INCORRECT POSITIONAL PARAMETERS IN ARECIBO FITS-HEADERS
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A new version of FITS-headers for spectral line data coming from the
WAPPs at Arecibo was introduced 16 September 2004. An interpolation
routine was added as part of the upgrade that would interpolate
telescope position parameters so that these parameters would
correspond to the exact time when the data was taken. Unfortunately,
this routine was rushed into service before having been properly
tested. As it turned out there were several severe bugs present in
this routine. The result is that a number of FITS-header keywords
dealing with telescope position information always contain erroneous
values. This problem affects all FITS-files generated between 16
September and 9 November 2004.

A corrected interpolation routine was installed 15 November 2004.


AFFECTED KEYWORDS:

The affected keywords are:

CRVAL2A and CRVAL3A             (beam position on sky in RA/Dec)
CRVAL2B and CRVAL3B             (beam position on sky in Az/ZA)
CRVAL2G and CRVAL3G             (beam position on sky in l/b)
CROFF2 and CROFF3               (map center offset in RA/Dec)
CROFF2B and CROFF3B             (map center offset in Az/ZA)
BEAM_OFFRAJ and BEAM_OFFDECJ    (total beam offset in RA/Dec)
PARA_ANG                        (parallactic angle)


SYMPTOMS

All the above-mentioned keywords are based on the telescope position
calculated as the commanded position with the tracking error offset
added to it. The problem has been with the calculation of the
interpolated tracking error offset. The values may thus at first
glance look reasonable, while they in fact always suffer from the
addition of a more or less random offset. The offset is often less
than an arcminute but is also rather frequently larger than one
degree, especially at the beginning of a scan.

Instead of interpolating the tracking errors, the routine made huge
extrapolations of them, thereby inflating small changes in the tracking
errors into large offsets. It is not uncommon that errors that should
have been a few arcseconds have been blown up to offsets of several
degrees. This is typically worst during the first 10-20 seconds of a
scan since there are usually larger changes in the tracking errors to
inflate when a scan starts. This is further worsened by the fact that
the routine often was not using the proper values for the calculations
but instead using tracking errors that were valid 10 seconds earlier
(for example while the telescope was still slewing to the
source). This was done "randomly" thus causing a jitter in the
affected parameters.

There was also another "random" effect caused by a badly implemented
floating point to integer conversion which also adds a jitter by
"randomly" extrapolating in one direction or the other.

One effect of the way the extrapolation worked is that the errors
should be smaller for observations taken early in the day (local time)
and worst for observations made just before midnight.


REMEDIES

The FITS-headers do contain uninterpolated and thus undisturbed
positional information of where the telescope was pointing in the form
of the azimuth and zenith angle as read out by the telescope
encoders. The keywords are ENC_AZIMUTH and ENC_ELEVATIO. The read-out
time is also available as the keyword ENC_TIME. From this information
it is thus possible to recalculate the true values for the affected
keywords.

We are currently in the process of writing a small program that will
do these recalculations and replace the incorrect values in the
FITS-headers.

					Mikael Lerner

					Arecibo, 17 November 2004