Tuesday May 26th 2009. Day 12.


Today Herschel has passed yet another huge obstacle in its path to being completely operational. HIFI had what is called is “Short Functional Test”. This consisted of several hours of checking one mechanism after another to ensure that everything is completely functional. HIFI is a particularly complicated instrument that uses radio astronomy technology rather than the more familiar imagining detectors of SPIRE and PACS. As the frequencies that HIFI operates at are inherently undetectable for standard radio receivers it uses a heterodyne system: this sounds like an impressive word, but simply means that the very high frequency, short wavelength signal from the sky is mixed with a very carefully controlled local oscillator signal of slightly different frequency. The two mix together and produce a combined signal at a frequency that is the difference between the two frequencies (e.g. you mix signals at 500GHz and 499.5GHz and you get a resulting signal at 500 – 499.5 = 0.5GHz). This much lower frequency is easier to detect with standard radio techniques. However, it means that each channel of HIFI – and there are no less than 14 – needs its local oscillator and needs a mixer (to mix the signals) and if either does not work that channel is dead.


Before launch we had accepted that with so much electronics and so many things that could go wrong, it was almost inevitable that we would lose a channel. However, the tests yesterday showed that every single channel is working correctly, as are the chopper and the other HIFI mechanisms. A gleeful message came in late afternoon, when most people had left for home, that the score in the testing was 14-0 and that HIFI was fully operational. To have so many complex detectors and instruments all functioning just about perfectly is astonishing. Of course, the usual caveat applies: things may look very different in about two and a half weeks when we pop off the cryocover and start to look at real sources on the sky. However, the feeling is that we have cleared a whole series of increasingly difficult obstacles without falling yet. Over the next two and a half weeks we will get into a series of specific tests that will go into finer detail, but the feeling is that until we start to look at the sky the worst is past… for now! Life is good and, incredible as it may seem, the biggest danger may be overconfidence as the tension of the early days reduces.


Tonight Peter Birtwhistle passed me a series of new images of Herschel, the Sylda and the presumed Upper Stage fragments, which are now getting to around magnitude 21 and very difficult to detect, with long exposures needed in a crowded starfield. As Herschel continues to move south-east, deeper into the Milky Way, observing the faintest objects in the Herschel constellation will become impossible. The two, faint fragments, 2009-026E and 2009-026F are still fading more rapidly than they should if they are simply reflecting sunlight from a constant surface area.


Unofficial Herschel image of the day archive:




Frequent updates are provided during the day on the Herschel Twitter (ESAHerschel) here:




You can follow Herschel testing and observations in real time on the Twitter.