Equatorial, Ecliptic and Galactic Coordinates

 

Source positions may be given in image pixels, sky pixels, detector pixels, equatorial coordinates right ascension (degrees) and declination (degrees), ecliptic longitude (degrees) and latitude (degrees) and galactic longitude (degrees) and latitude (degrees).        

Sky pixels are specified relative to a pointing direction. This pointing direction in case of EXSAS is specified by the descriptors POINT_LONG and POINT_LAT. The sky coordinates of the pointing direction are given in the descriptors SKY_CEN_X and SKY_CEN_Y, cf. Table 5.32.    

Detector coordinates are specified with respect to the optical axis of the instrument. The detector center is given in the descriptors   DET_CEN_X and DET_CEN_Y. The length of a sky pixel in arc seconds is expressed by the descriptor SKY_PIX_SIZE and the length of a detector pixel by the descriptor DET_PIX_SIZE. In source tables provided by the source detection the coordinates of the source positions are expressed in sky pixels and/or in image pixels. They can be converted to sky pixels, equatorial, ecliptic and galactic coordinates with the command

TRANSFORM/COORDINATES in_table [out_table] [equinox] -
                      [ref_image] [dist_flag] [ampl]

 

• in_table is the name of the source table with the columns :INDEX, :X_SKY and :Y_SKY ( or :X_IMA and :Y_IMA ( or :X_POS and :Y_POS ) ) and with the EXSAS header descriptors. In case the sky pixel columns are existing, they are used for the coordinate transformation, if not the image pixel columns :X_IMA and :Y_IMA are used and if they are not existing the image pixel coordinates :X_POS and :Y_POS are accessed.
• out_table is the name of the coordinate table with columns :INDEX, :X_POS, :Y_POS, :X_SKY, :Y_SKY, :RA_DEG, :DEC_DEG, :RA_HMS and :DEC_DMS and with the EXSAS descriptors. The default is trans.tbl
• equinox is the equinox (in decimal centuries) for which the right ascension and the declination will be calculated. The default value is 2000.0.  
• ref_image is the name of the reference image from which Midas standard and EXSAS general descriptors are read. If no reference image is given the descriptors are read from in_table, which is the default case.
• dist_flag is a flag specifying whether the distance to the next neighbour (source), the off-axis angle and the FWHM of the source shall be calculated (y) or not (n). In case of big tables the distance calculation slows down the calculation enormously. Then the setting dist_flag=n is recommended.
• ampl is the calibrated amplitude for which the FWHM is calculated. The default is ampl = 7.  

Examples are:

TRANSFORM/COORDINATES solst coo\_solst
TRANSFORM/COORDINATES solst coo\_solst 1950.0
TRANSFORM/COORDINATES mytable mycoord 2000.0 image1
TRANSFORM/COORDINATES long\_table long\_coord 2000.0 image n

  In the third example the name of the source table is mytable.tbl, the name of the transformed table is mycoord.tbl, the equinox for the coordinates is 2000.0. A reference image image1.bdf has been specified which contains the general EXSAS descriptors.

A reference image has to be given in case the source table does not contain a set of general EXSAS descriptors and of general MIDAS descriptors. In the standard source detection case a reference image has not to be specified.

The transformed table contains the information listed in Table 5.23.

  
Table 5.23: Coordinate information created by TRANSFORM/COORDINATES

A description of the information provided by the coordinate tables is given below.

• INDEX is the source index number.
• X_POS is the source x-position in units of image pixels. Image pixels are always given with respect to a reference image.
• Y_POS is the source y-position in units of image pixels. This coordinate is flipped as NPIX(2) + 1 -y. Image pixels are always given with respect to a reference image.  
• X_SKY is the source x-position in units of sky pixels. Sky pixels are specified relative to a pointing direction given in the descriptors POINT_LAT and POINT_LONG. The x,y sky pixels coordinates of the pointing direction are given in the descriptors SKY_CEN_X and SKY_CEN_Y, respectively.
• Y_SKY is the source y-position in units of sky pixels. Sky pixels are specified relative to a pointing direction given in the descriptors POINT_LAT and POINT_LONG. The x,y sky pixels coordinates of the pointing direction are given in the descriptors SKY_CEN_X and SKY_CEN_Y, respectively.
• RA_DEG is the right ascension in decimal degrees for equinox specified in the descriptor EPOCH.
• DEC_DEG is the declination in decimal degrees for equinox specified in the descriptor EPOCH.
• RA_HMS is the right ascension in hours,minutes,seconds for equinox specified in the descriptor EPOCH. The representation is as 05H42M10.0S in case of 5 hours, 42 minutes and 10.0 seconds.
• DEC_DMS is the declination in degrees,minutes,seconds for equinox specified in the descriptor EPOCH. The representation is as -69D30M25.0S in case of -69 degrees, 30 minutes and 25.0 seconds.
• LAMDA is the ecliptic longitude in decimal degrees for equinox specified in the descriptor EPOCH.
• BETA is the ecliptic latitude in decimal degrees for equinox specified in the descriptor EPOCH.
• LII is the galactic longitude in decimal degrees.
• BII is the galactic latitude in decimal degrees.
• OFFAX is the off-axis angle (with respect to the position specified in the SKY_CEN_X and SKY_CEN_Y descriptors). This value is different from the true off-axis-angle in case the sky position SKY_CEN_X, SKY_CEN_Y is not mapped to the detector center.  
• FWHM is the FWHM of the point spread function in units of sky pixels in the Gaussian model. The FWHM is evaluated at the calibrated amplitude channel AMPL=7. The off-axis-angle is calculated with respect to the SKY_CEN_X, SKY_CEN_Y position.
• DIST is the distance of the source to the nearest neighbour source in the coordinate table minus ( the Gaussian ) of that source in units of sky pixels.

Equatorial coordinates can be converted to image pixel and sky pixel coordinates with the command

TRANSFORM/RADEC_TO_PIXEL in_table [out_table] [ref_image] [equinox]

 

• in_table is the name of the coordinate table with columns :RA_DEG for right ascension (degree) and :DEC_DEG for declination (degree).
• out_table is the name of the transformed coordinate table with the columns :INDEX, :RA_DEG, :DEC_DEG, :X_IMA, :Y_IMA, :X_SKY and :Y_SKY and with the general EXSAS descriptors from ref_image.
• ref_image is the name of the reference image with the EXSAS header descriptors.
• equinox is the equinox for which right ascension and declination are specified. The default value is 2000.0.

Example:

TRANSFORM/RADEC_TO_PIXEL equ_coord pix_coord myimage 2000.0

  In this example the name of the coordinate table is equ_coord.tbl and the equatorial coordinates are specified for equinox 2000.0, the name of the transformed coordinate table is pix_coord.tbl and the name of the reference image is myimage.bdf. A reference image has to be specified in any case. The equatorial longitude and latitude of the source must be specified in decimal degrees and has to be written to the standard columns :RA_DEG and :DEC_DEG.

Equatorial coordinates can be converted to ecliptic and galactic coordinates (and vice versa) with the command

TRANSFORM/EQU_ECL_GAL in_table [out_table] [equinox] [ref_sys]

 

• in_table is the name of the coordinate table with the columns :RA_DEG for right ascension (degrees) and :DEC_DEG for declination (degrees).
• out_table is the name of the transformed coordinate table with the columns :INDEX, :RA_DEG, :DEC_DEG, :LAMDA, :BETA, :LII and :BII.
• equinox is the equinox for which the right ascension and the declination are specified. The default is 2000.0.
• ref_sys is the coordinate reference system. Coordinates must be specified in this system. EQU = equatorial (columns :RA_DEG and :DEC_DEG are evaluated), GAL = galactic (columns :LII and :BII are evaluated), ECL = ecliptic (columns :LAMDA and :BETA are evaluated).

Example:

TRANSFORM/EQU_ECL_GAL equ_coord ecl_gal_coord
TRANSFORM/EQU_ECL_GAL equ_coord ecl_gal_coord ? GAL

 

Note:The coordinates have to be specified for equinox 2000.0. With the Midas command COMPUTE/PRECESS equatorial coordinates can be   precessed from any equinox to equinox 2000.  

Equatorial, ecliptic and galactic coordinates have to be specified in degrees. The standard column labels are listed in Table 5.24.

  
Table 5.24: Standard labels for the TRANSFORM commands

The format is not fixed if a user provides his own table. As the commands to transform coordinates works on Midas tables with default names for the column labels, the user has to provide a MIDAS table with those columns. This he can do with the command WRITE/RADEC_TABLE (see section 5.2.30 in case of ref_sys = EQU.