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Journal of Astronomical Instrumentation

World Scientific Publishing Company

1

The Design and Performance of the Gondola Pointing System for the Sunrise II Balloon-Borne

Stratospheric Solar Observatory

A. Lecinski†, G. Card‡, M. Knölker‡ and B. Hardy§ †High Altitude Observatory, National Center for Atmospheric Research, Boulder, CO 80307, USA, [email protected]

‡High Altitude Observatory, National Center for Atmospheric Research, Boulder, CO 80307, USA §Dynamics Analysis & Control, LLC, Longmont, CO 80503, USA, [email protected]

Received (to be inserted by publisher); Revised (to be inserted by publisher); Accepted (to be inserted by publisher);

At one meter, the Sunrise Balloon-Borne Stratospheric Solar Observatory is the largest solar telescope to leave the earth. Its aim

during its June 12 to June 17, 2013 flight was to study the magneto-convective processes of the sun at a resolution of better than

100 km. To obtain this goal, the gondola and telescope are required to point to an accuracy of better than 26 arc seconds for

extended periods of time. Pointing of the gondola and telescope was effected by the Sunrise Pointing System (PS). The PS

takes pointing error signals provided by a Lockheed Intermediate Sun Sensor (LISS) and passes the data through a cascade of up

to four digital biquadratic filters to calculate best voltages to move azimuthal and elevation motors. All filter settings can be

modified in flight to adapt to changing conditions. Using this design, the Sunrise Pointing System achieved the required goal,

pointing the gondola and telescope to better than 26 arc seconds for 60% of the flight and continuous time periods up to 99

minutes. In this paper we detail the design and performance of the PS during the 2013 flight.

Keywords: Balloon; pointing; attitude control.

1. Introduction

The second science flight of the Sunrise balloon-borne stratospheric solar observatory took place in June of 2013.

The stratospheric balloon flight began from ESRANGE (near Kiruna Sweden) and floated to northern Canada's

Boothia Peninsula. Floating above most of the earth's atmosphere at 36km, the Sunrise telescope is able to obtain

data in important, far ultra violet wavelengths (as low as 214 nm) and observe the sun without the detriment of

atmospheric seeing.

The Sunrise balloon-borne stratospheric solar observatory has been fully described by Barthol et al. (2011). The

Sunrise Pointing System (PS), developed by the High Altitude Observatory (HAO), has many functions: off-

pointing, flat fielding and engineering data collection. But the most critical function is to “coarsely” point the

gondola and telescope to a target on the sun within 26 arc seconds or better accuracy. Once the gondola and

telescope are stably pointing at better than ±26 arc seconds the Kiepenheuer Institute for Solar Physics (KIS)

Image Stabilization and Light Distribution (ISLiD) system and Correlating Wave-Front Sensor (CWS) can deliver

a stabilized image of a precision of 0.04 arc second (RMS) to the science instruments: the Sunrise Filter Imager

(SuFI) and the Imaging Magnetograph eXperiment (IMaX). Descriptions of ISLiD and SuFI are given in

Gandorfer et al. (2011). CWS is described by Berkefeld et al. (2011). Information for IMaX is given by

Martínez Pillet et al. (2011).

Pointing a 1920 kg balloon-borne telescope accurately and continuously to the sun is a non-trivial matter. In the

azimuthal direction there is nothing solid to push against. However, if one uses an appropriately sized flywheel*,

the reaction torque generated by its motor-driven acceleration (or deceleration) can be used to control the rotation

* The flywheel is also referred to as a reaction wheel.

2 A. Lecinski, et al.

of the gondola. Occasionally atmospheric disturbances require corrections that could force rotational velocities of

the flywheel beyond its capabilities. Thus the design of Sunrise uses both a flywheel (fine azimuth) and an

additional coarse azimuthal motor to minutely turn the entire gondola and allow the flywheel to slow down. In

the elevation direction pointing is much more straightforward, but care must be taken neither to induce nor

reinforce any pendulum motions of the gondola and telescope. The Sunrise elevation design uses an inclined

linear stage and a single motor to raise and lower the telescope. Fine detail pointing errors are measured by a

Lockheed Intermediate Sun Sensor (LISS). When PS first finds the sun, it employs less accurate sun sensors with

wide angle acquisition ranges: the ‘Precision Azimuth Sun Sensor’ (PASS), ‘Full Range Elevation Detector’

(FRED) and corner cells (azimuthal detectors). The LISS, PASS, FRED and corner cells are described in detail

below. A schematic of the design is shown in Figure 1.

Fig. 1 Schematic of the Sunrise Gondola.

The Design and Performance of the Gondola Pointing System for the Sunrise II Balloon-Borne Stratospheric Solar Observatory 3

2. Design

2.1. Pointing System program overview

The PS computer, a Diamond systems PLT-N270XT-2G, runs the highly optimized PS program written in C++.

In addition to pointing, the PS program must also simultaneously perform other administrative tasks, and hence is

multi-threaded. Threads consist of:

Artificial Intelligence (AI) thread

o Computes running means and standard deviations of sun sensor data and motor voltages.

o Determines which sun sensors have valid data, which pointing plan to implement and which

servos to activate.

o Sets “use=1.0” or “don’t use=0.0” variables for the Pointing thread to use in its servo

calculations.

o Runs every 9 seconds.

Pointing thread

o Collects all sun sensor data.

o Collects all motor encoder data.

o Collects environmental data for PS components.

o Executes all pointing servo calculations.

o Sends output of servo calculations (voltages) to the elevation motor (El_), fine azimuth motor,

(Azf) and coarse azimuth motor (Azc).

o Saves collected data to memory.

o Runs at 150Hz.

Incoming command thread.

o Processes incoming commands from the Instrument Control Computer (ICU).

o Interrupt based.

CWS thread

o Processes incoming commands from CWS.

o Interrupt based.

CWS PS pointing lock thread

o Signals CWS when PS pointing is within ±6 arc seconds.

o Interrupt based.

Write data thread

o Gathers all collected data from memory and sends data to ICU. Data are retained on the gondola.

o Runs every 5 seconds.

House Keeping thread

o Gathers snippet of most recent data and sends to ICU. ICU transmits House Keeping to ground.

o Runs every second. May be commanded to run every 15 seconds.

Thumbnail thread

o If commanded, sends thumbnails of data to ICU. ICU transmits thumbnails to ground.

Thumbnails consist of trimmed down sun sensor data and motor velocities and voltages. They

contain sufficient information to determine how well the filters in the servos are behaving.

Thumbnail data were pivotal for fine tuning pointing servos and improving pointing accuracy.

The workhorses of the PS are the AI thread and the Pointing thread. They are described in detail below.


2.2. Sun sensors

The ensemble sun sensors used by the PS to determine orientation are:

low resolution coarse pointing, azimuth only, used during initial sun acquisition pointing:

o azimuth: four corner cells

The corner cells are mounted on the four corners of the gondola. They are wide range

photovoltaic cells with acceptance cones of 60° in elevation and 105° in azimuth. The PS corner

cells were calibrated such that the positon of the sun is known within a degree or so of accuracy.

intermediate pointing:

o azimuth: ‘Precision Azimuth Sun Sensor’ (PASS)

The PASS is mounted to the gondola frame, facing forward.

It is a shadow sensor type detector generating an azimuth (yaw) difference signal and Sun

Present intensity signal. It has a linear range of ±3° in azimuth and can capture the sun at any

elevation angle. It has an accuracy of a few to ~10 arc seconds.

o elevation: ‘Full Range Elevation Detector’ (FRED)

The FRED is mounted to the telescope frame. It is a shadow sensor detector with a linear range

of ±15° in elevation. For angles from ~15° to 60° (+/-) it gives a saturated signal which indicates

if the Sun is above or below the current telescope elevation. Within the linear range its accuracy

is a few arcminutes. The FRED has an azimuthal capture range of ±5°.

precise pointing:

o azimuth and elevation: Lockheed Intermediate Sun Sensor (LISS).

The LISS is mounted to the telescope frame. It consists of 5 photodiodes beneath a square

aperture window. They are arranged such that the central diode provides a Sun Present signal,

the right/left diodes providing an azimuthal (yaw) difference signal, and the top bottom diodes an

elevation (pitch) difference signal. When the Sun Present signal is fully saturated, zero readings

from LISS yaw and LISS pitch indicate the sun is centered. The capture range of the LISS is ±3°

in azimuth and elevation. Its linear range is ±15 arcminutes. Within the linear range the

accuracy of the LISS is 1 to 2 arc seconds.

The LISS is mounted on a two axis motorized tip-/tilt stage with precise sub-arc second

resolution encoders. By moving the LISS stage, the telescope can be pointed in any direction up

to a distance of 4 degrees from sun center. This allows the telescope to observe features located

anywhere on the sun and even beyond the solar limb. Co-alignment between the LISS and the

telescope is accomplished by moving the LISS stage as CWS looks for the solar limb. Once the

location of the north, south, east and west limbs have been found, orientation and spatial scale is

computed, stored in memory and saved to the PS disk.

The data from these sun sensors is acquired at 150Hz as part of the Pointing thread, described in more detail

below.

2.3. AI thread

To acquire and point to the sun, the PS AI thread employs three pointing stages: low, intermediate and precise.

The thread initially determines the sun’s location in azimuth. Low resolution pointing mode then slowly rotates

the gondola towards the sun. Once the thread has determined that the gondola is stably pointing to the sun within

a few degrees in azimuth, intermediate azimuthal pointing mode begins. Simultaneously, light should now be

available on the elevation sensors, so the thread may now activate the intermediate elevation pointing mode.

When the AI thread determines that azimuthal or elevation pointing is stable to within a few arcminutes, it


activates the precise (arc second) pointing mode for that axis. The AI thread treats each axis independently. That

is, the precise elevation pointing mode may be activated while the intermediate azimuthal pointing mode may still

be engaged. Additionally, steps can be skipped so that the precise pointing mode can start immediately as long

as certain criteria are met.

The AI thread autonomously determines which pointing plan to activate. As mentioned above, the AI thread

computes running means and running standard deviations of all the sun sensor data. If the means and standard

deviations simultaneously meet certain criteria, a pointing plan is activated. A pointing plan sets “use=1.0” or

“don’t use=0.0” for sun sensors and servos. For example, for the intermediate azimuthal pointing mode, the “use”

settings are:

UseCC = 0.0 (corner cells)

UsePassAz = 1.0 (PASS Difference)

UseLissAz = 0.0 (LISS yaw)

UseAzcTrack = 0.0 (low resolution azimuthal pointing servo, see below)

UseAzfToAzc = 1.0 (medium and precise azimuthal pointing servo, see below)

UseAzfTrack = 1.0 (medium and precise azimuthal pointing servo, see below)

These “use” values are a key strategy to avoid any variation in the execution time of the highly optimized

Pointing thread, described below.

The AI thread also compensates for misaligned sensors. For example, it was not possible for the PASS to be

perfectly aligned with LISS yaw. Thus when the gondola was in intermediate azimuthal pointing mode, the AI

thread had to nudge the gondola, check if the statistics for LISS yaw improved, and if so, it would keep nudging

the gondola until LISS yaw statistics met the established criteria. If the statistics did not improve, the AI thread

would nudge the gondola in the reverse direction. The slight nudging is accomplished by adding small offsets to

the PASS Difference signal. Once a good offset was found, it was applied to the PASS Difference signal,

retained in memory and saved to a file on the PS disk. The PS also applied this strategy between the alignments

of the corner cells to PASS, corner cells to LISS yaw, and FRED to LISS pitch. Thus if any misalignment

between sun sensors existed or changed at launch or occurred during the flight, it was automatically handled by

the AI thread.

The offsets could be set via commands from ground. And similarly, threshold settings for statistics and pointing

offsets could be modified in flight via commands sent from ground. Auto-pointing could be commanded to be

enabled, or disabled and commanded to go into manual mode.

2.4. Pointing thread

The voltages seen by the above sun sensors are converted to digital units with a Diamond Systems Analog to

Digital (A/D) Converter / Counter Timer board, model DSC_DMM32DXAT. The total intensity and intensity

differences from these sun sensors are sampled and processed at a constant rate, fast enough to achieve the desired

pointing consistency and accuracy. The lowest rate to achieve the desired pointing accuracy was calculated to be

~100Hz. Faster rates yield improved performance. Code optimization allowed the PS Pointing thread to run

quickly, so the Diamond Counter/Timer board was set to run at 150Hz.

Each interrupt of the 150Hz rate calls the highly efficient C++ Pointing thread which captures the current sample

of all of the data provided by the A/D board:


Corner cell (cc) intensities: ccRearRight, ccRearLeft, ccFrontRight, ccFrontLeft

FRED difference signal

PASS sum (PASS sun present)

PASS difference signal

LISS yaw difference signal

LISS pitch difference signal

LISS sun present

Amplifier temperatures for: Azc, Azf and El_

Motor temperatures for: Azc, Azf and El_

Upper gondola accelerometer data in X, Y and Z directions

Lower gondola accelerometer data in X, Y and Z directions

Telescope accelerometer data in X, Y and Z directions

PS computer environmental data: temperature, pressure, humidity

and also queries the Azc, Azf and El_ encoders for current positions.

The Pointing thread distributes the sun sensor data and flywheel velocity through the appropriate servo suites. A

servo suite consists of up to four sequential user selectable digital biquadratic filters (low pass, high pass, lead,

lag, integrator, notch, peak or one-to-one). The output from a servo suite calculation is a voltage that is sent to

the appropriate gondola/telescope pointing motor:

coarse azimuth motor (Azc)

fine azimuth (flywheel) motor (Azf)

elevation motor (El_)

The Pointing thread is highly optimized and coded to insure that it consumes identical clock times and identical

CPU cycles every time it is trigged for execution. No ‘if’ or ‘case’ statements are used. Instead, all calculations

for all light sensors and all servos are computed in each sample. The outputs of the servos are multiplied by the

“use=1.0” or “don’t use=0.0” values provided by the AI thread, and then they are added all together and sent to

the appropriate motors. Since the Pointing thread just does sampling and numerical calculations, and never has

to choose what light sensor to use or what servos to use, the thread always runs in the same amount of time. The

CPU cycles used by the Pointing thread easily fit within 150Hz.

2.5. Pointing servos

The servo suites are:

AzcTrack coarse azimuth pointing using Corner Cell intensities for input.

This servo is only used during initial sun acquisition pointing.

The output from AzcTrack suite is a voltage to the Azc motor which rotates the entire

gondola.

AzfTrack intermediate azimuth pointing when using the PASS difference signal as input,

or precise azimuth pointing when using the LISS yaw difference signal as input. The output from the AzfTrack suite is a voltage to the Azf motor which accelerates or

decelerates the flywheel. The acceleration/deceleration of the flywheel rotates the

gondola by small, precise amounts. A desired Azf velocity of 10 RPM ensures that the

Azf motor is nearly always in motion.

AzfToAzc spins off excessive Azf (flywheel) velocity to Azc.

The input is the difference between the desired Azf velocity and the actual Azf velocity.


The output from the AzfToAzc suite is a voltage to the Azc motor which will very

slightly rotate the entire gondola. This very slight rotation allows the flywheel to slow.

El_Track intermediate elevation pointing when using the FRED difference signal as input,

or precise elevation pointing when using the LISS pitch difference signal as input.

The results from the El_Track suite are passed to the El_Local servo suite.

El_Local acts to reduce the effects of bearing friction and changes the output of El_Track from a

motor current (the effects of which will change with temperature) to a calibrated velocity.

The results from the El_Local suite are passed to the El_Frict function.

El_Frict Friction is reduced further using this Friction Compensation function.

This is not a digital biquadratic filter but rather a function which compensates for stiction

in the motor/drive as well as gravitational effects. It is detailed below.

The output from this function is a voltage applied to the El_ motor which then drives the

telescope lever arm along the inclined elevation stage. Movement of the lever arm raises

or lowers the telescope.

With the exception of the El_Frict function, the above servo suites are

all cascaded digital biquadratic filters. Settings for all of the servos and

functions can be modified in-flight via commands sent through the ICU.

Modification of the settings allows the PS to adapt to nuances of

evolving flight conditions. Thumbnails containing 6 seconds of 150Hz

sun sensor data, motor voltages and velocities provided detailed servo

performance information. Analysis on the ground by servo engineers

determined where improvements were needed in the servo settings.

Improved settings could quickly be sent up to the PS and activated. The

settings in Table 1 produced a continuous 99 minute period during

which pointing was within ±26 arc seconds of the target.

Selection of the optimal suite gains and filter frequencies ensure that a

servo remains stable and resonances are not excited in the mechanical

structure of the gondola. Vibrations of the telescope are undesirable as

they negatively affect the CWS performance. High amplitude vibrations

prevent the CWS from achieving a closed loop control lock of the image

stabilization system. And low amplitude vibrations can create residual

image smear.

The deleterious effect of too large a suite gain is shown in data from the

commissioning phase of the mission. During the commissioning phase,

servo settings similar to those of the 2009 flight were initially tried and

subsequently adjusted. A trial setting applied on 2013 June 12 at

10:08:24 used fairly high servo suite gain for AzfTrack (-400.). As seen

in Figure 2, this resulted in a very high noise level, particularly at 10Hz.

A downloaded thumbnail revealed the problem to the PS engineers, who

determined a smaller AzfTrack servo suite gain was required. At

10:08:38 an AzfTrack servo suite gain of -150.0 was uploaded to the PS,

and as seen in Figure 3, the smaller gain resulted in greatly improved

pointing and much less noise at 10Hz.

Table 1. Servo settings for the 2013 Sunrise flight.

Settings shown produced a 99 minute period where

pointing was continuously within ±26 arc seconds.

AzfTrack settings: suite gain -150.00

Order Type Settings

2nd Low pass Frequency=5. Quality=1.

1st Lead Zero=0.1 Pole=20.

1st Integrator Frequency=0.11

AzfToAzc settings: suite gain -900.00

Order Type Settings

1st Lead Zero=0.1 Pole=1.0


El_Track settings: suite gain -0.100

Order Type Settings

2nd Low pass Frequency=5. Quality=1.


2nd Notch Frequency=0.43 Quality=4.7

2nd Notch Frequency=5. Quality=5.

El_Local settings: suite gain 2.0

Order Type Settings

1st Integrator Frequency=100.

1st Lead Zero=5.0 Pole=20.

El_Frict settings: El_FrictSlope 10.0

El_FrictThresPlus 1.2

El_FrictThreshMinus -1.2


Fig. 2 Plot showing deleterious effects of too large servo suite gain for AzfTrack: a 10Hz gondola resonance has been excited.

Fig. 3 Reduction of the AzfTrack servo suite gain by a factor of ~2.7 greatly improves pointing and decreases noise, notably the

10Hz gondola resonance.


Unlike the above servos, the El_Frict friction compensation function is not a biquadratic filter, but rather a simple

function. For most motors, a non-linear friction region exists where it is more difficult to initiate the rotation of

the axle. In this area, more force, hence higher voltage must be used. If one does not compensate for this

behavior, any small voltage sent within the friction region will not move the motor the desired amount, and the

next servo loop will see the increased pointing error and send ever larger voltages to the motor. This typically

causes oscillating overshoot/undershoot (‘limit cycle’) and very poor pointing performance.

To compensate, a friction compensation function can be implemented. Friction compensation functions assume

many shapes depending on what details are included in the friction model. For Sunrise, a simple Coulomb

friction model was used and was designed to match the specifics of the Sunrise elevation motor and elevation

stage geometry. The Sunrise friction compensation function simply boosts the voltage within a small region

around the origin with a multiplicative factor, El_FrictSlope. Outside of the region offsets are added. The

equations follow:

𝑖𝑓 ( 𝑉𝑖𝑛 ≥ 𝑇ℎ𝑟𝑒𝑠ℎ𝑝𝑙𝑢𝑠

𝐸𝑙_𝐹𝑟𝑖𝑐𝑡𝑆𝑙𝑜𝑝𝑒) 𝑉𝑜𝑢𝑡 = 𝑉𝑖𝑛 + 𝑇ℎ𝑟𝑒𝑠ℎ𝑝𝑙𝑢𝑠 −

𝑇ℎ𝑟𝑒𝑠ℎ𝑝𝑙𝑢𝑠

𝐸𝑙_𝐹𝑟𝑖𝑐𝑡𝑆𝑙𝑜𝑝𝑒

𝑒𝑙𝑠𝑒 𝑖𝑓 ( 𝑉𝑖𝑛 ≤𝑇ℎ𝑟𝑒𝑠ℎ𝑚𝑖𝑛𝑢𝑠

𝐸𝑙_𝐹𝑟𝑖𝑐𝑡𝑆𝑙𝑜𝑝𝑒) 𝑉𝑜𝑢𝑡 = 𝑉𝑖𝑛 + 𝑇ℎ𝑟𝑒𝑠ℎ𝑚𝑖𝑛𝑢𝑠 −

𝑇ℎ𝑟𝑒𝑠ℎ𝑚𝑖𝑛𝑢𝑠

𝐸𝑙_𝐹𝑟𝑖𝑐𝑡𝑆𝑙𝑜𝑝𝑒 (1)

𝑒𝑙𝑠𝑒 𝑉𝑜𝑢𝑡 = 𝐸𝑙_𝐹𝑟𝑖𝑐𝑡𝑆𝑙𝑜𝑝𝑒 × 𝑉𝑖𝑛 .

Figure 4 shows the output voltage as a function of the input voltage for the cases in Eq. (1).

To accommodate evolving conditions,

telescope loading, temperature changes or

other issues affecting the friction

characteristics of the motor, El_FrictSlope,

Threshminus and Threshplus could be

individually modified. During the flight only

symmetric adjustments of Threshminus and

Threshplus were needed to maintain good

motor performance and good pointing.

These adjustments were likely necessary due

to the non-linearity of the elevation stage

design as well as diurnal temperature

variations.

As Figures 5 and 6 indicate, the use of the

El_Frict friction compensation function was

critical to maintaining good pointing. In

Figure 5, where no friction compensation

was in use, pointing was erratic, elevation

motor voltage was quite high, and elevation

motor velocities varied widely and rapidly.

In Figure 6 where friction compensation was

implemented, pointing was greatly improved,

motor voltages were smaller and velocities

excursions were greatly diminished.

Fig. 4 El_Frict function overcomes stiction within the elevation motor. Within the

static friction region the voltage is boosted to compensate for the additional force

needed to overcome static friction. El_Frict assumes a simple Coulomb friction model,

following Eq (1). In the above example, El_FrictSlope=10, ThreshMinus=-1.0 and

ThreshPlus=+1.0 .


Fig. 5. Friction compensation not in use. Note large pointing errors, voltages and velocity excursions.

Fig 6. Friction compensation in use. Pointing is greatly improved. Voltages and volocities no longer over shoot.


When the friction compensation function is not used, the extra push caused by the servo overshoot can induce

pendulum motions. As seen in Figure 5, the large excursions with a ~8 second periodicity appear to be due to an

overshoot/undershoot induced pendulum motion of the telescope. The smaller excursions with a ~2 second

periodicity exist in both Figures. These are likely caused by the short period oscillation of the gondola on its

suspension system (flight train).

3. Results

During the 2013 flight of ~120 hours, the

gondola/telescope was pointing within ±26

arc seconds for more than 50% of the flight.

If one adds all time periods during which

the gondola/telescope was pointing within

±26 arc seconds for at least 1 minute

continuously, the total sum is over 72 hours,

or 60% of the flight.

Long periods of continuous good pointing

are needed for scientific goals, e.g. long

exposures, calibrations and movies. Thus

continuous, long duration pointing periods

of accuracy better than ±26 arc second are

highly desirable. Table 2 shows the total

sum of time over the entire flight that the gondola/telescope was pointed continuously within ±26 arc seconds of

the target for time periods of 30 seconds, 1 minute, 2 minutes, 5 minutes or 10 minutes.

Shown in the fourth column of Table 2 are the statistics from the 2009 flight of Sunrise (Sunrise I). Comparison

shows Sunrise II pointing statistics are a factor of two better than Sunrise I. During Sunrise I, thumbnails were

not available, so servo engineers on the ground had very limited information to fine tune the servos. Having the

benefit of thumbnails and the ability to upload informed servo settings were pivotal in the improved performance

of the 2013 flight.

Shown in Figure 7 is the histogram of the durations of all good pointing periods in the 2013 flight. There are

several time periods greater than an hour where the gondola/telescope was continually pointing within ±26 arc

seconds of the target. Notably, a 99 minute period of continuous good pointing was achieved.

Scientific objectives required re-pointing of the gondola/telescope for geometric scaling, flat fielding and

targeting of interesting solar features. To save time, re-pointing was done quickly and these sudden offsets

disrupted the continuity of the good pointing. Without these perturbations, the PS could have attained longer

periods of continuous good pointing. To test this hypothesis, the statistics for long periods of good pointing were

rerun by setting all LISS errors to zero for 30 seconds after any re-pointing command was received. The re-

computed histogram is shown in Figure 8.

Table 2. Total duration of flight with the gondola/telescope pointing within ±26 arc

seconds of target. The sum only includes continuous times of a minimum duration.

Minimum

continuous time

period with

gondola/telescope

pointing within

±26 arc seconds.

Sum of time in

2013 (Sunrise II)

flight with

gondola/telescope

pointing within

±26 arc seconds.

(hours)

Percent of entire

2013 (Sunrise II)

flight with

gondola/telescope

pointing within

±26 arc seconds.

Percent of entire

2009 (Sunrise I)

flight with

gondola/telescope

pointing within

±26 arc seconds.

30 seconds 80.9 67% 48%

1 minute 72.5 60% 40%

2 minutes 62.5 52% 30%

5 minutes 47.0 39% 18%

10 minutes 33.3 28% 12%


As can be seen from Figure 8, adjusting for commanded re-pointing has combined several of the shorter periods

together. Remarkably, there are now two periods of good pointing that exceed 135 minutes. In comparison with

the 2009 flight, as reported by Barthol et al. (2011), the longest period with telescope pointing within ±46 arcsec

of the target was 45 minutes.

Termination of many good pointing

periods resulted from elevation

pointing failure, an example of which

is shown in Figure 9. Since these

failures were sporadic events,

thumbnails were not available for

engineers to diagnose the problem.

Post flight analysis revealed the failure

was due to the very high 100Hz

frequency setting for the integrator

filter in the El_Local servo suite.

Reducing the frequency by a few

percent should eliminate the failure

and still maintain pointing

performance.

Fig. 7 Histogram of the length of periods of continuous pointing

within ±26 arc seconds of the target during the 2013 flight of Sunrise.

Fig. 8 Recomputed histogram of the length of periods of continuous

pointing within ±26 arc seconds of the target during the 2013 flight of

Sunrise after compensating for commanded re-pointing.

Fig. 9 Termination of a good pointing period due to elevation pointing failure.


Occasionally, azimuthal pointing failure occurred as well. The AzfTrack servo did not include a friction

compensation function. Thus, under circumstances where the velocity of the flywheel approached zero,

overshoot/undershoot of the Azf motor voltage occurred. An example is given in Figure 10. Around 14:35:32, a

typical atmospheric disturbance has perturbed the pointing, causing the flywheel to slow. Around 14:35:46 the

velocity required by the servos was positive, but very close to zero. However, friction caused the flywheel to stall

and the pointing to further degrade. Larger LISS yaw error signals caused the servos to demand more voltage,

beginning the overshoot/undershoot pattern. Although not obvious from the Figure, the Azc motor also exhibits

stiction. The large Azc velocity excursions at 14:35:52 and 14:36:08 reveal the friction effects on the motor’s

performance.

These errors did not occur frequently since the Azf motor was nearly always moving. But the easy addition of

simple friction compensation functions to the output of the AzfTrack and AzfToAzc servos could completely

eliminate this failure mode.

Fig. 10 Pointing failure due to stiction effects of the Azf flywheel. Stiction effects in the Azc motor are also evident.


4. Conclusions

The Sunrise II Pointing System was able to maintain stable pointing within the required ±26 arc seconds of the

target for a substantial portion of the 2013 flight. With this success, ISLiD and CWS were able to perform well,

and the science instruments, IMaX, and SUFI were able to obtain high quality data. Using Sunrise II

observations, Riethmüller et al. (2013) and Danilovic et al. (2014) have presented the first high resolution images

of quiet and active regions of the Sun in the Mg II k 2796 Å line. An upcoming special issue of The

Astrophysical Journal (Supplement Series) will be devoted to Sunrise II and describe the many exciting results

stemming from its observations.

Were a third flight of Sunrise to occur, easily made improvements to the PS, e.g. El_Local filter adjustments and

friction compensation functions for the fine (AzfTrack) and coarse azimuth (AzfToAzc) servos, would provide

even better pointing and longer duration times of continuous good pointing.

Acknowledgments

This work was performed under NASA grant number NNX13AE95G.

The authors thank the talented staff of NCAR’s Earth Observing Laboratory, Design and Fabrication Services.

Their contributions to the pre-flight and post-flight efforts are greatly appreciated. Additionally the authors wish

to thank Peter G. Nelson and especially Clemens Halbgewachs who worked tirelessly in determining optimal

servo settings. The authors are very grateful to Piyush Agrawal, Justus Brosche, Rebecca Centeno, Courtney

Peck, and Jack Fox. Their wonderful help was invaluable in during PS set up and flight operations. The Sunrise

II flight would not have been a success without the expertise of the dedicated staff of CSBF, ESRANGE, MPS,

KIS, and IMaX. We thank them.

The National Center for Atmospheric Research is sponsored by the National Science Foundation.

References

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