STScI Logo

Observatory Support
Pointing



Chapter 1: Introduction

Observation Log Contents

Since 20 October 1994, a set of pointing and specialized engineering data, called observation logs, has been routinely generated. These files, often referred to as `jitter' files, are produced by the Observatory Monitoring System (OMS), an automated software system which interrogates the HST engineering telemetry and correlates the time-tagged engineering stream with the scheduled events as determined from the Mission Schedule, the seven-day command and event list which drives all HST activities. The observation log contains a plethora of pointing-related information, a necessity for the most accurate evaluation and reconstruction of pointing stability and environment during a given exposure. The intended recipients are General Observers as well as STScI scientists and engineers.

This documentation navigates through the observation logs and discusses their contents and how various parameters might be used to develop a better understanding of the environment during an exposure. However, the types and names of these observation log files, as well as the method for obtaining them, have changed in the past, so it is important that current documentation, such as this, be used for information. A more brief but useful discussion of obtaining and working with these files in IRAF, can also be found in the HST Data Handbook.

Observation log files are named according to the first 8 characters of the science root name, but with a `j' appended to the end, and each log reports the data in a time period encompassing that science exposure. They can be obtained from the HST DADS Archive, for any observation after 20 October 1994. Prior to this, no observation logs were routinely produced.

For observations between October 1994 and August 1995, three files were produced, rootnamej.cmh, rootnamej.cmi, and rootnamej.cmj. The CMI and CMJ files are in STSDAS binary table format and can be manipulated using the IRAF/STSDAS table tools (ttools). The CMI table contains data averaged over 3 second intervals, while the CMJ table contains points at very high time resolution (a few points a second). The CMH file is an ascii file that accompanies the tables and provides keyword information. At the end of August 1995, the files formats and names changed. The rootnamej.cmh file became rootnamej.jih. Although very similar, the JIH file can now contain more keywords (discussed later). The rootnamej.cmi low time resolution table is now renamed rootnamej.jit, and the CMJ file has been replaced with rootnamej.jid, a 2-dimensional IRAF image of jitter excursions during the observation. The JIH, JID, and JIT files remain the current.

Note however that when de-archiving any observation log files, they will arrive as FITS files *.fits. These files are in FITS format and for observations prior to February 1997, they can easily be broken out into the GEIS format header, image and table, by using the IRAF/STSDAS STRFITS task. However for post-Feb 97 observation log files, the _jif.fits and _jit.fits files cannot be converted to GEIS format, and must be used as FITS. Additionally, the FITS format supports multiple extensions, and the new observation log files can have multiple extensions corresponding to the exposures within a STIS or NICMOS association.

Below is a table summarizing the formats:


SuffixContents
October 94 through August 95
.cmh
ascii header file
.cmj
high time resolution IRAF table
.cmi
3 second average IRAF table
_cmh.fits
Archived FITS file of cmh
_cmj.fits
Archived FITS file of cmj
_cmi.fits
Archived FITS file of cmi
August 95 through February 97
.jih/.jid
2 Dimensional histogram image & header (GEIS)
.jit
3 second average IRAF table
_jif.fits
Archived FITS file bundling the jih/jid files.
_jit.fits
Archived FITS file of jit
February 97 onward
_jif.fits
2 dimensional histogram image (FITS)
_jit.fits
Archived 3 second average table (FITS)

In the examples of header values below, and in subsequent discussions, please be aware that the information provided in the observation log headers has evolved over time, and that a particular header on hand may not look exactly like the examples given.

A representative _jif.fits header: The FITS header contains keywords providing information regarding: the file structure, observation details, modeled background light, point control system, jitter summary, orbital geometry, and problem flags and warnings.The extension 0 header will contain parameters relating to the entire association or dataset, ending at the "ASSOCIATION KEYWORDS" block. The header values for extensions 1 and beyond will contain all the group 0 keywords as well as additional blocks of parameters relating to that particular exposure in the association.

Zeroeth extension header parameters:

EXTEND  =                    T /File may contain standard extensions
NEXTEND =                   15 /Number of standard extensions
GROUPS  =                    F /image is in group format
FILENAME= 'N45I04030_JIF.FITS' /name of file
FILETYPE= 'JIM               ' /type of data found in data file

TELESCOP= 'HST               ' /telescope used to acquire data
INSTRUME= 'FGS               ' /identifier for instrument used to acquire data
EQUINOX =               2000.0 /equinox of celestial coord. system

OMS_VER = '19.3F             ' /OMS Version Used to Process This Observation
PDB_VER = '52A               ' /Project Database version in use
TARGNAME= 'MS1512+36-        ' /proposer's target name
PROC_TYP= 'Normal            ' /type of pipeline processing
CONFIG  = 'NIC3              ' /proposed instrument configuration
PROPOSID=                07448 /PEP proposal identifier
PROGRMID= '45I               ' /program id (base 36)
PARALLEL= 'YES-PRIMARY       ' /indicates if observation taken in parallel
PRIMESI = 'NICMOS            ' /instrument designated as prime
OPERATE = '1997.227:23:40:50 ' /Predicted Time Instrument Entered Operate mode
TSTRTIME= '1997.227:23:57:52 ' /Predicted Association Window Start Time
TENDTIME= '1997.228:05:19:48'  /Predicted Association Window End Time

                                / BACKGROUND LIGHT

ZODMOD  =                 22.9 /Zodiacal light - model (V mag/arcsec2)
EARTHMOD=                 22.7 /Peak Earth stray light - model (V mag/arcsec2)
MOONMOD =                 35.6 /Moon stray light - model (V mag/arcsec2)
GALACTIC=                 -1.0 /Diffuse galactic light - model (V mag/arcsec2)

                                / POINTING CONTROL DATA

GUIDECMD= 'FINE LOCK         ' /Commanded Guiding mode
GUIDEACT= 'FINE LOCK         ' /Actual Guiding mode at end of GS acquisition
GSD_ID  = '0256900412        ' /Dominant Guide Star ID
GSD_RA  =            228.67397 /Dominant Guide Star RA (deg)
GSD_DEC =             36.89218 /Dominant Guide Star DEC (deg)
GSD_MAG =               13.514 /Dominant Guide Star Magnitude
GSR_ID  = '0256900508        ' /Roll Guide Star ID
GSR_RA  =            228.79103 /Roll Guide Star RA (deg)
GSR_DEC =             36.50085 /Roll Guide Star DEC (deg)
GSR_MAG =               13.674 /Roll Guide Star Magnitude
GSACQ   = '1997.227:23:53:44 ' /Actual time of GS Acquisition Completion
PREDGSEP=             1448.774 /Predicted Guide Star Separation (arcsec)
ACTGSSEP=             1448.680 /Actual Guide Star Separation (arcsec)
MGSSPRMS=   1.200000000000E+00 /maximum Guide Star Separation RMS in assoc.
TNLOSSES=                    0 /Number of loss of lock events
TLOCKLOS=   0.000000000000E+00 /Total loss of lock time
TNRECENT=                    0 /Number of recentering events
TRECENTR=   0.000000000000E+00 /Total recentering time

                                / LINE OF SIGHT JITTER SUMMARY

TV2_RMS =   5.033554079045E+00 /total association V2 Axis RMS
MV2_P2P =   4.640000000000E+01 /maximum association V2 Axis peak to peak
TV3_RMS =   4.460418515491E+00 /total association V3 Axis RMS
MV3_P2P =   4.310000000000E+01 /maximum total association V3 Axis peak to peak
TRA_AVG =   2.286271220000E+02 /total association Average RA
TDEC_AVG=   3.662605000000E+01 /total association Average DEC
TROLLAVG=   2.771877700000E+02 /total association Average Roll

                                / PROBLEM FLAGS, WARNINGS, and STATUS MESSAGES
                                (present only if problem exists in an extension


                                / ASSOCIATION KEYWORDS

ASN_ID  = 'N45I04030'          /unique identifier assigned to association
ASN_TAB = 'n45i04030_asn.fits' /name of the association table
ASN_STAT= 'COMPLETE'           /status of association (COMPLETE/INCOMPLETE)
ASN_PROD=                    T /product created (T/F)
PRODNAM0= 'N45I04030'          /rootname of the zeroth association product
PRODNAM1= '                  ' /rootname of first association product
PRODNAM2= '                  ' /rootname of second association product
PRODNAM3= '                  ' /rootname of third association product
PRODNAM4= '                  ' /rootname of fourth association product
PRODNAM5= '                  ' /rootname of fifth association product
PRODNAM6= '                  ' /rootname of sixth association product
PRODNAM7= '                  ' /rootname of seventh association product
PRODNAM8= '                  ' /rootname of eighth association product
PRODNAM9= '                  ' /rootname of ninth association product

Non-zero extensions contain the above keywords as well as the following, which concern that individual exposure within the association.

PCOUNT  =                    0 /number of group parameters
GCOUNT  =                    1 /number of groups
TFIELDS =                    0 /number of fields in each table row
INHERIT =                    T /inherit the primary header
EXTNAME = 'jit               ' /extension name
EXTVER  =                    3 /extension version number
ROOTNAME= 'N45I04030'          /rootname of the observation set
EXPNAME = 'N45I04AKJ         ' /9 character exposure identifier
DATAMIN =                   0. /the minimum value of the data
DATAMAX =                 464. /the maximum value of the data
BUNIT   = 'COUNTS            ' /brightness units
ASN_MTYP= 'EXP-TARG'           /Role of the Exposure in the Association

                                / World Coordinate System and Related Parameter

CRPIX1  =                  32. /x-coordinate of reference pixel
CRPIX2  =                  32. /y-coordinate of reference pixel
CRVAL1  =                 0.00 /first axis value at reference pixel
CRVAL2  =                 0.00 /second axis value at reference pixel
CTYPE1  = 'RA---TAN          ' /the coordinate type for the first axis
CTYPE2  = 'DEC--TAN          ' /the coordinate type for the second axis
CD1_1   =       -0.3918828E-06 /partial of first axis coordinate w.r.t. x
CD1_2   =       -0.3937890E-06 /partial of first axis coordinate w.r.t. y
CD2_1   =       -0.3937890E-06 /partial of second axis coordinate w.r.t. x
CD2_2   =        0.3918828E-06 /partial of second axis coordinate w.r.t. y

                                / IMAGE PARAMETERS

COORDSYS= 'NIC3              ' /Image coordinate system
XPIXINC =                  2.0 /Plate scale along X in mas per pixel
YPIXINC =                  2.0 /Plate scale along Y in mas per pixel
PARITY  =                    1 /Parity between V2V3 frame and image frame
BETA_1  =               224.86 /Angle from +V3 to image +x(toward +V2)
BETA_2  =               314.86 /Angle from +V3 to image +y(toward +V2)

                                / OBSERVATION DATA

OBSET_ID= '04                ' /observation set id
OBSERVTN= 'AK                ' /observation number (base 36)
PROCTIME= '1997.229:15:26:44 ' /Date-Time When OMS Processed Observation
                                / Date-Time Format is YYYY.DDD:HH:MM:SS
STARTIME= '1997.228:00:16:36 ' /Predicted Observation Window Start Time
ENDTIME = '1997.228:00:25:10 ' /Predicted Observation Window End Time

                                / SCIENTIFIC INSTRUMENT DATA

TLMFORM = 'HN                ' /Telemetry Format
APERTURE= 'NIC3FIX           ' /Aperture
APER_V2 =             -235.590 /V2 Aperture position in Vehicle frame (arcsec)
APER_V3 =              224.340 /V3 Aperture position in Vehicle frame (arcsec)

                                / SPACECRAFT DATA

ALTITUDE=               614.26 /Average altitude during observation (km)
LOS_SUN =                76.40 /Minimum Line Of Sight to Sun (deg)
LOS_MOON=                79.16 /Minimum Line Of Sight to Moon (deg)
SHADOENT= '1997.227:22:52:54 ' /Earth shadow last entry (predicted)
SHADOEXT= '1997.227:23:28:03 ' /Earth shadow last exit (predicted)
LOS_SCV =                94.50 /Minimum line of sight to S/C veloc. (deg)
LOS_LIMB=                 62.6 /Average line of sight to Earth limb (deg)

                                / POINTING CONTROL DATA

GSSEPRMS=                  1.1 /RMS of Guide Star Separation (milli-arcsec)
NLOSSES =                    0 /Number of loss of lock events
LOCKLOSS=                  0.0 /Total loss of lock time (sec)
NRECENT =                    0 /Number of recentering events (sec)
RECENTR =                  0.0 /Total recentering time

                                / LINE OF SIGHT JITTER SUMMARY

V2_RMS  =                  5.0 /dominant guide star V2 axis RMS (mas)
V2_P2P  =                 40.6 /dominant guide star V2 axis peak to peak (mas)
V3_RMS  =                  4.5 /dominant guide star V3 axis RMS (mas)
V3_P2P  =                 32.7 /dominant guide star V3 axis peak to peak (mas)
RA_AVG  =            228.62711 /Average RA (deg)
DEC_AVG =             36.62600 /Average DEC (deg)
ROLL_AVG=            277.18777 /Average Roll (deg)

                                / PROBLEM FLAGS, WARNINGS, and STATUS MESSAGES
                                /(present only if problem exists)

A representative jih/cmh file: These ASCII headers contain much of the same information as the current FITS headers: file structure, observation details, modeled background light, point control system, jitter summary, orbital geometry, and problem flags and warnings.

SIMPLE  =                    F / data conforms to FITS standard               !
BITPIX  =                   32 / bits per data value                          !
DATATYPE= 'INTEGER*4         ' / datatype of the group array                  !
NAXIS   =                    2 / number of data axes                          !
NAXIS1  =                   64 / length of the 1st data axis                  !
NAXIS2  =                   64 / length of the 2nd data axis                  !
GROUPS  =                    T / image is in group format                     !
GCOUNT  =                    1 / number of groups                             !
PCOUNT  =                    0 / number of parameters                         !
PSIZE   =                    0 / bits in the parameter block                  !
OMS_VER = '16.2C             ' / OMS version used to process this observation
PROCTIME= '1994.133:06:24:18.35' / date-time OMS processed observation
                                 / date-times format (yyyy.ddd:hh:mm:ss.ss)

                               / IMAGE PARAMETERS
CRVAL1  =                  0.0 / right ascension of zero-jitter pixel (deg)     
CRVAL2  =                  0.0 / declination of zero-jitter pixel (deg)         
CRPIX1  =                   32 / x-coordinate of zero-jitter pixel              
CRPIX2  =                   32 / y-coordinate of zero-jitter pixel              
CTYPE1  = 'RA---TAN          ' / first coordinate type                        
CTYPE2  = 'DEC--TAN          ' / second coordinate type                       
CD1_1   =                  0.0 / partial of ra w.r.t. x (deg/pixel)
CD1_2   =                  0.0 / partial of ra w.r.t. y (deg/pixel)
CD2_1   =                  0.0 / partial of dec w.r.t. x (deg/pixel)
CD2_2   =                  0.0 / partial of dec w.r.t. y (deg/pixel)
COORDSYS= 'WFPC2             ' / image coordinate system         
XPIXINC =                  2.0 / plate scale along x (mas per pixel)         
YPIXINC =                  2.0 / plate scale along y (mas per pixel)         
PARITY  =                   -1 / parity between V2V3 frame and image frame   
BETA1   =               134.72 / angle from +V3 to image +x (toward +V2)   
BETA2   =               224.72 / angle from +V3 to image +y (toward +V2)   

                               / OBSERVATION DATA                        
PROPOSID=                05233 / PEP proposal identifier                      
PROGRMID= '288               ' / program id (base 36)
OBSET_ID= '02                ' / observation set id
OBSERVTN= '03                ' / observation number (base 36)
TARGNAME= 'NGC3379-PO        ' / proposer's target name 
STARTIME= '1994.133:06:24:18.35' / predicted observation window start time 
ENDTIME = '1994.133:06:39:18.35' / predicted observation window end time 
SOGSID  = 'U2880203          ' / SOGS observation name                        !
                                                                         
                               / SCIENTIFIC INSTRUMENT DATA              
CONFIG  = 'WFPC2             ' / proposed instrument configuration 
PRIMARY = 'SINGLE            ' / single, parallel-primary, parallel-secondary
OPERATE = '1994.133:06:22:46.91' / predicted time instr. entered operate mode 
TLMFORM = 'PN                ' / telemetry format                               
APERTURE= 'UWFALL            ' / aperture name 
APER_V2 =                1.565 / V2 aperture position in vehicle frame (arcsec) 
APER_V3 =                7.534 / V3 aperture position in vehicle frame (arcsec) 
                                                                         
                               / SPACECRAFT DATA                         
ALTITUDE=               593.23 / average altitude during observation (km)       
LOS_SUN =               106.08 / minimum line of sight to Sun (deg)             
LOS_MOON=                77.11 / minimum line of sight to Moon (deg)            
SHADOENT= '1994.133:05:11:29.00' / predicted Earth shadow last entry 
SHADOEXT= '1994.133:05:42:45.00' / predicted Earth shadow last exit
LOS_SCV =                12.46 / minimum line of sight to S/C veloc. (deg)      
LOS_LIMB=                 58.0 / average line of sight to Earth limb (deg)      

                                                                         
                               / BACKGROUND LIGHT                        
ZODMOD  =                 22.3 / zodiacal light - model (V mag/arcsec2)         
EARTHMOD=                 20.2 / peak Earth stray light - model (V mag/arcsec2) 
MOONMOD =                 35.5 / moon stray light - model (V mag/arcsec2)       
GALACTIC=                 -1.0 / diffuse galactic light - model (V mag/arcsec2) 
                                                                         
                               / POINTING CONTROL DATA                   
GUIDECMD= 'FINE LOCK         ' / commanded guiding mode                         
GUIDEACT= 'FINE LOCK         ' / actual guiding mode at end of GS acquisition   
GSD_ID  = '0084900235        ' / dominant guide star id                         
GSD_RA  =            161.70720 / dominant guide star RA (deg)                   
GSD_DEC =             12.45407 / dominant guide star DEC (deg)                  
GSD_MAG =               12.867 / dominant guide star magnitude                  
GSR_ID  = '0085201189        ' / roll guide star id                             
GSR_RA  =            161.93314 / roll guide star RA (deg)                       
GSR_DEC =             12.78141 / roll guide star DEC (deg)                      
GSR_MAG =               12.977 / roll guide star magnitude                      
GSACQ   = '1994.133:06:31:02.92' / actual time of GS acquisition completion
PREDGSEP=             1420.775 / predicted guide star separation (arcsec)       
ACTGSSEP=             1421.135 / actual guide star separation (arcsec)          
GSSEPRMS=                  3.8 / RMS of guide star separation (milli-arcsec)    
NLOSSES =                    0 / number of loss of lock events                  
LOCKLOSS=                  0.0 / total loss of lock time (sec)                  
NRECENT =                    0 / number of recentering events                   
RECENTR =                  0.0 / total recentering time (sec)                   
                                                                         
                               / LINE OF SIGHT JITTER SUMMARY            
V2_RMS  =                  4.5 / V2 axis RMS (milli-arcsec)                     
V2_P2P  =                 51.6 / V2 axis peak to peak (milli-arcsec)            
V3_RMS  =                 20.9 / V3 axis RMS (milli-arcsec)                     
V3_P2P  =                267.3 / V3 axis peak to peak (milli-arcsec)            
RA_AVG  =            161.85226 / average RA (deg)                               
DEC_AVG =             12.58265 / average dec (deg)                              
ROLL_AVG=            293.01558 / average roll (deg)                             

                               / PROBLEM FLAGS, WARNINGS and STATUS MESSAGES 
                               / (present only if problem exists)
ACQ2FAIL= '                 T' / target acquisition failure                   
GSFAIL  = 'DEGRADED          ' / guide star acquisition failure  (*1)
TAPEDROP= '                 T' / possible loss of science data                
TLM_PROB= '                  ' / problem with the engineering telemetry       
TM_GAP  =               404.60 / duration of missing telemetry (sec)          
SLEWING = '                 T' / slewing occurred during this observation     
TAKEDATA= '                 F' / take data flag NOT on throughout observation 
SIPROBnn= '                  ' / problem with specified science instrument (*2) 

END                                                                             
-----------------------------------------------------------------------------

notes 
*1 - GSFAIL appears only once in a single header file.  
     The following table lists all current possible values for the GSFAIL
     keyword:
     -------------------------------------------------------------
     GSFAIL   |DEGRADED         | / guide star acquisition failure
              |IN PROGR         | / guide star acquisition failure
              |SSLEXP           | / guide star acquisition failure
              |SSLEXS           | / guide star acquisition failure
              |NOLOCK           | / guide star acquisition failure
              |                 |                                 
              |SREXCS?          | / guide star acquisition failure
              |SREXCS1          | / guide star acquisition failure
              |SREXCS2          | / guide star acquisition failure
              |SREXCS3          | / guide star acquisition failure
              |                 |                                 
              |SREXCP?          | / guide star acquisition failure
              |SREXCP1          | / guide star acquisition failure
              |SREXCP2          | / guide star acquisition failure
              |SREXCP3          | / guide star acquisition failure
              |                 |                                 
              |UNKNOWN          | / guide star acquisition failure
              |VEHSAFE          | / guide star acquisition failure
     -------------------------------------------------------------

*2 - The SIPROBnn keywords appear in the header file with nn = 01 - 99.  
     The following table lists all current possible values for the SIPROBnn 
     keyword:
     -------------------------------------------------------------------------
     SIPROBnn |DCF_NUM unchanged| / This observation may not have been taken 
              |FOS Safing!      | / This observation affected when FOS Safed!
              |HRS Safing!      | / This observation affected when HRS Safed!
              |WFII Safing!     | / This observation affected when WFII Safed!
              |FOC Safing!      | / This observation affected when FOC Safed! 
              |Shut             | / FOS aperture door is not Open!            
              |FAILED           | / FGS astrometry target acquisition failed  
     -------------------------------------------------------------------------

Column description of the .jit / _jit.fits / .cmi tables: The layout and content of the low time resolution 3-second average jitter table has been largely unchanged since its introduction It includes the same information as the obsolete .cmj table but also includes orbital data (e.g., latitude, longitude, limb angle, magnetic field values, etc.) and instrument-specific items. This table is best suited for quick-look assessment of pointing stability and for studying trends in telescope or instrument performance with orbital environment. Listed are the table column headings, units and a brief definition.

Description of columns in the .cmj table: The table presented the data at the highest time resolution for the telemetry mode in use. It contains the reconstructed pointing, guide star coordinates, derived jitter at the SI aperture, and pertinent guiding-related flags. The intent is twofold: (1) to provide high-time resolution jitter data for deconvolution calculations or for assessing small aperture pointing stability, and (2) to display the slew and tracking anomaly flags with the highest resolution. As discussed, this table has been replaced with the .jid high resolution jitter image for observations after August 95.

Description of the .jid / _jif.fits image: This is an IRAF image and is used in conjunction with its corresponding header information. It is a fixed 64x64 array containing the sum of events occurring in that pixel. The plate scale in both dimensions is 2 milliarcsec per pixel. The data axes will be coincident with the detector coordinate frame of the instrument/mode combination. The image is a histogram of the SI_V2 and SI_V3 parameters. If the jitter value (V2,V3 position), transformed to SI detector space, is within the limits of the image array, the value will be included by incrementing the value of the corresponding image array position (x,y) by one. So the "brightness" of the image at various points represents how often the telescope was at that particular location. The uses of this image are similar to the uses for the old high-time resolution table, but the 2-D image format, and the axes oriented with the SI rather than the spacecraft, make it more useful for PSF analyses and deconvolutions.