Chapter 3: CMJ and CMI Table Definitions

CMJ and CMI Tables

The following documentation describes the data in the CMJ and CMI tables. The CMJ table contains information at the highest time resolution for the telemetry format in use. The CMI table contains 3-second averages. Some entries in both tables are duplicated however the CMI table contains additional orbital and environmental data, and unique SI flags.

Seconds

The number of seconds from the start of the observation window (keyword STARTIME in the CMH file). The observation window encompasses the exposure and includes some small amount of instrument overhead. The science headers should be consulted to determine the actual start of an exposure. The time resolution in the CMJ table is dependent on the format of the telemetry transmitted during the observation. See the keyword TLMFORM for an explanation of formats and contents. The CMI table presents the data in intervals of ~3 seconds.

Temporal discontinuities in the table could be caused by one of the following:

Telemetry format switches during an observation (e.g., TLMFORM= AN/FN or FN/AN/FN, etc.) result in loss of telemetry. This telemetry cannot be reconstructed.
Bad data transmissions.
Incomplete telemetry coverage due to loss of TDRSS contacts, TDRSS hand-overs, etc. Usually these are periods are accommodated by recording the engineering data on the engineering tape recorders for later downlink. Occasionally, there are periods when this is not possible.

CMJ/CMI: The time is available in both CMJ and CMI tables.

V2_dom

The V2 coordinate of the dominant guide star in arcsec as a function of time. The dominant guide star controls the pitch and yaw of the HST. The V2V3 coordinates are calculated from the FGS star selector encoder positions and transformed to the vehicle coordinate frame by taking into account the alignment of the FGS, corrections for optical field distortion and for the effect of differential velocity aberration between the target and the dominant guide star.

Note: If the actual guiding mode is GYRO during any part of the observation, the corresponding entries will default to "INDEF."

FGS Telemetry Update Rates

CMJ/CMI: The parameter is available in both CMJ and CMI tables. The CMI table contains 3-second averages. Note that the header keywords V2_RMS, V3_RMS, V2_P2P, and V3_P2P are derived from V2_dom and V3_dom.

V3_dom

The V3 coordinate of the dominant guide star (arcsec) as a function of time. The dominant guide star controls the pitch and yaw of the HST. The V2V3 coordinates are calculated from the FGS star selector encoder positions and transformed to the vehicle coordinate frame by taking into account the alignment of the FGS, corrections for optical field distortion and for the effect of differential velocity aberration between the target and the dominant guide star. No filtering algorithms are used. If the actual guiding mode is gyro during any part of the observation, the corresponding entries will be defaulted to "INDEF." See V2_dom for details on update rate.

CMJ/CMI: The parameter is available in both CMJ and CMI tables. The CMI table contains 3-second averages.

V2_roll

The V2 coordinate of the sub-dominant or roll guide star (arcsec). The roll guide star controls the motion around the V1 axis. The V2V3 coordinates are calculated from the FGS star selector encoder positions and transformed to the vehicle coordinate frame by taking into account the alignment of the FGS, corrections for optical field distortion and for the effect of differential velocity aberration between the target and the dominant guide star. No filtering algorithms are used. If the actual guiding mode is gyro during any part of the observation, the corresponding entries will be defaulted to "INDEF." See V2_dom for details on update rate.

CMJ/CMI: The parameter is available in both CMJ and CMI tables. The CMI table contains 3-second averages.

V3_roll

The V3 coordinate of the sub-dominant or roll guide star (arcsec). The roll guide star controls the motion around the V1 axis. The V2V3 coordinates are calculated from the FGS star selector encoder positions and transformed to the vehicle coordinate frame by taking into account the alignment of the FGS, corrections for optical field distortion and for the effect of differential velocity aberration between the target and the dominant guide star. No filtering algorithms are used. If, during any portion of the observation, the actual guiding mode is single FGS plus gyro or gyro, the corresponding entries will be defaulted to "INDEF." See V2_dom for details on update rate.

CMJ/CMI: The parameter is available in both CMJ and CMI tables. The CMI table contains 3-second averages.

SI_V2

The vehicle motion in the direction of V2 at the nominal aperture reference position in arcsec. The jitter is derived from the combined motion of the dominant and roll guide stars (when guiding in two FGS mode) or the single guide star (in single FGS guiding mode). The rms jitter during times of quiescence is usually 2-3 milliarcsec. Passes into and out of the Earth's shadow increase rms vehicle jitter (5-8 milliarcsec on average). The Day/Night flag in the tables and the header keywords SHADOEXT and SHADOENT indicate the times of day/night passages.

A threshold filtering algorithm is applied to the calculation to remove bad telemetry data. Our experience has shown that a point which deviates from its neighbor by more than 200 milliarcsec is spurious. These points are defaulted to "INDEF." Cautions:

The jitter data are referenced to the first valid data point in the set. This necessary normalization has unfortunate consequences for moving target data, spatial scans, and data obtained during slews: one would see a drift in the jitter data as a function of time. This motion will also contaminate the calculation of the standard deviations presented in the header. Because of the drift, the filtering algorithm is not applied if a slew was in progress. A slew motion is easily identified using the SLEW flag contained in the tables and noting the presence of a special keyword in the header.
The differential velocity aberration between the target and the guide stars is removed correctly when the telemetry is in AN, PN, or HN format, but is not removed when in FN format because the S/C velocity vector is not available. In this case, a drift will be seen in the jitter data as a function of time.

Note: The SI_V2 and SI_V3 columns are defaulted to "INDEF" if the actual guiding mode is GYRO or during periods of loss of lock.

CMJ/CMI: The SI_V2 and SI_V3 parameters are available in the CMJ table. The CMI table includes three corresponding data items which describe the jitter: 3-second averages of the jitter, peak to peak amplitude, and the rms (SI_V2_AV, SI_V2_RM, SI_V2_P2).

SI_V3

The motion (or vehicle jitter) at the nominal aperture reference position V3 in arcsec. The jitter is derived from the combined motion of the dominant and roll guide stars (when guiding in two FGS mode) or the single guide star (in single FGS guiding mode). The magnitude of the jitter during times of quiescence is usually 2-3 milliarcsec. Passes into and out of the Earth's shadow increase vehicle jitter (5-8 milliarcsec on average). The Day/Night flag in the tables and the header keywords SHADOEXT and SHADOENT indicate the times of day/night passages.

See SI_V2 for cautions and additional details.

SI_V2_AVG

The motion (or vehicle jitter) at the nominal aperture reference position along the V2 direction in arcsec. SI_V2_AVG is the average over a 3 second interval and resides in the CMI table. The jitter is derived from the combined motion of the dominant and roll guide stars (when guiding in two FGS mode) or the single guide star (in single FGS guiding mode). The magnitude of the jitter during times of quiescence is usually 2-3 milliarcsec. Passes into and out of the Earth's shadow increase vehicle jitter (5-8 milliarcsec on average). The Day/Night flag in the tables and the header keywords SHADOEXT and SHADOENT indicate the times of day/night passages.

Note: The SI_V2_AVG and SI_V3_AVG default to "INDEF" if the actual guiding mode was GYRO or during periods of loss of lock. Please refer to table item SI_V2 for a description of the calculation of the jitter. Update Rate: 3-second averages.

CMJ/CMI: The SI_V2_AVG and SI_V3_AVG are available in the CMI table only. Comparable information is provided in parameters SI_V2 and SI_V3 in the CMJ table.

SI_V3_AVG

The motion (or vehicle jitter) at the nominal aperture reference position along the V3 direction in arcsec. SI_V3_AVG is the average over a 3-second interval and resides in the CMI table. The jitter is derived from the combined motion of the dominant and roll guide stars (when guiding in two FGS mode) or the single guide star (in single FGS guiding mode). The magnitude of the jitter during times of quiescence is usually 2-3 milliarcsec. Passes into and out of the Earth's shadow increase vehicle jitter (5-8 milliarcsec on average). The Day/Night flag in the tables and the header keywords SHADOEXT and SHADOENT indicate the times of day/night passages.

Note: The SI_V2_AVG and SI_V3_AVG are defaulted to "INDEF" if the actual guiding mode was GYRO or during periods of loss of lock. Please refer to table item SI_V2 for a description of the calculation of the jitter. Update Rate: 3-second averages.

CMJ/CMI: The SI_V2_AVG and SI_V3_AVG are available in the CMI table only. Comparable information is provided in parameters SI_V2 and SI_V3 in the CMJ table.

SI_V2_RMS

The standard deviation of the jitter in the V2 direction (arcsec) within a 3-second interval and is found in the CMI table. The jitter is derived from the combined motion of the dominant and roll guide stars (when guiding in two FGS mode) or the single guide star (in single FGS guiding mode). The magnitude of the jitter during times of quiescence is usually 2-3 milliarcsec. Passes into and out of the Earth's shadow increase vehicle jitter (5-8 milliarcsec on average). The Day/Night flag in the tables and the header keywords SHADOEXT and SHADOENT indicate the times of day/night passages.

Note: The SI_V2_RMS and SI_V3_RMS are defaulted to "INDEF" if the actual guiding mode was GYRO or during periods of loss of lock. Please refer to table item SI_V2 for a description of the calculation of the jitter. Update Rate: 3-second averages.

CMJ/CMI: The SI_V2_RMS and SI_V3_RMS are available in the CMI table only. Comparable information is provided in parameters SI_V2 and SI_V3 in the CMJ table.

SI_V3_RMS

The standard deviation of the jitter in the V3 direction (arcsec) within a 3-second interval and is found in the CMI table. The jitter is derived from the combined motion of the dominant and roll guide stars (when guiding in two FGS mode) or the single guide star (in single FGS guiding mode). The magnitude of the jitter during times of quiescence is usually 2-3 milliarcsec. Passes into and out of the Earth's shadow increase vehicle jitter (5-8 milliarcsec on average). The Day/Night flag in the tables and the header keywords SHADOEXT and SHADOENT indicate the times of day/night passages.

CMJ/CMI: The SI_V2_RMS and SI_V3_RMS are available in the CMI table only. Comparable information is provided in parameters SI_V2 and SI_V3 in the CMJ table.

SI_V2_P2P

The maximum peak-to-peak amplitude of the jitter over a 3 second interval in the V2 direction (arcsec). The parameter is resident in the CMI table. The jitter is derived from the combined motion of the dominant and roll guide stars (when guiding in two FGS mode) or the single guide star (in single FGS guiding mode). The magnitude of the jitter during times of quiescence is usually 2-3 milliarcsec. Passes into and out of the Earth's shadow increase vehicle jitter (5-8 milliarcsec on average). The Day/Night flag in the tables and the header keywords SHADOEXT and SHADOENT indicate the times of day/night passages.

Note: The SI_V2_P2P and SI_V3_P2P are defaulted to "INDEF" if the actual guiding mode was GYRO or during periods of loss of lock. Please refer to table item SI_V2 for a description of the calculation of the jitter. Update Rate: 3-second averages.

CMJ/CMI: The SI_V2_P2P and SI_V3_P2P are available in the CMI table only. Comparable information is provided in parameters SI_V2 and SI_V3 in the CMJ table.

SI_V3_P2P

The maximum peak-to-peak amplitude of the jitter over a 3 second interval along the V3 axis (arcsec). The parameter is resident in the CMI table. The jitter is derived from the combined motion of the dominant and roll guide stars (when guiding in two FGS mode) or the single guide star (in single FGS guiding mode). The magnitude of the jitter during times of quiescence is usually 2 to 3 milliarcsec. Passes into and out of the Earth's shadow increase vehicle jitter (5-8 milliarcsec on average). The Day/Night flag in the tables and the header keywords SHADOEXT and SHADOENT indicate the times of day/night passages.

Note: The SI_V2_P2P and SI_V3_P2P keywords default to "INDEF" if the actual guiding mode was GYRO or during periods of loss of lock. Update Rate: 3-second averages.

CMJ/CMI: SI_V2_P2P and SI_V3_P2P are available in the CMI table only. Comparable information is provided in parameters SI_V2 and SI_V3 in the CMJ table.

RA

The right ascension (J2000) at the aperture reference position in degrees. Three methods are used to calculate the pointing, depending on the guiding mode in use. These are described in detail below. We emphasize that the pointing is referenced to the nominal aperture position, and the name and coordinates of that aperture are provided in the header. Offsets from this aperture reference position, e.g., POS TARGS, real-time maneuvers, are not taken into account by the observation log software.

Note: When TLMFORM=FN and the guiding mode is single FGS+GYRO or GYRO, the keyword will be blank: the absence of the velocity information in FN format would result in a +/- 20 arcsec error in the pointing calculation.

There are several reasons why the observation log RA, Dec, and roll may be different from the commanded values. Some of these reasons depend on the guiding mode used for the observation. The reasons for some discrepancies are summarized here:

If pre-planned or real-time offsets were commanded (all guiding modes);
The relative guide star positional error (Taff et al., 1990), 0.33" for northern hemisphere plates and 0.43" for southern hemisphere plates (two FGS guiding mode);
Mis-identified guide stars (two and single FGS guiding modes);
Inherent error of tenths of an arcsec in the telemetered gyro data (gyro and single FGS guiding modes);
Gyro drift which ranges from 1-5 milliarcsec/second depending on the gyro bias update and slew activities prior to the observation (gyro and single FGS guiding modes);
Thermal expansion, contraction and jitter. The peak-to-peak amplitude of the breathing in the direction of pitch and yaw is less than 15 milliarcsec. Jitter is on average about 2-8 milliarcsec (single and two FGS guiding modes);
In TLMFORM=FN where the absolute and differential velocity aberration corrections are not applied. The absolute velocity variation is xb1 20 arcsec and the differential aberration is xb1 50 milliarcsec (all modes);
Slight differences in the method of calculation between the observation log software and the operations ground and onboard flight software systems (all modes).

The causes for the target missing the aperture reference position include the following:

Relative guide star (0.33-0.43 arcsec) and target coordinate error (single and two FGS guiding modes);
Mis-identified guide stars or binary guide stars;
Calibration errors including: (1) FGS/FGS alignment (0.2-0.3 arcsec); (2) FGS/SI aperture calibration errors which depend on the target acquisition method used and the relative errors between target acquisition aperture and science aperture. Values range from 0.1 arcsec to 0.5 arcsec; (3) Calibration errors between the Fixed Head Star Trackers, Rate Gyro Assembly and FGSs;
Gyro drift of 1-5 milliarcsec/second (single FGS and gyro modes);
Thermal expansion and contraction, known as breathing (up to 15 milliarcsec).

The 3 methods used in the calculation of the RA, Dec, and roll are the following: Two FGS Fine Lock Guiding, GYRO Control, and single FGS Pitch and Yaw plus GYRO Roll Control.

Telemetry Update Rates

CMJ/CMI: The RA, Dec, and roll in degrees are available in the CMJ and CMI tables. Their averages are displayed in the header keywords.

DEC

The declination (J2000) at the aperture reference position in degrees. The RA, Dec, and roll are calculated in different ways depending on the guiding mode used during the observation.

See table entry RA for a detailed explanation for the methods, the errors, the cautions in interpretation, and the update rates.

CMJ/CMI: The RA, Dec, and roll in degrees are available in the CMJ and CMI tables. Their averages are displayed in the header keywords.

Roll

The roll angle (J2000) in degrees calculated at the position of the aperture reference position. The roll angle is defined as the angle from north to the +V3 axis, in the direction of east. Note that this angle is calculated at the position of the aperture. In practice, it differs only slightly from the roll angle of the V1 axis (~ 100 arcsec in roll and dependent on the target coordinates). This angle is not to be confused with the ORIENT angle specified on the proposal. The ORIENT angle and the roll angle differ by 180 degrees.

The CMJ and CMI tables contain the roll angle tabulated as a function of time. See table entry RA for a detailed explanation for the methods, the errors, and the cautions in interpretation.

During a single observation, does the roll angle change as a function of time and why? Here again, the answers depend on the method used for guiding. The roll angle will change over the course of an observation for the following reasons:

The roll will change when the RA and Dec changes via spatial scans, offsets, moving targets, slews. All small angle maneuvers will change the direction of north with respect to the aperture reference position. The telescope itself does not adjust the roll during the observation; only the calculations reflect the change in position.
Gyro drift will cause a change in the roll angle. The drift rates are 1-5 milliarcsec/second depending on the gyro bias update and slew activities prior to the observation (gyro and single guide star modes);
Thermal expansion and contraction and jitter (single and two FGS guiding modes). The peak-to-peak amplitude of the breathing in the direction of pitch and yaw is less than 15 milliarcsec. The effects on the roll are yet to be determined.

CMJ/CMI: The RA, Dec, and roll in degrees are available in the CMJ and CMI tables. Their averages are displayed in the header keywords.

LimbAng

The angle between the V1 axis and the tangent to the Earth limb in degrees. The angle lies in the Earth-center/HST/target plane. A spherical Earth is assumed with a radius of 6378.14 km (mean equatorial value). Even though the HST is restricted from pointing too close to the limb, (15 and 7.1 degrees for the sunlit Earth limb and dark Earth limb respectively) the background does change significantly as the bright Earth limb is approached. The limb angle may be used to alert the observer to the possibility of enhanced background levels. The angle is used in the calculation of Earth Mod, an estimation of the background brightness levels.

The inputs to the limb angle calculation are the altitude of the HST (and hence the GCI coordinates of the HST position in her orbit), a telemetered parameter which is the cosine of the angle between the V1 axis and the Earth's center, and an assumption of a spherical Earth.

Note: This column is "INDEF" when the TLMFORM=FN because the spacecraft velocity and position are unavailable in that format. See definition for header keyword LOS_LIMB for more details.

Telemetry Update Rate

CMJ/CMI: LimbAng is provided in the CMI table only.

EarthMod

A semi-empirical model is used to estimate the stray light reaching the HST focal plane from an "average-albedo" bright Earth. This keyword is an estimate of the background due to the bright Earth during the observation window. The model is based on a pre-launch stray light analysis of the HST baffling system which has been adjusted by actual data from a dedicated in-flight HST engineering test. The stray light from the Earth is also tabulated as a function of time in the CMI table. Error bars and ranges of values are not yet determined. Future enhancements will provide the data in terms of instrument magnitudes.*1

Note: The calculation of this column entry requires the position of the spacecraft and so is not available in FN format.

Telemetry Update Rate

CMJ/CMI: EarthMod is available in the CMI table only.

TermAng

The angle between the V1 axis and the terminator, when the terminator is visible. The terminator is defined as a line approximately 90 degrees from the Earth-center to sun-center line. The intersection of the terminator is chosen as that which minimizes the terminator angle. The tangent point is the intersection of the horizon with the tangent to the Earth from HST in the Earth- center/HST/target plane. TermAng is a rough indicator of the extent of the bright Earth beyond the tangent point.

Note: TermAng defaults to "INDEF" when the terminator is not visible or when TLMFORM=FN.

Telemetry Update Rate

CMJ/CMI: TermAng is provided in the CMI table only.

LOS_Zen

The angle between the V1 axis and the local zenith in degrees. This parameter is of relevance in studying the interaction of the spacecraft with the upper atmosphere. The input parameters include the telescope attitude and GCI coordinates of the telescope position.

Note: LOS_ZEN defaults to "INDEF" in FN format because the velocity and position of HST are unavailable in that format.

Telemetry Update Rate

CMJ/CMI: This column is available in the CMI table only.

Latitude

The geographic latitude of the spacecraft sub-point in degrees, as defined by the radius vector from the center of the Earth to the spacecraft. The telemetered GCI coordinates of the telescope are used to calculate latitude.

Note: This column defaults to "INDEF" in FN format because the position of HST is unavailable in that format.

Telemetry Update Rate

CMJ/CMI: This column is available in the CMI table only.

Longitude

The geographic longitude of the spacecraft sub-point in degrees, as defined by the radius vector from the center of the Earth to the spacecraft. The parameter is tabulated as a function of time and expressed in degrees. The input parameters to the longitude calculation are the GCI coordinates of the telescope position. See Latitude for update rates.

Note: This column defaults to "INDEF" in FN format because the position of HST is unavailable in that format.

CMJ/CMI: This column is available in the CMI table only.

Mag(3)

The magnetic field strength, in gauss, along each of the 3 vehicle axes +V1, +V2, and +V3. The magnetic field is of interest when studying effects such as the FOS GIMP and other detector-related behaviors.

MAG(1): the Magnetic field strength (Gauss) in the direction of the line of sight (+V1 axis) during the observation, as measured by the on-board magnetometers.
MAG(2): Magnetic field strength (Gauss) in the direction of +V2
MAG(3): Magnetic field strength (Gauss) in the direction of +V3

Telemetry Update Rate

CMJ/CMI: These three columns are available in the CMI table only.

Recenter

Recenter is a flag whose value may be 0 (OFF) or 1 (ON). The toggle to 1 occurs when a recentering event is in progress. A recentering event is triggered when the gyroscopes have detected a pointing excursion that exceeds 20 milliarcsec. At this time, telescope pointing control is transferred from the FGSs to the gyros because of excessive jittering of the telescope. When the disturbance subsides, control is transferred back to the FGSs. Typical recentering events last for a few seconds, i.e., the spacecraft computer requires a minimum of 3 seconds to recover from a recentering event. In contrast, losses of lock (see LOCKLOSS) are at least minutes in duration. The science instruments will continue to acquire data during the recentering period. The capability was installed in the 1993.

A knowledge of the onset and duration of recentering events is important because the position of a target within an aperture could change significantly (due to gyro drift and spacecraft disturbances) thus degrading the pointing quality. The trade-off, i.e., degraded pointing versus loss of FGS lock and significant reduction in exposure time (3 minutes or more during lock losses), has been judged acceptable.

Telemetry Update Rate

CMJ/CMI: All flags are available in both the CMJ and CMI tables. If the Recenter flag is on or 1 at any time during the 3-second interval, it will appear a 1 in the corresponding row in the CMI table.

TakeData

The TakeData flag is an onboard pointing control system flag which, when ON, indicates that the spacecraft is ready for an observation and that the guide star tracking performance is nominal. The value of this field is 1 (ON) or 0 (OFF). The TakeData flag is used to determine the loss of lock status of the vehicle and is the input for the calculation of the header keywords NLOSSES and LOCKLOSS.

The flag is essentially a green light for the SIs to proceed with the data-take. With a few exceptions, the SIs will respond quickly (not instantaneously) when the TakeData flag toggles to the OFF position. The response is the interruption of an exposure and/or shutter closures. The exceptions are the following: (1) Usually, the WFPC2 does not acknowledge the TAKEDATA status during observations shorter than 300 seconds and continues to observe. (The observation would be over before the vehicle had a chance to recover from the loss of lock). (2) The FOS checks the TakeData flag at the start of an observation but not during the observation.

The TakeData flag typically signals a loss of lock of the guide stars, however it can also be triggered by vehicle SAFING events.

Telemetry Update Rate

Note: The update rate for the TakeData flag in the telemetry is slower than the vehicle response time to the indicator.

CMJ/CMI: All flags are available in both the CMJ and CMI tables. If the TakeData flag is OFF at any time during the 3-second interval, it will appear as OFF in the particular row in the CMI table.

DayNight

A flag indicating the day or night status of the HST in its orbit. The status is determined from analysis of the solar array current sensors. The voltages are very sensitive to the geometrical placement of the sun with respect to the arrays. The flag has a value of 1 for day and 0 for night.

Telemetry Update Rate

CMJ/CMI: All flags are available in both the CMJ and CMI tables. If the DayNight flag is triggered at any time during the 3-second interval, it will appear as such in the particular row in the CMI table.

Slew

A flag which indicates that the pointing control system is slewing the telescope while still tracking on guide stars. A 1 indicates a slew maneuver and 0 signifies stationary tracking. Small slews (or "small angle maneuvers) are commanded during target acquisitions and peak-ups, spatial scans, POS TARG activity, real-time maneuvers, and moving target observations.

A special keyword will appear at the end of the header file when slewing has occurred during the observation window. Because of the method of calculation, slewing during an observation will adversely affect the jitter statistics in the header and in the tables.

Telemetry Update Rate

CMJ/CMI: All flags are available in both the CMJ and CMI tables. If the SLEWING flag is ON at any time during the 3-second interval, it will appear as ON in the particular row in the CMI table.

SI-Specific Column

The SI-Specific Column in the CMI table reports a unique, instrument-specific parameter for the FOC, FOS, and HRS. The number of instrument-specific items may be increased in future enhancements.

FOC VPU Noise Indicator

The FOC Video Processing Unit (VPU) Noise Indicator reports the instantaneous sum, in volts, of the signal (targets plus background) seen by the FOC detectors. The parameter can range from 0 to 5.1 volts, however typical dark backgrounds generate 0.3 volts, and backgrounds obtained near the South Atlantic Anomaly can reach 1.5 volts. Although the FOC does not operate in the South Atlantic Anomaly, the SAA strength and boundaries vary with time. The brightness limitations imposed by FOC sensitivity should constrain the background+target sum from exceeding about 1.5 volts. The internal LED observations also generate about 1.5 volts. This parameter is very useful for tracking changes in the background during an observation. STScI engineers use the parameter for verifying that the SAA models used by the scheduling system still accommodate FOC requirements.

HRS On-The-Fly Adder

The HRS On-the-Fly Adder sums the counts in all 512 diodes over an integration period. The adder is initiated at the start of an ACCUM mode and continues until the completion of the observation. In all but Rapid Readout and the Pulse Height Analysis modes, the data transmitted in the telemetry is processed by HRS to remove bad diode contribution and the output from the corner (radiation sensitive) diodes. The parameter may indicate anomalies in pointing stability and instrument performance. The sum may be used to indicate changes in instrument performance and centering of the target in the aperture.

FOS Overlight Sum

The FOS Overlight Sum provides the sum of counts in all 512 diodes over a 60 second interval. Note that the 60-second interval is not timed with respect to the start or ending of an observation. The sum is generated continuously, e.g., during filter/grating changes, aperture wheel motions, darks, closing of shutters, etc. The Overlight Sum is interrogated by the flight software to verify that the maximum brightness (for instrument safety) has not been exceeded. The sum may be used to indicate changes in instrument performance and in centering of the target in the aperture.

Telemetry Update Rate

CMJ/CMI: The SI-Specific parameter column is available in the CMI table.

Focus_Change

This is a reserved field for a future implementation of an indicator of orbital changes in focus (called breathing).

Seconds
V2_dom
V3_dom
V2_roll
V3_roll
SI_V2
SI_V3
SI_V2_AVG
SI_V3_AVG
SI_V2_RMS
SI_V3_RMS
SI_V2_P2P
SI_V3_P2P
RA
Two FGS Fine Lock Guiding
GYRO Control
Single FGS Pitch and Yaw plus GYRO Roll Control
DEC
Roll
LimbAng
EarthMod
TermAng
LOS_Zen
Latitude
Longitude
Mag(3)
Recenter
TakeData
DayNight
Slew
SI-Specific Column
FOC VPU Noise Indicator
HRS On-The-Fly Adder
FOS Overlight Sum
Focus_Change

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