Hello Alex,
I absolutely agree with you; they should have done more observations of very-low-density regions such as intergalactic space and the voids, because of the extraordinary sensitivity of this telescope to the heat radiation (thermal emission, at far-infrared wavelengths) from extremely-low-temperature material in the universe.
A lot of the Herschel programmes revolved around observing the the details of the star formation process and observing the unbelievably complex 'goings on' in the interstellar medium
within galaxies;
which is very understandable, because trying to figure out star formation has been
an unsolved problem for a very long time...... and Herschel can peer into the middle of even the densest clouds of gas and dust - with minimal extinction.
Here is the list of the Herschel key programmes, which I assume they did carry out in full:
http://herschel.esac.esa.int/Key_Programmes.shtml
As you can see, the focus of Herschel observations was
not on
intergalactic space, which is a pity, as they might well have found something interesting there.
The
intergalactic medium has not been a very mainstream topic of research till recently, and it is
only recently becoming a part of the universe where actual objects are being detected by our telescopes; so perhaps the target list for Herschel would have included more intergalactic space, if the telescope had been launched today.
I do agree with you, they should have looked in the voids with Herschel, and in a few other 'unusual' places. But of course, the mission was too short for this.
This type of lack of imagination and 'thinking out of the box' in the targetting of the Herschel Space Observatory also applies very much to the targetting decisions that have been made for the Hubble Space Telescope. (as you will discover if you look at contents of the data archives for the HST)
Best regards,
Robert