Here's a graph that may help.
The graph represents the wavelength of light (along the horizontal axis) and the amount of light allowed to pass through the filter at any given wavelength (the height of the graph).
The bandpass for a typical Canon UV/IR filter is drawn in red. This particular company (although their filter is fairly typical amoung modified astro-imaging cameras ... and probably similar to the filter in a 60Da). This particular graph is showing the bandpass of a filter by a company called Baader-Planetarium.
If you look at the red line, it starts slightly curving down at 500nm and by 550nm it starts ramping down aggressively. The wavelength at 656.28nm (656.3) is the Hydrogen alpha emission line. That's a key color found in many deep-space emission nebulae.
You can see that at the H-alpha emission line, the filter is only allowing about 25% of the light (at that wavelength) to pass through. That means a normal unmodified camera (which mimmicks the human eye sensitivity) is blocking 75% of that light.
It is possible to modify a camera by simply removing that filter, but if you do that then you also let in all the IR light and the IR light focuses at a different distance and that would cause the images to look a bit out of focus. So when unfiltered cameras are used, there's general a seperate IR filter somewhere in the path (for example, a screw-on filter in front of the camera). Most imagers tend to replace the normal UV/IR filter with a new UV/IR filter that simply has a more abrupt cut-off at 700nm rather than the slow ramp.
This is basically what all the fuss is about for having a dedicated camera for astro-imaging.
Astro-imaging cameras can still be used for normal photography, but you would notice the images look really warm. You can always set a custom white balance to cool the images.
The hard-core astro-imagers will buy dedicated CCD imaging cameras that are cooled (the cooling system can usually drop the temperature of the imaging chip by at least 40-50ºC because there's a relationship between image "noise" and the physical temperature of the imaging chip.) These cameras are monochrome and have no filters, but a mechanical filter wheel is typically installed in front which minimally contains a "red", "green", "blue", and "luminance" filter (a luminance filter is simply a filter that has a UV/IR cut and allows all the light in the visible spectrum to pass but blocks everything else.) When they image, there's always a filter in front of the sensor... so they'll take a series of images with each filter and then assign the images to color channels in photoshop, which merges the image to produce a color images.
The dedicated CCD imaging cameras, filter wheel (a robotic device that rotates in the next filter between images), and the filters themselves are are rather expensive. A "cheap" setup would typically be at least $5k and it's not unusual for them to be well over $10k. One of the more accomplished imagers that I know considers any imaging camera that costs less than $50k to be a "cheap" camera (yeah, crazy... right?).
