Current Status of Amplitude Calibration in the EVN.
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Calibration diode temperatures (Tcal) derived during the CAL experiments are
used to produce the ANTAB (Tsys) files for every session. The following table
shows the median absolute amplitude error for EVN stations in 2004 (sessions 1
and 2). The number in brackets after each entry is the number of experiments
that were used to determine the median error for that entry. These results are
derived from the pipeline amplitude self-calibration results of all EVN
experiments recorded in 2004 which have been pipelined to date.


Station   C band      X band      18cm       21cm       K band      UHF
Cm        0.700 (2)               0.834 (2)  0.483 (1)  
Ef        0.098 (6)               0.206 (5)  0.029 (1)  0.345 (2)   0.369 (3)
Hh        0.250 (2)   0.072 (1)   0.155 (1)                         
Jb        0.183 (5)               0.474 (4)  0.080 (1)  0.321 (1)
Mc        0.172 (6)   0.050 (1)   0.381 (4)  0.063 (1)  
Mh                                                      0.275 (2)
Nt        0.105 (6)   0.091 (1)   0.400 (5)  0.376 (1)  0.191 (2)   1.155 (3)
On        0.345 (4)   0.080 (1)   0.321 (4)  0.230 (1)  0.553 (2)   1.740 (3)
Sh        0.185 (3)   0.109 (1)                         0.228 (2)   
Tr        0.794 (6)               1.500 (5)  2.585 (1)              0.801 (3)
Ur                    0.333 (1)   0.260 (1)  0.375 (1)              2.871 (1) 
Wb        0.085 (6)   0.049 (1)   0.156 (5)  0.041 (1)              0.218 (3)

The blank entries are where insufficient data were available. Statistics will
continue to be collected as experiments are pipelined. Note that the numbers
above are the absolute median amplitude error, and so are significantly higher
than the rms error. A value below 0.1 can be considered to indicate good
calibration, while values above 0.2 probably indicate a significant error. 

Points of note:

At C band and X band the overall calibration quality is quite good. However Tr,
On, Cm, Ur and Hh have significant errors. The Tr problem appears to be related
to an error in the estimation of the Gain Curve, which was discovered in the
summer of 2004. This has now been corrected and significant improvement is
expected for experiments in session 3 of 2004. The Hh problem at C-band is at
least partially the result of the fact that the two observations were performed
at low elevation where accurate calibration is significantly more difficult to
obtain. Results from the other wavelengths indicate that there is no
fundamental problem with the Hh calibration scheme. The poor calibration at Cm
is due to the fact that the microwave link prevents the measurement of system
temperatures, and nominal values of the SEFD are used to calibrate the
telescope. The reason for the poor Ur and On calibration is not yet understood,
but will be investigated in session 3 of 2004.

L band calibration is less accurate than C and X band, and this appears to be
due to RFI. This is supported by the fact that there is a large scatter in the
amplitude correction factors between experiements, which seems to be related to
the observing frequency. Experiments in the cleanest part of the 18 cm band
have significantly smaller errors than the median values given above.  Again,
the Tr values at L band are affected by the (now corrected) error in the gain
curve. 

K band is clearly a problem for all stations. This is probably related to
opacity effects and to the low sensitivity of many of the antennas at high
frequency. Pointing errors may also be an issue.

Calibration in the UHF band is very poor. This is because of the extremely
difficult RFI environment at these frequencies.

Comparison of Tcal tables from February 2003 and May 2004 sessions indicates
that the Tcal versus frequency dependence can vary over the timescale of a few
months. Continued monitoring of Tcal during every session is therefore
required.



ANTAB Scripts
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A new version of the ANTAB scripts has been distributed to stations for
the November 2004 session. Stations will attempt to make their own ANTAB files
for this session (to be checked afterwards at JIVE).


UVFLG
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Stations continue to provide off-source monitoring data which allow data taken
when the antenna was off-source to be flagged. This system continues to work
well at most stations. Exceptions in session 3 of 2004 were Westerbork and
Noto. Torun have been successfully using the off-source flagging since May
2004. Urumqi station has also introduced off-source flagging in session 3 of
2004, but the accuracy of this flagging has yet to be tested. Noto are reported
to be investigating possible solutions. This system is unlikely to be useful
for the tied-array at Westerbork.

Cormac Reynolds.