Chapter 5
Pointing Control and Aspect Determination System
5.1 Introduction
The system of sensors and control hardware that is used to point the
observatory, maintain the stability, and provide data for determining
where the observatory has been pointing is called the Pointing Control
and Aspect Determination (PCAD) system. As Chandra detectors are
essentially single-photon counters, an accurate post-facto history of the
spacecraft pointing direction is sufficient to reconstruct an X-ray
image.
In this chapter we briefly discuss the hardware that comprises the
PCAD system, how it is used, and its flight performance. Further
information can be found on the Aspect Information web page within the
main CXC Science web site
(http://cxc.harvard.edu/cal/ASPECT/).
5.2 Physical configuration
The main components of the PCAD system are:
- Aspect camera assembly (ACA) - 11.2 cm optical telescope, stray light
shade, two CCD detectors (primary and redundant), and two sets of
electronics
- Inertial reference units (IRU) - Two IRUs, each containing two
2-axis gyroscopes.
- Fiducial light assemblies (FLA) - LEDs mounted near each science
instrument (SI) detector which are imaged in the ACA via the FTS
- Fiducial transfer system (FTS) - The FTS directs light from the fid
lights to the ACA, via the retroreflector collimator (RRC) mounted at
the HRMA center, and a periscope
- Coarse sun sensor (CSS) - Sun position sensor, all-sky coverage
- Fine sun sensor (FSS) - Sun position sensor, 50 degree FOV and 0.02 degree accuracy
- Earth sensor assembly (ESA) - Conical scanning sensor, used
during the
orbital insertion phase of the mission
- Reaction wheel assembly (RWA) - 6 momentum wheels which change
spacecraft attitude
- Momentum unloading propulsion system (MUPS) - Liquid fuel thrusters
which allow RWA momentum unloading
- Reaction control system (RCS) - Thrusters which change spacecraft
attitude
Since data from the CSS, FSS, and ESA are not normally used in the
processing of science observations these are not discussed. However,
in the unlikely event of a complete failure of the ACA, we would
attempt to use CSS and FSS data.
The aspect camera assembly (Figure 5.1) includes a
sunshade ( ∼ 2.5 m long, ∼ 55 cm in diameter), a 11.2 cm, F/9
Ritchey-Chretien optical telescope, and light-sensitive CCD detector(s). This assembly and its related components are mounted on
the side of the HRMA. The camera's field of view is 1.4×1.4
deg and the sun-shade is designed to protect the instrument from the
light from the Sun, Earth, and Moon, with protection angles of 47, 20
and 6 deg, respectively.
Figure 5.1: Aspect camera assembly
The aspect camera focal plane detector is a 1024×1024 Tektronix
CCD chip operating between -16°C and -19.7°C, with 24×24 micron (5′′×5′′) pixels,
covering the spectral band between 4000 and 9000 Å. The optics of
the camera are defocused (point source FWHM = 9 arcsec) to spread
the star images over several CCD pixels in order to increase accuracy of
the centering algorithm, and to reduce variation in the point response
function over the field of view. There is a spare identical CCD chip, which can be illuminated by inserting a rotatable mirror.
The ACA electronics track a small pixel region (either 4×4,6×6, or 8×8 pixels) around each fiducial light and star
image. There are a total of eight such image slots available for
tracking. Typically five guide stars and three fiducial lights
(section 5.2.2) are tracked. The average background
level is subtracted on-board, and image centroids are calculated by a
weighted-mean algorithm. The image centroids and magnitudes are used
on-board by the PCAD, and are also telemetered to the ground along
with the raw pixel data.
The spectral response of the CCD detector
(Figure 5.2) is such that faint cool stars (e.g. type
N0), with visual magnitudes much fainter than selected guide stars
(i.e., 10.5 mag) can produce large numbers of counts. These
so-called "spoiler stars" are effectively avoided in the mission
planning stage.
Figure 5.2: Spectral response of the
ACA CCD. The same signal-to-noise is achieved for a V=11.7 magnitude
N0 star as for a V=10 magnitude G0V star. Also shown are the spectra
and the standard visual response for the two stars.
5.2.2 Fiducial lights and Fiducial Transfer System
Surrounding each of the SI detectors is a set of light emitting
diodes, or "fiducial lights", which serve to register the SI focal
plane laterally with respect to the ACA boresight. Each fiducial light
produces a collimated beam at 635 nm which is imaged onto the ACA
CCD via the RRC, the periscope, and the fiducial transfer mirror
(Figure 5.3).
Figure 5.3: Fiducial Transfer System
Two Inertial Reference Units (IRU) are located in the front of the
observatory on the side of the HRMA. Each IRU contains two gyros,
each of which measures an angular rate about 2 gyro axes. This gives
a total of eight gyro channels. Data from four of the eight channels
can be read out at one time. The gyros are arranged within the IRUs
and the IRUs are oriented such that all 8 axes are in different
directions and no three axes lie in the same plane. The gyros' output
pulses represent incremental rotation angles. In high-rate mode, each
pulse nominally represents 0.75′′, while in low-rate mode (used
during all normal spacecraft operations) each pulse represents
nominally 0.02′′.
5.2.4 Momentum control - RWA and MUPS
Control of the spacecraft momentum is required both for maneuvers and
to maintain stable attitude during science observations. Momentum
control is primarily accomplished using 6 Teldix RDR-68 reaction wheel
units mounted in a pyramidal configuration. During observing, with the
spacecraft attitude constant apart from dither, external torques on
the spacecraft (e.g. gravity gradient, magnetic) will cause a buildup
of momentum in the RWA. Momentum is then unloaded by firing the MUPS
and simultaneously spinning down the reaction wheels.
5.3 Operating principles
The Chandra aspect system serves two primary purposes: on-board
spacecraft pointing control and aspect determination and post-facto
ground aspect determination, used in X-ray image reconstruction and
celestial location.
The PCAD system has 9 operational modes (6 normal and 3 safe modes)
which use different combinations of sensor inputs and control
mechanisms to control the spacecraft and ensure its safety. These
modes are described in Section 5.7.1. In the normal
science pointing mode, the PCAD system uses sensor data from the ACA
and IRUs, and control torques from the RWAs, to keep the target attitude
within ∼ 30" of the telescope boresight. This is done using a
Kalman filter which optimally combines ACA star centroids (typically
5) and angular displacement data from two 2-axis gyroscopes. On short
time scales ( ∼ seconds) the spacecraft motion solution is
dominated by the gyroscope data, while on longer timescales it is the
star centroids that determine the solution.
Post-facto aspect determination is done on the ground and uses more
sophisticated processing and better calibration data to produce a more
accurate aspect solution. The suite of CXC tools to perform this
processing is called the aspect pipeline. The key improvements over
PCAD aspect come from better image centroiding and using Kalman
smoothing (which uses all available data over the observation period
- as opposed to historical data). In addition, the aspect pipeline
folds in the position of the focal-plane instrument as determined by
the fiducial light data.
5.4 Performance
5.4.1 Post-Facto and On-Orbit Aspect Determination
The important PCAD system performance parameters and a comparison to
the original requirements are shown in Table 5.1. In
each case the actual performance far exceeds the requirements.
Celestial location accuracy measures the absolute accuracy of Chandra
X-ray source locations. Based on observations of 587 point sources
detected within 2′ of the boresight and having accurately known
coordinates, the 90% source location error circle has a radius of
less than 0.6′′ overall, and less than 0.7′′ for each SI
(Figure 5.4). Approximately 1.6% of sources are outside a 1′′ radius. The difference in
astrometric accuracy for different SIs is a function of two factors:
number of available data points for boresight calibration and
accuracy of the fiducial light SIM-Z placement1. The plotted level
of accuracy applies for observations that have been processed or reprocessed
after Dec. 31, 2003. This condition applies to all Chandra observations
currently in the archive. For pre-2004 observations it is recommended that
users download the latest version of the observation from the archive.
Figure 5.4: Cumulative histogram of celestial accuracy for
Chandra X-ray source locations for each SI. Radial offset is
the distance in arcsec between the optical coordinate, typically from
the Tycho-2 catalog, and the Chandra position.
The image reconstruction performance measures the effective blurring
of the X-ray PSF due to aspect reconstruction. A direct measure of
this parameter can be made by determining the time-dependent jitter in
the centroid coordinates of a fixed celestial source. Any error in
the aspect solution will be manifested as an apparent wobble in the
source location. Unfortunately this method has limitations. ACIS data
are count-rate limited and we find only an upper limit: aspect
reconstruction effectively convolves the HRMA PSF with a Gaussian
having FWHM of less than 0.25′′. HRC observations can produce
acceptably high count rates, but the HRC photon positions (at the chip
level) have systematic errors due to uncertainties in the HRC de-gap
calibration2.
These errors exactly mimic the expected dither-dependent
signature of aspect reconstruction errors, so no such analysis with
HRC data has been done. An indirect method of estimating aspect
reconstruction blurring is to use the aspect solution to de-dither the
ACA star images and measure the residual jitter. We have done this
for 350 observations and find that 99% of the time the effective
blurring is less than 0.20′′ (FWHM).
Absolute celestial pointing refers to the accuracy with which an X-ray
source can be positioned at a specified location on the detector, and
is about 6′′ in radius. This is based on the spread of apparent
fiducial light locations for observations through 2012. Because of
the excellent celestial location accuracy, the
fiducial light locations are a very accurate and convenient predictor
of where an on-axis X-ray source would fall on the detector. It should
be noted that the 6′′ value represents the repeatability of
absolute pointing on timescales of less than approximately one year.
During the first 4 years of the mission, there was an exponentially
decaying drift in the nominal aimpoint of about 10′′, probably
due to a long-term relaxation in the spacecraft structural alignment.
Since that time
the drift rate has been less than about 1′′/year in the spacecraft Y and Z
directions
apart from large jumps associated with the cooling the ACA CCD in
Jul-2003 and Dec-2006 and the safemode in Jul-2011.
The continued drift at this time is believed to be directly related to the
increasing temperatures of the ACA housing and the ACA telescope mount.
These drifts have been a driving factor in updates to the ACIS
default aimpoint offsets.
The PCAD 10-second pointing stability performance is measured by
calculating the RMS attitude control error (1-axis) within successive
10 second intervals. The attitude control error is simply the
difference between the ideal (commanded) dither pattern and the actual
measured attitude. Flight data show that 95% of the RMS error
measurements are less than 0.06′′ (pitch) and 0.07′′ (yaw). Systematic offsets are not included in this term.
Table 5.1: Aspect System Requirements and Performance
| Description | Requirement | Actual |
| Celestial location | 1.0′′ (RMS radius) | 0.5′′ |
| Image reconstruction | 0.5′′ (RMS diameter) | 0.3′′ |
| Absolute celestial pointing | 30.0′′ (99.0%, radial) | 6.0′′ |
| PCAD 10 sec pointing
stability | 0.12′′ (95% RMS) | 0.06 ′′ (pitch) |
| 0.07′′ (yaw) |
In addition to the four key performance requirements listed in
Table 5.1 we also measure the relative astrometric accuracy
which is achieved with Chandra data. This refers to the residual astrometric
offsets assuming that the X-ray coordinates have been registered using
well-characterized counterparts of several X-ray sources in the field. The most
comprehensive dataset for measuring relative astrometry is based on the
900 ksec ACIS-I observation of the Orion Nebula. The members of COUP (Chandra
Orion Ultradeep Project) have kindly provided us with a data file for over 1300
X-ray sources listing the offset from a 2MASS counterpart and the off-axis
angle3.
The left plot of Figure 5.5 shows a scatter plot of offset
(arcsec) versus off-axis angle (arcmin). The right side of the figure shows
cumulative histograms of the fraction of sources with relative offset below the
specified value. This is broken into bins of off-axis angle as listed in the
plot. In the "on-axis" (0 - 2′) bin, 90% of sources have offsets less
than 0.22′′. After, accounting for the ∼ 0.08′′ RMS uncertainty in
2MASS coordinates, this implies the intrinsic 90% limit is 0.15′′. See
http://www.ipac.caltech.edu/2mass/releases/allsky/doc/.
Figure 5.5: Left: scatter plot of offset (arcsec) versus off-axis angle (arcmin)
for sources in the ultra-deep ACIS-I Orion observation. Right: cumulative histograms of the fraction of sources with relative offset below the
specified value.
5.4.2 On-Board Acquisition and Tracking
As described in Section 5.8, in normal operations, the
ACA is used to acquire and track stars and fiducial lights.
Occasional failures in
acquisition and difficulties in tracking are expected due to
uncertainties in star position and magnitude, the presence of spoiler stars, CCD dark
current noise (see Section 5.6.3), and other factors.
Table 5.2 summarizes success statistics for
star acquisition and tracking4.
Table 5.2: Star Acquisition and Tracking Success
| Catalog Star Magnitude | Acquisition Success
| Tracking Success |
| All stars | 97% | 99% |
| 10.3 - 10.6 mag stars | 83% | 94% |
| 10.6 - 10.9 mag stars | 68% | 85% |
5.5 Heritage
The Chandra aspect camera design is based on the Ball CT-601 star
tracker, which is currently operating on the RXTE mission. The
Chandra IRUs are nearly identical to the SKIRU V IRUs, some 70 of
which have been built by the manufacturer - Kearfott. These IRUs are
similar to those used on CGRO.
5.6 Calibration
5.6.1 Pre-launch calibration
IRU component testing at Kearfott provided calibration data necessary
for accurate maneuvers and for deriving the aspect solution. The key
parameters are the scale factor (arcsec/gyro pulse) and the drift rate
stability parameters. The stability parameters specify how quickly
the gyro readout random-walks away from the true angular
displacement. These terms limit the aspect solution accuracy during
gyro hold observations (Section 5.8.2).
ACA component testing at Ball provided calibration data necessary for
on-orbit pointing control and for post-facto ground processing.
On-orbit, the ACA uses CCD gain factors, the plate scale factor, and
temperature dependent field distortion coefficients to provide the
control system with star positions and brightnesses. In ground
processing, the CXC aspect pipeline makes use of those calibration
data as well as CCD read noise, flat-field maps, dark current maps,
and the camera PSF in order to accurately determine star positions.
5.6.2 Orbital activation and checkout calibration
Orbital activation and checkout of the PCAD occurred during the
first 30 or so days of the Chandra mission. During the first phase
of OAC, before the HRMA sunshade door was opened, it was possible to
use the ACA to observe the fiducial lights (period 1). After the
sunshade door was opened it was possible to fully check the aspect
camera using star light (period 2).
Chandra activation produced the following aspect system calibration data:
- Bias, alignment, scale factor of the CSS and FSS (period 1)
- Coarse gyro bias (period 1)
- ACA CCD dark current map (period 1)
- Fiducial light intensity, image, and centroid at nominal voltage
(periods 1 and 2)
- IRU bias, alignment, scale factor (period 2)
- ACA alignment and field distortion coefficients (period 2)
5.6.3 On-orbit calibrations
During the Chandra science mission, aspect system components require
on-orbit calibration to compensate for alignment or scale factor
drifts, and to evaluate ACA CCD degradation due to cosmic radiation. The IRU-1
calibration coefficients were updated once (Jul-2002) based on
analysis of PCAD data for 3105 maneuvers during the course of the
mission. Following the swap to IRU-2 in Jul-2003, new coefficients
have been updated as needed (6 times between 2003 and 2012).
The following ACA calibrations are performed, as-needed, based on the
trending analyses of aspect solution data.
Dark current
Cosmic radiation damage will produce an increase in both the mean
CCD dark current and the non-Gaussian tail of "warm" (damaged)
pixels in the ACA CCD. This is illustrated in Figure 5.6,
which shows the distribution of dark current shortly after launch
and in 2012-Oct. The background non-uniformity caused
by warm pixels (dark current > 100 e−/sec) is the main
contributor to star centroiding error, though the effect is
substantially reduced by code within the aspect pipeline which detects
and removes most warm pixels.
Figure 5.6: Differential histogram of dark current distribution for the
ACA CCD in 1999-Aug and 2012-Oct
Currently the fraction of warm CCD pixels is 7% and is increasing
linearly at 0.4% per year. The impact of this continued radiation
damage has been analyzed within a study of ACA lifetime issues. The
analysis shows that we expect no significant degradation of X-ray
image quality (due to aspect) even in the worst case scenario out to
20 years after launch. One key issue is the loss of power margin in
the ACA thermoelectric cooler which maintains the CCD at -19 C. It
is expected that the CCD temperature at 20 years will be somewhere
between -15 C to -18 C, thus increasing the fraction of warm pixels
by up to a factor of 65% beyond the simple linear extrapolation.
Current data and analysis indicate that this increase in the fraction
of warm pixels will cause negligible degredation of post-facto image
reconstruction and no significant reduction in on-board control
margin.
Dark current calibrations are performed approximately three times per
year. Because the ACA has no shutter, a dark current calibration must
be done with Chandra pointing at a star field which is as free from
optical sources as possible. Five full-frame CCD maps are collected,
each with slight pointing offsets in order to allow removal of field
stars. The entire calibration procedure takes approximately 3 hours.
Flickering pixels
The dark current of radiation damaged pixels is observed to fluctuate
by factors of up to 25% on time scales of 1 to 50 ksec. This
behavior was studied using a series of ACA monitor windows commanded
during perigee passes in 2002.
In 2008 and 2009, similar monitor window data was acquired and
analysed and the flickering pixel behavior was seen to be qualitatively
unchanged from that seen in 2002.
An important consequence of the flickering pixel
phenomenon is that the dark current pixel values obtained during the
dark current calibration may not be directly subtracted from
observation pixel data in post-facto processing. Instead, users of
monitor window data should use a warm pixel detection algorithm such
as the one implemented in ground processing5.
Charge transfer inefficiency (CTI)
Radiation damage degrades the efficiency with which charge is
transferred in the CCD by introducing dislocations in the
semiconductor which trap electrons and prevent their transfer. The
most important consequence is a "streaking" or "trailing" of star
images along the readout column(s), which can introduce systematic
centroid shifts. These shifts depend primarily on CCD transfer
distance to the readout and star magnitude.
The procedure for calibrating the mean CTI is to dither a faint star
across the CCD quadrant boundary and observe the discontinuity in
centroid (the CCD is divided electrically into four quadrants). In 2004, a
total of 20 calibration observations were performed during perigee, each with a guide
star dithering over a quadrant boundary.
Despite significant concerns prior to launch, as well as notable CTI
degradation in the ACIS front-illuminated chips, there is no evidence
of increased CTI in the ACA CCD.
Field distortion
The precise mapping from ACA CCD pixel position to angle relative to
the ACA boresight is done with the "ACA field distortion
polynomial". This includes plate scale factors up to third order as
well as temperature-dependent terms. In order to verify that no
mechanical shift in the ACA had occurred during launch, a field
distortion calibration was performed during the orbital activation and
checkout phase. The on-orbit calibration revealed no mechanical
shift. Such a shift would have caused degraded celestial location accuracy.
The calibration was done by observing a dense field of stars with the
spacecraft in normal pointing mode. Two reference stars were observed
continuously, while sets of 4 stars each were observed for
100 seconds. The calibration was completed after observing 64 stars
over the ACA field of view, taking roughly 60 minutes. There are
currently no plans to repeat this on-orbit calibration. Instead, the
field distortion coefficients are monitored by long-term trending of
observed star positions relative to their expected positions.
Responsivity
Contamination buildup on the CCD surface was predicted in pre-launch
estimates to result in a mean throughput loss of 9% after 5 years
on-orbit, though the calculation of this number has significant
uncertainties. The buildup of contaminants is tracked by a trending
analysis of magnitudes for stars which have been observed repeatedly
throughout the mission (e.g. in the AR LAC field). To date, these
trending analyses show no indication of contamination build-up. In
the unlikely event that future contamination occurs and causes significant
operational impact, we will consider "baking-out" the CCD on-orbit. In this
procedure the current to the CCD thermo-electric cooler is reversed
so as to heat the device to approximately 30 C for a period of
several hours. After bake-out the
CCD would be returned to its nominal operating temperature of
-19 C.
5.7 Operations
5.7.1 PCAD modes
The PCAD system has 9 operational modes (6 normal and 3 safe) which
use different combinations of sensor inputs and control mechanisms to
control the spacecraft and ensure its safety. These modes are listed
in Table 5.3. Normal science observations are carried
out in Normal Pointing Mode (NPM), while slews between targets are
done in Normal Maneuver Mode (NMM).
Table 5.3: PCAD modes
| Mode | Sensors | Control | Description |
| |
| Standby | - | - | OBC commands to RWA, RCS, and SADA disabled, for initial deployment,
subsystem checkout, etc. |
| Normal Pointing | IRU, ACA | RWA | Point at science target, with optional
dither |
| Normal Maneuver | IRU | RWA | Slew between targets at peak rate of 2° per
minute |
| Normal Sun | IRU, CSS, FSS | RWA | Acquire sun and hold spacecraft -Z axis and
solar arrays to the sun |
| Safe Sun | IRU, CSS, FSS | RWA | Safe mode: acquire sun and hold spacecraft -Z
axis and solar arrays to the sun |
| Derived Rate Safe Sun | IRU, CSS, FSS | RWA | Similar to Safe Sun Mode, but
using only one gyro (two axes) plus sun sensor data |
| |
| Transfer orbit only - now disabled |
| |
| Powered Flight | IRU | RCS | Control Chandra during Liquid Apogee Engine burns |
| RCS Maneuver | IRU | RCS | Control Chandra using the RCS |
| RCS Safe Sun | IRU, CSS, FSS | RCS | Same as Safe Sun Mode, but using RCS
instead of RWA for control |
5.7.2 Operational constraints
The ACA will meet performance requirements when the ACA line of sight
is separated from: the Sun by 45 degrees or more; the limb of the
bright Earth by 10 degrees or more; and the dark Earth or Moon by 6
degrees or more. If these restrictions are violated, the star images
may be swamped by scattered background light, with the result that
added noise on the star position will exceed the the 0.360′′
requirement (1-σ,1-axis).
5.7.3 Output data
The important output data from the ACA are the scaled raw pixel
intensities in regions (4×4, 6 ×6, or 8×8
pixel) centered on each of the star and fiducial light images. These data are
placed in the engineering portion of the telemetry stream, which is
normally allocated 8 kbit s−1. During an ACA dark current
calibration (Section 5.6.3), Chandra utilizes a
512 kbit s−1 telemetry mode in real-time contact to enable
read-out of the entire CCD (1024 ×1024 pixels). The key data
words in telemetry from the IRU are the 4 accumulated gyro counts (32
bits every 0.256 sec).
5.8 Performing an Observation
5.8.1 Star acquisition
After maneuvering at a rate of up to 2°/minute to a new celestial
location using gyroscope data and the reaction wheels, Chandra begins the star acquisition sequence, a process which typically takes
from 1 to 4 minutes. First the OBC commands the ACA to search for up
to 8 acquisition stars, which are selected to be as isolated from
nearby stars as possible. The search region size is based on the expected uncertainty in attitude,
which is a function of the angular size of the slew. If two or more acquisition stars are found,
an attitude update is performed using the best (brightest) pair of
stars. This provides pointing knowledge to 3′′ (3 σ per
axis). Next the guide star search begins. Depending on the
particular star field configuration, the star selection algorithm may
choose guide stars which are the same as the acquisition stars. In
this case the guide star acquisition time is somewhat reduced. When at least two
guide stars have been acquired and pointing control errors converge,
the on-board Kalman filtering is activated and the transition to
Normal Point Mode is made, at which point sensing of the fiducial
lights begins.
5.8.2 Science pointing scenarios
The on-board PCAD system is flexible and allows several different
Chandra science pointing scenarios, described in the following
sections.
Normal Pointing Mode Dither
The large majority of observations are performed using Normal Point
Mode, with dither selected. In this case the Chandra line-of-sight
will be commanded through a Lissajous pattern. Dithering distributes
photons over many detector elements (microchannel pores or CCD pixels) and serves several purposes: reduces uncertainty due to pixel
to pixel variation in quantum efficiency (QE); allows sub-sampling of
the image; and, in the case of the HRC, distributes the total exposure
over many microchannel pores - useful since the QE of a pore degrades
slowly with exposure to photons. The dither pattern parameters are
amplitude, phase, and period for two axes. Each of the six parameters
is separately commandable and differ for the two different instruments
(See Chapters 6 and 7). The default values
for these parameters are given in Table 5.4. Dither can be disabled for ACIS
observations, while the minimum
dither rate required to maintain the health of the HRC is
0.02′′/sec. The maximum dither rate, determined by PCAD stability
requirements, is 0.22′′/sec.
Table 5.4: Default dither parameters
| Parameter | HRC |
ACIS |
| Phase (pitch) | 0.0 rad | 0.0 rad |
| Phase (yaw) | 0.0 rad | 0.0 rad |
| Amplitude (pitch) | 20.0 arcsec | 8.0 arcsec |
| Amplitude (yaw) | 20.0 arcsec | 8.0 arcsec |
| Period (pitch) | 768.6 sec | 707.1 sec |
| Period (yaw) | 1087.0 sec | 1000.0 sec |
Normal Pointing Mode Steady
This mode is identical to NPM dither, but without the dither.
Pointing at solar system objects
Observations of moving solar system objects are done using a sequence
of pointed observations, with the object moving through the field of
view during each dwell period. Except in special circumstances, each
pointing is selected so that the object remains within 5′ of
the Chandra line-of-sight. Most solar system objects move slowly
enough that a single pointed observation will suffice.
Raster scan
Survey scans of regions larger than the instrument field of view are
specified simply with a list of target coordinates giving the field
centers. The fields can optionally overlap, depending on the science
requirements of the survey.
Offset and gyro hold
In special circumstances it will be necessary to perform observations
without tracking guide stars. It may occur that a field has no
suitable acquisition and guide stars, although this situation has not been encountered
to date. A more likely situation is that a very bright object, such as
the Earth or Moon, saturates the ACA CCD and precludes tracking
stars. In this case Chandra will first be maneuvered to a nearby
pointing which has guide stars to establish fine attitude and a gyro
bias estimate. A dwell time of approximately 25 minutes is needed to
calibrate the bias estimate, which is the dominant term in the drift
equation below. Chandra will then be maneuvered to the target. The
default automatic transition to NPM will be disabled, and the
spacecraft will hold on the target attitude in NMM.
While holding on gyros only, the spacecraft attitude will drift due to
noise in the gyros, which results in an aspect solution error. The
variance of the angle drift for each gyro axis, in time t, is given
by
|
σ2 = σb2 t2 + σv2 t + σu2 t3 / 3 |
|
Ground test data for gyro noise parameters indicate worst case values
of σu = 1.5×10−5 arcsec sec−3/2 and σv = 0.026 arcsec sec−1/2. Analysis of the residual Kalman filter
bias estimate gives σb = 0.002 arcsec sec−1. This
results in 1-σ angle drift errors of: 0.3′′ for 0.1 ksec;
2.2′′ for 1 ksec; 11′′ for 5 ksec; and 22′′ for
10 ksec. After a maximum of 7.2 ksec, Chandra will be maneuvered back
to the nearby field with guide stars in order to re-establish fine
attitude and update the gyro drift rate.
5.8.3 PCAD capabilities (advanced)
Monitor star photometry
The ACA has the capability to devote one or more of the eight image
slots to "monitor" particular sky locations. This allows
simultaneous optical photometry of one or more targets in the ACA
field of view. These optical sources can be slightly fainter than the
ACA guide star limit of mACA = 10.2 mag. The bright-end limit
for monitor star photometry is mACA=6.2 mag. However, since
there are a fixed number of image slots, devoting a slot to photometry
instead of tracking a guide star results in a degradation of the image
reconstruction and celestial location accuracy
(Section 5.4). Using one monitor slot represents
a 15 - 25% increase in the aspect image reconstruction RMS diameter,
depending on the particular guide star configuration. Two monitor
slots would increase the diameter by about 50 - 60%, but this configuration is
not operationally allowed under normal circumstances. The photometric accuracy
which can be achieved depends primarily on the star magnitude,
integration time, CCD dark current, CCD read noise, sky
background, and the CCD dark current uncertainty.
Dark current uncertainty ultimately limits the photometric accuracy at
the faint end, and results from uncalibrated pixel-to-pixel changes in
dark current due to radiation damage. This includes both changing
background pixels as Chandra dithers, as well as intrinsic flickering
in the radiation-damaged CCD pixels. This flickering, which occurs
on time scales from less than 1 ksec to more than 10 ksec, poses
fundamental problems for accurate photometry since the background dark
current is a strong random function of time. With straightforward
data processing, the noise introduced by the dark current variations
(both spatial and temporal) is approximately 300 e-/sec. A star with
an ACA magnitude of 12.0 produces about 1100 e-/sec, giving a S/N of
3.7. This represents the practical faint limit for ACA monitor star
photometry. Somewhat improved S/N could be obtained with a more
sophisticated analysis which tracks the time-dependent dark current of
each pixel. Users interested in processing ACA monitor window data are advised to
contact the CXC HelpDesk for assistance.
The zero instrument magnitude is defined as the Aspect Camera response
to a zero magnitude star of spectral class G0V. The conversion from V
and B magnitude to ACA instrument magnitude, based on flight data, is
given approximately by
|
mACA = V + 0.426 − 1.06(B−V) + 0.617(B−V)2 − 0.307(B−V)3 |
|
5.9 Ground Processing
For each science observation, the aspect system data described in
Section 5.7.3 are telemetered to the ground to allow
post-facto aspect determination by the CXC aspect pipeline, as part
of the standard CXC data processing pipeline. The important
components of the pipeline are:
- Gyro process: Filter gyro data, gap-fill, and calculate raw
spacecraft angular rate
- ACA process: Filter bad pixels, make CCD-level corrections
(e.g. dark current), find spoiler stars, centroid, make camera-level
corrections, convert to angle
- Kalman filter and smooth: Optimally combine ACA and gyro
data to determine ACA celestial location and image motion
- Combine ACA and fids: Derive fid light solution and
combine with ACA solution to generate image motion and celestial
location at the focal plane science instrument.
5.9.1 Data products
The data products which are produced by the aspect solution pipeline
are listed in Table 5.5. Key data elements
include: IRU accumulated counts; raw pixel data for 8 images; observed
magnitudes, pixel positions of the aspect stars and fiducial lights
versus time; and aspect solution versus time.
The star data are used to determine the RA, Dec, and roll (and
corresponding uncertainties) of the HRMA axis as a function of time.
The fid light images are used to track any drift of the SIM away from
the nominal position. One cause of such drift is thermal warping of
the optical bench assembly. The Kalman filtering routines also
calculate an optimal estimate of the gyro bias rate as a function of
time.
Table 5.5: Aspect pipeline data products
| Product | Description |
| ASPSOL | Final aspect solution with errors |
| ASPQUAL | Aspect solution quality indicators |
| AIPROPS | Aspect Intervals |
| ACACAL | ACA calibration data from ODB and CALDB |
| GSPROPS | Guide star properties, both from the AXAF Guide and
Acquisition Star Catalog, and as actually observed with the ACA |
| FIDPROPS | Fiducial light properties, as commanded and as observed |
| ACADATA | Aspect camera telemetry (including ACA housekeeping), and
images after CCD -level correction |
| ACACENT | Image centroids and associated fit statistics |
| GYROCAL | Gyro calibration data from ODB and CALDB |
| GYRODATA | Gyro raw and gap-filled, filtered data |
| KALMAN | Intermediate and final data in Kalman filter and
smoother |
5.9.2 Star catalog
The Aspect system uses the AGASC (AXAF Guide and Aspect Star Catalog) version
1.6. Further information about the AGASC, as well as access to catalog data,
can be found on the CXC AGASC web page http://cxc.harvard.edu/agasc).
The AGASC was prepared by the CXC Mission Planning and Operations &
Science Support groups, and is a compilation of the Hubble Guide Star Catalog,
the Positions and Proper Motion Catalog and the Tycho Output Catalog.
Last modified:12/13/12
