Image Stabilization System (ISS)

1. HMI00424 Image Stabilization System (ISS) Chris Edwards (T.Tarbell,S. Sour,V.Duong) ISS Lead chris.edwards@lmco.com

2. Image Stabilization System – Agenda • Driving Requirements • ISS Design • Design Overview - Block Diagram • Limb Sensor • Active Mirror • Design Performance • Remaining Design Steps • Future Plans

3. Driving Requirements • HMI stabilization requirement is 0.10 arc-second (3-s) in each axis pitch and yaw. • S/C environment up to 5 arcsecond, PSD TBD. • Jitter reduction by at least a factor of 100 at 1Hz, 10 at 10Hz, and 2 at 50Hz is required. • The required offset range of tilt mirror is ±14 arc-seconds. The first resonance should be >500Hz, much higher than the first structural mode. • Limb sensor must be insensitive to solar phenomena and must be able to accommodate small offsets in order to center the solar disk on the CCD camera. • Flight S/W must be able to open and close the servo loop depending on observing mode.

4. Operational Approach Objective: Keep solar disk centered on the same pixels of the CCD during long-term scientific observations. • The ISS uses the image of the solar limb projected onto four orthogonal detectors at the limb sensor focal plane. Each detector consists of a redundant photodiode pair. The electronic limb sensor photodiode preamplifier has 2 gains, test mode and sun mode, and selectable prime or redundant photodiodes. Analog electronics converts diode outputs to X and Y error signals. • The error signals coming from the limb sensors are adjusted in gain and offset and then filtered with an analog filter. These 2 axis signals are converted to 3 axis pzt drive signals which are changed by the PZT driver circuit from +/-15 Volts to 0-72 Volts for the low voltage PZT’s. This voltage moves the PZT’s on the mirror which moves the light towards center on the limb sensor, and the error signals closer to zero. • If the error offsets come close to the end of the offset range or the average PZT voltages come close to the end of their range, the alignment legs will be used to bring the image back to center by ground operator intervention. The error and mirror signals are continually sampled and down-linked to monitor jitter and drift. For special calibrations, these signals can be sampled at a higher rate.

5. ISS Design Overview - Block Diagram ISS ELECTRONICS PZT MIRROR DRIVER ERROR AMPLIFIER THREE STAGE FILTER TWO AXIS TO 3 ACTUATOR CONVERSION MIRROR DRIVE ELECTRONICS MIRROR DRIVE ASSEMBLY X & Y ERROR OFFSETS X & Y GAIN ADJUSTMENTS A,B & C ACTUATOR OFFSETS M1 LIMB SENSORS & PREAMP LIMB TRACKER IMAGE FROM SUN CMDS Control Computer RAD6000 CCD CAMERAS S/C HK TLM ALIGNMENT LEGS

6. ISS Design Overview • Contains 5 components: Limb Sensor, Limb Sensor Preamplifier, Active Mirror, Limb Tracker interface board, PZT Driver board. The Limb Sensor, Limb Sensor Preamplifier, and the Active Mirror are located in the HMI Optics package. The Limb Tracker Interface board and the PZT Driver board are in the HMI Electronics Box. • ACTIVE MIRROR • The active mirror uses PZT actuators, similar to MDI.Calculations and characterization from this design are used to derive current estimated performance. • ELECTRONICS • As in MDI the Limb Tracker Interface board contains the digital interface to the electronics to adjust the gains, offsets, open/close servo loop, etc. The PZT driver is a slightly modified MDI design to decrease power requirements and shape the drive transfer function as appropriate for the HMI disturbance spectrum. • LIMB SENSOR • System similar to MDI, TRACE, and Program F, consisting of bicell photodiodes mounted at 90 degree intervals around a full disk image. The limb sensor image is beamsplit from the main image before it goes through the filter oven to the CCD cameras.

7. Limb Sensor & Preamp • The Limb Sensor is similar to the MDI design. • It is scaled approximately 3:2 bigger than MDI because of the difference in image scale. • It contains four bicells and an occulter that masks the central part of the solar disk. • It has a preamp (copy of updated MDI design for Program F) that enables it to have a switchable high gain (for test with a lamp) and low gain (for use with the sun). The preamp also selects the use of the prime or redundant diodes. • The MDI Limb Sensor has worked on orbit for 8 years, TRACE Limb Sensor for 5 years.

8. Active Mirror Design: • Flat mirror mounted on 3 low voltage PZT’s Requirements: • Range: +/- 100 arc seconds (mirror space) to meet the +/- 14 arc second (object space) offset requirements. • First mode: 500Hz to simplify control system for 50Hz bandwidth control loop. Performance (MDI): • Range: +/- 100 arc seconds (mirror space) or +/- 24 arc seconds (object space). Translates to +/-15 arc seconds in X and +/-18 arcseconds in Y in HMI optical system (object space). • MDI has worked for 8 years on-orbit. TRACE for 5 years (slightly different design) • First mode: 1kHz

9. Design Performance • MDI Servo loop performance on SOHO is shown below. The actual reduction factor for HMI will be determined by how much gain is allowed to be applied without stimulating resonances in the instrument and its mount to the spacecraft as well as external disturbances that would do the same. In this case our DC jitter reduction was a factor of approximately 300. The zero dB cross over point was approximately 250 Hz. The zero dB cross over will be adjustable from 10Hz to a few hundred Hz.

10. Design Performance - HMI Limb Sensor • The predicted HMI Limb Sensor performance is shown below as Error Signal in volts per unit of image shift in arcseconds. The linear range is approximately +/-50 arcseconds. The maximum voltage per arcsecond is determined by diode size/location and occulter overlap.

11. Design Performance - MDI Limb Sensor • MDI Limb Sensor performance in the presence of a very large sunspot region is shown in the next two charts. In the plot on the next page you can see that two parts of this active region cause two separate levels as they rotate onto the diode over a couple day period (11/1/03 - 11/2/03). Then they slope off as the region disappears over the West limb (11/3/03). This huge region only affects the error signals by 3.6 arcseconds, and the peak rate is about .4 arcsec/hour (.007 arcsec/minute).

12. Design Performance - MDI Limb Sensor

13. Design Performance - MDI Servo Loop • MDI Servo loop frequency domain performance on SOHO is shown in the presence of 10Hz UVCS motion. The left two plots are error signal and pzt mirror signal with the loop open, the right two are with the loop closed. It is quite apparent that the system removes all the low frequency error and still has a small effect at even the 100Hz peak.

14. Design Performance - MDI Servo Loop • MDI Servo loop time domain performance on SOHO is shown in the presence of UVCS motion. The left two plots are error signal and pzt mirror signal with the loop open, the right two are with the loop closed. With the loop open the error signal is approximately .3 arcsec peak, closed its less than .05, less than the resolution of the monitor ADC.

15. Design Performance - from MDI images • MDI Servo loop performance on SOHO is shown below. This shows the actual pointing stability measured from the “jitter” in a series of HiRes images (exposure time =~.25sec). The images were coaligned using the very accurate Solar-B algorithm, then solar rotation subtracted to show residuals of .021 arcsec and .027 arcsec in each axis rms. These include errors in coalignment so the actual stability is better than this.

16. Remaining Design Steps • Continue to refine/update the schematics for the Limb Tracker Interface and PZT Driver boards of electronics. • Finish the mechanical drawings for the updates to the Active Mirror and Limb Sensor. • Update the electrical packaging for the Limb Sensor Preamp. • Do the performance analysis/filter design based on S/C inputs of disturbance and structural modes in the instrument and S/C mount.

17. Future Plans • Fabrication of a brassboard ISS. • Fabrication of the structural model OP. • Test of brassboard ISS. • Test of structural model OP to determine modes. • Receive data from S/C about flight disturbance PSD and modes to set filter parameters. • Test of flight ISS in flight OP with correct filter parameters.