INAF Osservatorio astronomico di Torino Technical Report nr. 152

Transcription

1 INAF Osservatorio astronomico di Torino Technical Report nr. 152 LCTF Wavefront error G. Capobianco 1, S. Vives 2, E. Hugot 2 1 INAF- Osservatorio Astronomico di Torino 2 Laboratoire d Astrophysique de Marseille Pino Torinese, 25 th july 211

2 2 Index Index... 2 Index of Figures... 2 List of Acronyms... 2 Revision Log... 2 Introduction... 3 Set up... 4 Data Acquisition Procedure... 5 Data Analysis... 6 Conclusionss... 8 Index of Figures Figure 1 Mechanical assembly of the LCTF. From left: the pre filter, the LCPR and the multistage LC Lyot filter... 3 Figure 2 Schematic view of the set up... 4 Figure 3 Picture of the set up... 4 Figure 4 Interferogram [OPD] of the LCTF... 5 Figure 5 Variations of the 2 nd order Zernike parameters... 7 Figure 6 Imaging quality normalized to the set up aberrations... 7 List of Acronyms LCPR Liquid Crystals Polarization Rotator LCTF Liquid Crystals Tunable Filter OPD Optical Path Difference WFE Wavefront Error Revision Log Date Issue Release Released by G.Capobianco 1 S.Vives 1 G. Capobianco Comment First issue Document revision Document formatted for OATo TR

3 3 Introduction The purpose of this test is to characterizee the WFE of the LCFP and measure its evolution in function of the voltage applied to the LCFP (i.e. when the LCFP scans in wavelength). The LCFP has been designed for the ASPIICS\PROBA 3 mission: its properties are summarized in Table 1, and the mechanical assembly is shown in Figure 1. Measurements have been performed at LAM. LCTF General Pre filter LCPR Length [mm] 9 Diameter [mm] 6 Aperture [mm] 2 Number of stages 4 FSR [nm] 2,7 FWHM [nm],15 Center wavelength [nm] 528,64 533,38 Tuning step [nm],1 Manufacturer Andover Corp. Center wavelength [nm] 53,69 FWHM [nm] 1,89 Manufacturer MLO Rotation angles [deg] 18 Table 1 LCTF properties Figure 1 Mechanical assembly of the LCTF. From left: the pre filter, the LCPR and the multistage LC Lyot filter

4 LCTF Wavefro ont errorr cha aracterizzation 4 Set up The LCFP has h been teested in a double passs configuraation. The set up s (Figure 2 and Figgure 3) wass composed by the ADEE Phase Shiift MiniFIZ a Fizeau intterferometeer with a reeference flat mirror. In n a the LCP PR have beeen remove ed from thee order to perform theese measureements thee pre filter and LCTF. Notee that duringg the measu urement, th he LCFP wass not contro olled in tem mperature. The objecttive lens is used u as redu ucer to fit th he clear ape erture of th he LCFP. Figure 2 Scchematic view of the set up Figure 3 Picture of th he set up

5 5 Data Acquisition Procedure After the alignment, three sets of data have been recorded: 1. Reference: without the LCFP to get the intrinsic errors of the setup; 2. LCFP turned off: to get the intrinsic errors of the LCFP in its static state; 3. LCFP turned on: different voltages are applied to the LCFP to get the evolution of the WFE as a function of the voltage. The interferometer is equipped with a HeNe red laser (628nm) while the LCFP has been optimized for the green Fe XIV line (532nm). So the LCFP has not been tested in its optimal configuration. Due to the previous limits of the used set up, only 3 voltages have been tested. The selected voltages are referred to the nominal wavelengths of 5312Å, 5325Å and 5338Å. The resultantt interferogram is obtained as a resultant of 1 measurements (5x2). In Figure 4 is shown an example of the interferogram obtained with the LCTF. Figure 4 Interferogram [OPD] of the LCTF

6 6 Data Analysis The data acquired are summarized in Table 2. Note that the LCFP has been tested in a double passs configuration. Corrected RMS X Astig [2] Y Astig [2] X Coma [2] Y Coma [2] Spherical [2] X Trefoil [3] Y Trefoil [3] X Astig [3] Y Astig [3] X Coma [3] Y Coma [3] Spherical [3] X Tetrafoil [4] Y Tetrafoil [4] X Trefoil [4] Y Trefoil [4] X Astig [4] Y Astig [4] X Coma [4] Y Coma [4] Spherical [4] X Pentafoil [5] Y Pentafoil [5] X Tetrafoil [5] Y Tetrafoil [5] X Trefoil [5] Y Trefoil [5] X Astig [5] Y Astig [5] X Coma [5] Y Coma [5] Spherical [5] REF No Voltage 533, nm nm , , , , , ,631 5, ,677-15, ,4282-1, , ,424 -, ,22 17,9827 8, ,388 28,181 7,541-16, ,567 2,791-7,4575-2, ,6961 8,93 4, ,367-5, ,8614-1,823-5,1594-6,4237 5,9478-6, , ,181 29, , ,4517-4,184 17,821-83,456-53,37 6, , , ,16-12,915 79,381 72,875 9, ,13-59, , ,9359-1, , ,7457-7, , , , , , , ,365 2, , , ,26-148, , , , , , , , , , ,283-2,13 52, , , , , , ,68-16,3471-8,342-41,858 32,4387-7, , , , , ,412-12,44 19, ,93 116, , ,8811 3, , , ,4276-2,5 13,4766 9,956-13, , , ,2641-1, ,4891 3,41 -,1694-8,966 7, , , ,8549 4,8716-5,359 1,19-8,139 -,8858-2,757 1,9782-1,9158 Table 2 Data Acquired. The values are in nm nm , , ,473 29, ,321-93,949-3, , ,9867-5,8485 5, ,788 43,83-77, ,367 91,48 68,2233 6, , ,9166 8,755 27,4626-9, , , ,44-18, ,347 65, , , , , ,348 The first column shows the name of the optical aberrations and in the square bracket the order of the corresponding Zernike polynomial. The variation of the 2 nd order aberrations vs. the nominal wavelength of the LCTF is in Figure 5. The dominant aberration is the spherical one.

7 7 25 WFE (2nd order Zernike coeff) 2 15 Aberration [nm] X Astig [2] Y Astig [2] X Coma [2] Y Coma [2] Spherical [2] 15 2 Nominal LCTF wavelength [ nm] Figure 5 Variations of the 2 nd order Zernike parameters The resultant imagingg quality is shown in Figure 6. The variations are of order of ¾ λ. With the controller turned off, the imagingg quality is λ/2, as expected. 7 Imaging quality normalized 6 Aberration [nm] Nominal LCTF Wavelength [nm] Figure 6 Imaging quality normalized to the set up aberrations

8 8 Conclusions This test shows that the most relevant contribution to the imaging quality of the LCTF is the spherical aberration. The variations of the wave front error are of order of ¾ λ. The non accurate repeatability tests have to be performed to confirmed the possibility to calibrate the LCTF. The contributions of the pre filter and of the LCPR are not taken into account in this analysis. Further investigationss are suggested to confirm the large variation of the WFE while tuning and to evaluate the possibility to calibrate the LCTF. uniformity of this parameterr suggest us a more accurate calibration. However,