetry Working Group Recommendations on Telescope Calibration and Polarization Modulator Location David Elmore ATST Instrument Scientist 1 November 2011 Advanced Technology Solar Telescope 950 N. Cherry Avenue Tucson, AZ 85719 Phone 520-318-8102 atst@nso.edu http://atst.nso.edu Fax 520-318-8500
1. Date: 28 April 2011 Revision: Draft A1 Changes: Initial release REVISION SUMMARY: 2. Date: 7 June 2011 Revision: Draft A2 Changes: Text clean up and addition of references following review by Project Scientist 3. Date: 1 November 2011 Revision: Draft A3 Changes: Removed sub-aperture method critique of registration it is statistical RPT-0046, Draft A3 Page ii
Table of Contents Abstract... 1 1. INTRODUCTION... 2 2. PROCESS... 3 3. REASONING AND CONCLUSIONS... 4 3.1 OVERVIEW... 4 3.2 WHAT TYPE OF POLARIZATION MODULATION IS TO BE EMPLOYED AND WHERE IS POLARIZATION MODULATION PERFORMED?... 4 3.3 HOW IS TELESCOPE POLARIZATION CALIBRATION PERFORMED... 6 4. ONGOING WORK OF THE PWG... 8 5. REFERENCES... 9 RPT-0046, Draft A3 Page iii
Abstract The ATST Polarimetry Working Group has been charged with answering three questions. 1. What type of polarization modulation is to be employed and where is polarization modulation performed? 2. How is polarization calibration performed? 3. How is telescope polarization calibration performed? This report presents recommendations by the PWG on the first and third questions. To address these questions the PWG has divided effort among work packages. Different groups within the PWG have produced reports addressing several of the work packages related to these two questions. The group as a whole reviewed and discussed the reports in order to arrive at two recommendations simply stated here. Polarization modulation for instruments sharing adaptive optics should be performed at the instruments themselves. Telescope polarization will be inferred using the correlation method. RPT-0046, Draft A3 Page 1 of 9
1. INTRODUCTION Science objectives of ATST require high precision polarimetry. To perform this function, there must be polarimeters, in the case of ATST using polarization modulation and analysis optics, calibration optics for characterization of polarimeters, and a technique to measure of infer the polarization of the telescope. The Polarimetry Analysis and Calibration (PA&C) system provides, among other services, support for performing these functions. Guidance on selection of optics, location of optics, and techniques to be utilized by PA&C is provided by the ATST Polarimetry Working Group (PWG). The PWG is charged with answering three questions. 1. What type of polarization modulation is to be employed and where is polarization modulation performed? 2. How is polarization calibration performed? 3. How is telescope polarization calibration performed? This is a report on recommendations addressing the first and third questions. RPT-0046, Draft A3 Page 2 of 9
2. PROCESS The polarimetry group consists of stakeholders in the success of polarimetric measurements at ATST including representatives of all polarimetric instruments. To answer the three key questions, effort has been divided into work packages following a discussion of the issues pertaining to those questions presented in RPT-00137 Polarization Issues (Elmore). Those work packages that directly address answering of the first and third questions have been emphasized. A web site was set up to list the questions, work packages, reports addressing those packages, and log meetings. http://atst.nso.edu/pwg Those work packages and the reports addressing them are as follows. WP5: Haosheng Lin, IfA: Use data from instruments at the DST. Apply polarimeter response calibrations then infer the telescope matrix. Use this matrix to correct data and compare to Hinode. Use the matrix to infer a model for the DST and determine how well this inferred matrix correctly varies as a function of telescope geometry. Use of the cross correlation method to calibrate FIRS data and compare telescope and polarimeter response calibrated data to Hinode. David will compare the inferred telescope matrix elements to a DST model. RPT-0045, Inference of telescope polarization: Dunn Solar Telescope using Facility Infrared Spectropolarimeter Observations (Draft 1) - Elmore WP6: Alexandra Tritschler, David Elmore, NSO: Use a solar model to create stokes profiles. Apply a model ATST telescope matrix to these profiles to create output profiles. Determine how well the telescope matrix can be recovered using the output profiles. Introduce errors into the output profiles of a magnitude set by the error matrix and determine how well the telescope matrix can be recovered. Apply a full seeing model to the input vectors. Now how well can the telescope be recovered? Alexandra is modeling how well one can recover input Stokes vectors through a polarizing ATST telescope and an uncalibrated polarimeter located at Gregorian or Coudé stations. David is evaluating the same but for a perfect polarimeter response calibration. Alexandra will next add the effect of residual image degradation after AO to evaluate induced crosstalk RPT-0044, Inference of telescope polarization: ATST telescope model applied to synthetic Stokes profiles of a model solar atmosphere (Draft 3) Elmore WP8: Model the influence of seeing induced crosstalk on different types of polarimeters and located at the GOS or Coudé station. Include calibration errors guided by the error matrix. Include evaluation of dual beam mis-registration. Roberto Casini, Alfred de Wijn, HAO Analytic study of seeing induced crosstalk (WP8) by Robert and Alfred - related to dual beam registration, the type of modulation scheme Alexandra Tritschler, Jose Moreno, Friedrich Wöger, David Elmore, NSO Use a solar model to create stokes profiles. Apply a model ATST telescope matrix to these profiles to create output profiles. Add the effect of residual image degradation after AO to evaluate induced crosstalk On the Detection of Polarized Light: A Case Study for the ATST: Part 1, Part 2, Part 3 Tritschler, Elmore, Moreno, Rimmele, Uitenbroek, & Wöger Seeing-Induced Polarization Cross-Talk and Modulation Scheme Performance (v.2) Casini, de Wijn, & Judge RPT-0046, Draft A3 Page 3 of 9
3. REASONING AND CONCLUSIONS 3.1 OVERVIEW Reports addressing the questions were distributed to the group, and archived on the web site. The PWG discussed the reports with numerous e-mail exchanges, then met via teleconference to resolve the questions on 19 and 26 April 2011. Key points from the reports and brought up during the discussions and listed in the meeting notes are given here. 3.2 WHAT TYPE OF POLARIZATION MODULATION IS TO BE EMPLOYED AND WHERE IS POLARIZATION MODULATION PERFORMED? The baseline design calls for modulation at the Gregorian Optical System (GOS) and analysis at each instrument. Although numerous polarimeters at various solar telescopes have produced excellent science using a polarization modulator at the instrument, the reasoning that led to the ATST baseline was a concern that residual image motion even after AO correction would lead to unacceptably high levels of cross talk among polarization parameters. This concern is based, in part, on papers by Lites 1 and Judge 2 that infer crosstalk among Stokes parameters using tip-tilt corrected residual image motion measurements performed at the Dunn Solar Telescope (DST). There are science drivers for modulating at each instrument instead of using a single modulator for all instruments. Optimization of signal to noise for as many instruments as possible is key. With a modulators located at the instrument Modulator parameters can be optimized for efficiency of each observation. For example a modulation scheme may intentionally not be balanced to emphasize sensitivity to the Hanle effect or for coronal Zeeman polarimetry. Modulation rates can be optimized for each observation. For example when one instrument is observing in the core of the Ca-K line, it will be able to modulate slowly enough to avoid sensor read noise. Modulation efficiency change due to telescope geometry does not go through zero for modulation at the instrument and therefore does not need compensation such as by rotation of the modulator at the GOS or of a half-wave retarder at the instrument - neither technique producing as high a modulation efficiency throughout the day as modulation at the Coudé. (shown in Part2 and TN- 0115) There is the flexibility of performing polarization calibration at the instrument. With clear scientific advantages for modulating at the instrument, the concern about errors in measuring Stokes parameters behind polarizing optics and in the presence of residual seeing after AO correction becomes the issue to resolve. The study On the Detection of Polarized Light: A Case Study for the ATST: Part 1, Part 2, Part 3, addresses this question. Features of this forward model are Synthetic solar atmosphere for active network creating Stokes profiles for the FeI 630.25nm line spanning a 7 arc second patch Atmospheric model with time varying correlation lengths for a 4-m aperture telescope Modeled correction by a high order adaptive optics (HOAO) system Several instrument applications different types of polarization modulators and locations (rotating retarder and electro-optic, GOS and Coudé) Model ATST polarizing telescope optics preceding and following the modulator RPT-0046, Draft A3 Page 4 of 9
An important outcome from this modeling effort is that residual tip-tilt plays a relatively small role in image changes compared to changes in the shape of the point spread function when HOAO is used for such a large aperture telescope and observations are performed close to the lock point. A consequence of that modeling outcome when applied to the various instrument applications leads to the following conclusion in Part 3 of that report. Polarized crosstalk V Q,U We do not find direct and obvious evidence of a detectable systematic correlation between the Stokes signals in the selected instrument applications. A systematic crosstalk from V Q,U should show up as a distinct arrow-like shaped pattern in diagrams of Stokes Q,U versus Stokes I (which it does not), and a systematic correlation of data points that shows up as a rotation against the reference points when Stokes V is shown over Stokes Q,U (see Sect. 6.) (which is not observed either). The solar model shows higher V polarization than Q and U but since modulators encode signals similarly for all polarization parameters, this conclusion can be extended to crosstalk from Q and U to V. The concern about crosstalk using a modulator at Coudé (one of the instrument applications) was just that, a concern, and not a problem. This removes the only significant reason for modulation at the GOS leaving the weight of the science drivers calling for modulation at each instrument. An assumption that goes into this conclusion is that the polarimeter and the telescope are perfectly calibrated. To minimize the amount of telescope optics that cannot be directly calibrated polarization calibration optics be located as close to the sun as possible, meaning in practice at the GOS. There are two parts to the question, the type of modulator and the location. If the location is at the instrument, the type of modulator is then up to each instrument. This allows for any type of modulation, but we know from both experience and from discussion with partners that for facility instruments this will be time multiplexed modulation using electro-optical retarders or rotating retarders. Synchronization of any variety of time-multiplexed modulation has a lot of commonality with any other. By modulating at the instrument following a slit or mono-chrometer, solar flux will be reduced to the extent that a broader range of modulator materials can be considered in the design as compared to the limited choices available for a modulator in the hot f/13 beam at the GOS. There is one application that for technical reasons leads to the need polarization modulation at the GOS. This is the Cryo-NIRSP. That instrument has a large, 5 arc minute diagonal field of view compared to 2.8 arc minutes for instruments sharing AO. The smallest beam diameter in the light feed for the Cryo- NIRSP is at the GOS and, though still large, polarization modulators of that size have been quoted. A larger beam diameter challenges the size of available IR birefringent materials. To ensure flexibility of the ATST for future polarimetric applications and since polarization calibration and modulation optics must already exist at the GOS it makes sense to maintain ability of the current GOS design to accommodate additional modulators in the future though only the Cryo-NIRSP would use the GOS at commissioning. Optical design modeling of modulators meeting beam deflection specifications of currently implemented designs excessively degrade optical performance at visible wavelengths. Designs that compensate this deflection have been demonstrated for smaller apertures. Also there are trade-offs between beam deflection and fringing. Conclusion: The Polarimetry Working Group recommends that the ATST baseline approach to polarization modulation be changed so that instruments sharing AO will perform polarization modulation at the instrument. The ability to modulate at the GOS will be maintained in case instruments or multiple instruments require it (Modulation at the GOS is required by the Cryo-NIRSP). To be able to serve RPT-0046, Draft A3 Page 5 of 9
instruments utilizing AO that modulate at the GOS, beam deflection and its effect on downstream optics needs to be addressed. 3.3 HOW IS TELESCOPE POLARIZATION CALIBRATION PERFORMED Current generation telescopes such as the DST, German VTT, and Swedish Vacuum Solar Telescope have used polarizers or polarizer arrays over the entrance aperture to directly measure telescope polarization. The 4-m aperture prohibits that approach for ATST. An approximation to the direct measurement was proposed by Hector Socas-Navarro 3, which we call the sub-aperture method and is the current baseline for ATST. In this method a sub-aperture is isolated using a pupil mask. There are polarization calibration optics in front of M1 matching this aperture. It is therefore possible to directly calibrate this sub-aperture end to end including M1 and M2. After calibrating the sub-aperture a polarized feature is observed with one of the ATST polarimeters and immediately thereafter the same feature is observed with full aperture. The full aperture observation can be directly calibrated as far as the GOS so any differences in the measured polarization from the solar feature must be due to M1 and M2 as seen using the full aperture. Modeling in the publication and using the ViSP flux budget show that an adequate signal to noise can be obtained. The sub-aperture method, though promising, is completely untried. A program verifying the validity of this approach would be costly and time consuming possibly involving one of the new solar telescopes such as the NJIT NST. If insurmountable obstacles were with the method were encountered, then ATST would be without a calibration procedure. Given this uncertainty, exploration of other techniques to measure telescope polarization have been explored. November 4 proposed a technique using solar observations themselves that relies upon symmetry and asymmetry of Zeeman lines. Kuhn et. al. 5 developed this technique for fully split Zeeman lines in the infrared. This symmetry technique has become the standard for some infrared instruments including TIP, Collados 6. There are two ATST reports (RPT-0044 & RPT-0045) that extend this technique to the Zeeman weak field regime expected in general for visible wavelength lines. Rather than depending upon symmetric or anti-symmetric signatures, in these studies cross-correlations among Stokes parameters observed in active regions are utilized to infer the magnitude of crosstalk among polarization parameters. There is a weighting applied with a Doppler corrected shape of an anti-symmetric Zeeman profile that isolates the region of the spectrum at which to perform the cross-correlations. We call this the correlation method. The first study, RPT-0044, uses the same synthetic Stokes profiles for a patch of active network used for the seeing study. A model telescope Mueller matrix (M1 and M2) is applied to these synthetic profiles. Then using only the modified Stokes profiles, the correlation technique infers elements of the telescope Mueller matrix. A sufficient number of elements of the telescope matrix can be inferred allowing construction of the full matrix of the product of M2 acting upon M1. There are, in fact, only two parameters needed to describe the product of two aligned mirrors and five elements can be inferred so there is some redundancy. This exercise was able to reproduce the model telescope applied to the synthetic profiles to the accuracy required by ATST (error matrix given in DL- and Cryo-NIRSP ISRDs). This study permitted examination of details of the cross correlation method and optimization to minimize the uncertainty of the fits. The next study, RPT-0045, utilized what was learned from the synthetic model and applied the cross correlation method to Facility Infrared Spectro-polarimeter (FIRS) observations performed at the DST. Elements of the telescope matrix of the DST were inferred using the correlation method for observations over the course of a day. A DST telescope model was constructed using the same codes as used for the direct telescope measurement and its parameters adjusted to fit the observations. It was possible to infer the parameters for a single telescope model that could fit the observations for the range of telescope geometries over the day and that fit was within approximately a factor two of the requirements of the RPT-0046, Draft A3 Page 6 of 9
ATST error matrix. Since the ATST polarization due to M1 and M2 is expected to be a factor of 3 (diattenuation) to 6 (retardation) smaller than that of the DST, one could conclude that the ATST requirements can be met by the correlation technique. The ATST telescope is the matrix product of two aligned mirrors. The DST contains two windows that are assumed to be net retarders, but the actual form of the matrices for these optics under stress is not known and it is only due to some asymmetry that these windows can have a net retardation. The fact that the correlation technique can produce a model at all for the DST is encouraging for its application to the much simpler and less polarizing ATST. Since the correlation technique can be applied to any telescope without the addition of additional equipment, we plan to continue to refine it through several approaches. One plan is to expand the wavelength range utilizing Zeeman sensitive lines from the blue to the NIR. While observing in the blue at least one Hanle line could be included. There will also be work on fitting a mirror coating model to the telescope matrix to see if it is possible to analytically determine the telescope matrix for any wavelength avoiding interpolations. In an e-mail to this author and to the PWG, Haosheng Lin presented his reasoning that the baseline telescope calibration method be changed to the correlation method. The baseline should be the most plausible method that's available. This may be the case for the sub-aperture method years back, but it has never progressed beyond a concept. With your recent progress on the correlation method, and its potential to be extended to the 'weak field' lines in the blue, I think it is fair to say that this is our most advanced and most promising method to meet the telescope polarization calibration requirements. The PWG feels that the sub-aperture not be ruled out in the design of the ATST though its implementation should await an identified need and someone with the funding to do so. The PWG reaffirmed the requirement that polarization calibration should be as close to the sun as possible (the GOS) to minimize the amount of optics needed to be inferred by the correlation method. Conclusion: The Polarimetry Working Group recommends changing the baseline telescope polarization calibration technique to the 'correlation method'. Infrared instruments would use the equivalent Kuhn et. al. method. The Polarimetry Working Group will continue to develop other observation-based techniques. The TMA design should not be changed in that it provides mounting locations for a telescope calibrator and sub-aperture mask in case they are needed in the future. RPT-0046, Draft A3 Page 7 of 9
4. ONGOING WORK OF THE PWG The second question, how will polarization calibration be performed is not yet complete. We know that polarization calibration optics will reside as close to the sun as possible (at the GOS). The calibration technique that will be explored uses polarization calibrations over a range of telescope geometries to break the polarimeter response into three matrix groups with time varying rotations between. The complete polarimeter response is the product of these groups with appropriate rotations for the time of the observation. Addressing details of the polarization calibration is a development program that is quite unlikely to affect the design of ATST in contrast to the significant impact of questions 1 and 3. Another potential observation-based technique to infer telescope polarization is continuing to be developed by the Institute for Astronomy and the University of Hawaii. This technique uses sky polarization and a sensitive spectro-polarimeter such as the Cryo-NIRSP to utilize that polarization, which can be modeled, to infer the contribution of the telescope to a measurement calibrated to the GOS. The rate at which modulation must be performed influences the Camera System. Is it necessary to perform polarization modulation at a high rate (khz) necessitating the development of charge-caching detectors? Part 3 (Figure 45) of the forward model report shows that modulation in the 30Hz to 100Hz range for a spectrograph avoids most residual measurement errors and that there is no gain going to 2kHz. The analytical model is continuing to be worked with more realistic modeling of residual image motion to see if it comes to the same conclusion. RPT-0046, Draft A3 Page 8 of 9
5. REFERENCES [1] Lites, B. W.: Applied Optics (ISSN 0003-6935), vol. 26, Sept. 15, 1987, p. 3838-3845 [2] Judge, Philip G.; Elmore, David F.; Lites, Bruce W.; Keller, Christoph U.; Rimmele, Thomas: Applied Optics IP, vol. 43, Issue 19, pp.3817-3828 [3] November, L. J.: in L. November (ed.), Solar Polarimetry; Proceedings of the 11 th NSO Workshop, p. 149 (1991) [4] Socas-Navarro 2005, JOSA-A, 22, 907 [5] Kuhn, J.: Solar Physics (ISSN 0038-0938), vol. 153, no. 1-2, 454 p. 143 (1994) [6] Collados, M.: Proceedings of the SPIE, 4843-09 (2003) RPT-0046, Draft A3 Page 9 of 9