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20.4 Headers, Keywords, and Relationship to Phase II

As with previous HST instruments, the FITS header keywords in STIS data files store important information characterizing the observations and telemetry received during the observations and describe the post-observation processing of your dataset. Each keyword follows FITS conventions and is no longer than eight characters. Values of keywords can be integer, real (floating-point), or character string. Many are HST and STIS specific. Knowledge of the keywords and where to find them is an important first step in understanding your data. By examining your file headers, using either infostis, catfits, imhead, or hedit, you will find detailed information about your data including:

Figure 20.3: Using infostis to Display Header Keywords


cl> infostis o3xi03a1m_raw.fits
--------------------------------------------------------------------------------

                                   S  T  I  S

--------------------------------------------------------------------------------

 

            Rootname: O3XI03A1M                    Detector: CCD

         Proposal ID: 7071                         Obs Type: IMAGING

         Exposure ID: 3.031                        Obs Mode: ACQ

                                                       Lamp: NONE

         Target Name: GD153-1                      Aperture: F28X50LP

     Right Ascension:  12:57:02.3                    Filter: Long_Pass

         Declination: +22:01:53.2               Opt Element: MIRVIS

             Equinox: 2000.0                        CCD amp: D

                                                       Gain: 4

      Axis 1 binning: 1                            CR-split: 1

      Axis 2 binning: 1

            Subarray: yes

 

     Total Exp. Time: 0.3 sec

    Number of imsets: 3

STIS takes CCD and MAMA spectroscopic and imaging data, as well as acquisitions and acq/peakups. The keywords relevant for one of these data types will not necessarily be relevant to another. Accordingly, you will find that the header on your particular file contains a unique combination of keywords appropriate for your type of observation. Long definitions for the keywords can also be accessed from the following Web page, which provides detailed explanations of the contents and algorithm for populating the keywords. This site also provides sample headers for different STIS file types:

http://archive.stsci.edu/keyword

Keywords that deal with a particular topic, such as the instrument configuration, are grouped together logically throughout the headers. Table 20.5 lists a useful subset of these groups of keywords, indicates the name of the grouping, and where applicable, shows their relationship to the corresponding information from the Phase II proposal.

Table 20.6 summarizes the possible calibration switch keywords, and indicates whether they are present for a particular observation; it also indicates the reference file keyword corresponding to the particular calibration step. A calibration switch keyword is populated with one of OMIT, COMPLETE, or PERFORM. Similarly, Table 20.7 summarizes the reference file group of keywords that identify the files used by the pipeline during calibration (see Chapter 21 for a detailed description of pipeline processing).


Selected Keywords and Relationship to Phase II

Keyword

Phase II Equivalent

Description

NEXTEND

Number of image extensions in the file.

Target Information (Primary Header)

TARGNAME

Target Name

Name of target.

RA_TARG

DEC_TARG

RA

Position

DEC

Right ascension of the target (deg) (J2000).

Declination of the target (deg) (J2000).

Proposal Information (Primary Header)

PROPOSID

4 digit proposal number

LINENUM

Indicates the visit and exposure number from the phase II proposal; format vv-nnnn, visit number, exposure num..

Summary Exposure Information (Primary Header)

TDATEOBS

UT date of start of first exposure in file (a character string)

TTIMEOBS

UT start time of first exposure in file (a character string); primary header.

TEXPSTRT

Start time (MJD) of 1st exposure in file (a real number).

TEXPEND

End time (MJD) of last exposure in the file (a real number); primary header.

TEXPTIME

Time_per_Exposure

Total exposure time (a real number); summed for an association; primary header.

Science Instrument Configuration (Primary Header)

OBSTYPE

Observation type (IMAGING or SPECTROSCOPIC).

OBSMODE

Opmode

Operating mode (ACQ,ACQ/PEAK,ACCUM,TIME-TAG).

DETECTOR

Config

Detector in use: NUV-MAMA, FUV-MAMA, or CCD.

OPT_ELEM

Sp_Element

Optical element in use (grating name or mirror).

MIRROR

Sp_Element

Mirror element used for imaging observations.

CENWAVE

Centra_Wavelength

Central wavelength for grating settings.

APERTURE

Aperture

Aperture name.

FILTER

Derirved from Aperture

Filter in use.

APER_FOV

Aperture field of view.

WAVEMIN

Minimum wavelength of the data.

WAVEMAX

Maximum wavelength of the data.

PLATESC

Plate scale (arcsec/pixel).

SCLAMP

Lamp status, NONE or name of lamp which is on.

LAMPSET

Spectral cal lamp current value (milliamps).

NRPTEXP

Number_of_Iterations

Number of repeat exposures in set: default 1.

SUBARRAY

Data from a subarray (T) or full frame (F).

CRSPLIT

CR-SPLIT (optional parameter)

Number of cosmic ray split exposures.

Readout Definition Parameters

SIZAXIS1

SIZEAXIS1

Subarray axis 1 size in unbinned detector pixel.

SIZAXIS2

SIZEAXIS2

Subarray axis 2 size in unbinned detector pixel.

BINAXIS1

BINAXIS1

Axis 1 data bin size in unbinned detector pixel.

BINAXIS2

BINAXIS2

Axis 2 data bin size in unbinned detector pixel.

CCDAMP

CCD amplifier read out (A,B,C,D).

CCDGAIN

GAIN (optional parameter)

Commanded gain of CCD.

CCDOFFST

Commanded CCD bias offset.

ATODGAIN

Calibrated CCD amplifier gain value.

Exposure Information (Extension header only)

DATE-OBS

UT date of start of observation (dd/mm/yy); extension header.

TIME-OBS

UT time of start of observation (hh:mm:ss); extension header.

PA_APER

Position angle of slit.

ORIENTAT

Position angle of detector.

CRPIX et al.

DGESTAR

FGS ID(F1,F2,F3) concat. w/ dom. gd. star id.



Calibration Switch Keywords

Keyword

Ref. File Keywords

Explanation

Spectra

Images

CCD

MAMA

STATFLAG

N/A

Calculate image statistics?

·

·

·

·

DQICORR

BPIXTAB

Initialize data quality image

·

·

·

·

LORESCORR

N/A

Convert MAMA image to low-res

·

·

·

DARKCORR

DARKFILE

Dark image correction

·

·

·

·

FLATCORR

LFLTFILE

Flatfield corrections

·

·

·

·

PFLTFILE

DFLTFILE

SGEOCORR

SDSTFILE

Small-scale distortion correction

·

·

·

·

RPTCORR

N/A

Add individual repeat observations

·

·

·

·

ATODCORR

ATODTAB

A-to-D correction

·

·

·

BLEVCORR

N/A

Correct for CCD bias level (overscan)

·

·

·

BIASCORR

BIASFILE

Bias image (structure) correction

·

·

·

CRCORR

CRREJTAB

Cosmic ray rejection

·

·

·

EXPSCORR

N/A

Process individual observations after CRCORR

·

·

·

SHADCORR

SHADFILE

Shutter shading correction

·

·

·

PHOTCORR

PHOTTAB

Populate header photometric keywords

·

·

·

GEOCORR

N/A

Geometric correction

·

·

·

X2DCORR

SDCTAB

Rectify 2-D spectral image

·

·

·

DISPTAB

·

·

·

INANGTAB

·

·

·

APDESTAB

·

·

·

MOFFTAB

·

·

·

SPTRCTAB

·

·

·

X1DCORR

SPTRCTAB

Extract 1-D Spectrum

·

·

·

XTRACTAB

BACKCORR

XTRACTAB

Subtract spectral background

·

·

·

DISPCORR

DISPTAB

Apply dispersion solution

·

·

·

INANGTAB

APDESTAB

MOFFTAB

WAVECORR

N/A

·

·

·

FLUXCORR

APERTAB

Convert to absolute flux

·

·

·

PHOTTAB

HELCORR

N/A

Convert to heliocentric wavelengths

·

·

DOPPCORR

N/A

Doppler correction needed? (TIMETAG mode only)

·

·

GLINCORR

N/A

Global detector non-linearities

·

·

LFLGCORR

N/A

Flag pixels for local and global non-linearities

·

·



Reference File Keywords

Keyword

File

Suffix

Format

Explanation

Spectra

Images

CCD

MAMA

BPIXTAB

_bpx

Table

Bad pixel

·

·

·

·

ATODTAB

_a2d

Table

A-to-D correction

·

·

·

CCDTAB

_ccd

Table

CCD parameters

·

·

·

BIASFILE

_bia

Image

Bias (structure)

·

·

·

CRREJTAB

_crr

Table

Cosmic ray rejection parameters

·

·

·

SHADFILE

_ssc

Image

Shutter shading correction

·

·

·

DARKFILE

_drk

Image

Dark

·

·

·

·

LFLTFILE

_lfl

Image

Low-order flat

·

·

·

·

PFLTFILE

_pfl

Image

Pixel-to-pixel flat

·

·

·

·

DFLTFILE

_dfl

Image

Delta-flat

·

·

·

·

SDSTFILE

_ssd

Image

Small-scale distortion correction

·

·

·

MLINTAB

_lin

Table

Flux linearity table

·

·

·

PHOTTAB

_pht

Table

Photometric conversion

·

·

·

·

APERTAB

_apt

Table

Aperture throughput

·

·

·

LAMPTAB

_lmp

Table

Template CAL lamp spectra

·

·

·

APDESTAB

_apd

Table

Aperture descriptions

·

·

·

IDCTAB

_idc

Table

Image distortion correction

·

·

·

SDCTAB

_sdc

Table

2-D spectrum distortion correction

·

·

·

INANGTAB

_iac

Table

Incident angle correction

·

·

·

MOFFTAB

_moc

Table

MAMA Offset correction

·

·

DISPTAB

_dsp

Table

Dispersion coefficients

·

·

·

SPTRCTAB

_1dt

Table

1-D Spectrum trace

·

·

·

XTRACTAB

_1dx

Table

1-D Extraction parameter

·

·

·



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