FITS Header Configuration

Astra automatically generates comprehensive FITS headers for all captured images using metadata from your observatory devices. This system ensures every image contains the scientific metadata required for analysis.

The FITS header system uses a CSV configuration file that maps device properties and observatory parameters to standard FITS keywords. This approach provides flexibility while maintaining consistency across observations.

Configuration File Format

The CSV configuration file defines how FITS headers are constructed:

• header: FITS header keyword

• dtype: Data type (string, integer, float, boolean)

• fixed: Whether the value is set at the beginning of an imaging sequence (true) or filled later from polling (false)

• device_type: Source device type or data source

• device_command: ASCOM command or data source identifier

• comment: Description of the header keyword

Example Configuration

Example FITS Header Configuration

header	dtype	fixed	device_type	device_command	comment
SIMPLE	bool	true	astropy_default		conforms to FITS standard
BITPIX	int	true	astropy_default		array data type
NAXIS	int	true	astropy_default		number of array dimensions
NAXIS1	int	true	astropy_default		# of pixels/row
NAXIS2	int	true	astropy_default		# of rows (also # of scan lines)
EXTEND	bool	true	astra_fixed	1	Extensions may be present
BZERO	float	true	astropy_default		value = fits-value*BSCALE+BZERO
BSCALE	float	true	astropy_default		value = fits-value*BSCALE+BZERO
BUNIT	str	true	astra_fixed	ADU	Physical unit of array values
BLANK	int	true	astra_fixed	-32768	Value used for NULL pixels
ORIGIN	str	true	astra_fixed	ORIGIN	Observatory
TELESCOP	str	true	astra_fixed	TELESCOPE	Telescope Name
INSTRUME	str	true	astra_fixed	INSTRUMEMENT	Instrument used
PI-COI	str	true	astra_fixed	PI-COI	Name of the PI/Co-I
OBSERVER	str	true	astra_fixed	Astra	Name of the observer
OBJECT	str	true	astra		Target as given by the user
RADESYS	str	true	astra_fixed	FK5	Reference system
EQUINOX	float	true	astra_fixed	2000	Catalog equinox of the object coords
DATE	str	false	astra		UTC datetime file written
MJD-OBS	float	false	astra		Modified Julian Date at start of exposure
DATE-OBS	str	false	astra		UTC datetime start of exposure
TIMESYS	str	true	astra_fixed	UTC	Time system used
EXPTIME	float	true	astra		[s] Exposure time
LST	float	false	astra		[s] Local Sidereal Time at start of exposure
HA	str	false	astra		[hms] Hour Angle
UTC	float	false	astra		[s] Coordinated Universal Time at start of exposure
CHECKSUM	str	false	astra		HDU checksum
DATASUM	str	false	astra		Data unit checksum
XBINNING	int	true	Camera	BinX	Binning level along the X-axis
YBINNING	int	true	Camera	BinY	Binning level along the Y-axis
XPIXSZ	float	true	astra		Pixel Width in microns (after binning)
YPIXSZ	float	true	astra		Pixel Height in microns (after binning)
XORGSUBF	int	true	Camera	StartX	Subframe X position in binned pixels
YORGSUBF	int	true	Camera	StartY	Subframe Y position in binned pixels
CAM-SNAM	str	true	Camera	SensorName	Camera sensor name
CAM-STYP	str	true	Camera	SensorType	Camera sensor type
CAM-DNAM	str	true	Camera	Name	Short name of Camera driver
CAM-DVER	str	true	Camera	DriverVersion	Camera driver version
SET-TEMP	float	true	Camera	SetCCDTemperature	[Celsius] CCD temperature setpoint
CCD-TEMP	float	false	Camera	CCDTemperature	[Celsius] CCD temperature at start of exposure
CAM-STAT	int	false	Camera	CameraState	Camera status
FILTER	str	true	astra		Filter name
FW-POS	str	false	FilterWheel	Position	FilterWheel position
FW-NAME	str	true	FilterWheel	Name	FilterWheel name
FW-DVER	str	true	FilterWheel	DriverVersion	FilterWheel driver version
FOCUSPOS	float	false	Focuser	Position	[steps] Focuser position
FOCUSSSZ	float	true	Focuser	StepSize	[micron] Focuser step size
FOCUSTEM	float	false	Focuser	Temperature	[Celsius] Focuser temperature
RA	float	false	Telescope	RightAscension	[deg] Target Right Ascension
DEC	float	false	Telescope	Declination	[deg] Target Declination
APTAREA	float	true	astra	ApertureArea	[m^2] Aperture area of telescope
APTDIA	float	true	astra	ApertureDiameter	[m] Aperture diameter of telescope
FOCALLEN	float	true	astra	FocalLength	[m] Focal length of telescope
ALT-OBS	float	true	Telescope	SiteElevation	[m] Altitude above mean sea level
LAT-OBS	float	true	Telescope	SiteLatitude	[deg +N WGS84] Geodetic latitude
LONG-OBS	float	true	Telescope	SiteLongitude	[deg +E WGS84] Geodetic longitude
ALTITUDE	float	false	Telescope	Altitude	[deg] Telescope altitude in horizontal coordinates
AZIMUTH	float	false	Telescope	Azimuth	[deg] Telescope azimuth in horizontal coordinates
AIRMASS	float	false	astra		Averaged airmass
TEL-DNAM	str	true	Telescope	Name	Short name of Telescope driver
TEL-DVER	str	true	Telescope	DriverVersion	Telescope driver version
TRACKING	bool	false	Telescope	Tracking	Tracking
SLEWING	bool	false	Telescope	Slewing	Slewing
TELPARK	bool	false	Telescope	AtPark	Telescope at park
PIERSIDE	bool	false	Telescope	SideOfPier	ASCOM Telescope side of pier (1 = West 0 = East)
DOMEAZ	float	false	Dome	Azimuth	[deg] Dome azimuth in horizontal coordinates
DOMESTAT	int	false	Dome	ShutterStatus	Dome shutter status
DOM-DNAM	str	true	Dome	Name	Short name of Dome driver
DOMPARK	bool	false	Dome	AtPark	Dome at park
IMAGETYP	str	true	astra		Type of image
DEWPOINT	float	false	ObservingConditions	DewPoint	[Celsius] Dew point
HUMIDITY	float	false	ObservingConditions	Humidity	[%] Ambient atmospheric humidity
AMBTEMP	float	false	ObservingConditions	Temperature	[Celsius] Ambient temperature
WINDSPD	float	false	ObservingConditions	WindSpeed	[m/s] Wind speed
SKYTEMP	float	false	ObservingConditions	SkyTemperature	[Celsius] Sky temperature
SKYLUX	float	false	ObservingConditions	SkyBrightness	[lux] Sky brightness
OBC-DNAM	str	true	ObservingConditions	Name	Short name of ObservingConditions driver
OBC-DVER	str	true	ObservingConditions	DriverVersion	ObservingConditions driver version
SAF-DNAM	str	true	SafetyMonitor	Name	Short name of SafetyMonitor driver
SAF-DVER	str	true	SafetyMonitor	DriverVersion	SafetyMonitor driver version
JD-OBS	float	false	astra		Julian Date at start of exposure
HJD-OBS	float	false	astra		Heliocentric Julian Date at start of exposure
BJD-OBS	float	false	astra		Barycentric Julian Date at start of exposure
JD-END	float	false	astra		Julian Date at end of exposure
MJD-END	float	false	astra		Modified Julian Date at end of exposure
DATE-END	str	false	astra		[ISO 8601] UTC date/time at end of exposure
ASTRA	str	true	astra		Version of Astra

How FITS Headers Are Created

Astra creates FITS headers in two stages to ensure complete and accurate metadata:

1. Base Headers (at the beginning of an imaging sequence)

When an imaging sequence begins, Astra immediately creates an initial FITS header containing (if fixed=true in the configuration):

• Observatory information: Site name, coordinates, altitude

• Optical system: Aperture diameter, area, focal length

• Instrument details: Camera settings, filter name, pixel scale

• Observation metadata: Object name, exposure time, image type placeholders

• Software information: Astra version number

• Fixed device values: Properties marked as fixed=True in the configuration

TODO: note about IMAGETYP EXPTIME

2. Final Headers (after exposure)

After an imaging sequence, Astra completes any missing header values:

• Identifies images with incomplete headers

• Retrieves device polling data from ±10 seconds around each exposure time (stored in a local sqlite database)

• Interpolates device readings (temperature, pointing, focus position, etc.) to match exposure timestamps

• Updates FITS files in place while preserving original header structure

• Adds calculated values such as:

  • Various time scales and reference frames

  • Derived parameters like airmass

This two-stage approach ensures headers contain accurate, time-synchronized metadata, while minimizing time loss between exposures.

Customizing FITS Headers

You can customize the FITS headers by modifying the CSV configuration file:

• Adding headers: Add new rows to include additional metadata

• Removing headers: Delete rows to exclude certain keywords

• Timing control: Set fixed=True for immediate values, fixed=False for post-exposure interpolated values