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The Herschel Telescope Close To Blast Off
pha7boy writes "The Herschel space observatory, the European Space Agency's answer to the Hubble Telescope, is about to be sent into orbit. With a mirror 1.5 times the size of the Hubble mirror, the Herschel will look at the universe in the infrared and sub-millimeter range. This 'will permit Herschel to see past the dust that scatters Hubble's visible wavelengths, and to gaze at really cold places and objects in the Universe — from the birthing clouds of new stars to the icy comets that live far out in the Solar System.'"
This instrument is capable of great science, but spatial resolving power is not it's strong suit. Since it is measuring at wavelenghts much greater than hubble (100-1000x), the 3.5 meter mirror doesn't give you anything like hubble.
Redshift, probably.
When you're looking at things really really far away, the frequencies shift towards the red end of the spectrum due to the doppler effect of the Hubble Expansion. If we only looked in the visible spectrum, we wouldn't see anything, because the light had already shifted out of the proper range. Thus, but looking towards the infrared and longer wavelengths, we can actually detect things that originally light emitted in the visible spectrum but are reaching us in a heavily stretched state.
In fact, there is no case for very long exposures like the Hubble Deep Field or the Chandra Deep Fields (X-rays) with infrared telescopes, because the maximum depth reachable is not limited by sensitivity (exposure time) but by confusion (resolving power).
The confusion limit is reached when you can not detect any more sources because the field is so crowded that they start overlapping with each other. This limit is usually reached in infrared telescopes long before the detection limit (a few minutes), because the wavelength of the light in this spectral range is so big that the resolving power is very poor.
Note that resolving power is proportional to the diameter of the main mirror and inversely proportional to the observing wavelength, so a ~4.5m telescope like Herschel operating at 100 microns has aproximately half resolving power of an amateur 6cm telescope operating in visible light.
This also implies that the "ESA response to Hubble" statement is absurd and misleading
As described in here, the point of putting the observatory in a Lissajous orbit around the Earth-Sun L2 Lagrange point is to have the three nearest and largest sources of infrared light pollution (the earth, the moon, and the sun) sufficiently far away and in the same hemisphere relative to the observatory, allowing for a clear viewing angle anywhere in the other hemisphere.
In a certain respect, it is related to the focal length, if you consider the eyepiece or sensor fixed. If you have a fixed pixel size on your CCD, then changing the focal length will change the angular size of each pixel, and thus change the resolution, although I think this kind of result is usually called magnification. Similarly, when using an eyepiece, the magnification is related to the ratio of the focal lengths, so a longer focal length will change the magnification
When the CCD pixels get small enough though, that size is no longer the limit on the resolution. Instead (neglecting atmospheric effects) you run into the fact that photon impacts are defined by probability densities that behave like waves, and you get a certain 'spreading' around the nominal impact location. The diameter of this spreading (the Airy disk) means that two sources that are too close together cannot be distinguished from each other, and this is called the diffraction limit. (There are other equally valid explanations for this effect, particularly coming from a wave perspective, this is just the one that I started typing.)
Now, in order to reduce this you want to bring in more photons from further seperated distances, meaning you want a larger aperture in order to improve the diffraction limited resolution. Generally the limiting angular resolution is given by theta_r = 1.4 lambda/D. Of course, if you have too strong of aberrations in the optical system, have to deal with atmospheric 'seeing' effects, the system is not diffraction limited, and the point spread function spreads out more.
Of course, its dangerous to compare the capabilities of telescopes at different wavelengths (Hubble is visible, Herschel is infrared to millimeter wave), because the total amount of light available changes, the angular resolution changes, and the engineering requirements change. Really, Hubble is about the maximum size optical space telescope you can make with current launch vehicles without moving to a completely new kind of telescope (active feedback with wavefront sensing like JWST). Herschel is able to be bigger easily because it requires significantly lower precision, due to the larger wavelengths.