PROPOSID (jih) / PROPOSAL (cmh)

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.

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.

• FOC operate--The FOC is most affected by the amount of time since high voltage turn-on. In order to achieve an optical and thermal stability, the FOC high voltage must remain on for 75 minutes prior to the start of an observation, however the longer the stabilization period, the better. In this mode, all electronics units and high voltages of one chain are turned on. The FOC can remain in this state for several hours prior to activation. If 12 hours of idle time occurs, the FOC must be returned to a hold (or high voltage-off state) for the sake of the operational lifetime of the instrument.

• WFPC2 operate--The WFPC2 low voltage is always on and so the relevance of using this field to study thermal stabilization is limited. Only after a decontamination process where heaters are cycled and after an instrument safing event does this parameter become an important indicator of the stability of the instrument. After the completion of a decontamination or safing event, the WFPC2 will require several hours before thermal and optical stabilization is achieved.

• FOS operate--When in operate mode, the FOS will have the logic supply, quiet supply, and high voltage supplies turned on for the side of the instrument that is active (Red or Blue). Only one side of the instrument may be active at a time. The high voltage will be brought up to its nominal value for that side at least 30 minutes prior to the beginning of an observation.

• HRS operate--In the HRS operate mode, the low voltage, high voltage and Detector Electronics Box (DEB) for one of the two detector sides are on. The low voltage is always on for the Side 2 detector. Because of this, an extended warm-up time is not needed for either side. The DEB generates a great deal of heat and therefore, the HRS is really never thermally stable. As a result, a periodic, frequent, and automatic re-calibration of the detector coordinate location of the apertures is performed (DFCAL).

• HSP operate--The HSP operate mode refers to a high voltage state but is slightly different for the 4 image dissector tubes and photomultiplier tube (PMT). For the POL and VIS detectors, this state refers to the lowest high voltage state prior to the ramp-up to the highest or operating high voltage state. A 30-minute delay is built in prior to the ramp-up. For the UV detectors and the PMT, the ramp-up has already occurred and a 30-minute delay is built in prior to the start of an observation.

• WFPC operate--Not relevant in the context of stability after a decontamination event. This parameter is for engineering purposes only.

TLMFORM

APERTURE

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.

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.

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.

• FINE LOCK = two FGSs in fine lock,

• COARSE TRACK = two FGSs in coarse track,

• FINE LOCK/GYRO = one FGS in fine lock (pitch/yaw) and gyro (roll),

• COARSE/GYRO = one FGS in coarse track (pitch/yaw) and
  gyro (roll),

• GYRO = GYRO control only.

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.

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.

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.

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.

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.

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.

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

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

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

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.

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.

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 in this mode can vary over time as calibrations and geometries change, but can be expected usually within the range 0.2 - 1 arcsec. The nominal operational FGS search radii have been between 15 and 30 arcsec. If either guidestar (correct expected brightness) is not found within this radius the acquisition will fail to achieve 2GSFL. If both FGSs acquire guidestars, the predicted minus measured separation between them must pass the coarse mode angle check which is set at 6 arcsec. If the difference is greater than 6 arcsec the acquisition will fail. It is possible given an unfortunate confluence of worst case FGS calibrations and worst case GSC error that the proper guidestars could be locked up while exhibiting a predicted-measured value of nearly 2 arcseconds. This is rare however and the value is usually a few tenths of an arcsec. It is possible for spoiler guidestar(s) or guidestars with high proper motions however to pass the brightness check and separation check while still producing an absolute pointing error of up to 6 arcseconds.

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)

• Relative guide star position error (~ 0.3 arcsec) and target coordinate error such as proper motion or target coordinates specified from a catalog containing a rotation or zero point offset with respect to the Guide Star Catalog (single and two FGS guiding modes);
• Fine lock of an unintended or binary dominant guide star (single and two FGS guiding modes). Only the dominant guidestar is important in the case of target positioning, since the roll about that guidestar (appropriate for the target positioning) is made by the pointing control system without FGS input, and thus is not affected by the location of the subdominant guidestar in the FGS;
• "False lock" of the correct dominant guide star. (single and two FGS modes). The FGS's interferometric tracking will occasionally jump into a local minimum in the S-curve, resulting in a pointing offset (0.3 to >1 arcsec) and an abnormally high jitter.
• Focal Plane calibration errors including:
• FGS/FGS alignment (currently time dependent; has been as high as 0.3 arcsec, but after a recent calibration should be < 0.050);
• FGS/SI aperture calibration errors which can depend on the target acquisition method used and the relative errors between target acquisition aperture and science aperture, as well as other internal instrument calibrations. Values range from 0.1 arcsec to 0.5 arcsec;
• PCS calibration errors between the Fixed Head Star Trackers, Rate Gyro Assembly and FGSs
• Gyro drift of 1-5 milliarcsec/sec (single FGS and gyro modes);
• Thermal expansion and contraction, known as breathing (up to 15 milliarcsec).

• If pre-planned or real-time offsets were commanded (all guiding modes). The jitter file reports Ra, Dec and Roll always with respect to an aperture reference whose name and V2V3 position is given in the jitter file header. If an observation consists of a POS TARG offset from, say the PC2 aperture reference, the Obs Log will report the RA,Dec of the PC2 reference pixel position and not the POS TARG pixel position. In other words any motion on the sky is tracked by the Obs Log, but always with respect to the aperture reference position. This is not a source of numerical error but a point of interpretation the user of the Obs Log should be aware of;

• Positional calibration errors at the SI. The Observation Log maps a reference V2V3 position to the sky based on the attitude determination. The accuracy to which that relates to a given detector pixel depends on the calibration of that SI: (position in focal plane, distortions, etc);

• The relative guide star positional error with GSC1 over scales comparable to HST field of view, 0.33" for northern hemisphere plates and 0.43" for southern hemisphere plates (two FGS guiding mode). Since the guidestars' GSC celestial coordinates are fit to the FGS measured positions to find the vehicle attitude, the determination is affected by this error;

• FGS to FGS alignment error (currently time dependent; has been as high as 0.3 arcsec, but after recent calibrations should be < 0.050). This, like the guidestar celestial coordinates, affects the pointing calculation, in this case by introducing error in the FGS XY to V2V3 transformation;

• Fine lock of an unintended or binary guide star (two and single FGS guiding modes). This error is closely related to the first in its affect on the attitude determination;

• "False Lock" of the dominant or subdominant guidestar. (Single and two FGS 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/sec 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 RMS (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 up to 20 arcsec and the differential aberration is up to 50 milliarcsec (all modes);

• Negligible differences in the method of calculation between the observation log software and the operations ground and onboard flight software systems (all modes).

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

It is sometimes 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.