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Chapter 3: Table Definitions

CMJ & JIT/CMI Tables

This chapter describes the data in the CMJ and JIT/CMI tables. The CMJ table contains information at the highest time resolution for the telemetry format in use, and was not produced after August 1995, at which time it was replaced with a JID jitter image. In the large majority of cases, however, the most useful file is the JIT (formerly and identical to the CMI) table, which is of manageable size and contains data averaged at 3-second intervals. Some entries in both JIT and CMJ tables are duplicated, however the JIT 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). 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 JIT table presents the data in intervals of ~3 seconds.

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

CMJ/JIT: The time is available in both CMJ and JIT 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."

CMJ/JIT: The parameter is available in both CMJ and JIT tables. The JIT 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."

CMJ/JIT: The parameter is available in both CMJ and JIT tables. The JIT 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."

CMJ/JIT: The parameter is available in both CMJ and JIT tables. The JIT 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."

CMJ/JIT: The parameter is available in both CMJ and JIT tables. The JIT 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:

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/JIT: The SI_V2 and SI_V3 parameters are available in the CMJ table. The JIT 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_P2P).

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 JIT 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/JIT: The SI_V2_AVG and SI_V3_AVG are available in the JIT 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 JIT 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/JIT: The SI_V2_AVG and SI_V3_AVG are available in the JIT 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 JIT 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.

CMJ/JIT: The SI_V2_RMS and SI_V3_RMS are available in the JIT 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 JIT 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.

CMJ/JIT: The SI_V2_RMS and SI_V3_RMS are available in the JIT 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 JIT 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.

CMJ/JIT: The SI_V2_P2P and SI_V3_P2P are available in the JIT 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 JIT 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.

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

RA

NOTE: For information on accuracy, calculations, and sources of errors on the following 3 table columns (RA, Dec, and Roll at the reference aperture) please see elsewhere in Chapter 2.

The right ascension (J2000) at the aperture reference position in degrees. 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.

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.

CMJ/JIT: The RA, Dec, and roll in degrees are available in the CMJ and JIT 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.

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 will differ increasingly from the roll angle of the V1 axis as the declination of the pointing increases. This angle should not be confused with the ORIENT angle specified on the proposal. The ORIENT angle and the roll angle differ by 180 degrees.

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.



Telemetry Update Rate

CMJ/JIT: LimbAng is provided in the JIT 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 JIT 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/JIT: EarthMod is available in the JIT 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/JIT: TermAng is provided in the JIT 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/JIT: This column is available in the JIT 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/JIT: This column is available in the JIT 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/JIT: This column is available in the JIT 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.



Telemetry Update Rate

CMJ/JIT: These three columns are available in the JIT 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 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/JIT: All flags are available in both the CMJ and JIT tables. If the Recenter flag is on or 1 at any time during the JIT's 3-second data interval, it will appear a 1 in the corresponding row of the 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/JIT: All flags are available in both the CMJ and JIT tables. If the TakeData flag is OFF at any time during the JIT's 3-second data interval, it will appear as OFF in the particular row of the 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/JIT: All flags are available in both the CMJ and JIT tables. If the DayNight flag is triggered at any time during the JIT's 3-second data interval, it will appear as such in the particular row of the table.

SlewFlag

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

SI-Specific Column

The SI-Specific Column in the JIT table reports a unique, instrument-specific parameter for the first generation SIs: FOC, FOS, and GHRS, and for an astrometer FGS. For interpreting populated SI-Specific values in jitter files for the first generation instruments, contact help@stsci.edu.

For astrometry observations, the SI_Specific column will comprise a decimal point followed by three digits. These digits represent the Step Size Limit Exceeded (SSLE), Search Radius Limit Exceeded (SRLE), and Stop flags respectively of the astrometer FGS.

For successful astrometry observations, these flags will be all zeros and no recenterings or losses of lock will be indicated in the JIF files. If an anomaly occurs with the astrometer FGS, then two of these flags should be set to 1. One of the set flags will always be the STOP flag, while the other will be the SRLE or SSLE depending on the kind of failure. If only one flag remains set, or both SSLE and SRLE are set, there is a problem with OMS, the FGS, or HST. An example of these flags (using the IRAF tread command, which resides in the ttools package) follows:

Column    22        23         24         25        26        27        28
Label  _Mag_V3__ EarthMod_ SI_Specific DayNight_ Recenter_ TakeData_ SlewFlag_
    30 -0.0756   INDEF     0.          0.        0.        1.        0.
    31 -0.072    INDEF     0.          0.        0.        1.        0.
    32 -0.072    INDEF     0.          0.        0.        1.        0.
    33 -0.072    INDEF     0.101       0.        0.        1.        0.
    34 -0.072    INDEF     0.101       0.        0.        1.        0.
    35 -0.0708   INDEF     0.101       0.        0.        1.        0.
    36 -0.0672   INDEF     0.101       0.        0.        1.        0.
    37 -0.0672   INDEF     0.101       0.        0.        1.        0.
    38 -0.0672   INDEF     0.101       0.        0.        1.        0.

Values of 0.011 refer to a SRLE failure and 0.101 is a SSLE failure.

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/JIT: The SI-Specific parameter column is available in the JIT 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
DEC
Roll
LimbAng
EarthMod
TermAng
LOS_Zen
Latitude
Longitude
Mag(3)
Recenter
TakeData
DayNight
SlewFlag
SI-Specific Column
FOC VPU Noise Indicator
HRS On-The-Fly Adder
FOS Overlight Sum
Focus_Change