2.1 Engineering Telemetry Data

The three FGSs are a vital part of HST’s pointing control system (PCS). Whether operating or not, either as a science instrument or guiding HST, data from all three FGSs are continuously downlinked on the engineering stream (rather than the science stream). The contents and rates of the FGS data are determined solely by the engineering telemetry format.

For each FGS, the fundamental data of interest for both astrometry and guiding are the photon counts from the four photomultiplier (PMT) tubes, the instantaneous values of the two Star Selector servo angles, and the 13 status and flags bits. The first two items are sampled every 25 msec (40 Hz), while the status flag bits are reported every 150 msec (6.67 Hz) for both telemetry format FN (default for astrometry observations) and for telemetry format HN (default for all other times). Additional FGS mnemonics present in the telemetry either can be reconstructed from those listed above or are of interest only to the ­system engineering staff.

When an FGS is operated as an astrometer, post observation processing performed at STScI builds the astrometry data products by extracting the PMT data, Star Selector position angles, and FGS status and flags bits for all 3 FGSs from the engineering telemetry. This processor accesses the Mission Support Schedule (MSS) for additional information needed to populate keywords in header files that facilitate the proper identification and interpretation of the associated data files. These products are similar to the GEIS format files produced for WFPC2 and the legacy science instruments (see Chapter 2 for more on GEIS file format).

Before November 1997, generation of the astrometry data products was the responsibility of Astrometry and Engineering Data Processing system (AEDP) based at the Goddard Space Flight Center. However, in early 1998, the STScI Astrometry Pipeline, part of STScI’s Observatory Management System (OMS) became operational, and the use of AEDP was discontinued for FGS astrometry data. As of the writing of this document, OMS has been incorporated into part of the OPUS telemetry processing system at STScI.

Although containing the same information, the AEDP science products and OMS science products differ in format and number. The AEDP generated files with 17 groups of data for each FGS, with separate GEIS format header and data products for each FGS for each astrometry observation. OMS products consist of a single FITS format product per FGS per astrometry observation with only 7 groups of data (the remaining 10 groups were not useful for astrometric observations). Each OMS FITS product contains both the header and data information previously contained in the GEIS format products. Specifically, each astrometry FITS file contains a primary header, a primary binary data array containing telemetry data specific to a particular FGS, an ASCII table extension header, and an ASCII table containing summary information about the primary telemetry data array.

Each FGS astrometry observation generates header and data files (.a*h/.a*d) for all three FGSs whenever any one FGS is used as an astrometer. The contents of the data files for the guiding FGSs record the photometry and positions of the two guide stars over the same interval of time as that covered in the products for the astrometry FGS. The contents of the header files contain both information common to all FGSs and information specific to the FGS associated with that file, such as the values assigned to uplinked control parameters.

2.1.1 File Description

When an FGS is operated as a science instrument, the four PMT, two Star Selector Servo position angles, and thirteen status flags are extracted from the engineering telemetry. The data are recorded in GEIS files and are grouped by type; groups 1 through 4 contain PMT data, groups 5 and 6 contain the position angle of Star Selectors A and B respectively, and group 7 contains the thirteen status flags. There will be three GEIS data files, one per FGS, each containing 7 groups of data.

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 in the Introduction to HST Data Analysis. 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 from real time and tape recorded telemetry (using which ever source was of superior quality). Here, ’PPP’ and ’SS’ correspond to the HST program ID and visit ID respectively, while ’OO’ corresponds to the exposure number (as specified in the HST Phase 2 proposal for the program). For example, a set of FGS observations during a hypothetical visit would have the rootnames f42n0201m, f42n0202m, f42n0203m, f42n0204m, and f42n0205m.

For each observation, there should be seven total files: one header-data pair for each FGS, and one *.dmh file containing scheduling and support data relevant to the observation. Note there is no data file associated with a *.dmh file. The complete list of files associated with an observation are given in Table 2.1.

Table 2.1: GEIS Files in an FGS Dataset

File NameContents
Header file, FGS1, exposure 1
Data file, FGS1
Header file, FGS 2, exposure 1
Data file, FGS 2, exposure 1
Header file, FGS 3, exposure 1
Data file, FGS 3, exposure 1


Support schedule for observation 01

In the example given above, if FGS1r is the astrometer and measures the position of a star in a Position Mode exposure of id f42n0201m. The files f42n0201m.a1h and f42n0201m.a1d contain the astrometry data. The guide star data gathered during this astrometry observation are recorded into files f42n0201m.a2h, f42n0201m.a2d and f42n0201m.a3h, f42n0201m.a3d for FGS2 and FGS3 respectively.

2.1.2 Group Structure and Group Contents

Each data file contains the same number of data groups (7) and each group is of the same size, having the same number of samples. The duration of the observation and the rate of the most frequently read out mnemonic determine the sizes of the groups for a particular observation. For example, if an observation spans 100 seconds, then each data group will have:

100 sec * 40 Hz = 4000 samples,

governed by the 40 Hz sampling of the photo-multiplier tubes (PMTs) and the Star Selector A and B positions. Unlike the other six groups, the flags/status bits group is sampled every 150 msec (6.67 Hz). In this situation, valid data are packed into the beginning 1/6 of the group, with the remaining 5/6 of the group padded with fill data. Table 2.2 lists the details of the seven FGS groups available.

Table 2.2: Groups in FGS GEIS Files


Group Mnemonic


Sample Period

Sample Frequency



Photon counts, channel A, x-axis

25 msec

40 Hz



Photon counts, channel B, x-axis

25 msec

40 Hz



Photon counts, channel A, y-axis

25 msec

40 Hz



Photon counts, channel B, y-axis

25 msec

40 Hz



Star selector A encoder position

25 msec

40 Hz



Star selector B encoder position

25 msec

40 Hz



Indicates specific FGS activity

150 msec

6.67 Hz

The first four groups 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.

Table 2.3: Status/Flag Bits














Slewing the IFOV to star’s location







Begin spiral search for star







Spiral search located star







Star detected, begin Coarse Tracking







Coarse Track Acquisition successful







Attempt FineLock acquisition







Tracking star in FineLock

The PCS status flag was added to the astrometry products in November 2000. It shows a value of 1 (ON) when HST is being guided under control of at least one FGS (plus gyros). A value of 0 (OFF) indicates that HST’s pointing is not maintained by guide stars.

The fifth and sixth groups record the position (to 0.6 mas) of the IFOV in the FGS's full Field of View (FOV), and therefore in the HST focal plane. If the FGS is guiding, the measured position of the guide star is accessed by 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 3.

The flags/status bits group records the value of the thirteen flags and indicators, each of which is a single bit which is either set to 1(ON) or set to 0 (OFF). If the FGS is guiding and is tracking its guide star in FineLock, then the FineLock, DataValid, Star Presence, and PCS flags will be set to 1 (ON) and all others set to 0 (OFF). The astrometer FGS will display a sequence of flags/status bits settings which reflects the current activity of the FGS (but its PCS flag will also be set). If the FGS is operating in Position Mode, then the sequence of flag/status bits shown in Table 2.3 occurs for a successful acquisition.

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

  • SSM: FGS under control of the SSM (486) 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).
  • PCS: The pointing control system uses data from the guiding FGSs to control the spacecraft’s pointing. This can be set to 0 when HST is being guided on gyros only, such as during a re-centering event or after loss of lock on guide stars.

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.
  • 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 Chapter 3 and Chapter 4.

2.1.3 FITS Header Keywords

The GEIS header files for FGS data contain keywords that will help to interpret the data files. The most important keywords contained in the header files are:

NAXIS       dimensions in the data file (always=1
NAXIS1      # of samples (or pixels) in each data group
GCOUNT      # of groups in the data file (7 after 11/9)
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 (UT)
PUTOS       predicted start time of the observation (UT)
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
TARGNAME    proposer’s target name 
TARGETID    target identification
PX_POS      HST state vector (position and velocity) at
PY_POS      beginning of observation (at PUTOS)
FGSREFV2    FGS aperture reference V2 position (asec)    
FGSREFV3    FGS aperture reference V3 position (asec)    
FGS_PAV3    FGS aperture position angle of V3-axis (deg)   
FGSOFFV2    V2 offset from the FGS aper ref position (asec)
FGSOFFV3    V3 offset from the FGS aper ref position (asec)
CAST_FLT    filter used for the observation.

The values assigned to these keywords are used in the calibration pipeline processing discussed in Chapter 3. To print an entire header to the screen, you can use the IRAF imheader task as in the following example:

cl> imheader f42n0201m.a1h long+ | page