[daip] Reference spectral pixel in AVSPC

[Eric Greisen]

Richard Rand wrote:
> 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'

The result is correct.  The pixel counter is an integer in the center of 
the pixel - so the ref pixel goes from 0.5 to 1.5 and the 2nd pixel goes 
from 1.5 to 2.5.  In the new data set, pixel 1 goes from 0.5 to 1.5 and 
the ref frequency which is not changed is 1/4 of a new pixel from the 
low end or 0.75.  The full formula is
          TMP = NCHAVG
          CATR(KRCRP+JLOCF) = ((CATR(KRCRP+JLOCF) - 1.0 -
      *      ((TMP-1.0)/2.0)) / TMP) + 1.0

Eric Greisen