6 FITS headers

The FITS headers give additional information about the observation required for ORAC-DR to reduce the data. In many cases there is an equivalent header in the original GSD data file.

6.1 Telescope Specific

TELESCOP

TEL_NAME (C1TEL).

ORIGIN

’Joint Astronomy Centre’ (default).

OBSGEO-X, OBSGEO-Y, OBSGEO-Z

The cartesian coordinates of the telescope are calculated using the values stored in TEL_LATITUDE (C1LAT), TEL_LONGITUDE (C1LONG), and TEL_HEIGHT (C1HGT).

ALT-OBS

Convert TEL_HEIGHT (C1HGT) from km to m.

LAT-OBS

TEL_LATITUDE (C1LAT).

LONG-OBS

TEL_LONGITUDE (C1LONG).

ETAL

Convert ETAL (C8EL) from percentage to decimal.

6.2 OMP and ORAC-DR Specific

PROJECT

PROJECT (C1PID).

RECIPE

’REDUCE_SCIENCE’ (default).

DRGROUP

AST__UNDEFS (string bad value).

MSBID

AST__UNDEFS (string bad value).

SURVEY

AST__UNDEFS (string bad value).

RMTAGENT

AST__UNDEFS (string bad value).

AGENTID

AST__UNDEFS (string bad value).

6.3 Observation, Date and Pointing Specific

OBSID

The OBSID is generated using the BACKEND (C1BKE) header, the observation number, and the date of the observation. For observations after February 2nd, 2003, the observation number found in the GSD file’s NOBS (C1SNO) header can be used, prior to this date the user provides the observation number as an input to GSD2ACSIS (see OBSNUM). The date of the observation provided by OBS_UT1D (C3DAT) and OBS_UT1H (C3UT) headers are stored in UT1 time and must be converted to UTC time using OBS_UT1C (C3UT1C).

OBSIDSS

The OBSIDSS header simply appends the subsystem number to the OBSID.

OBJECT

GSD files store two strings for the source name: CENTRE_NAME_1 (C1SNA1) and CENTRE_NAME_2 (C1SNA2). These two names are concatenated to produce the OBJECT header.

STANDARD

The STANDARD flag indicates whether or not the observation was of a spectral line standard. The OBJECT name is compared with a lookup table of standards to determine if STANDARD is true.

OBSNUM

Prior to Feb 2nd, 2003, observation numbers were assigned to individual projects which could span several nights, and data files are stored in subdirectories according to project name. After Feb 2nd, 2003, observation numbers changed from night to night, and data files are stored in subdirectories by date. The observation number for data collected prior to Feb 2nd, 2003 should be determined from the project name and date, and must be supplied as a user input to GSD2ACSIS, whereas for later observations the GSD file’s NOBS (C1SNO) header can be used.

NSUBSCAN

For longer observations split over several files, the NSUBSCAN would represent this file’s place in the sequence of data. For GSD2ACSIS converted files we would expect this number to always be 1 as only one output file per subsystem would be written.

OBSEND

The OBSEND flag indicates whether or not the file is the last in the current observation. For GSD2ACSIS converted files we would expect this number to always be 1 as only one output file per subsystem would be written.

UTDATE

The UTDATE is a 8 digit integer represnting the UT date of the observation in YYYYMMDD format. It is obtained from the OBS_UT1D (C3DAT) header.

DATE-OBS

The DATE-OBS header is a string representation of the UTC datetime of the start of the observation in the format YYYY-MM-DDTHH:MM:SS (eg. 2004-01-01T01:01:01). The GSD OBS_UT1D (C3DAT) and OBS_UT1H (C3UT) headers are used to create the DATE-OBS header, but as these GSD headers are in UT1 time, the OBS_UT1C (C3UT1C) header is used to convert to a UTC datetime.

DATE-END

The DATE-END is derived by adding the length of the observation to the DATE-OBS. The observation length is estimated by finding the difference between the recorded LSTs of the first and last time steps in the SCAN_TABLE1 (C12SCAN_TABLE_1) array.

DUT1

OBS_UT1C (C3UT1C).

INSTAP

AST__UNDEFS (string bad value).

**NOTE: Possibly this should be the same as the value stored in REFRECEP.

INSTAP_X

0.0 (default).

INSTAP_Y

0.0 (default).

AMSTART

The airmass at observation start corresponds to the JCMTSTATE TCS_AIRMAS of the first time step.

AMEND

The airmass at observation end corresponds to the JCMTSTATE TCS_AIRMAS of the last time step.

AZSTART

The azimuth at observation start corresponds to the JCMTSTATE TCS_AZ_AC1 of the first time step.

AZEND

The azimuth at observation end corresponds to the JCMTSTATE TCS_AZ_AC1 of the last time step.

ELSTART

The elevation at observation start corresponds to the JCMTSTATE TCS_AZ_AC2 of the first time step.

ELEND

The elevation at observation end correspond to the JCMTSTATE TCS_AZ_AC2 of the last time step.

HSTSTART

The HST at observation start is simply the DATE-OBS converted to HST.

HSTEND

The HST at observation end is simply the DATE-END converted to HST.

LSTSTART

The LST of the start of the observation is the first LST stored in the SCAN_TABLE1 (C12SCAN_TABLE_1).

LSTEND

The LST of the end of the observation is the last LST stored in the SCAN_TABLE1 (C12SCAN_TABLE_1).

6.4 Integration Time Specific

INT_TIME

For rasters, the total integration time is the time per scan, SCAN_TIME (C3SRT) multiplied by the number of complete scans, NSCAN (C3NSAMPLE). For other observations, the total integration time is the sum of the integration times found in the INTGRN_TIME (C3INTT) array.

6.5 ACSIS Specific

BACKEND

BACKEND (C1BKE).

MOLCULE, TRANSITI

The MOLECULE and TRANSITI headers are found by comparing the rest frequency against a lookup table of molecular transition lines.

DRRECIPE

AST__UNDEFS (string bad value).

BWMODE

The bandwidth mode takes the format ’$<$BANDWIDTH$>$MHzx$<$CHANNELS$>$’ (e.g. ’250MHzx2048’) where BANDWIDTH is the value for this subsystem in the BES_BANDWIDTH (C12BW) array, and CHANNELS is the value for this subsystem in the NO_BES_O_CH (C3LSPC) array.

SUBSYSNR

The number of the current subsystem.

SUBBANDS

Same as BWMODE.

NSUBBAND

1 (default).

SUBREFP1

Same as REFCHAN (see below).

SUBREFP2

AST__UNDEFF (float bad value).

NCHNSUBS

The number of channels for this subband in the NO_BES_O_CH (C3LSPC) array.

REFCHAN

The reference channel is calculated as follows:

where NUMCHANNELS, TOTIF, and FREQ_RESOLUTION are the values for this subband in the NO_BES_O_CH (C3LSPC), BES_TOT_IF (C3BETOTIF) and BES_DELTANU (C12FR) arrays, respectively, and IF is as defined in IFFREQ below.

IFCHANSP

Value from the BES_DELTANU (C12FR) array for this subband.

FFT_WIN

’truncate’ (string bad value).

BEDEGFAC

For the DAS backend, BEDEGFAC is 1.15, for AOSC it is 1.0.

MSROOT

AST__UNDEFS (string bad value).

6.6 Front End Specific

INSTRUME

FRONTEND (C1RCV).

**NOTE: Receiver RxG is not currently supported by GSD2ACSIS.

SB_MODE

C3SBMODE (C3SBMODE).

IFFREQ

The IF for special observations for each group of subbands to be merged is defined to be the average of the values in the BES_TOT_IF (C3BETOTIF) array for those subbands. **NOTE** currently GSD2ACSIS treats each subband in a special observation separately and so each subband uses its corresponding value in BES_TOT_IF (C3BETOTIF) for its IF. For non-special observations, the IF is 4.0 GHz, with the exception of the MRI frontend which used an IF of 3.5 GHz.

N_MIX

The number of mixers can be determined from the frontend. For RxB and RxW frontends, the N_MIX is 2, for all others N_MIX is 1.

OBS_SB

If the sideband sign found in the BES_FE_SB_SIGN (C3BEFESB) array for this subband is positive, OBS_SB = ’USB’, otherwise OBS_SB = ’LSB’.

LOFREQS, LOFREQE

The value found in the BES_FE_NULO (C3BEFENULO) array for this subband.

Because the GSD data does not record both a starting and ending LO frequency, the same value is used for LOFREQE as for LOFREQS.

RECPTORS

Receptor names come from the name of the FRONTEND, and the mixers used in the observation as listed for each subband in the DAS_MIXER (C3MIXNUM) array. All receptors used in the observation are listed in a comma separated list in RECEPTORS (eg. ’BA, BB’ would indicate that both receptors of an RXB frontend were used).

**NOTE: Receiver RxG is not currently supported by GSD2ACSIS.

REFRECEP

REFRECEP is the first of the receptors listed in RECPTORS.

MEDTSYS

For sample observations, MEDTSYS is AST__UNDEFF (float bad value), for all other observations it is the value stored in the BES_T_SYS (C12SST) array for this subsystem.

TEMPSCAL

’TA*’ (default).

DOPPLER

VEL_DEFN (C12VDEF).

SSYSOBS

’TOPOCENT’ (default).

6.7 Environment Specific

ATSTART, ATEND

TAMB (C5AT).

Because the GSD data does not record both a starting and ending air temperature, the same value is used for ATEND as for ATSTART.

HUMSTART, HUMEND

HAMB (C5RH).

Because the GSD data does not record both a starting and ending relative humidity, the same value is used for HUMEND as for HUMSTART.

BPSTART, BPEND

PAMB (C5PRS).

**NOTE: GSD files state that PAMB (C5PRS) is in units of mmHg, although it is actually in mbar, so no conversion is required.

Because the GSD data does not record both a starting and ending air pressure, the same value is used for BPEND as for BPSTART.

WNDSPDST

AST__UNDEFF (float bad value).

WNDSPDEN

AST__UNDEFF (float bad value).

WNDDIRST

AST__UNDEFF (float bad value).

WNDDIREN

AST__UNDEFF (float bad value).

TAU225ST, TAU225EN

CSO_TAU (C7TAU225).

Because the GSD data does not record both a starting and ending TAU225 value, the same value is used for TAU225EN as for TAU225ST.

TAUDATST, TAUDATEN

CSO_YYMMDDHHMM (C7TAUTIME).

Because the GSD data does not record both a starting and ending TAU225 value, the same value is used for TAUDATEN as for TAUDATST.

TAUSRC

’CSO225GHZ’ (default).

WVMTAUST

AST__UNDEFF (float bad value).

WVMTAUEN

AST__UNDEFF (float bad value).

WVMDATST

AST__UNDEFF (float bad value).

WVMDATEN

AST__UNDEFF (float bad value).

SEEINGST, SEEINGEN

SAO_SEEING (C7SEEING).

Because the GSD data does not record both a starting and ending seeing, the same value is used for SEEINGEN as for SEEINGST.

SEEDATST, SEEDATEN

SAO_YYMMDDHHMM (C7SEETIME).

Because the GSD data does not record both a starting and ending seeing, the same value is used for SEEINGEN as for SEEINGST.

FRLEGTST

AST__UNDEFF (float bad value).

FRLEGTEN

AST__UNDEFF (float bad value).

BKLEGTST

AST__UNDEFF (float bad value).

BKLEGTEN

AST__UNDEFF (float bad value).

6.8 Switching and Mapping Details

SAM_MODE

If the GSD header OBS_TYPE (C6ST) is ’RASTER’, SAM_MODE is ’raster’. If OBS_TYPE (C6ST) is ’SAMPLE’, SAM_MODE is ’sample’. In all other cases, SAM_MODE is ’grid’.

SW_MODE

The Switch Mode header is generated by checking the following GSD headers:

SWITCH_MODE (C6MODE)
CHOPPING (C4SM)
REFERENCE_X (C4RX)
REFERENCE_Y (C4RY)

SKYREFX

SKYREFX takes the form ’[OFFSET] $<$REFX$>$ [LOCL_CRD]’ (eg. [OFFSET] 180 [J2000]’) where REFX is the value found in REFERENCE_X (C4RX) and LOCL_CRD is the value from the FITS LOCL_CRD header.

SKYREFY

SKYREFY takes the form ’[OFFSET] $<$REFY$>$ [LOCL_CRD]’ (eg. [OFFSET] 0 [J2000]’) where REFY is the value found in REFERENCE_Y (C4RY) and LOCL_CRD is the value from the FITS LOCL_CRD header.

OBS_TYPE

If the GSD OBS_TYPE (C6ST) is ’FIVEPOINT’, the ACSIS OBS_TYPE is ’pointing’. If the GSD OBS_TYPE (C6ST)is ’FOCUS’, the ACSIS OBS_TYPE is ’focus’. For all other observations, the ACSIS OBS_TYPE is ’science’.

The following 4 headers are only defined for observations where SAM_MODE is ’grid’ and SW_MODE is either ’chop’ or ’sample’.

CHOP_CRD

The name of the chopping coordinate is translated to the modern name from the value stored in COORDS (C4SMCO).

• ’AZ’: ’AZEL’
• ’EQ’: ’HADEC’
• ’RB’: ’B1950’
• ’RJ’: ’J2000’
• ’RD’: ’APP’
• ’GA’: ’GAL’

CHOP_FRQ

FREQUENCY (C4FRQ).

CHOP_PA

POSANG (C4POSANG).

CHOP_THR

THROW (C4THROW).

ROT_CRD

AST__UNDEFS (string bad value).

ROT_PA

AST__UNDEFF (float bad value).

JIGL_CNT

AST__UNDEFI (integer bad value).

JIGL_NAM

AST__UNDEFS (string bad value).

JIG_PA

AST__UNDEFF (float bad value).

JIG_CRD

AST__UNDEFS (string bad value).

JIG_SCAL

AST__UNDEFF (float bad value).

The following 11 headers are only defined for observations where SAM_MODE is ’raster’ and SW_MODE is ’pssw’.

MAP_HGHT

The map height is:

(NO_X_MAP_PNTS (C6XNP), CELL_X (C6DX))

MAP_PA

The MAP_PA is defined to be the SCAN_PA - 90.0${}^{\circ }$.

MAP_WDTH

The map width is:

(NO_Y_MAP_PNTS (C6YNP), CELL_Y (C6DY))

LOCL_CRD

The name of the local offset coordinate frame is identified by the GSD CELL_CODE (C6FC) and translated into the modern name:

• 1: ’AZEL’
• 3: ’HADEC’ (Currently unsupported)
• 4: ’APP’
• 6: ’B1950’
• 7: ’J2000’
• 8: ’GAL’

MAP_X

CENTRE_OFFSET_X (C4SX).

MAP_Y

CENTRE_OFFSET_Y (C4SY).

SCAN_CRD

Same as LOCL_CRD.

SCAN_VEL

For observations where the OBS_DIRECTION (C6SD) is ’HORIZONTAL’, the SCAN_VEL is:

For observations where the OBS_DIRECTION (C6SD) is ’VERTICAL’, the SCAN_VEL is:

(CELL_X (C6DX), CELL_Y (C6DY), NO_X_MAP_PNTS (C6XNP), NO_Y_MAP_PNTS (C6YNP), SCAN_TIME (C3SRT))

SCAN_DY

For observations where the OBS_DIRECTION (C6SD) is ’HORIZONTAL’, the SCAN_DY is CELL_Y (C6DY). For observations where the OBS_DIRECTION (C6SD) is ’VERTICAL’, the SCAN_DY is CELL_X (C6DX).

SCAN_PA

CELL_V2X (C6MSA).

SCAN_PAT

If SCAN_REVERSAL (C6REV) is 1 (true), SCAN_PAT is ’BOUSTROPHEDON’, otherwise SCAN_PAT is ’RASTER’.

6.9 Secondary Mirror Specific

ALIGN_DX

DX (C2FV).

ALIGN_DY

DY (C2FL).

FOCUS_DZ

DZ (C2FR).

DAZ

OFFS_EW (C4OFFS_EW).

DEL

OFFS_NS (C4OFFS_NS).

UAZ

SDIS(36) (UAZ).

UEL

SDIS(37) (UEL).

6.10 JOS Parameters

STEPTIME

For observations where SAM_MODE is ’raster’ and OBS_DIRECTION (C6SD) is ’HORIZONTAL’, STEPTIME is:

For observations where SAM_MODE is ’raster’ and OBS_DIRECTION (C6SD) is ’VERTICAL’, STEPTIME is:

For all other observations, STEPTIME is SCAN_TIME (C3SRT).

(SCAN_TIME (C3SRT), NO_X_MAP_PNTS (C6XNP), NO_Y_MAP_PNTS (C6YNP))

NUM_CYC

NCYCLE (C3NCYCLE).

NUM_NODS

1 (default).

JOS_MULT

AST__UNDEFI (integer bad value).

JOS_MIN

For observations where SAM_MODE is ’sample’, JOS_MIN is NSCAN (C3NSAMPLE). For all other observations, JOS_MIN is 1.

NCALSTEP

AST__UNDEFI (integer bad value).

NREFSTEP

When OBS_CONTINUOUS (C3FLY) is 1, NREFSTEP is calculated as follows:

For all other observations, NREFSTEP is the value found in SCAN_TIME (C3SRT).

(NO_SCAN_PNTS (C3MXP), SCAN_TIME (C3SRT))

STBETREF

If SW_MODE is ’chop’, STBETREF is undefined (AST__UNDEFI).

When OBS_CONTINUOUS (C3FLY) is 1, STBETREF is NO_SCAN_PNTS (C3MXP). For all other observations, STBETREF is 1.

STBETCAL

AST__UNDEFI (integer bad value).

STARTIDX

For observations where SAM_MODE is ’sample’ or ’raster’, STARTIDX is 1. In ’grid’ observations, the STARTIDX is the index into the grid of the first pair of coordinates found in the MAP_TABLE (C14PHIST).

FOCAXIS

AST__UNDEFS (string bad value).

NFOCSTEP

AST__UNDEFI (integer bad value).

FOCSTEP

AST__UNDEFF (float bad value).

6.11 Miscellaneous

OCSCFG

AST__UNDEFS (string bad value).

SIMULATE

0 (default).

SIM_CORR

0 (default).

SIM_SMU

0 (default).

SIM_TCS

0 (default).

SIM_RT

0 (default).

SIM_IF

0 (default).

STATUS

’NORMAL’ (default).

6.12 ROVER Polarimeter Specific

POL_CONN

0 (default).

POL_MODE

AST__UNDEFS (string bad value).

ROTAFREQ

AST__UNDEFF (float bad value).

POL_CRD

AST__UNDEFS (string bad value).

POLFAXIS

AST__UNDEFF (float bad value).