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A short (2-hr) test was made on the morning of 12 November 2001
for position switching with a specified OFF position, i.e. in contrast
with a "standard OFF position" which gives tracking over the same
segment of the dish as traversed by the corresponding ON. For these
tests, short integrations of either 2 or 4 minutes in the ON position
were used. "Standard" ON/OFFs" were alternated with ON/OFF scan pairs
having specified OFF positions. The Sun rose late on into the test,
and some data were taken post-sunrise. These data are also presented
below as they reveal interesting results.
The observations were made with 4 x 25-MHz bandwidths centered
(with frequency overlaps) at 1382.5, 1387.5, 1407.5 and 1412.5 MHz, and
using 9-level recording. This range of frequencies was essentially
unaffected by RFI during the measurements (except for the Sun). Doppler
correction was switched off. The ON position used was chosen randomly
(R.A. = 09.0hr; Dec = +16.0d), and no HI emission was seen, apart from that
of our own Galaxy which lay at the high-frequency end of Subcorrelator
(SBC) 4. A number of offsets were used for the "specified OFF
positions", ranging over pure declination offsets of between 2*HPBW
(7'12") and 2d30', and an offset in R.A. equal to that for "standard
position switching", but of opposite sign.
For 9-level correlation, an rms value for (DelT/Tsys) of about 8.43x10-4 was expected for an unsmoothed (ON-OFF)/OFF analysis, i.e. 2/sqrt(channel-width * total integration time). The values obtained from the observations (in units of 10-4) for the central 15 MHz of each SBC were as follows:
-------------------------------------------------------------------- Offset Poln A Poln B SBC1 SBC2 SBC3 SBC4 SBC1 SBC2 SBC3 SBC4 -------------------------------------------------------------------- Standard 8.28 8.34 8.06 8.49 8.82 8.42 8.18 8.34 -------------------------------------------------------------------- -7'12"Dec 10.39 9.93 10.37 10.06 10.13 10.80 11.11 12.30 -1d15'Dec 12.10 12.86 12.11 11.51 11.51 13.65 12.42 11.68 --------------------------------------------------------------------
Here, the theoretical rms value is about 5.96x10-4. The values (in units of 10-4) for pre-sunrise scans were;
-------------------------------------------------------------------- Offset Poln A Poln B SBC1 SBC2 SBC3 SBC4 SBC1 SBC2 SBC3 SBC4 -------------------------------------------------------------------- Standard 6.46 6.07 6.01 6.33 6.04 6.12 6.12 6.33 Standard 6.28 6.25 5.87 6.14 6.22 6.09 6.43 6.41 -------------------------------------------------------------------- -1d15'Dec 9.16 8.69 9.18 9.36 11.23 10.76 9.68 10.02 -1d15'Dec 8.89 8.01 11.45 10.69 9.19 9.83 10.20 11.38 -2d30'Dec 9.53 9.73 10.29 9.84 10.49 10.58 10.20 10.18 -5mRA 8.02 8.22 8.36 8.47 9.29 10.96 8.91 8.47 --------------------------------------------------------------------
Two 8-min ON integrations were obtained by summing, a) the pair of Standard ON/OFFs, and b) the pair of scans with the OFF position:
SBC1 SBC2 SBC3 SBC4 SBC1 SBC2 SBC3 SBC4 -------------------------------------------------------------------- Standard 6.33 6.56 6.42 6.46 7.19 6.54 6.63 6.59 Standard 6.69 6.67 6.77 6.88 7.37 7.02 6.80 7.08 -------------------------------------------------------------------- -1d15'Dec 51.19 34.91 29.89 64.27 13.54 12.84 12.83 13.65 -1d15'Dec 24.04 24.81 25.41 19.83 29.26 42.29 25.99 22.15 --------------------------------------------------------------------
Although this was a short test, and a longer, more comprehensive test may be useful, a few conclusions can be drawn from these data.
1) The "standard ON/OFF" procedure for position switching performs well, producing results close to the theoretical signal-to-noise ratio for the integration times used here.
2) Even for the rather short integrations of 2 and 4 min used here, position switching using specified OFF positions seem to result in degraded signal-to-noise ratios compared to theoretical expectations. Further observations need to be made to investigate what the situation is for different integration times (shorter, as well as longer). The effect of varying the magnitude of the offset also needs more investigation, although the data here suggest that there is little difference for 4-min integrations between declination offsets of 1.25 and 2.5 deg.
3) Sunrise effects seem to cause vastly degraded response for a
specified OFF position when compared with "standard position
switching".
Last updated: Friday, 14-Feb-2003 11:36:48 AST