DBCONing Data Sets From Different Epochs

If you are an archive monger like me, you might find yourself reducing one J2000. data set and one B1950. data set, and then wanting to concatenate them together. If you try to do this, DBCON will yell at you. So...

You will have to run EPOSWTCH on the 1950 data set to bring it into epoch 2000. However, if you try to run DBCON now, DBCON will still yell at you and say:

tsingt> DBCON1: DEC      AXIS HAS UNEQUAL ATTRIBUTES
tsingt> DBCON1: Purports to die of UNNATURAL causes

As explained by Eric Greisen here , this is because the image you EPOSWTCHed now has a Declination axis with a rotation imposed upon it. For example:

AIPS 1: Image=NGC1569   (UV)         Filename=N1569_3B    .SC_P1 .   1
AIPS 1: Telescope=VLA                Receiver=VLA
AIPS 1: Observer=AA116               User #=   53
AIPS 1: Observ. date=21-AUG-1990     Map date=06-JUN-2008
AIPS 1: # visibilities    135940     Sort order  TB
AIPS 1: Rand axes: UU-L-SIN  VV-L-SIN  WW-L-SIN  BASELINE  TIME1
AIPS 1:            WEIGHT  SCALE
AIPS 1: ----------------------------------------------------------------
AIPS 1: Type    Pixels   Coord value     at Pixel     Coord incr   Rotat
AIPS 1: COMPLEX      1   1.0000000E+00       1.00  1.0000000E+00    0.00
AIPS 1: STOKES       4  -1.0000000E+00       1.00 -1.0000000E+00    0.00
AIPS 1: FREQ         1   8.4149000E+09       1.00  5.0000000E+07    0.00
AIPS 1: IF           2   1.0000000E+00       1.00  1.0000000E+00    0.00
AIPS 1: RA           1    04 30 49.498       1.00       3600.000    0.00
AIPS 1: DEC          1    64 50 59.587       1.00       3600.000   -0.60
AIPS 1: ----------------------------------------------------------------
AIPS 1: Coordinate equinox 2000.00
AIPS 1: Rest freq      0.000         Vel type: OPTICAL wrt YOU
AIPS 1: Alt ref. value  0.00000E+00  wrt pixel    1.00
AIPS 1: Maximum version number of extension files of type FQ is   1
AIPS 1: Maximum version number of extension files of type HI is   1
AIPS 1: Maximum version number of extension files of type AN is   1
AIPS 1: Maximum version number of extension files of type WX is   1
AIPS 1: Maximum version number of extension files of type FG is   1

You can get rid of this rotation using UVSRT:

AIPS 1: UVSRT:  Task which sorts UV data
AIPS 1: Adverbs     Values                 Comments
AIPS 1: ----------------------------------------------------------------
AIPS 1: INNAME     'N1569_3B'              Input UV file name (name)
AIPS 1: INCLASS    'SC_P1'                 Input UV file name (class)
AIPS 1: INSEQ         1                    Input UV file name (seq. #)
AIPS 1: INDISK        1                    Disk unit # of input UV data
AIPS 1: OUTNAME    ' '                     Sorted UV file name (name)
AIPS 1: OUTCLASS   'ROTAT'                 Sorted UV file name (class)
AIPS 1: OUTSEQ        0                    Sorted UV file name (seq. #)
AIPS 1: OUTDISK       0                    Disk unit # of sorted UV data
AIPS 1:                                      0 => highest with space
AIPS 1:                                    If outfile spec. equals the
AIPS 1:                                    infile spec., output will
AIPS 1:                                    overwrite the input
AIPS 1: BADDISK    *all 0                  Disk drive #'s to avoid
AIPS 1: SORT       ' '                     Two char. sort order, eg.'XY'
AIPS 1:                                    blank => 'XY'
AIPS 1: ROTATE        0.6                  Angle thru which to rotate.
AIPS 1:                                    positive rotates CCW
AIPS 1: DEFER         0                    Defer output file creation?

Note that the ROTATE parameter is the negative of the rotation noted in the header. Now you should be able to DBCON the EPOSWTCHed data with the other J2000 data set!

(You may have to UVSRT the DBCONed data so that it is in TB order again.)