ANDICAM FITS Header Specification
Updated: 2003 November 10
This document describes the FITS headers of the CCD and IR images produced by the ANDICAM, breaking them into their basic components to aid in interpreting their contents.
This set of FITS header cards is common to all ANDICAM images (CCD and IR). Header cards containing data specific to CCD or IR images are described in subsequent sections.
SIMPLE = T / Fits standard BITPIX = -32 / Bits per pixel NAXIS = 2 / Number of axes NAXIS1 = 1051 / Axis length NAXIS2 = 1028 / Axis lengthRaw CCD images are written as 16-bit signed integer data values (BITPIX=16), but pipeline processed files are 4-byte floating-point (BITPIX=-32). All IR images are binned on-site and in 4-byte floating-point data values.
IMAGETYP= 'OBJECT ' / Image Type Code OBJECT = 'OB03423 ' / Object name FILENAME= 'ccd031108.0003' / Image filename UNIQNAME= '970203F7.3sd' / Data system unique filename for image OBSERVER= 'David Gonzalez' / Observer names OWNER = 'OSU-03A-0001' / SMARTS Project ID code TIMESYS = 'UTC ' / Time system (usually UTC) TIME-OBS= '00:11:57' / UTC time at start of exposure [hhmmss.s] DATE-OBS= '2003-11-09' / UTC date at start of exposure [ccyy-mm-dd] JD = 2452952.50831 / Julian Day NumberThe IMAGETYP card records the type of image being acquired. Four image type codes used with the ANDICAM:
OBJECT - science targets or standard stars FLAT - flat field images BIAS - bias (aka "zero") images DARK - dark imagesOBJECT records the object name given in the observing template file that was executed to acquire the observations.
FILENAME is the name of the raw data file, and is used as the basis for the subsequent processed data files (see ANDICAM Data Products). The UNIQNAME is a coded, sequential unique name assigned to all ANDICAM images, that has no meaning for users.
The OBSERVER card records the name of the on-site observer who took the data.
The OWNER card records the SMARTS project ID code of the program that "owns" the data file. This is a unique code assigned to all approved SMARTS ANDICAM projects. At the Yale Data Repository, all incoming data are ultimately sorted by this "OWNER" code to keep the data for all of the currently executing SMARTS projects separate.
The remaining 4 cards record the date and time of the observation in Coordinated Universal Time (UTC). These all refer to the START of the observation. Times are acquired from the 1.3m telescope control system (TCS). If the TCS is not responding (see Telescope Information Cards below), the time is taken from the data-taking computer system clock. Both the TCS and the data-taking computers are synched to UTC via a mountain-top network time server tied to a Global Positioning Satellite (GPS) receiver. See Keywords Describing Time below for details on the time info.
TCSLINK = 'Enabled ' / TCS Link Status [Enabled|Disabled|Idle] RA = '18:04:14.15' / Telescope Right Ascension [hhmmss.ss] DEC = '-31:06:29.2' / Telescope Declination [ddmmss.s] EQUINOX = 2000.000 / Equinox of RA and DEC ST = '22:39:47' / Local Sidereal Time [hhmmss.s] HA = '+04:35:14' / Hour Angle [hhmmss.s] SECZ = 1.90 / Airmass [Sec ZD] TELFOCUS= -7 / Telescope focus TELTEMP = 18.6 / Telescope temperature [degrees C]These data should be self-explanatory. On rare occasions when the TCS communication link is temporarily disabled (i.e., the TCS does not respond to remote queries), the TCSLINK keyword is set to "Disabled" and appropriate "blank" values are inserted in place of the other parameters. The starting time is always recorded from a reliable source (see above), so in principle it is possible to recover this information after the fact from the DATE-OBS and DATE-OBS cards.
OBSERVAT= 'CTIO ' / Observatory name TELESCOP= 'ct13m ' / 1.3m ex-2MASS Telescope LATITUDE= '-30:09:54' / Telescope Latitude [degrees] LONGITUD= '+70:48:54' / Telescope Longitude [degrees W] ALTITUDE= 2215 / Telescope Altitude [meters]
These header cards record information about CCD images
EXPTIME = 300.0 / Exposure time [seconds] DARKTIME= 302.04 / dark current integration time [seconds]EXPTIME records the total time the shutter was open and the CCD exposed to light. DARKTIME is roughly 2-seconds longer than EXPTIME, and includes some setup overhead that is incurred between the time when the CCD is erased and the shutter is opened and the exposure timer started.
INSTRUME= 'ANDICAM-CCD' / Instrument name DETECTOR= 'Fairchild 447' / Detector nameThese record that the data are indeed from the CCD channel of ANDICAM, and give the name of the detector.
FORMAT = 'DELACED ' / Data interlacing & coadding info ERASE = 1 / Number of CCD erase cycles before integration READOUT = 'UL ' / Readout mode used MPPMODE = 'OFF ' / Multi-Pinned Phase mode on or off CBBAGAIN= 0 / Analog gain jumper setting on clock bias board PA_TAU = 0.0000E+0 / Charge integrator time constant, seconds PIXITIME= 9.9000E-6 / Pixel charge integration, seconds GAINDL = 32 / Pixel charge integration, sequencer clocks XAMP = 1 / Number of amplifiers along fast-readout axis YAMP = 1 / Number of amplifiers along slow-readout axisThese cards record engineering data about the CCD erase and readout process. They are not of much use to observers, but could help if there are problems.
COMMENT ob03423ih.obs CCDFILT = 0 / CCD Channel Filter Wheel position CCDFLTID= 'I ' / CCD Channel Filter IDThe COMMENT card is used here to record the name of the Observing Template File that was used to execute this observation. CCDFLTID records the name of the filter used, whereas CCDFILT records the CCD filter wheel position (numbers 0-9) where that filter is located.
These header cards record information about CCD images
EXPTIME = 50.04 / Exposure time [seconds] NCOADDS = 1 / Number of coadded framesEXPTIME records the total integration time on the IR array, which is the time elapsed between array reset and readout (the IR array has no shutter, it is operated in a "staring" mode). The smallest integration time is roughly 4 seconds (the time required to reset the array). If more than one image is being co-added together to form a single "exposure", NCOADDS records the number of frames, and EXPTIME in that case is the exposure time per co-add, not the cumulative exposure time. NCOADDS should always be 1, unless you are doing something special.
INSTRUME= 'ANDICAM-IR' / Instrument name DETECTOR= 'NICMOS 4' / Detector nameThese record that the data are indeed from the IR channel of ANDICAM, and give the name of the detector.
FORMAT = 'DELACED ' / Data interlacing & coadding info NSAMPLES= 1 / Number of samples READMODE= 'RESET-PRE-POST' / Readout type: Post-Reset-Pre or Reset-Pre-Post SAMPLED = 'FRAMES ' / Sample type: frame, line or column GAINDL = 35 / Pixel integration time, sequencer clocks PIXITIME= 1.0800E-5 / Pixel integration time, seconds RESETDL = 1 / Line reset time, sequencer clocksThese cards record engineering data about the IR array reset and reaout procedure. They are not of much use to observers, but could help if there are problems.
COMMENT ob03423ih.obs IRFILT = 2 / IR Channel Filter Wheel position IRFLTID = 'H ' / IR Channel Filter ID TILT1 = 1388 / IR T-T Mirror Mechanism 1 position TILT2 = 2015 / IR T-T Mirror Mechanism 2 position TILT3 = 2120 / IR T-T Mirror Mechanism 3 position FOCUS = 1841 / IR Mirror focus positionThe COMMENT records the name of the Observing Template File that was used to execute this observation. IRFLTID records the name of the filter used, whereas IRFILT records the IR filter wheel position (numbers 0-7) where that filter is located.
The TILTn keywords record the positions of the 3 actuators used to tip/tilt/focus the IR channel's tip-tilt (T-T) mirror. This is the mirror used to internally dither the images if "DITHER" is enabled in the observing template. The units are in native encoder units, and are relative to the nominal "centered" position. They are useful only if you are trying to verify that the particular IR image was indeed tilted as required. FOCUS records the nominal IR mirror focus position, which is used to conjugate the focii of the IR and CCD channels.
All of the raw data (CCD and IR) undergo some minimum post-processing through IRAF before delivery to investigators, as described in the ANDICAM Data Products document. The header keywords written into the headers by this processing are as follows:
ORIGIN = 'NOAO-IRAF FITS Image Kernel July 1999' / FITS file originator EXTEND = F / File may contain extensions DATE = '2003-11-09T14:43:24' IRAF-TLM= '09:43:24 (09/11/2003)'ORIGIN is self-explanatory. The EXTEND keyword shows that the original data format (FITS image with no extensions) has been retained by IRAF when it created the new (processed) file.
The DATE keyword gives the UTC date and time when the processed image was created (NOT the observing time, these are in DATE-OBS and TIME-OBS). This date is coded in ISO8601-conformal long date format. The IRAF-TLM gives the same information for local time in its own, non-ISO8601 format).
WCSDIM = 2 LTM1_1 = 1. LTM2_2 = 1. WAT0_001= 'system=physical' WAT1_001= 'wtype=linear' WAT2_001= 'wtype=linear' OVERSCAN= 'Nov 9 8:30 Overscan section is [3:14,1:1024] with mean=191.6306' ZEROCOR = 'Nov 9 8:30 Zero level correction image is ccd031108.bias.fits' FLATCOR = 'Nov 9 8:30 Flat field image is ccd031105.skyflatI.fits with scale=' CCDSEC = '[1:1051,1:1028]' CCDPROC = 'Nov 9 8:30 CCD processing done'The first 6 keywords (WCSDIM through WAT2_001) are IRAF imposing a default world-coordinate system (WCS) on the image: here just physical coordinates in pixels. No astrometric calibration of ANDICAM data is performed by the pipeline.
The remaining keywords are written by the ccdproc task, and allow end-users of the data to reconstruct which bias section was used for overscan correction (OVERSCAN), and the exact "zero" (aka "bias") and flat field images were used (ZEROCOR and FLATCOR, respectively).
FIXPIX = 'Nov 9 5:38 Bad pixel file is /home/observer/iraf/badmap.pl' WCSDIM = 2 LTV1 = 0.25 LTV2 = 0.25 LTM1_1 = 0.5 LTM2_2 = 0.5 WAT0_001= 'system=physical' WAT1_001= 'wtype=linear' WAT2_001= 'wtype=linear'The FIXPIX keyword records the bad pixel map used by the IRAF fixpix task to fix bad pixels. The following 8 keywords (WCSDIM through WAT2_001) are IRAF imposing a default world-coordinate system (WCS) on the image: here recording that the units of the image are 2x2 binned pixels.
Beware: Some IRAF tasks (and other programs) will use these WCS keywords in processing, others will not. It can confuse issues when making measurements and translating "pixels" to "arcsec" using the detector pixel scale, as programs which use the WCS keywords can report sizes in the original unbinned pixel coordinates if WCS usage is not explicitly disabled, whereas programs that don't care about WCS keywords will report them in binned pixels.
The basis of this specification is The FITS Standard (NOST 100-2.0), Section 5.4.2.1 (Keywords Describing the History or Physical Construction of the [Header]).
CCYY-MM-DDwhere
CCYY is the calendar year
MM is the ordinal number of a calendar month (1=Jan to 12=Dec)
DD is the ordinal number of a day within a calendar month
Dates are to be expressed in UTC for dates since 1972, UT before.
CCYY-MM-DDThh:mm:ss[.sss...]where:
T = ISO 8601 time designator (literal T) hh = hour in the day mm = minutes after the hour ss[.sss...] = seconds after the minute. The fraction is optional
For example, IRAF writes a DATE card in the header (see Post-Processing Header Cards above) whenever it handles a FITS file, and uses this long-format date specification to record the time and date. Similarly, the Arcon data-taking system in use at the CTIO 1.5m and 0.9m telescopes use this long format for the DATE-OBS header card.
The set of time keywords adopted for ANDICAM headers are as follows:
Note that all times are referenced to the start of the observation. Some data-analysis programs are reported to implicitly modify these header cards in reduced data to reflect the time at mid-observation. This conversion must not be done for any ANDICAM data, as the archive software assumes the times are for the start of the observation and makes the conversion to mid-observation time explicitly.
Example:
DATE-OBS= '2003-08-07' / Date of start of observation in UTCIndicates that the integration began on 2003 August 7 UTC.
Example:
TIME-OBS= '20:54:49' / Time of start of observation in UTCThe time system used (UTC) is explicitly noted in the header, as well as in the TIMESYS keyword.
Example:
TIMESYS = 'UTC ' / Time scale is UTCIndicates that all times and dates in the FITS header are UTC, unless the comments associated with that keyword specify otherwise explicitly.
ANDICAM has made adoption of UTC timesystem mandatory for all data.
Example:
JD = 2451033.37140 / Julian Date of start of obsNote that 5-digits of precision on the JD implies a relative time precision of 0.86 seconds, which is what is delivered by the CTIO 1.3m TCS.
Keywords Describing Coordinates
These keywords give the celestial coordinates of the object being observed. Except for EQUINOX, the NOST FITS standard does not reserve keywords for this purpose, but a set of keywords have entered into common use. The coordinate keywords defined for ANDICAM headers are as follows:
Example:
RA = '18:18:00.0' / Object RA at EQUINOX DEC = '-33:30:00' / Object DEC at EQUINOX
NOTE:
For the CTIO 1.3m telescope control system, RA and DEC
refer to the coordinates where the telescope
was pointing, not the celestial coordinates of the object
referred to by the OBJECT keyword.
Example:
EQUINOX = 2000.0 / Equinox of coordinatesUse of the EQUINOX is specifically defined by the NOST FITS standard, and is not equivalent to the deprecated keyword EPOCH, which does not appear in ANDICAM FITS headers. If the epoch of the observations is desired, you need to reference the DATE-OBS and TIME-OBS keywords.