HST Keyword Dictionary

Space Telescope Science Institute

3700 San Martin Drive

Baltimore, MD 21212

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Keyword Definitions for the

dated 14-APR-94

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BITPIX
bits per data value 
Datatype: INTEGER*2           
Units:    bits
The value field shall contain an integer. The absolute value is used in computing the sizes of data structures. It shall specify the number of bits that represent a data value. The only valid values of BITPIX for the FITS standard are: 
		Value	Data Represented                                                        
		_____	________________                                                        
		   8	Character or unsigned binary integer                                     
		  16	16-bit twos-complement binary integer                                    
		  32    32-bit twos-complement binary integer                                 
		 -32    IEEE single precision floating point                                  
  		 -64    IEEE double precision floating point
It should be noted that PODPS produces files in GEIS format which does not conform to this standard. Floating point, in particular, will be in VAX format for the GEIS files. See also the keyword DATATYPE

Source : PODPS

CDi_j
transformation matrix 
Datatype: REAL*4              
Units:    various
The CD matrix is used in the transformation from pixel coordinates to other units. For the two-dimensional case, the transformation is to a tangent plane of sky at the reference pixel, and the CD matrix includes both the pixel scaling and the rotation by ORIENTAT. In two dimensions, it has units of degrees of great circles. For one dimensional time series, the units are seconds per pixel. For other dimensional data, it is unity and no transformation occurs when CRVAL1 equals CRPIX1. 
                                                                                
       coord_i = CRVALi + Sum_j( CDi_j * (pixel_j - CRPIXj) )

Source : PODPS

CLKDRFTR
spacecraft clock drift rate 
Datatype: REAL*8              
Units:    SEC/COUNT^2
A POCC generated value. Checked daily. Stored onboard and dumped with the science telemetry. This value is as input from the ground, it is updated only after spacecraft safings and/or when the difference in spacecraft time from a POCC time `obtained from White Sands' is > 10msec. Used in PODPS translation of spacecraft generated time tags. See EXPSTART for usage.

Source : POCC via HST

CLKRATE
spacecraft clock rate 
Datatype: REAL*8              
Units:    SEC/COUNT
A POCC generated value. Checked daily. Stored onboard and dumped with the science telemetry. This value is as input from the ground, it is updated only after spacecraft safings and/or when the difference in spacecraft time from a POCC time `obtained from White Sands' is > 10msec. Used in PODPS translation of spacecraft generated time tags. See EXPSTART for usage.

Source : POCC via HST

CRPIXi
x or y coordinate of reference pixel 
Datatype: REAL*4              
Units:    pixels
For image data, it defines the pixel location of the tangent point for the celestial coordinates at CRVALi. For one-dimensional, time-series data, it is the origin of the rescaled coordinate. For other one-dimensional data, always set to 1.

Source : SCIOPSDB

CRVALi
reference value at reference pixel 
Datatype: REAL*8              
Units:    degrees
CRVALi is the physical value that corresponds to the CRPIXi. 
		 FOC: right ascension/declination of the reference pixel                      
		 FOS: same as CRPIX1 - no change                                              
		 HRS: same as CRPIX1 - no change                                              
		 HSP (IMG): right ascension/declination of the reference pixel                
		 HSP (SCI): time at reference pixel                                           
		 WFPC: right ascension/declination of the reference pixel                     
		 WFPC2: right ascension/declination of the reference pixel
For image data, the position (RA,DEC) is based on RA_TARG and DEC_TARG, unless this is an internal calibration, in which case this position is based on RA_V1 and DEC_V1.

Source : PODPS

CTYPEi
unitless, line, pixel, channel, sample, time 
Datatype: CHARACTER*08        
Units:
Type of coordinate associated with axis i.

Source : PODPS

DATAMAX
maximum value of the data 
Datatype: see DATATYPE keyword
Units:
Maximum value for all dimensions.

Source : PODPS

DATAMIN
minimum value of the data 
Datatype: see DATATYPE keyword
Units:
Minimum value for all dimensions.

Source : PODPS

DATATYPE
datatype of the group array 
Datatype: CHARACTER*10        
Units:
Examples: REAL*4, INTEGER*2

Source : PODPS

DATE
date this file was written (dd/mm/yy) 
Datatype: CHARACTER*08        
Units:    DATE
Creation date of this file (not the exposure date, not the archive date).

Source : PODPS

DCFOBSN
DCF observation number in SHP 
Datatype: INTEGER*4           
Units:
This is the same as the SMS word 11/14 number. See WRD11_14.

Source : HST

DEFAULTS
default values used in reformatting these data 
Datatype: LOGICAL*4           
Units:
Normal processing is without defaults.

Source : PODPS

ERRCNT
number of segments containing errors 
Datatype: INTEGER*4           
Units:    COUNTS
Science data is divided for telemetry purposes into lines and frames. A line is a maximum of 965 16-bit words of science data, and a frame is a collection of one or more lines. For transmission purposes, a line of data is accommodated in a packet, which in turn is divided into from 1..16 segments (each of which has 64 16-bit words.)

Source : PODPS

FILETYPE
shp, ext, edq, sdq, sci 
Datatype: CHARACTER*03        
Units:    Filetype acronym
Indicates the type of data to be found in the data file: 
                                                                                
		SCI	Science Data                                                              
		IMG	Image Data                                                                
		SHP	Standard Header Packet                                                    
		UDL	Unique Data Log                                                           
		SHL	Science Header Line (FOS)                                                 
		STL	Science Trailer Line (FOS)                                                
		RTB	Return to Brightest Data (HRS)                                            
		EXT	Extracted Engineering Data

Source : PODPS

FILLCNT
number of segments containing fill 
Datatype: INTEGER*4           
Units:    COUNTS
Science data is divided for telemetry purposes into lines and frames. A line is a maximum of 965 16-bit words of science data, and a frame is a collection of one or more lines. For transmission purposes, a line of data is accommodated in a packet, which in turn is divided into from 1..16 segments (each of which has 64 16-bit words.)

Source : PODPS

GCOUNT
number of groups 
Datatype: INTEGER*2           
Units:    COUNT
The value of this keyword indicates the total number of groups present. From this and other required values in the header one can calculate the total number of bits in the data file: 
                                                                                
		NBITS = GCOUNT*(|BITPIX|*(NAXIS1*NAXIS2*...*NAXISn) +                         
				PSIZE*PCOUNT)

Source : PODPS

GROUPS
data has groups 
Datatype: LOGICAL*1           
Units:    T/F
Although it is standard FITS, the random group structure has been used almost exclusively for applications in radio interferometry and the Hubble Space Telescope. This keyword indicates whether the data conform to the random group structure.

Source : PODPS

INSTRUME
instrument in use 
Datatype: CHARACTER*04        
Units:                        
		FGS: Fine Guidance System                                                     
		FOC: Faint Object Camera                                                      
		FOS: Faint Object Spectrograph                                                
		HRS: Goddard High Resolution Spectrograph                                     
	     	HSP: High Speed Photometer                                               
	       WFPC: Wide Field/Planetary Camera                                       
	      WFPC2: Wide Field/Planetary Camera II

Source : HST

NAXIS
number of data axes 
Datatype: INTEGER*2           
Units:    COUNT
The value field shall contain a non-negative integer no greater than 999, representing the number of axes in an ordinary data array. A value of zero signifies that no data follow the header.

Source : PODPS

PCOUNT
number of group parameters 
Datatype: INTEGER*2           
Units:    COUNT
The total number of group parameters which FOLLOW the data array in the science data set. A bit offset to the first of the group parameters may be calculated as: 
		NBITS = |BITPIX|*(NAXIS1*NAXIS2*...*NAXISn) + 1                               
                                                                                
	The bit offset to the Nth group of data is calculated as:                      
                                                                                
		NBITS = (N-1)*(|BITPIX|*(NAXIS1*NAXIS2*...*NAXISn) +                          
				PSIZE*PCOUNT) + 1

Source : PODPS

PHOTBW
RMS Bandwidth of the Filter and Detector 
Datatype: REAL*4              
Units:    ANGSTROMS
PHOTBW is a measure of the root-mean-square width of the passband P(lam). The rms bandwidth is defined as in the WF/PC instrument handbook: 
		rmslam^2 = barlam^2 [[ln(lam/barlam)]^2] ,                                    
        where barlam is the bar wavelength, defined by Schneider,               
        Gunn and Hoessel ( 1983 ApJ 264,337 ) as:                               
                                                                                
               barlam = exp([ln(lam)]) ,                                        
        and [ ] denotes an average over the passband in ln(lam)                 
                                                                                
	 [X(lam)] = ( Int P(lam) X(lam) dlam/lam ) / ( Int P(lam) dlam/lam ) .         
	Note that the dimensionless bandwidth is:                                      
                                                                                
		(rmslam/barlam)^2 = [ [ln( lam/barlam)]^2 ]                                   
	                          = [ [ln(lam) - [ln(lam)]]^2 ],                       
	                          = [ln(lam)^2] - [ln(lam)]^2 .                        
                                                                                
	 The latter formula is used in the numerical calculation below.

Source : PODPS

PHOTFLAM
Inverse Sensitivity 
Datatype: REAL*4              
Units:
PHOTFLAM is the flux-density that produces a count rate of 1 per second with the HST observing mode. Multiply an observed count rate by PHOTFLAM to get the mean broad-band flux-density of the target in the passband with which the target was observed. In terms of the spectral energy distribution of the target FLAM(LAM), and the passband sensitivity vs wavelength P(LAM), the mean broad-band flux density is given by: 
                  INTEGRAL ( FLAM(LAM) * P(LAM) * LAM * DLAM )                  
        FLAM(P) = -------------------------------------------                   
                        INTEGRAL ( P(LAM) * LAM * DLAM )
Even when the spectral energy of the target is unknown, this mean flux density is exactly determined by the measured count rate in the passband.

Source : PODPS

PHOTMODE
observation mode 
Datatype: CHARACTER*68        
Units:
PHOTMODE is a character string giving a list of keywords that specify the HST observing mode. For example, 
		WF,1,F336W                                                                    
		FOC,F/96,F195W                                                                
		FOS,BLUE,G160L,1.0                                                            
		HSP,UV1,F248M,A
are all valid PHOTMODEs. The PHOTMODE is used by the STSDAS.SYNPHOT package to look up the passband P(LAM) (sensitivity vs wavelength) of the observing mode.

Source : PODPS

PHOTPLAM
Pivot Wavelength 
Datatype: REAL*4              
Units:    ANGSTROMS
The pivot wavelength is the wavelength at which a broad-band flux converts exactly between FNU and FLAM units. It does not depend on the spectrum of the target observed, but only on the shape of the observing passband P(LAM). The definition of the pivot wavelength is 
		LAM_P = SQRT( [ LAM ] / [ 1/LAM ] )                                           
        where:                                                                  
		 [ LAM ] = ( INTEGRAL P(LAM)*LAM*DLAM ) /                                     
				( INTEGRAL P(LAM)*DLAM )                                                    
        and:                                                                    
		 [ 1/LAM ] = ( INTEGRAL P(LAM)*DLAM/LAM ) /                                   
				( INTEGRAL P(LAM)*DLAM )

Source : PODPS

PHOTZPT
ST magnitude zero point 
Datatype: REAL*4              
Units:
PHOTZPT is the zero-point of the ST magnitude system. The ST magnitude is STMAG = -2.5 * LOG_10( FLAM ) + 21.10, thus constant STMAG corresponds to constant FLAM, and the zero-point is PHOTZPT = -21.10. This value is chosen so that Vega has an ST magnitude of zero for the Johnson V passband. If the absolute calibration of Vega changes, this value may also change slightly.

Source : PODPS

PKTTIME
time from the packet ancillary data 
Datatype: REAL*8              
Units:    DAYS
Each packet contains ancillary information including a spacecraft time code value. This value contains the spacecraft time associated with the SHP (standard header packet) which contains ancilliary data. Modified Julian Day.

Source : HST

PODPSFF
0=(no podps fill); 1=(podps fill present) 
Datatype: INTEGER*2           
Units:
An indicator of fill data inserted for missing data by PODPS.

Source : PODPS

PROC_TYP
level of reprocessing (NORMAL, MINOR, MAJOR) 
Datatype: CHARACTER*06        
Units:
If the keyword is set to MINOR the proprietary date in DADS will not be changed and the data will not be automatically distributed to the Guest Observers (GOs). If the keyword is set to MAJOR the proprietary date in DADS will be reset as if this were new data the data will be automatically distributed to the GO as if it were new data.

Source : PODPS

PSIZE
bits in the parameter block 
Datatype: INTEGER*2           
Units:    Bits
This is the total number of bits which are included in the group parameter block. It is the sum of the PSIZEi for each of the PCOUNT parameters in the block.

Source : PODPS

PSIZEi
bits in the ith group parameter 
Datatype: INTEGER*2           
Units:    Bits
The number of bits in the ith group parameter. The total number of bits in the group parameter block, PSIZE, is the sum of all the PSIZEi values for each of the PCOUNT parameters in the block.

Source : PODPS

ROOTNAME
rootname of the observation set 
Datatype: CHARACTER*12        
Units:    IPPPSSOOT
Follows the ipppssoot convention where: 
		i: instrument identifier:                                                     
			V:hsp, W:wfpc, X:foc, Y:fos, Z:hrs                                           
		ppp: program_id                                                               
		ss : obset_id                                                                 
		oo : observation number                                                       
		t  : T:tape, R:realtime.

Source : PODPS

RSDPFILL
bad data fill value for calibrated images 
Datatype: INTEGER*2           
Units:    FILL_VALUE
Indicates the fill value used after calibration.

Source : PODPS

RTAMATCH
above f&i match RTA 
Datatype: CHARACTER*18        
Units:    N/A
Real Time Analysis files are no longer sent to PODPS. This is an obsolete item. This keyword should alway be 'NO RTA FI AVAILABL'.

Source : PODPS

SIMPLE
data conforms to the fits standard 
Datatype: LOGICAL*1           
Units:    T/F
The value in this field shall contain a logical constant with th value T if the file conforms to the FITS standard. This keyword is mandatory only for the primary header. A value of F signifies that the file does not conform to this standard in some significant way. The output of the RSDP pipeline always sets the value of this keyword to F. The main reason is that the headers are kept in separate files from the data which is definately inconsistant with the FITS standard.

Source : PODPS

SPCLINCN
spacecraft clock at UTC0 
Datatype: INTEGER*4           
Units:    MJD
Value of the counter which is keeping count of the number of 0.125 second ticks of the spacecraft clock. This value is as input from the ground. It is updated only after spacecraft safings and/or when the difference in spacecraft time from a POCC time `obtained from White Sands' is > 10msec.

Source : POCC via HST

STDCFFF
0=(no ST DCF fill); 1=(ST DCF fill present) 
Datatype: INTEGER*2           
Units:    none
A flag indicating the presence of fill data generated at ST Data Capture Facility.

Source : PODPS

STDCFFP
ST DCF fill pattern (hex) 
Datatype: CHARACTER*04        
Units:    none
The binary value expressed as four hexidecimal digits for the 16-bit fill pattern used at ST Data Capture Facility.

Source : PODPS

UTC0
Coordinated Universal Time (Mod Julian Date) 
Datatype: REAL*8              
Units:    days
The UTC at the reference S/C clock SPCLINCN. The integer portion is the UTC day since the beginning of Nov 17, 1858. The fractional portion is the fraction of 24 hours.

Source : POCC via HST

UTCO1
bytes 5-8 of UTC0 
Datatype: INTEGER*4           
Units:    100 nanoseconds
Part of the UTC at the reference S/C clock SPCLINCN. This is the format stored in Standard Header Packets. Least significant 4 bytes of 8-byte integer containing the number of 100-nanoseconds units since the beginning of Nov 17, 1858. Formatted as a signed integer but must be used as an unsigned integer. The most significant bit is set when the value is negative and the remaining bits are the absolute value (ala DEC signed integer format).

Source : POCC via HST

UTCO2
bytes 1-4 of UTC0 
Datatype: INTEGER*4           
Units:    (see UTCO2)
Part of the UTC at the reference S/C clock SPCLINCN. This is the format stored in Standard Header Packets. Most significant 4 bytes of 8-byte integer containing the number of 100-nanoseconds units since the beginning of Nov 17, 1858.

Source : POCC via HST

HST Keyword Dictionary: 14-APR-94: (rose@stsci.edu)