10.2 FGS Astrometry Files

File Description

Each individual FGS astrometry observation produces one GEIS header and data file pair for each of the three FGSs. A typical FGS POSITION mode observing sequence or visit consists of an HST orbit filled with observations of several stars, usually the science target and a few reference stars. Therefore, each visit will yield a number of GEIS file sets for three FGSs, each set corresponding to an observation of an individual star. These file sets contain all the FGS tracking and photometry data from the time interval when the specific observation was made.

The rootnames of FGS data sets adhere to the IPPPSSOOT convention described in Chapter 2. The first letter is always an F, identifying the file as FGS data, and the last letter is an M, indicating that the data were merged real time and tape recorded. For example, an FGS observation might be named f42n0203m. If 5 individual FGS observations were made in this hypothetical visit, their rootnames would be f42n0201m, f42n0202m, f42n0203m, f42n0204m, and f42n0205m. The 02 in each case is the visit number, which is important to note because all observations from a given visit must pass through the data reduction pipeline together (see Chapter 3). The two digits following the visit number give the observation numbers, in the sequence they were obtained.

For each observation, there should be six files, one header-data pair for each FGS. Data from the first observation will appear in the following files

Header	Data	Sensor
f42n0201m.a1h	f42n0201m.a1d	FGS 1
f42n0201m.a2h	f42n0201m.a2d	FGS 2
f42n0201m.a3h	f42n0201m.a3d	FGS 3

In this example, if FGS 3 is the astrometer and measures the position of a star in exposure id = F42N0201M, then the f42n0201m.a3h and f42n0201m.a3d files contain the astrometry data. The guide star data gathered during this astrometry observation are recorded into files f42n0201m.a1h, f42n0201m.a1d and f42n0201m.a2h, f42n0201m.a2d for FGS 1 and FGS 2 respectively.

Group Structure and Group Contents

100 * 40 = 4000 samples

Some groups, such as the flags/status bits group, are readout less frequently (once every 150 msec). In this situation the valid data are packed from the beginning and the remaining 5/6 of the group is padded with fill data.

For FGS astrometry science data processing, the groups of interest in each GEIS data file are groups 1 through 6 and group 17. Table 10.1 lists the contents of these groups. :

Groups in FGS GEIS Files

Group	Contents	Description	Sample Period
1	PMTXA	Photon counts, channel A, x-axis	25 msec
2	PMTXB	Photon counts, channel B, x-axis	25 msec
3	PMTYA	Photon counts, channel A, y-axis	25 msec
4	PMTYB	Photon counts, channel B, y-axis	25 msec
5	SSENCA	Star selector A encoder position	25 msec
6	SSENCB	Star selector B encoder position	25 msec
17	FLAGS/STATUS BITS	Indicates specific activity of FGS	150 msec

Groups 1 through 4 contain the photometry data for the 5" x 5" patch of sky observed by the FGS. If the FGS is guiding and tracking its guide star, then it registers the photon counts from the guide star. If the FGS is operating as an astrometer, there will be an astrometric target in its instantaneous field of view (IFOV) only after the IFOV has slewed to the target's position and the FGS has successfully acquired the star. While the slew is underway, the FGS records the sky background and serendipitous field stars. These background data are used in the data reduction pipeline.

Groups 5 and 6 record the position (to 0.6 mas) of the IFOV in the FGS's pickle and therefore in the HST focal plane. If the FGS is guiding, the measured position of the guide star is fed into the Pointing Control System so that the spacecraft's pointing can be maintained or corrected. These guide star data are used in the pipeline processing discussed in Chapter 11.

The flags/status bits group records the value of the 12 flags and indicators, each of which is a single bit which is either set (=1) or not set (=0). If the FGS is guiding and is tracking its guide star in FineLock, then the FineLock, DataValid, and Star Presence flags will be set (=1) and all others not set (=0). The astrometer FGS will display a sequence of flags/status bits settings which reflects the current activity of the FGS. If the FGS is to be operated in POSITION mode, then the following sequence of flag/status bits would be displayed for a successful acquisition:

Status/Flag Bits

SSM	SR	CT	FL	DV	SP	Action
ON						Slewing the IFOV to star's location
	ON					Begin spiral search for star
	ON				ON	Spiral search located star
		ON			ON	Star detected, begin CoarseTracking
		ON		ON	ON	CoarseTracking successful
			ON		ON	Attempt FineLock acquisition
			ON	ON	ON	Tracking star in FineLock

The flags and status bits have the following meanings when set to 1:

• SSM: FGS under control of DF 224 computer.
• SR: FGS in autonomous (FGE) control, performing a spiral search .
• CT: FGS in CoarseTrack mode.
• FL: FGS in FineLock mode, if DV=0, then in WalkDown phase, if DV=1, then tracking in FineLock.
• DV: CoarseTrack or FineLock was successful.
• SP: Star presence, photon counts summed from all four PMTs fell within the commanded range (LOCOUNT < PMTSUM < HICOUNT). When determining the position of an astrometric object observed in FineLock, the data of interest to the astrometry pipeline are:

• The slew to the star's expected location (provides background data).
• The WalkDown to FineLock (provides critical PMT data to better locate interferometric null) and to support FGS photometry.
• The FineLock tracking of the object (provides the FGS's measured location of star in focal plane). More details on interpreting and making use of the FGS data are provided in the sections "Plate Overlays" on page 13-3 and "Resolving Structure with TRANSFER Mode" on page 13-5.

NAXIS   :    number of dimensions in the data file (=1 of FGS)

NAXIS1  :    number of samples (or pixels) in each data group

GCOUNT  :    number of groups in the data file (=17 for FGS)

BITPIX  :    bits/pixel (=32 for FGS)

DATATYPE:    datatype (= 32 bit integer for FGS)

FGSID   :    identifies astrometer FGS

FGSNO   :    identifies FGS associated with this header file

PASTMODE:    observing mode, POSITION or TRANSFER

AUTOS   :    actual start time of observation (Universal time)

PRAV1   :    predicted right ascension of HST's V1 axis

PDECV1  :    predicted declination of HST's V1 axis

PROLLV3 :    predicted roll orientation of HST 

PRATGT  :    predicted right ascension of target

PDECTGT :    predicted declination of target

PX_POS  :    HST state vector (position and velocity) at

PY_POS  :    beginning of observation (at PUTOS)

PZ_POS  :

PX_VEL  :

PY_VEL  :

PZ_VEL  :

CAST_FLT:       filter used for the observation.

cl> imheader f42n0201m.a1h long+ | page