Helioseismic and Magnetic Imager Calibration Status Jesper Schou and the HMI Calibration Team Stanford University and Other Places Abstract The HMI instrument is planned to be delivered shortly and launched in August 008. During the integration and testing of the instrument we have performed an extensive series of calibrations. In this poster we will start with a brief summary of the instrument status. We will then describe the calibrations performed so far including some of the results. Finally we will describe what remains to be done, including on-orbit calibrations, and how well we expect to be able to calibrate the final data. 1
Instrument Overview and Status The HMI instrument is designed to observe Doppler shift, intensity, magnetic fields (line of sight and vector) and other observables. HMI will use the 617.3 nm FeI line and produce 4096 images with 0.5 pixels and 1 optical resolution at a cadence of 40s-50s (TBD) for most observables. Wideband Michelson Beam control lens Blocking filter Image stabilization mirror Telecentric lens ¼ Waveplate ½ Waveplates Telescope lens set Aperture stop Polarizer Lyot Narrowband Michelson Tuning waveplates BDS beamsplitter ISS beamsplitter and limb tracker assembly Calibration lenses and focus blocks Front window filter Relay lens set CCD Shutter assemblies CCD Fold mirror Optical characteristics: Focal length: 495 cm Focal ratio: f/35. Final image scale: 4 m/arcsec = 0.5 /pixel Primary to secondary image magnification: Focus adjustment range: 16 steps of 1.07 mm CCD fold mirror CCD fold mirror Filter characteristics: Central wavelength: 617.3 nm FeI Front window rejects 99% solar heat load Final filter bandwidth: 0.0076 nm Tuning range: 0.069 nm All polarization states measurable Figure 1: A cartoon of the optical layout of HMI. A cartoon of the optical path of HMI is shown in Figure 1. Sunlight travels through the instrument from the front window at the upper right to the cameras at the lower right. The front window is a 50Å bandpass filter that reflects most of the incident sunlight. It is followed by a 14 cm diameter refracting telescope.
Two focus mechanisms, three polarization selectors and the image stabilization system tip-tilt mirror are located between the telescope and the polarizing beamsplitter feeding the tunable filter, which is mounted in a precisely temperature controlled oven. The oven contains the following elements: A telecentric lens A blocking filter A Lyot filter with a single tunable element A beam control lens Two tunable Michelson interferometers Reimaging optics Following the oven is a beam splitter feeding two shutters mounted at a pupil image and two CCD/camera assemblies. There are two mechanisms external to the optics package: a front door to protect the front window during launch and an alignment mechanism to adjust the optics package pointing. Usually the instrument is operated in what is known as obsmode where the Sun is imaged onto the CCDs, but by using a pair of lenses in the wheels holding the focus blocks it is possible to obtain so-called calmode images where the front window is imaged onto the CCDs. The HMI optics package is currently undergoing final integration and calibration at the LMSAL facility in Palo Alto. From an optical point of view the only item left to install is the final front window, as discussed later. There are also a few outstanding issues in the electrical and software areas. Finally the instrument needs to go through the environmental test program including Thermal Vacuum/Balance, vibration testing and electromagnetic testing. Delivery is scheduled for late summer 007 and launch onboard the SDO spacecraft for August 008. 3
Calibration Overview The calibration activities are summarized in Figures and 3 and have been divided into several phases: Sun tests: These were performed in 006 with a partially assembled instrument in order to learn how to operate and calibrate the instrument, to discover gross errors in design or workmanship and to develop procedures and software. First vacuum calibration: This was performed in early 007 when the instrument was almost completely assembled. In-air calibration: This will be the first formal calibration of the instrument. Vacuum calibration: Most calibrations will be repeated with the instrument back in the vacuum tank. Having the instrument in a vacuum tank enables the CCD to be cooled and thus results in a dramatically lower noise level. Initial Setup Jitter Characterization 1 1 Wavelength Flat Field CCD Linearity Camera Crosstalk CCD and Camera Noise Filter Filter Thermal Thermal Filter Thermal 3 1 Image Scale 1 Wavelength Flat Field Vignetting, Contamination, 6Scattered Light Angular Focus, MTF, CCD Alignment 3 Thermal Effects: Pointing, Focus 8 Front Window Thermal 6 Polarization Vignetting, Contamination, 4Scattered Light 4 Wavelength 4 Angular Filter Turn-on Transients Duration, in hours PCU Doppler Observables Line of sight Observables Vector Observables Lamp Laser Sun Figure : Overview of the vacuum calibration activities. 4
1 3 4 5 6 7 8 9 10 11 1 13 14 15 16 17 18 19 0 1 3 4 5 6 7 8 9 30 31 3 33 34 35 36 37 38 39 A B C F G Test Group Property Sun Light Source Test 1 Image quality Distortion Yes Lamp Image scale Yes Sun 3a MTF Yes Lamp 3b MTF Yes Sun 4a Focus Yes Lamp 4b Field curvature Yes Lamp 4c Focus Yes Sun 4d Field curvature Yes Sun 5 Relative alignment of No Lamp cameras 6 Relative focus of No Lamp cameras 7 Ghost images No Various 8 Scattered light No Various 9 CCD and Camera Noise No None 10 CCD and Camera Flat field Yes Lamp 11 Linearity Yes Lamp 1a Quadrant crosstalk No Lamp 1b Quadrant crosstalk No Laser 13a Contamination Yes Lamp 13b Contamination Yes Sun 13c Contamination No Laser 14 Image motions Offset and distortion Yes Lamp + Sun 15 Filter transmission Wavelength and Yes All spatial dependence 16 Angular dependence Yes Laser + Sun 17 Stability No Sun 18 Turn-on transients No Sun 19 Throughput Yes Sun 0 Contrast Yes Laser + Sun 1a Polarization Various Yes Lamp 1b Polarization Various Yes Sun Observables Doppler Yes Sun performance 3 Line of sight Yes Sun 4 Vector No Sun 5 Thermal effects Pointing No Lamp 6 Focus No Lamp 7 ISS Range and stability No Lamp 8 Alignment legs Range and step size Yes Lamp 9 Repeatability Yes Lamp Figure 3: Top level list of calibrations to be performed, as developed before the First Sun Test. 5
On the other hand some calibrations can only be performed in air. In particular the measurement of the telescope polarization would be compromised by the presence of the vacuum tank window, as might various aspects of the image quality. Once on orbit a comprehensive set of calibrations will be run as soon as the instrument is operational followed by periodic measurements of a variety of properties. One test setup is shown in Figure 4. Laser light and various test targets can be projected into the instrument using the stimulus telescope or the Sun can be observed using a heliostat. The Polarization Calibration Unit (PCU) is described later. In the following sections a few of the results of the calibrations will be shown ordered by subject. HOP Line of Sight Entrance to cleanroom PCU Entrance to gowning area Inside cleanroom Outside cleanroom Heliostat HEB Spacecraft Simulator Work area Figure 4: Layout of the test facility. 6
Image Quality The category covers a wide range of properties, including focus, distortion, field curvature and Modulation Transfer Function (MTF)/Point Spread Function (PSF). Figure 5: Theoretical MTF of HMI, the MTF without a front window, and the MTF with one of the candidate front windows in different orientations. The two line styles show the effect of astigmatism, the solid line is in the best azimuthal direction, the dashed one in the worst. Figure 5 shows the MTF as determined by projecting targets with the stimulus telescope. (Image quality measurements using sunlight are difficult to perform due to poor seeing in Palo Alto and heliostat defects.) Clearly this candidate front window is degrading the image quality substantially below the 7
theoretically expected MTF. The dependence on the window orientation indicates wavefront errors unrelated to the window. Different stimulus telescope orientations as well as interferometer measurements can be used to separate the errors in the stimulus telescope from those in the instrument. Figure 6: Best focus in focus steps as a function of image position. The field curvature and gradient caused by the stimulus telescope has been removed. One focus step is about /3 depth of focus. For the case without a front window it is likely that part of the deviations from being diffraction limited are caused by the stimulus telescope, air currents and other deficiencies in the test setup, as well as scattered light. We are currently working on understanding these deviations. We are also working on various options for improving the front window, including correcting 8
the internal errors by polishing the surface and by modifying the manufacturing process. Figure 6 shows the best focus as a function of image position. Both the field curvature and the focus gradients are very small. Figure 7: The optical distortion derived from offset images. Figure 7 shows a distortion map derived by offpointing the instrument relative to the stimulus telescope using the alignment legs. While the distortions are significant in some places, this is easily correctable since the images will be remapped as part of the ground processing. 9
Wavelength Another critical area is to determine the wavelength response of the instrument. This is done using sunlight, a tunable laser and white light. Selected results are shown in Figures 8-10. Among the interesting features seen are the weak fringes in the front window seen in calmode. Both the phase maps and contrasts are excellent and should not cause significant degradation of the quality of the calibrated data. 10
Figure 8: Calmode phase maps and line parameters obtained with sunlight. 11
Figure 9: Calmode phase maps and contrasts measurements made with the tunable laser. 1
Figure 10: Obsmode phase maps and contrasts measurements made with the tunable laser. 13
Polarization The third major area of calibration is polarization. To measure this the PCU shown in Figure 4 was designed and assembled by HAO. The PCU is placed between the stimulus telescope (or heliostat) and the instrument. The PCU consists of a polarizer followed by a (roughly 1/4λ) waveplate, both mounted on linear and rotary stages allowing them to be moved in and out of the beam and rotated to arbitrary angles. Using various PCU configurations combined with various settings of the internal mechanisms it is possible to infer the retardance of the telescope and internal waveplates as well as their absolute orientations. Figure 11 shows various polarization properties. As can be seen the artifacts in the properties of the waveplates are extremely small. The retardance map of the front window is dominated by the stress birefringence introduced by the vacuum tank window. Maps made in air show very little telescope birefringence and the resulting depolarization is negligible. Unfortunately a software error caused difficulties for the polarization calibration. This and the fact that the window has to be replaced means that the quality of the polarization results shown is somewhat degraded. Once these two problems have been corrected we expect to obtain results significantly better than the target polarization accuracy. 14
Figure 11: Maps of various polarization properties. Top row shows the retardance of the three waveplates, relative to the median values. Scale is +/- 0.001 waves. The first first maps in the bottom row show φ 1 φ and φ 3 φ, where φ 1 through φ 3 are the rotation angles of the waveplates relative to their reference position (the individual angles are nearly degenerate). Scale is +/- 0.. The final map shows the retardance of the combination of the front window and the vacuum tank window on a scale of 0 to 0.1 waves. 15
Miscellaneous In addition to the general calibration topics listed in the previous sections, several other properties have been or will be measured. These include items such as properties of the CCDs and cameras, checking for ghost images and contamination, thermal effects, ISS performance and alignment leg repeatability. Finally it is possible to obtain real observing sequences and to calculate observables. However, due to the poor seeing, limitations of the instrument, lack of solar activity and the weather during the first Sun test few of these were taken. For entertainment first light Doppler and line of sight magnetograms are shown in Figure 1. Figure 1: First light (almost) Doppler and line of sight magnetograms obtained with HMI. 16
Conclusion We have by now done initial calibrations of all parts of the assembled instrument and have developed the procedures necessary to perform the final calibrations. The only known issue remaining from an optical point of view is that of the image quality degradation caused by the front window, which will be corrected before launch. Apart from this it appears that the HMI instrument will produce excellent data on orbit. Acknowledgements The first author is grateful to the large number of people who have helped build HMI or worked on the calibrations. The HMI calibration team members include Sebastien Couvidat, Cristina Rabello-Soares, Richard Wachter, Tom Duvall, Juan Manuel Borrero and Jesper Schou. Other people providing significant help include: Steve Tomczyk, Aimee Norton, David Elmore, Greg Card, Jack Harvey, Phil Scherrer, Jim Aloise, Jeneen Sommers, Todd Hoeksema, Keh-Cheng Chu, Hao Thai, Karen Tian, Rock Bush, Yang Liu, Rick Bogart, Ted Tarbell, Dick Shine, Barbara Fischer, Brett Allard, Brett Pugh, Carl Yanari, Claude Kam, Dave Kirkpatrick, Dave Sabolish, Gary Linford, Gil Mendelilla, Glenn Gradwohl, Hank Hancock, Jerry Drake, John Miles, JP Riley, Keith Mitchell, Louis Tavarez, Roger Chevalier, Ron Baraze, Rose Navarro, Tom Cruz, Tom Nichols and Tracy Niles. People interested in helping with the calibrations or with ideas for what to do are encouraged to contact the first author of this poster or other members of the HMI team. Details about HMI can be found at http://hmi.stanford.edu. HMI is funded through NASA contract NAS5-0139 to Stanford University. 17