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Summary of 74 MHz Test with “Power Down” at Parkes Site (2003-01-29)

Summary of 74 MHz Test with “Power Down” at Parkes Site (2003-01-29) New Data file: 2003-01-29_1120_z20s.rpf (apparently no matched load data taken this time) Matched Load Data: for passband comparison was taken at an earlier session (2003-01-09) OVERVIEW & CONCERNS :

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Summary of 74 MHz Test with “Power Down” at Parkes Site (2003-01-29)

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  1. Summary of 74 MHz Test with “Power Down” at Parkes Site (2003-01-29) • New Data file: 2003-01-29_1120_z20s.rpf (apparently no matched load data taken this time) • Matched Load Data: for passband comparison was taken at an earlier session (2003-01-09) • OVERVIEW & CONCERNS: • Still pretty cluttered up with indications of spurious intermods being cycled in • frequency with ~25 minute cycle • Strongest RFI spikes at 2048, 4096, 6144 (and some intermediate at N*1024). Is • correlator in good health? • Total power passband on sky is much different than the passband with matched load • (is the feed working right? We have been using the MITEC amps, and maybe Bill’s • amps did more in the way of matching?) • Description of Plots (that follow): • 1. Comparison of spectra for one 5 second integration with scan average passband shape • for a long integration on a previous date. • 2. Total power versus time. Black lines are straight averages over frequency; red lines • result from applying a median filter across each 5 sec spectrum first (to remove spikes) • before averaging over frequency. • 3. Another measure of Total Power variation with time -- obtained by choosing a narrow, • apparently-empty frequency band between the spikes at channels 3280 and 3350.

  2. Description of Plots (continued): 4. Dynamic spectra: First 50 minutes. Upper image is the low frequency half of the band 72-74 MHz; lower image is upper half 74-76 MHz. A passband shape was divided out; the passband was constructed by (1) median filtering each spectrum in frequency, (2) skipping those that jumped by 3% in total power, (3) averaging the spectra from the time step range 20 to 6000 (during which they appear to be well behaved in total power). 5. Dynamic spectra: 50 minutes near center of the recording. 6. Dynamic spectra: the entire range of the recording. Only 1 in 20 spectra are included. Note: sawtooths & gentle arcs. The total power doubles during the recording so that a smoothly varying total power level was subtracted to keep the image values within a useful grey scale… But the noise level increases with time through the image, and the passband calibration becomes inaccurate, since the sky power experiences a different passband than the amplifier/ground-pickup noise power (causing the white patches in the extreme freq channels at late times). 7. Example spectrum: Normalized to Tsys=1.0. Strongest spikes at N*2048 at ~35 Tsys 8. Example spectrum: Normalized to Tsys=1.0. Different gains and zooms. Lower panels show channels 2049:4096 (left) and 4097:6144 (right). Looks like N*1024 and possibly some N*512 coming through.

  3. Description of Plots (continued): • Comparison Total Power vs. Time for site-powered (2003-01-09) and site-unpowered • (2003-01-20). • 10. & 11. Dynamic spectra: comparison of Site-Powered and Site-Unpowered on • ~same stretch (normalized to unity Tsys). The other possibly important influence on the • appearance of the dynamic spectra is whether the Parkes dish is allowed to move or • not. When it changes its position with respect to the 74MHz feed (which is sitting • on the ground), the reflected ground/sky radiation changes, and the total power and • passband shape changes. This is probably the cause of the bright and dark bands in the • image for 2003-01-09. The other possibility could be a time-variable source of • broadband noise. • 12. Comparison of Spectra from averages Time Steps 1500:2099 (Galactic Pole) and • 5000:5599 (elevated total power as creep into Plane) [Galactic Center transited during • the daytime, and RFI levels became prohibitive.] • Difference between spectra in Fig 12. This is the ‘Gain(f)’ curve. • Quotient of spectra in Fig 12… Flatness of curve again implies that the • sinusoidal power level in spectrum is a gain of the feed effect. • New data recorded by JR – analyzed by FB.

  4. Comparison of Passbands measured with Matched Loads: Greenand Red with one 5 second integration on Sky: Blue and White

  5. Total Power vs. Time Avg of entire band: Black Median filtered band: Red

  6. Total Power vs. Time Step [@ 100 sec/step] * alternate approach * based on avg of frequency channels 3280:3350 from band of 8192 channels

  7. Dynamic Spectra: first 50 min(time increases upward) 72 MHz 74 MHz 74 MHz 76 MHz

  8. Dynamic Spectra: 50 min(17:00-17:50UT - time increasing upward) 72 MHz 74 MHz 74 MHz 76 MHz

  9. Dynamic Spectra: entire dataset(11:20-23:00 UT - time increasing upward) * passband determined from average of median-filtered spectra from 11:23-20:00 UT * plots show 1 in 20 spectra * all dynamic spectra are Polarization 1 72 MHz 74 MHz 74 MHz 76 MHz

  10. Time: 12:06 UT

  11. Time: 17:25 UT 5120 3584 2560 3072

  12. Powered UP Powered DOWN

  13. Powered UP + telescope movement (2003-01-09)

  14. Powered Down + NO telescope movement (2003-01-29)

  15. Comparison of Galactic Pole with low Galactic Latitude

  16. Difference between Galactic Pole and low Galactic Latitude

  17. Quotient: low Galactic Latitude divided by Galactic Pole

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