Chapter 2: Header Keyword Definitions

Header Keywords


Below is a listing of keywords which can be found in the image headers. The .jih file is detailed here. The old .cmh header contains a subset of these keywords. In some cases, the same keyword's name changes between the .cmh and .jih files. In these cases, both names are listed before the description. The _jif.fits file contains nearly all of the same keywords as the .jih ASCII header file, but with the exception of some keywords affected by the new style FITS associations.

Note: The calculation of some keywords below requires the position of the spacecraft or the velocity vector, parameters which are not available in the FN telemetry format (see TLMFORM). For exposures taken in FN format, these keywords will be unpopulated. If telemetry switches occur during an observation, such keywords will be blank when switching from AN or HN to FN and may or may not be blank when the switch is from FN to AN/HN. In this latter case, if the AN/HN portion is long enough to encompass an update of the telemetered HST position data, then a value will be provided; otherwise the keyword is blank.


PROPOSID (jih) / PROPOSAL (cmh)

The proposal processing and transformation identification number extracted from the Mission Schedule.

TARGNAME

The first 10 characters of the target name as it appears in the proposal. The name is extracted from the Mission Schedule.

STARTIME and ENDTIME

The STARTIME and ENDTIME keywords delineate the boundaries of an exposure window. All jitter, attitude and pointing information are referenced to this time period. These times are determined by a set of rules, unique to each science instrument (SI) mode, and are obtained from analysis of the Mission Schedule. Note that these times are not to be confused with the actual exposure start and stop times available in the science data headers. The window often includes instrument overheads. For exact exposure start times, the science headers and explanations in the instrument handbooks should be consulted. Once the exact exposure start time has been identified, the offset is determined by comparison with the observation log STARTIME.

Note: The HRS has the unique capability of pausing (idling) during an observation for an occultation or South Atlantic Anomaly (SAA) passage. These idle periods are pre-planned. When idling occurs during an HRS observation, the duration as indicated by the STARTIME and ENDTIME will encompass the entire observation including the occultation or SAA passage. However, the tables will only contain data during non-idle periods.

SOGSID

The SOGSID is similar to the ROOTNAME of the observation. It consists of the first 8 characters of the ROOTNAME. Encoded in the rootname is the instrument mnemonic (X=FOC, Y=FOS, V=HSP, W=WFPC, U=WFPC-2, Z=HRS,F=FGS), a 3-character program ID assigned by the scheduling system, an observation set id (used as a unit for scheduling purposes), and a running count of the exposure within the alignment (not to be confused with the exposure number in the proposal).

CONFIG (jih) /INSTRUME (cmh)

The acronym of the instrument used for the observation: WFPC2= WFPC 2, WFC= Wide Field Camera, PC= Planetary Camera, FOS= Faint Object Spectrograph, FOC= Faint Object Camera, HRS= Goddard High Resolution Spectrograph, HSP=High Speed Photometer, FGS= Fine Guidance Sensor (astrometry observations).

PARALLEL (jih) / PRIMARY (cmh)

This keyword indicates whether multiple SIs are in use at the same time. In the case of the JIH file, the values of this keyword will either be NO, YES-Primary, or YES-Secondary. For the CMH file, the keyword will either be SINGLE, PARALLEL-PRIMARY, or PARALLEL-SECONDARY. If PRIMARY, the observation dictated the pointing but another SI was operating in parallel. If SECONDARY, another observation dictated the instrument pointing.

One important characteristic to note when analyzing pointing data associated with PARALLEL-SECONDARY is the effect of the differential velocity aberration. The telescope (and so the observation log software) computes a differential velocity aberration correction for the PRIMARY aperture only. Therefore, the secondary will experience a small drift (cyclic as the telescope moves in its orbit). The effect of the error is a deviation in the positioning of the secondary target of up to 50 mas peak to peak for a full orbit (~ 96 minutes).

Note that when WFPC2 (or WFPC) is the parallel instrument, the aperture used in the calculations of jitter and pointing in the WFPC2 observation log is always the UWFALLFIX aperture.

OPERATE

OPERATE is the predicted time when the instrument was commanded to an "operate" state. For some SIs, the operate state refers to high voltage turn-on. The length of time between "operate" and "observe" is an indication of instrument stability. In general, the rules for warm-up and stabilization for all the science instruments have been carefully scrutinized and tested to ensure thermal stability and optimize efficiency. However, as the instruments age, the warm-up times may require adjustment. This field can assist the HST engineers in performing trend analysis studies by giving the time interval during which they may examine voltage and temperature fluctuations, etc.

TLMFORM

TLMFORM is the format of the engineering telemetry transmitted during the observation. The formats differ not only in their rate of update, but in their content. On occasion, multiple telemetry switches can occur during a single observation and some telemetry will be lost during these switches (seen as a time jump in the tables). The TLMFORM keyword will indicate single and multiple telemetry formats (e.g., TLMFORM = 'AN/FN/AN'). If data dropouts from the
tape recorder issue are large enough, the TLMFORM keyword value may be left blank.

Given below are the rate possibilities of TLMFORM, the quantization of the time tags in the observation logs, and brief comments on the primary use of the format.


TLMFORMRate (sec)Display Rate (sec)Comment
AN0.50.5Common format
FN0.050.125Older format, mostly replaced with HN
PN0.150.125Pointing and gyro observations
HN0.050.125Most commonly used format

Note: The rate column indicates how often the engineering telemetry is updated for a given format. The display rate column indicates the time resolution of the Observation Log files, the time separating lines of data in the file.

CONTENT:

APERTURE

The 10-character name of the target aperture used for the exposure. The vehicle coordinates (V2,V3) of this aperture are used as input to the calculation of the RA, Dec, and roll. For parallel observations, the observation log correctly populates this keyword, whether the aperture is the primary or secondary.

APER_V2

The V2 coordinate of the aperture reference position in arcsec as specified in the latest version of the operational data base. This reference position defines the fiducial in the focal plane's coordinate system. Positions with respect to the V1 axis, and the guidestars in the FGSs, are needed to calculate the RA, Dec, and roll (at the aperture position), and to correctly apply the differential velocity aberration. APER_V2 is used as input to the calculation of CRVAL1 and CRVAL2, available in the science data header.

Note: Aperture positions may be refined and updated or sometimes re-defined; each Observation Log's calculations are done using what was the operational value for the aperture in question, at that time.

APER_V3

The V3 coordinate of the aperture reference position. See above.

ALTITUDE

The average altitude of the HST above the surface of the Earth in kilometers. This keyword is calculated using the Geocentric Inertial Coordinates (XYZ) transmitted in the telemetry and a model for the radius of the Earth. The telemetered spacecraft position is generated from the onboard ephemeris (rather than the definitive orbit ephemeris) implying uncertainties of a few kilometers.

The eccentricity of HST's orbit oscillates between ~0.0002 and 0.0012 over a period of about 56 days. Taking into account the oscillation of the orbital eccentricity and assuming a spheroidal Earth, the altitude range is 580 to 630 kilometers, The algorithm for the Earth radius assumes a spheroid (elliptical cross-section):*1

Where:

LOS_SUN

This keyword is the minimum angle in degrees between the HST V1 axis and the center of the sun during an observation. The angle is calculated using the on-board solar ephemeris and spacecraft attitude and is reported as the arc_cosine in the telemetry. Errors associated with the telemetered parameter are a few tenths of a degree. This parameter is useful if investigating the temperature changes of the environment of HST as a function of attitude.

HST is generally restricted from pointing within 50 degrees of the sun. The tolerance for off-nominal roll angles on a particular date is dependent on the value of the angle between the sun and the line of sight. In general, the HST aft shroud is hottest when this angle is close to 135 degrees (maximum surface area exposured) and coolest when the angle is 180 degrees.

LOS_MOON

The minimum angle in degrees between the HST V1 axis and the center of the moon. The angle is calculated using the on-board lunar ephemeris and spacecraft attitude and is reported as the arc_cosine in the telemetry. Errors associated with the keyword are a few tenths of a degree. The HST is restricted from pointing closer than 9 degrees from the center of the moon while using the FGSs.

SHADOENT

The predicted time of the closest entry into Earth shadow with respect to the start of an observation window. It is possible that the entry and exit times precede the start of an observation. The time refers to entry into the umbra. The penumbral-umbral transition occurs much more rapidly than the time resolution of the orbit records. This keyword is useful in investigations of stray light, thermal effects, and spacecraft jitter. The approximate actual time of the shadow entry or exit is determined from the solar array voltages and the DAY/NIGHT flag is based on these voltages. The accuracy of the entry and exit times depends on the telemetry format: 6 seconds for HN, FN and PN, and 10 seconds for AN.

SHADOEXT

The predicted time of the exit from the Earth shadow. It is possible that the entry and exit times precede the start of an observation. The time refers to exit from the umbra. The penumbral-umbral transition occurs much more rapidly than the time resolution of the orbit records. This keyword is useful in investigations of stray light, thermal problems, and spacecraft jitter. The approximate actual time of the shadow entry and exit can be determined from the solar array voltages which are available in the telemetry. The approximate actual time of the shadow entry or exit is determined from the solar array voltages and the DAY/NIGHT flag is based on these voltages. The accuracy of the entry and exit times depends on the telemetry format: 6 seconds in HN,FN,PN and 10 seconds in AN.

LOS_SCV

The minimum angle between the HST V1 axis and the spacecraft velocity vector. The inputs are the velocity vector in the GCI reference frame and the spacecraft attitude. This angle is of interest when investigating background or aging effects caused by the interaction of the HST with the upper atmosphere. Tests performed with the HST in orbit have shown that the effect is undetectable at the HST nominal altitude.*2

LOS_LIMB

The average of the angle between the HST 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 limb is approached.

The inputs to the limb angle calculation are the altitude of the HST (and hence the GCI coordinates of the HST position in its 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 spheroidal Earth. This CMI table contains the limb angle as a function of time.

ZODMOD

This keyword gives the zodiacal light background (V mag/arcsec2) at the end of an exposure. A model for the zodiacal light background at the position of the target is provided as a function of time in the CMI table. This model is interpolated from the results of Levasseur, Regourd and Dumont 1980.*3 The inputs in the calculation are the Julian date, right ascension, and declination of the V1 axis; the ecliptic longitude and latitude of the V1 axis and the Sun are calculated at the specified date and time. ZODMOD is a coarse estimate of the zodiacal light background and no error bars are available. A future enhancement will present the background in units of instrument counts.

EARTHMOD

This keyword is an estimate of the peak stray light reaching the HST focal plane from an "average-albedo" bright Earth in V mag/arcsec2 during an exposure. 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. *4 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 yet to be determined. Future enhancements will provide the data in terms of instrument counts.

MOONMOD

This keyword is an estimate of the stray light reaching the HST focal plane from a full moon in V mag/arcsec2 at the end of the observation. 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. *5 Error bars and ranges of values are yet to be determined.

GUIDECMD

The commanded guiding mode as extracted from the Mission Schedule. The values are:

If an anomaly should occur during the acquisition process, the actual guiding mode achieved at the end of the acquisition (reported in keyword GUIDEACT) will be different from the commanded mode. An anomaly keyword will appear at the end of the header, indicating a guide star acquisition failure or degradation. Cases will occur where the GUIDECMD=GUIDEACT, even though the spacecraft may be in a LOSS OF LOCK or RECENTERING state during part or all of the observation. The reason is that GUIDEACT reports the state of the pointing attitude control system immediately after the guide star acquisition, and not during the observation. To determine the guiding integrity during the observation, see the keywords
NLOSSES, LOCKLOSS, NRECENT, and RECENTR, as well as the table column TAKEDATA.

GUIDEACT

The actual guiding mode achieved at the end of the guide star (GS) acquisition. The possible values are the same as those for GUIDECMD.

Because of acquisition anomalies, the guiding mode initiated may not be used throughout the observation. The default guiding modes used in cases where fine-lock was not achieved has varied since HST launch. Until 1993, FGSs that did not achieve a commanded fine-lock could default to coarse track. Early in 1993, as a conservation measure, FGS2 was restricted from coarse tracking; a few months later, on day 104, FGSs 1 and 2 were similarly restricted. Currently, if one or both FGSs failed to achieve fine lock, then the pointing would fall back to either FINE LOCK/GYRO or pure GYRO.

GSD_ID

The identification of the dominant guide star. The number is the same as the HST Guide Star Catalog identifier. The dominant guide star controls the pitch and yaw of the telescope (V2 and V3 motions). When guiding with one FGS, this keyword is the ID of the corresponding (single) guide star.

If an anomaly occurs during the guide star acquisition process, the guide star keywords will be populated, however GUIDECMD and GUIDEACT will be different, the guide star separation keywords (PREDGSEP, ACTGSSEP) may differ, and an anomaly keyword will appear at the end of the header record warning of a guiding failure or degradation.

GSD_RA

The right ascension (J2000) of the dominant guide star in degrees. The dominant guide star controls the pitch and yaw of the spacecraft. When guiding with one FGS, this parameter is the RA of the corresponding (single) guide star. The accuracies expected from the HST Guide Star Catalog vary from center to edge of a plate and in the two hemispheres.*6 Positional errors are of order 0.5-1.1 arcsec in the northern hemisphere and 1.0-1.6 arcsec in the southern hemisphere. Relative errors are magnitude and plate location dependent. The relative guide star error is on average 0.33" for northern hemisphere plates and 0.43" for southern hemisphere plates. This field is blank when the predicted guiding (GUIDECMD) is GYRO.

GSD_DEC

The declination (J2000) of the dominant guide star in degrees. The dominant guide star controls the pitch and yaw of the spacecraft. The accuracies expected from the HST Guide Star Catalog vary from center to edge of a plate and in the two hemispheres.*7 Positional errors are of order 0.5-1.1 arcsec in the northern hemisphere and 1.0-1.6 arcsec in the southern hemisphere. Relative errors are magnitude and plate location dependent. The relative guide star error is on average 0.33" for northern hemisphere plates and 0.43" for southern hemisphere plates. This field is blank when the predicted guiding (GUIDECMD) is GYRO.

GSD_MAG

The V magnitude of the dominant guide star. Errors on Guide Star Catalog magnitudes are a few tenths of a magnitude.

GSR_ID

Identification of the guide star used for roll control when guiding with two FGSs.

GSR_RA

The right ascension (J2000) of the roll guide star in degrees. The sub-dominant or roll guide star controls the roll motion around the dominant guide star. When guiding with only one FGS, this parameter is defaulted (to zero) and the roll is controlled by the gyroscopes. When guiding solely on GYROS this parameter is blank. The positional accuracies expected from the HST Guide Star Catalog are described under keyword GSD_RA.

GSR_DEC

The declination (J2000) of the roll guide star in degrees. The sub-dominant or roll guide star controls the roll motion around the dominant guide star. When guiding with only one FGS, this parameter is defaulted (to zero) and the roll is controlled by the gyroscopes. When guiding solely on GYROS, this keyword is blank. The positional accuracies expected from the HST Guide Star Catalog are described under keywords GSD_DEC.

GSR_MAG

The V magnitude of the roll guide star. Errors on the Guide Star Catalog magnitudes are a few tenths of a magnitude.

GSACQ

The UT time of the actual completion of the guide star acquisition or re-acquisition.

PREDGSEP

The predicted guide star separation in arcsec. The value is calculated using the FGS encoder positions specified in the Mission Schedule. A comparison of the predicted and actual separations facilitates verification (in most cases) that the correct guide stars were acquired or is an immediate indication of a problem with the acquisition.

Note: If the guide star acquisition failed, PREDGSEP will still be populated with the predicted separation, however the actual separation (ACTGSSEP) will be blank. If GYROS are used (GUIDECMD=GYRO) or single guide star guiding (GUIDEACT=FINE LOCK/GYRO, COARSE/GYRO), PREDGSEP will be blank.

ACTGSSEP

The measured guide star separation in arcsec, as calculated from the telemetered FGS encoder positions. The star selector encoder positions are extracted from the telemetry at the time when the guide star acquisition is complete and the spacecraft signals to the science instruments that it is ready to observe (toggling of the TAKEDATA flag). The differential velocity aberration correction is applied in the calculation. The errors associated with ACTGSSEP are due to FGS/FGS alignment errors (Vary with time and calibration, but have been as high as 0.5 arcsec. 1996 calibrations put this value at <0.1) and errors in the distortion calibration (milliarcsec level).

Note: If the commanded guiding (GUIDECMD) is set to a mode with a single guide star only (FINE LOCK/GYRO, COARSE TRACK/GYRO, GYRO), this field will be blank. If a two-FGS guide star acquisition failed into GYRO mode, this field will also be blank and an anomaly keyword will appear at the end of the header record.

Note on the actual minus predicted guidestar separation: The PREDGSEP and ACTGSSEP keyword values will differ. The difference is predominantly due to the positional errors in the Guide Star Catalog which will typically result in differences between these keyword values of 0.1-0.3 arcsec. FGS to FGS calibrations affect this as well. Depending on when the last FGS calibrations were done, there can be another 0.1 to 0.2 arcsec total contribution. For reference, the nominal operational FGS search radii are between 15 and 30 arcsec. If either guidestar is not found within this radius the acquisition will fail. Even if both FGSs acquire guidestars, the predicted minus measured separation must pass the coarse mode angle check which is set at 6 arcsec. If the difference is greater than 6 arcsec the acquisition should fail. It is possible given an unfortunate confluence of older FGS calibrations and worst case GSC error that the proper guidestars could have been locked up while exhibiting a predicted-measured value of nearly 1 arcsecond. This is rare however and in recent times this value is usually less than 0.2 arcsec. For differences between about 1 arcsec and 6 arcsec, the observer should suspect a "spoiler" field star, or a binary guidestar. This will grossly affect the target positioning.

One will also sometimes encounter the problem of an FGS "false lock". This peculiarity of the FGS interferometric tracking results in accidental guiding on a local minimum in the S-curve, and can be identified by an offset pointing and abnormally high jitter. When multiple observations are made with the same guidestars, the ACTGSSEP keyword will be very constant. An observation affected by false lock will exhibit a 0.3 to 1 arcsecond offset with respect to the normal observations' ACTSGSSEP keyword, as well as an unusually high GSSEPRMS

GSSEPRMS

The standard deviation of the separation of the guide stars in milliarcsec during an exposure. The separation is box-averaged with a window size of 121 data points, or one minute of time in AN telemetry format, and 6 seconds in PN and FN formats. GSSEPRMS is an indicator of any smooth, continuous long-term drift between the guide stars (rather than the rms of high frequency jitter of the guide stars--which is given in the tables). The slow drift is due to the orbital focus change and thermal effects in the FGS. The guide star separation can vary up to 20 milliarcseconds (peak to peak for a full orbit). In a future enhancement of the observation log, GSSEPRMS will be used in an algorithm which models the breathing or orbital thermal drift of the telescope and guiding system.

Note: When comparing this value from a set of observations with the same guidestars, "false locks" discussed above can be identified. Look for unusually high GSSEPRMS keywords compared to the others in the set. As discussed above, the ACTGSSEP will also be an outlier if false lock is the culprit. The value of GSSEPRMS can also vary when there are large slews during the observation or during moving target exposures due to residual optical distortion errors in the FGSs. This keyword can be blank for the following reasons: (1) For short observations, GSSEPRMS is not meaningful in this context and the keyword is blank, (2) If the commanded guiding is set to a mode with a single guide star only (FINE LOCK/GYRO, COARSE TRACK/GYRO, GYRO), this field will be blank, (3) If a two-FGS guide star acquisition failed, this field will also be blank and an anomaly keyword will appear at the end of the header record.

NLOSSES

The total number of loss of lock events that occurred within the observation window. Loss of lock occurs when the FGS is unable to track on a guide star because of excessive jitter or telescope disturbances. From loss to recovery, each event usually lasts 3 to 4 minutes and therefore will affect the quality of the science data. During a loss of lock, the control of the vehicle pointing is transferred from the FGSs to the gyros. A summary of the loss of lock status as a function of time is available in the tables under the heading TAKEDATA. When the flag is ON, the guiding performance is nominal. The TAKEDATA flag is the input for the calculation of the header keywords NLOSSES and LOCKLOSS.

LOCKLOSS

The accumulated duration in seconds of all losses of lock during an observation. Each event typically lasts 3 to 4 minutes and will affect the quality of the science data. During a loss of lock, the control of the vehicle pointing is transferred from the FGSs to the gyroscopes. A summary of the loss of lock status as a function of time is available in the tables under the heading TAKEDATA. When the flag is ON, the guiding performance is nominal. The TAKEDATA flag is the input for the calculation of the header keywords NLOSSES and LOCKLOSS. The TAKEDATA flag will toggle to OFF when HST is slewing under FGS control (e.g., during spatial scans, moving target observations, peak-ups).

The HRS and FOC are interrupted during their data-taking when a loss of lock event occurs. If enough time remains in the exposure, the data accumulation will resume when lock is re-established. The WFPC and WFPC2 close their shutters during loss of lock provided that the exposure time exceeds 3 minutes (otherwise the instrument continues to expose even though the star may be drifting out of the field). The FOS, on the other hand, continues to observe. For all SIs but the FOS, the actual exposure time is the commanded time minus the LOCKLOSS. Note that the LOCKLOSS is accumulated over the duration of the observation window.

Loss of lock is the result of the limited dynamic range of the FGS interferometry mode and is normally detected on-board when the total flux received is not as expected or the star is suddenly found too far from the nominal position (an adjustable parameter set currently at 100 milliarcsec). A typical single loss of lock event lasts about 3 minutes, which is the time it takes to re-acquire the guide stars and re-initialize guiding. These events are managed by the onboard computers through the use of flags.

NRECENT

The total number of unique recentering events during an observation. 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 the excessive jittering and potential for a loss of lock. When the disturbance subsides, control is transferred back to the FGSs. In contrast to LOCKLOSS, where each individual event requires at least 3 minutes for recovery, typical recentering events last for a few seconds. The science instruments will therefore continue to acquire data during this short period. The capability was installed in 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.

RECENTR

The total time in seconds during the observation when the recentering status was positive. A RECENTER flag is available in the CMJ, CMI, and JIT tables. The toggle from 0 to 1 occurs when a recentering event is in progress. The event is triggered when the gyros detect 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. In contrast to LOCKLOSS, where each individual event requires at least 3 minutes for recovery, typical recentering events last for a few seconds.

V2_RMS

The standard deviation of the jitter of the dominant (or single) guide star along V2. Units are milliarcsec. The rms jitter during times of quiescence is usually 2-3 milliarcsec and entries into and exits from the Earth's shadow will increase the rms jitter to 5-8 milliarcsec on average. The Day/Night flag in the tables and the header keywords SHADOEXT and SHADOENT indicate the times of day or night passages. A threshold filtering algorithm is applied to the rms 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 default to "INDEF." Currently, the rms is not calculated for fewer than 60 points.

Caution: The jitter is referenced to the first data point. Therefore, slews (small angle maneuvers during target acquisitions, peakups, spatial scans) and moving target tracking will introduce an apparent drift. This motion will also contaminate the calculation of the jitter statistics presented in the header and artificially inflate the values (hundreds of milliarcsec or more). To determine whether a slew was in progress, the SLEW flag in the tables may be consulted (a 1 will appear in the SLEW flag column). The filtering algorithm is not applied if slews occurred during the observation.

Note: This field is blank when in GYRO mode. Also, when TLMFORM=FN, no compensation for differential velocity aberration is performed and therefore a small drift will be included in the calculation of the jitter. This problem will be corrected in a future enhancement.

V2_P2P

The maximum amplitude (peak-to-peak) of the jitter in the V2 direction The motion of the dominant (or single) guide star is used for the calculation. Units are milliarcsec. This field is blank when in GYRO mode.

See keyword V2_RMS for a discussion of cautions in interpretation when the vehicle is slewing.

V3_RMS

The standard deviation of the jitter of the dominant or single guide star in the V3 direction. Units are milliarcsec. This field is blank when in GYRO mode. The rms jitter during times of quiescence is usually 2-3 milliarcsec and entries into and exits from the Earth's shadow will increase the rms jitter to 5-8 milliarcsec on average. The Day/Night flag in the tables and the header keywords SHADOEXT and SHADOENT indicate the times of day or night passages.

See keyword V2_RMS for a discussion of cautions in interpretation when the vehicle is slewing.

V3_P2P

The maximum amplitude (peak-to-peak) of the jitter in the V3 direction The motion of the dominant (or single) guide star is used for the calculation. Units are milliarcsec. This field is blank when in GYRO mode.

See keyword V2_RMS for a discussion of cautions in interpretation when the vehicle is slewing.

RA_AVG

The average right ascension (J2000) at the aperture reference position in degrees over the time of the observation. The RA, Dec, and roll of the reference aperture at each point in the file are also tabulated in the observation log tables. (See next chapter). The _avg keywords provide absolute pointing information, the average offset between the commanded RA & Dec and the actual resulting pointing, and a means to assess suspicious vehicle pointing.

See below for important details on the calculation, accuracy, and cautions in interpretation of the attitude.

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.

DEC_AVG

The average declination (J2000) at the aperture reference position in degrees. The RA, Dec, and roll are also tabulated in the CMI,CMJ, and JIT tables. These keywords indicate the direction of the jitter with respect to the celestial sphere, absolute pointing, and a means to assess suspicious vehicle pointing. 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.

ROLL_AVG

The average roll angle in degrees calculated at the position of the aperture reference fiducial. 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. It can 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.

The CMJ, CMI and JIT tables contain the roll angle tabulated as a function of time.

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.


Pointing Calculations and Sources of Error

The methods used to calculate the pointing depend on the guiding mode in use, and are described below. We emphasize that the pointing is referenced to the nominal aperture position, and the name and V2V3 coordinates of that aperture are provided in the header.

The 3 methods used in the calculation of the RA, Dec, and roll depend on the 3 current guiding scenarios: Two FGS Fine Lock Guiding, Gyro Control, and Single FGS plus gyro.

Two FGS Fine Lock Guiding: This mode will produce the most accurate pointing. Absolute accuracy of the jitter files' calculation of the aperture RA and Dec in this mode can vary over time as calibrations improve and geometries change, but can be expected 0.4 - 2 arcsec. Relative accuracy throughout an exposure, and even throughout a visit with the same guidestars will be ~1-50 milliarcsec. The inputs are the FGS star selector encoder positions, four Photomultiplier Tube Counts per FGS, the RA and Dec of the two guide stars (from the Mission Schedule and originally from the HST Guide Star Catalog), the spacecraft velocity, and the V2V3 coordinates of the aperture in use. The software first calculates the V2V3 coordinates of the guide stars using the current FGS alignment and distortion calibrations. These data are then fit to the RA and Dec of the guide stars. From the fit, the telescope attitude (V1 pointing) and the RA, Dec, and roll of the aperture reference position are derived. For related information on the guidestar separation errors, see above

Gyro Control: Pitch, yaw, and roll control is performed by the Rate Gyro Assembly. The pointing is calculated using the data from the gyros. These vectors, (quaternions), are corrected for the differential velocity aberration between the V1-axis (for which the vectors are reported in the telemetry) and the aperture reference position. Because of the gyro drift, this mode produces the least accurate absolute and relative pointing. Absolute accuracy of the aperture RA and Dec will be 2-50 arcsec (15 arcsec 1 sigma) and positional drift will be of order 1-5 milliarcsec per second of observation time.

Single FGS Pitch and Yaw plus Gyro Roll Control: In this mode, a single FGS is used to control the pitch/yaw of the spacecraft and the roll control is handled by the gyros. The absolute pointing is calculated from the Rate Gyro Assembly output. The jitter information from the guide star in the FGS is then factored in to the absolute pointing. Finally, the combined pointing profile is determined at the position of the aperture (i.e., absolute and differential velocity aberration corrections are applied in all telemetry formats except FN). This guiding mode will produce less accurate pointing than the two FGS guiding mode because of gyro drift, in this case, a drift in roll about the single guide star. Because of the drift, relative pointing during an observation will be affected, but absolute pointing can also be impacted due to possible roll biases at the time of acquisition. Expect absolute accuracies 0.5 - 5 arcseconds. The error due to the roll drift during the observation will be 1-5 milliarcsec/sec of roll about the dominant guidestar. This introduces a much smaller translation drift at the SI than the pure gyro mode. Note however that the roll drift continues to build during occultations as well as visibility periods. Example: During a single guidestar WFPC observation, if the roll drift about the guidestar 700 arcsec away from WFPC2 is 1.5 mas/sec, this results in a translational drift at WFPC2 of 0.005 mas/sec. In a 10 minutes exposure, that produces 3 mas. After 3 orbits the uncorrected drift totals 87 mas or 2 PC pixels.

Roll angle changes (roll drift) during an observation:

Roll Repeatability

Users may note small shifts in images from multiple visits at the same pointing, with the same guidestars, and SAME ORIENT specified. HST sets up a given roll for an observation with gyros, and acquires the dominant guidestar. For visits with the same guidestars, target, and roll specified, the Pointing Control System (PCS) will place the dominant guidestar at the same location within the FGS to high precision (1 or 2 milliarcseconds). The subdominant (roll control) guidestar is then acquired and tracked in fine lock. The source of the small roll non-repeatability is in this step. If the subdominant guidestar is not found in exactly the predicted location, the PCS will not adjust the roll to place the subdominant in the same location. This is due to the fact that the PCS, once calibrated via the Fixed Head Star Trackers (FHSTs), can normally execute a commanded roll better than if it relied on the FGS subdominant guidestar location, with its ~0.3 arcsecond catalog error.

The accuracy with which HST is rolled for a given visit depends then not on the subdominant guidestar but on the amount of gyro bias in the PCS. Typically, an FHST update is made some time prior to a full guidestar acquisition. In an FHST update, the PCS reconciles the accumulated gyro drift with FHST (fixed head star tracker) star fields, providing a "reference update" and zeroing out most of the PCS error. In most cases a successful FHST update is made within a reasonable time before an acquisition, and under these circumstances the roll accuracy will be around 0.003 degrees. This angle of roll around the dominant guidestar works out to a positional shift of about 73 mas at the subdominant guidestar (assuming 1400 arcsecond separation), which, when this number is compared to the error in the guidestar catalog positions, illustrates why the guidestars are not used to "correct" the roll. At the WFPC2, this shift is 38 mas and just under a WFPC2 PC pixel. Therefore, multiple visits at the same specified roll, target, and with the same guidestars will under nominal circumstances show repeatability at this level. It is not uncommon for scheduling constraints to affect the time between FHST updates and GS acquisitions. Such acquisitions that take place with on older FHST update, or an inaccurate one, can suffer from larger roll deviations, 0.01 upward can occasionally be seen. Note: The jitter files, which determine roll based on guidestar locations in the FGSs can be used in the case of visits with the same guidestars and same roll, to determine quite accurately the amount of actual roll change incurred between visits, though the commanded or specified roll would be the same for each visit. Sources of Error:

    Here we discuss two types of errors:

  1. Errors on positioning a target at the intended reference. (commanded positions vs. independent measurement from an image)
  2. Errors between the Observation Logs' calculated target position and a measured value (Obs Log vs. independent measurement from image)

1.) Causes for the target missing the aperture reference position include the following:

2.) Causes for the Obs Log determination to differ from an image measurement:

Calculated (jitter file) Positions versus Commanded (science headers)

It is often useful to use pointing information to correlate images taken as part of a proposal for various reasons (dithering, image subtractions). Usually these observations are made with the same guidestars and are often but not always within a visit. When wishing to register or align such a set of observations without convenient point sources allowing an empirical correlation, commanded or calculated pointing information is the alternative. Here the interest is in shifts relative to some initial pointing, and one can use the commanded values (CRVALs, RA / Dec_APER, ORIENTAT) in the science header, or the jitter file's calculated RA / Dec / Roll_AVG to describe these relative motions.

What are the practical differences between the commanded and calculated shifts when used in this application? Assuming the same guidestars are used, and the shifts are correspondingly small compared to the FGS Field of View, within a single visit both the commanded and calculated positions describe well the image shifts relative to an initial point. RMS agreement to better than 10 mas is common. From visit to visit over a number of days, the agreement rises to ~20 mas for the commanded values and ~15 mas for the calculated values. The jitter file's calculated value knows the amount of the roll error introduced each full-acquisition from visit to visit, since it calculates this based on measurement of the same guidestars each time. This gives better agreement with images taken over multiple visits. Because the jitter file's evaluation of the roll is affected by guidestar catalog error, the commanded roll from the science header is usually closer to what was achieved than the jitter file's calculation (see Roll Repeatability), and the jitter file will exhibit some absolute offset of the calculated target location. However, when using the jitter files to describe relative motion given the same guidestars, this offset it removed, leaving only the roll changes due to errors in achieving the commanded roll, which the jitter file can measure sensitively.

Finally, independently of how accurately the jitter file calculation is performed, even in the case of describing relative shifts between observations with the same guidestars, much of the ~15 mas rms level of disagreement is affected by thermal motions in the focal plane: motions within the SI, within or between FGSs, and from the OTA plate scale, and probably represents close to a limiting accuracy.


PROPOSID
TARGNAME
STARTIME and ENDTIME
SOGSID
INSTRUME
PRIMARY
OPERATE
TLMFORM
CONTENT:
APERTURE
APER_V2
APER_V3
ALTITUDE
LOS_SUN
LOS_MOON
SHADOENT
SHADOEXT
LOS_SCV
LOS_LIMB
ZODMOD
EARTHMOD
MOONMOD
GUIDECMD
GUIDEACT
GSD_ID
GSD_RA
GSD_DEC
GSD_MAG
GSR_ID
GSR_RA
GSR_DEC
GSR_MAG
GSACQ
PREDGSEP
ACTGSSEP
GSSEPRMS
NLOSSES
LOCKLOSS
NRECENT
RECENTR
V2_RMS
V2_P2P
V3_RMS
V3_P2P
RA_AVG
DEC_AVG
ROLL_AVG
Pointing Calculations and Sources of Error