Thursday, June 14, 2007

Reference Frequencies (AIPS v. Miriad)

I went back and reviewed my early post; this is an update, with a few more details.

I've been working with GMRT data reduced in AIPS, and ATCA data reduced in MIRIAD.
It turns out that (at least for GMRT observations) AIPS gives the reference frequency (in imhead or prtan) as the frequency in the middle of the first channel, whereas Miriad gives the reference frequency (in prthd or uvlist, options=spectral) as that measured at the end of the first channel. But don't freak out yet--the channels still contain the same data, they are just identified differently in AIPS vs Miriad. See "read more" for details.

It's relatively easy to see from reading the headers, so I'm not sure why this caused me trouble when I first posted it (notice the "at Pixel" in the AIPS header):

From AIPS:

AIPS 1: Image=NGC2997 (UV) Filename=15JAN .UVLIN . 1
AIPS 1: Telescope=GMRT Receiver=GMRT
AIPS 1: Observer=PISANO User #= 43
AIPS 1: Observ. date=14-JAN-2007 Map date=18-APR-2007
AIPS 1: # visibilities 325734 Sort order TB
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 2 -1.0000000E+00 1.00 -1.0000000E+00 0.00
AIPS 1: FREQ 128 1.4129999E+09 0.50 3.1250000E+04 0.00
AIPS 1: IF 1 1.0000000E+00 1.00 1.0000000E+00 0.00
AIPS 1: RA 1 09 45 38.811 1.00 3600.000 0.00
AIPS 1: DEC 1 -31 09 27.768 1.00 3600.000 0.00
AIPS 1: ----------------------------------------------------------------
AIPS 1: Coordinate equinox 2000.00
AIPS 1: Rest freq 0.000 Vel type: RADIO wrt YOU
AIPS 1: Alt ref. value 0.00000E+00 wrt pixel 1.00
AIPS 1: Max version number of extension files type HI is 1
AIPS 1: Max version number of extension files type FQ is 1
AIPS 1: Max version number of extension files type AN is 1


Filename: 15jan.uvlin/
Telescope: GMRT
Object: ngc2997 Observer: PISANO
First time: 07JAN15:18:31:06.8
Number of antennae: 30
Polarisations Present: RR,LL
Type of correlations present: crosscorrelation
Spectral Correlations:
Spectrum Channels Freq(chan=1) Increment Restfreq
1 128 1.41302 0.000031 1.42041 GHz
Total number of correlations: 83387904
Correlations are stored in 16-bit form
J2000 Source RA: 9:45:38.811 Dec: -31:09:27.76
Apparent Source RA: 9:45:58.554 Dec: -31:11:18.61

As a side note, when checking the reference freq in AIPS check the outputs of both imhead and prtan--an AIPS bug is that occassionally they don't match!
From Chapter 8 of the AIPS cookbook:
"Another, more serious, problem is that two locations in the FITS file give the "reference frequency" -- the header and the antenna table. In principle these should agree. However there are instances when they will differ. Task FITS will issue a warning when this happens, and will use the antenna table reference frequency as the true reference frequency. In some cases this will be the correct frequency. In others it will not." 8-7

**Note, this is now consistent with one of Laura's later posts (Edited by KMH 9/6/07).

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