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For a public outreach talk I wanted to start with the importance of the Cosmic Microwave Background (CMB) for our understanding of the evolution of the universe. Then I remembered this anecdote that you can “see” the CMB radiation in the noise of your analog TV (if you still have one) and started wondering if that is actually true…
On the web there are several sources claiming that 1% of the noise between the channels of your TV programme are CMB, but I couldn’t find any actual calculation. So I tried my own.
Since I’m not an antenna expert and couldn’t immediately find some meaningful numbers for the radiation for analog terrestrial TV, I used this to come up with an order of magnitude number:
The old analog sender on top of mount Wendelstein in Upper Bavaria radiated at 0.4 kW in channel 48 according to the Wikipedia page. Channel 48 is at about 680 MHz and has a bandwidth of about 8 Mhz. This sender was apparently only used to broadcast to Bayrischzell which is about 2 km away. The sender had a round radiation pattern, i.e. it radiated in all directions, but only in one plane. However, since the town of Bayrischzell has a finite length, the half-power beam-width cannot have been too small. I assume here an H/R of ~ 0.2, i.e. at 2 km distance the half-power beam-width should be ~ 400m (i.e. illuminating an area of 5e6 m^2 at that distance). This is of course only a crude estimate, but for an order of magnitude calculation it should be OK. The specific flux received in Bayrischzell from this sender is thus
0.4 kW / (5e6 m^2 * 8e6 Hz) = 1e-11 W(m^2 Hz) or about 1e15 Jansky (in radio astronomers’ units)
OK, now let’s look at the specific flux of the CMB. This is easy, we just need to substitute proper values (2.7 K, 680 MHz) in the Planck equation and arrive at a specific flux of about 5.4e5 Jansky at the frequency used for the local TV station.
Now we need to compare this not to the signal strength estimated above, but to the estimated noise level of a typical analog TV receiver system. This is a bit tricky, but typical carrier-to-noise ratios (CNR) should be of some help. The carrier-to-noise ratio is the signal-to-noise ratio of a modulated signal. I didn’t find any recommended CNR values for terrestrial analog TV, but other CNR values for TV might give some guidance. In an article published with the Society of Cable Telecommunications Engineers, this guidance is given:
The FCC’s minimum CNR is 43 dB, which, in my opinion, is nowhere near good enough in today’s competitive environment. Indeed, most cable operators have company specs for end-of- line CNR somewhere in the mid to high 40s, typically 46 to 49 dB.
The National Association of Broadcasters Engineering Handbook (p. 1755) has a similar statement:
Noise will become apparent in pictures as the carrier-to-noise ratio (CNR) approaches 43-44 dB […]; a good design target is 48-50 dB.
So let’s assume the sender on mount Wendelstein is calibrated such that is achieves a CNR of 50 dB for the terrestrial analog broadcasting of the local TV station in nearby Bayrischzell. This means the signal is a factor 100,000 (1e5) stronger than the noise.
In this case, the noise level in the TV is about 1e10 Jy or about 200.000 times larger than the CMB signal. So, if this calculation is correct, it seems unlikely that you would be able to see the CMB signal while searching for signal with your analog terrestrial TV.
Recently I submitted another paper to astro-ph and I wanted to play the “first” game and try to get my paper on top of the daily listing. Now, it is well documented that the deadline for the daily submissions is 16.00 EST meaning that if you submit just after that deadline you’re paper is likely to appear on top of the next day’s mailing. (NB: the announcement is inverse on the web page)
But how likely do you get your paper on top if you submit right in time? For the above mentioned paper I had prepared everything before hand. This took a bit of time since arxiv.org didn’t accept a
after the abstract but only spewed out weird errors about lines that aren’t ending etc. So I had prepared everything, even submitted the paper and un-submitted it again.
Then, at 20.59.59 GMT and a split second I hit the submit button. It took more than three minutes for the server to handle this request. Still, I was quite amazed that the paper appeared only as #6 (#19 if counting also the cross-lists) from the bottom of the webpage (i.e. top of presumably more important mailing), despite having been registered by the server at 21.00.04 GMT. So I used my much-loved bash tools curl, awk and grep and extracted the submission times of all astro-ph submissions this year until October and found this:
There is a clear spike at 20.00 UT (which is 16.00 EDT = UTC-4) and a smaller one at 21.00 UT (corresponding to 16.00 EST = UTC-5) — most of the year so far has had (Northern hemisphere) summer time. I don’t have an explanation for the smaller and wider peaks at about 9.00 and 16.00 UT. Now we can also zoom in to the region around 20 / 21:
We see that the peak submission time (which is when the submission is registered by the server) is at about 12 seconds after the deadline. Going back to my case — submitting right at the deadline, registered 4 seconds after the deadline (despite server only replying 3 minutes later) — we can ask: what are the chances of getting on top if you submit within 4 seconds? Over these 10 months (ca. 200 submission days), there have been 26 submissions in this timeframe, i.e. your chances of getting on top if submitting so close in time should be almost 100%. It just so turns out, however, that on the particular day when I submitted, there were five papers submitted even closer to the deadline. Tough luck. 😉 Hopefully, however, this will play less of a role in the future as more and more people read their daily astro-ph through voxcharta or similar services where the announcement order is either randomized or sorted according to your preferences.
As the big observatories of the world observe ever more astronomical objects, their archives become powerful research tools. Finding out whether an object has been observed with a certain instrument is just a few mouse clicks away, if the observatory has a public archive like ESO provides for all VLT instruments.
For a research project, I recently needed to find all local AGNs ever observed with a certain instrument (SINFONI at the VLT). Since I didn’t know the target names or programmes, I got all unique observed coordinates, resolved them via Simbad (which also gives the class of an object) and then selected the AGNs among all the targets.
Since ESO unfortunately does not provide direct access to the archive database, a query like “give me all unique observed coordinates” is not possible per se. So I had to download all headers, parse the relevant information and build my own database (SQLite for the moment).
I have a script to collect the metadata, which does this:
- query the ESO archive for all observations of a day
- then parse the resulting html file for the unique identifiers of each dataset (“data product ID” or DPID, e.g. XSHOO.2015-04-13T04:46:11.730)
- download the header for the given DPID
- parse the header for relevant information and construct an SQL insert statement
- insert all into a database
There are also scripts that
- query the database for all programmes and search metadata (PI/CoI names, titles) for them
- get atmospheric data for all observations (querying the ambient conditions server)
- And there is a top-level script that calls all of these scripts in a meaningful way and that I call about once a month or when needed to update the database.
My database consists of one table for each ESO instrument that I am interested in (currently MIDI, SINFONI and X-SHOOTER), a table with programme meta data (PI/CoI names and titles), a table with atmospheric data as well as tables with basic information about calibrators and science objects that I use for matching up observations and building LaTeX tables in a scripted way. This has become quite handy over the recent years and has helped me in building the largest sample of interferometrically observed AGNs with MIDI (Burtscher et al. 2013) as well as the largest sample of local AGNs observed with SINFONI (Burtscher et al. 2015) and a follow-up paper (submitted).
In case you are interested in tables that I have already compiled and am maintaining, please contact me and I will be happy to share the database with you. It is currently about 700 MiB and I update it every month.
Apart from nice science, one can also use this database to create other plots of interest, like a map of the exposure depth of SINFONI for example:
Interestingly, the Galactic Center (at 17:45h, -29 deg) is not the field with the deepest SINFONI integration time (“just” about 400 hours). Instead the Extended Chandra Deep Field South is the deepest SINFONI field with about 600 hours of integration time. Another field with deep coverage is the COSMOS south field (10:00h, +02 deg). About 300 hours of total integration time have been spent on this field.
Update 12 Jan 2016: I have now put my codes and the database online. Please see the github project page for further details on how to use these.
…that are actually refereed articles or made it at least to astro-ph:
- Gates, V.; Kangaroo, E.; Roachcock, M.; Gall, W. C.: Stuperspace – a classic. If you don’t know it, read it immediately. That is, hold on a minute… First read a quantum field theory article like this to fully value the former article…
- Douglas Scott, Ali Frolop: Down-sizing Forever. An April’s fool article on astro-ph that reaches the astonishing conclusion that “the early Universe was in fact a giant galaxy”.