Since the application of a band-pass filter in spring 2010 ensured interference-free observations, we started a complete survey of the Milky Way's hydrogen emission, covering all galactic longitudes accessible from Illkirch (about 0° to 250° in steps of 2°) and the latitude band between -30° and +30° also in steps of 2°. Data was taken in the form of vertical scans at each longitude.
Although the system temperature was still rather high (between 700 and 1000 K), the first trial survey showed all the essential features. Since then, we have continually improved the system temperature, and repeated the survey several times. Here is a short history of events:
- Survey0 are data taken between March and June 2010, as the first interference-free observations. Each longitude took about 5 hours to complete, with 10 minutes for each latitude, giving about 56 spectra with 1.5 MHz span. System temperatures were between 600 and 2000 K.
- Survey1 was the first try of systematic observations, in June 2010.
Done with a system temperature of something more than about 1000 K, it was
necessary to smooth the raw data over 2 bins in frequency in order to suppress
the strong noise.
- Survey2 was done with a system temperature of about 1000 K or somewhat
less. Frequency smoothing was no longer necessary. The observations were done
between July and September 2010, but they were terminated when the elevation
pushrod eventually got stuck.
The data may be viewed interactively with the JavaScript tool here.
- Survey3 has a system temperature of about 700 K. It was executed
from November 2010 until February 2011.
The data may be viewed interactively with the JavaScript tool here.
- Survey4 are basically various tests with different filters, from
March to June 2011. System temperatures were about 500 K. Observations were
done with a large frequency span of 4 MHz, in search of a good definition of
the baseline. Some were done with a rather narrow filter, resulting in a
strongly curved baseline. Hence, subtraction of the baseline became more
difficult. Some observations (Survey4.5) were done with a system
temperature of about 300 K and with a frequency span of 4 MHz. 23 spectra
were taken at each position.
- Survey5 started in June 2011, with a
system temperature between 300 and 200 K. A frequency span of 1.5 MHz is
used, and staying at each position for 10 minutes gives 56 spectra. Hence
the signal-to-noise ratio is almost eight times better than that of a single
spectrum.
The data may be viewed interactively with the JavaScript tool here.
In summary, we may conclude that even with a poor system temperature of 1000 K one may successfully map the hydrogen emission in the Milky Way, by averaging over a sufficient number of spectra for each position. Below we show from Survey0 a raw single spectrum, a single baseline-subtracted spectrum, and the average over all 56 spectra:
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last update: July 2011 J.Köppen