[daip] Reference spectral pixel in AVSPC

[Richard Rand]

Hi,

I am trying to use AVSPC to rebin uv data in frequency.  I do not 
understand what the task assigns as the output reference spectral pixel.
For instance, if the input file header says:

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       2  -1.0000000E+00       1.00 -1.0000000E+00    0.00
AIPS 1: FREQ         2   1.4148917E+09       1.00  4.8828125E+04    0.00
AIPS 1: IF           1   1.0000000E+00       1.00  1.0000000E+00    0.00
AIPS 1: RA           1    12 21 37.600       1.00       3600.000    0.00
AIPS 1: DEC          1    14 36 07.000       1.00       3600.000    0.00

And I try to bin by a factor of 2, the output header looks like:

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       2  -1.0000000E+00       1.00 -1.0000000E+00    0.00
AIPS 1: FREQ         1   1.4148917E+09       0.75  9.7656250E+04    0.00
AIPS 1: IF           1   1.0000000E+00       1.00  1.0000000E+00    0.00
AIPS 1: RA           1    12 21 37.600       1.00       3600.000    0.00
AIPS 1: DEC          1    14 36 07.000       1.00       3600.000    0.00


Why is the reference pixel on the freq axis now a fraction?  I doubt 
that AVSPC does a fractional pixel shift, so I'm not sure it is correct. 
  Below are my inputs to AVSPC.  Is spectral pixel 1.00 the center of 
the pixel or an edge?

Thanks for your help.

Rich Rand

***************************************************************************


 >inp
AIPS 1: AVSPC     Task to average uv data in frequency.
AIPS 1: Adverbs     Values                 Comments
AIPS 1: ----------------------------------------------------------------
AIPS 1: INNAME     'N4302AEREE'            Input UV file name (name)
AIPS 1: INCLASS    'UVLIN2'                Input UV file name (class)
AIPS 1: INSEQ         2                    Input UV file name (seq. #)
AIPS 1: INDISK        2                    Input UV file disk unit #
AIPS 1: SOURCES    *all ' '                Source name
AIPS 1: QUAL         -1                    Calibrator qualifier -1=>all
AIPS 1: CALCODE    ' '                     Calibrator code '    '=>all
AIPS 1: STOKES     ' '                     Stokes of output
AIPS 1: TIMERANG   *all 0                  Time range to use
AIPS 1: SELBAND      -1                    Bandwidth to select (kHz)
AIPS 1: SELFREQ      -1                    Frequency to select (MHz)
AIPS 1: FREQID        1                    Freq. ID to select.
AIPS 1: SUBARRAY      0                    Sub-array, 0=>all
AIPS 1: BIF           1                    Low IF number to do
AIPS 1: EIF           0                    Highest IF number to do
AIPS 1: DOCALIB       0                    > 0 calibrate data & weights
AIPS 1:                                    > 99 do NOT calibrate weights
AIPS 1: GAINUSE       0                    CL (or SN) table to apply
AIPS 1: ** press RETURN for more, enter Q or next line to quit print **
#
AIPS 1: DOPOL        -1                    If >0.5 correct polarization.
AIPS 1: BLVER        -1                    BL table to apply.
AIPS 1: FLAGVER       0                    Flag table version
AIPS 1: DOBAND        0                    If >0.5 apply bandpass cal.
AIPS 1:                                    Method used depends on value
AIPS 1:                                    of DOBAND (see HELP file).
AIPS 1: BPVER        -1                    Bandpass table version
AIPS 1: SMOOTH     *all 0                  Smoothing function. See
AIPS 1:                                    HELP SMOOTH for details.
AIPS 1: DOACOR       -1                    Include autocorrelations?
AIPS 1: OUTNAME    'N4302AEREE'            Output UV file name (name)
AIPS 1: OUTCLASS   'UVLIN3'                Output UV file name (class)
AIPS 1: OUTSEQ        0                    Output UV file name (seq. #)
AIPS 1: OUTDISK       2                    Output UV file disk unit #.
AIPS 1: ICHANSEL   *all 0                  Array of start and stop chn
AIPS 1:                                    numbers, plus a channel
AIPS 1:                                    increment and IF to be used
AIPS 1:                                    for channel selection in the
AIPS 1:                                    averaging. See HELP ICHANSEL.
AIPS 1:                                    Default = center 75% of band.
AIPS 1: AVOPTION   'SUBS'                  Averaging option '  ','SUBS'
AIPS 1: ** press RETURN for more, enter Q or next line to quit print **
#
AIPS 1: CHANNEL       2                    # chans to average together
AIPS 1:                                    used when AVOPTION = 'SUBS'
 >