Reference Documents
	Key	Local Link	Link
	FITS_home		FITS NASA Support Office Home Page
	FITS	standard_dict.html	FITS Keyword Standard
	WCS	~jsoc/keywords/FITS_and_WCS/	Representations of world coordinates in FITS
	SOLAR	coordinates.pdf	Bill Thompson WCS for Solar
	SSW	ssw_standards.html	SolarSoft Keyword Description
	MDI	MDI_lev1.5.txt	MDI Level1.5 Keywords
	MDI_2	l2_vel.txt	MDI level2 Keywords, older than "MDI" list above but more complete documentation
	SOHO	SoHO.FITS_keywords.pdf	Original SOHO Keywords
	Base set of FITS keys that define external format				n means 1 or more digits
	Class	DRMS Keyword	FITS Export for SSW	Ref	Note	Meaning
			SIMPLE	FITS		FITS required, file type tag. Always "T" for DRMS
			NAXIS	FITS		FITS required, rank
			NAXISn	FITS		FITS required, nth axis dimension
			EXTEND	FITS		Not used by DRMS FITS writer
			BITPIX	FITS		FITS required, data storage type
			BSCALE	FITS		FITS required, for int types, scale to physics units
			BZERO	FITS		FITS required, for int types, offset to physics units
			BLANK	FITS		FITS required, for int types missing data value
			END	FITS		FITS required, end of header
	FITS Standard keywords that describe the data processing
	Class	DRMS Keyword	FITS Export for SSW	Ref	Note	Meaning
			COMMENT	FITS	can have multiple instances	FITS commonly used, comment text, may be multiple instances
			DATE	FITS	ISO8601	FITS commonly used, Date file written, not data date.
			ORIGIN	FITS		FITS commonly used, Insititution, e.g. JSOC
			FILENAME	FITS		FITS commonly used, redundant on disk or tar images
			HISTORY	FITS	can have multiple instances	FITS commonly used, processing comments
	FITS Standard keywords that describe the data acquisition
	Class	DRMS Keyword	FITS Export for SSW	Ref	Note	Meaning
			DATE-OBS	FITS	MDI DATE-OBS is mean time as if at Earth.  See T_OBS	The time and date of the start of the observation, in UT at the observatory or spacecraft, in CCSDS ASCII Calendar Segmented (ISO 8601). Example: 1988-01-18T17:20:43.123
			DATE_OBS	SOHO	Not used	DATE_OBS was DATE-OBS + EARTH_DT, i.e. time as if SOHO was at the center of the Earth.  For SDO same as DATE-OBS.
			DATE-END	FITS	not used, DATE-OBS + EXPTIME
		duration	EXPTIME			The duration of the observation, in seconds (floating point).
			TELESCOP	FITS		set to SDO
			INSTRUME	FITS		one of HMI, HMI_V, HMI_M, AIA, AIA_xx where xx is 1,2,3,4 for camera number or code for wavelength, TBD
			OBJ_ID	SSW
			SCI_OBJ	SSW
			OBSERVER	FITS
			OBS_PROG	SSW
			WAVELNTH	SSW		HMI: 617.3, AIA: list options here.
			OBJECT	FITS
		T_OBS		MDI	MDI DSDS data uses T_OBS same as DATE-OBS - EARTH_DT	Same as DATE-OBS + EXPTIME/2.0, but may be corrected past level-0.  I.e. DATE-OBS will be the onboard time but T_OBS may be corrected if onboard time dirfts.
	FITS Standard general array index mapping to physical coordinate units				i is axis in "world space", j is axis in raw image space, c means 0 or 1 letter for the case where multiple mappings are given.
	Class	DRMS Keyword	FITS Export for SSW	Ref	Note	Meaning
			CDi_j	WCS	NOT used, use PCn_n and CRPIX
			CDELTi	WCS	float	unit of increment on ith axis after any rotation.
			CRPIXjc	WCS	int	Reference pixel, center of rotation and applicable pixel for CRVAL.
			CRVALic	WCS	float	world coordinate value at CRPIX
			CTYPEic	WCS	string	Axis type, should be from CTYPE list below.
			CUNITic	WCS	string	Physical units of CRPIX, CRVAL
			PCi_jc	WCS	floats	Rotation matrix
			CROTAj	WCS	float, consider PCi_j instead.	Amount, in degrees, to rotate the image plane, + is CCW.  Note this is opposite to the astronomical position angle convention.
			WCSAXES	WCS
	SSW Solar image location in array
	Class	DRMS Keyword	FITS Export for SSW	Ref	Note	Meaning
			XCEN	SSW	This defn implies CTYPE1 = SOLAR_X	East-West FOV center of image relative to sun center in CDELT1 units, positive West. If SDO_ROLL = 0 then XCEN is related to the above FITS keywords by: XCEN = CRVAL1 + CDELT1 * [(NAXIS1+1)/2 - CRPIX1 ] - show full formula here
			YCEN	SSW	This defn implies CTYPE2 = SOLAR_Y	North-South FOV center of image relative to sun center in CDELT2 units, positive North. If SDO_ROLL = 0 then YCEN is related to the above FITS keywords by: YCEN = CRVAL2 + CDELT2 * [(NAXIS2+1)/2 - CRPIX2 ] - show full formula here
	MDI Solar image location in array
	Class	DRMS Keyword	FITS  MDI	Ref	Note	Meaning
		R_SUN	R_SUN
		X0	X0
		Y0	Y0
	SDO Attitude and FDS data, Instrument constants.
	Class	DRMS Keyword	FITS Export for SSW	Ref	Note	Meaning
		SDO_ROLL	SDO_ROLL	JSOC		Commanded (lev < 1) or Actual (lev >= 1) SDO roll wrt the projection of the solar rotation axis onto the "y" axis of the instruments CCD.  In degrees, positive is CCW roll of SDO when viewed from behind looking toward the Sun
		D_SUN				Distance from Sun center to SDO in km
		SUN_RAD				Assumed radius of Sun in km TBD but ~ 696000
		IM_SCALE				Plate scale in arc-sec per pixel assuming square pixels.  For HMI is about 0.5.  Fixed for lev < 1 but computed from R_SUN, D_SUN, and SUN_RAD for lev >= 1.  For AIA fixed value per telescope.
	CTYPE list
	TYPE	Authority	Meaning			Reference
	SOLAR_X	SSW	east-west on the Sun in arc-sec from CM, + is to west.  For SOLAR_X to be used the rotation CROTAj or PCi_j must be such that the solar rotation axis projects onto a column of the rotated image.  SOLAR_X should be applied to CTYPE1.			http://www.lmsal.com/solarsoft/ssw_standards.html
	SOLAR_Y	SSW	north-south on Sun in arc-sec from disk center, + is north.  For SOLAR_Y to be used, the rotation CROTAj or PCi_j must be such that the solar rotation axis projects onto a column of the rotated image.  SOLAR_Y should be used for CTYPE2.			http://www.lmsal.com/solarsoft/ssw_standards.html