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Tests of Mode: 1N4-0A-12-9 at 1420 MHz
(4-Sampler, Single Quadrant)
R. Prestage, D. Hogg, and R. Maddalena
September 30,2002

Summary

These tests showed that the noise on each receiver at each IF integrates down properly, and that the measured spectrum of HI in a test galaxy agrees closely with the values reported by Fisher in the GBT HI Galaxy Survey. Although there is one small anomalous feature in the test spectrum we recommend that this configuration be released for general use at L Band.

Observing Details for Project TSPECTEST_06 Mode 1N4-0A-12-9

The data were acquired from 08:00 - 14:00 20 September 02 (EDT). The goal was to obtain 1 hour sets of test spectra on a range of "multi-IF" spectrometer modes.

Data was acquired using the "test" pre-release version of M&C 3.7. This was expected to have no impact on these tests, apart from improved balancing behavior. The GBT equipment was initially set up using "tiger_setup.g", however all devices were subsequently checked through cleo.

The primary source used was U3851o, with 3C147 used for pointing.

Three Spectrometer modes were checked; 1N4-0A-12-9, 1N4-0A-50-9 and 2W2-4XY-200. This report summarizes the tests on 1N4-0A-12-9.

1N4-0A-12-9: This is a four-sampler, single quadrant mode. Spectrometer low-speed samplers 0,1,4 and 5 were used (input ports 9,10,13,14). The relevant subset of the IF chain are given below: 

Receptor  LO2      ConvModule  CenterSky  Sampler  aips++ Pol  aips++ nif
Linear X  13030.25    CM1        1.421      0          1          1
Linear Y  13030.25    CM5        1.421      4          2          1
Linear X  13032.25    CM2        1.419      1          1          2
Linear Y  13032.25    CM6        1.419      5          2          2

Scan Summary:

1-12  DCR pointing scans
13/14 False start spectrometer scans

15/16 *  30 sec test * OffOn 1N2-0A-12-9 (Standard single IF checked-out
mode)
17/18 * 300 sec test * OffOn 1N2-0A-12-9 (Standard single IF checked-out
mode)

12.5MHz mode: 1N4-0A-12-9  (2 polns/2IF frequencies)

19/20 *  30 sec test * OffOn 1N4-0A-12-9
21/22 300 sec OffOn
23/24 300 sec OffOn
25/26 300 sec OffOn
27/28 300 sec OffOn
29/30 300 sec OffOn
31/32 300 sec OffOn

(all of the above show small linear baseline offsets in pol=2,nif=2)

Analysis of the Data

The data are all usable. In the first test, we examine how the noise integrates down with increased integration. The expected rms is calculated assuming that the receiver temperature is 19.0 K, and that the effective channel width is 3.05 kHz (0.64 km/s). 
Integration                            RMS, in mK
   Time                            IF#1                   IF#2
  (secs)        Expected     RX1    RX2    Both    RX1     RX2     Both
    300            19.9      18.6   19.0           18.6    19.1
    600            14.0      13.3   13.6   13.4    13.3    13.5    13.3
    900            11.5      10.7   11.5           10.7    11.2
   1200             9.93                    9.50                    9.37
   1800             8.11      7.84   8.59   7.95    7.70    8.00    7.76
   3600             5.73                    5.86                    5.46

The average scans for each channel and each IF are shown in Figure 1
(ttday6.pl1a.ps) and Figure 2 (ttday6.pl2a.ps) for IF#1 and IF#2, respectively. 



The individual values of the rms are given in Figure 3 (tday6.rms1.ps)
and Figure 4 (tday6.rms2.ps).
Each receiver/IF combination performs close to expectations, and all
agree with one another satisfactorily.





In the second test we compare the derived values from the HI profile
of the galaxy with those listed by Fisher in his catalogue. We use
the average of the two channels for each IF, and we convert from
antenna temperature to flux density using the conversion factor
found in early August by R. Maddalena, 1 Jy = 1.91 K.

Quantity                     Fisher             IF#1           IF#2

Maximum in profile (Jy)       2.5               2.39           2.37

Peak Intensity     (Jy)       2.33              2.24           2.25

Line Width @ 20%   (km/s)     112.6             111.1          111.4

Systemic Velocity  (km/s)     100.4             100.0          100.0

Flux Integral   (Jy km/s)     214.0             202.5          201.2

The spectra, expanded to be consistent with the display in Fisher's
catalog, are given in Figure 5 (ttday6.av1b.ps) and Figure 6 (ttday6.av2b.ps).
The agreement between the two IF's, and with the values from Fisher,
is excellent. The displacement of 2 MHz in the second IF has been
accounted for correctly.



Figure 7 shows the spectrum of UGC3851o from the Fisher survey.


There is only one remaining concern with this configuration. There
is an apparent absorption feature in the spectrum at a frequency
of 1419.903 MHz (see Figure 8 (ttday6.pl12.ps)). The feature appears
at the same frequency in each IF, and appears in the test spectrum
scan #18 (1N2-0A-12-9) but not in the 200 MHz spectrum (2W2-4XY-200)
taken later in this spectrometer test.

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