To begin with, back-lit CCDs are way more sensitive (no Bayer filter + higher QE => up to 5 times more sensitive for a given wavelength) but as important is that astronomical CCDs almost always are actively cooled to reduce the dark current. Dark current can easily be the most signicant source of noise for very faint sources that require long exposures. An additional problem is that the dark current fills up the electron well with time, reducing the dynamic range unless the photo-sites are periodically reset by making multiple shorter exposures, but the problem with multiple exposures is that each readout adds noise.
The advantage of the 60Da on the other hand is that you get all three colours simultaneosly, and at 18 Mpix, for a low cost. And you can easily use it with EOS lenses, something that can be a bit involved for astrophotography-dedicated CCDs.
I happen to be an astronomical researcher and an astrophotographer who use Canon extensively. Here are a few of my recent photos taken with a modified 5D2:http://www3.asiaa.sinica.edu.tw/~whwang/gallery/picutres/M42_2010.htmlhttp://www3.asiaa.sinica.edu.tw/~whwang/gallery/picutres/M101.htmhttp://www3.asiaa.sinica.edu.tw/~whwang/gallery/picutres/rosette-2011.htm
I would like to add a few words to the topic on DSLR vs cooled astronomical monochrome CCDs.
As epsiloneri pointed out, a big advantage of astro mono CCDs is their high QE. The fact they don't have Bayer filters allow them to adopt the LRGB color combination, in which they spend most of the time on L and only a little time on RGB. The L filter essentially uses the entire throughput range in the optical spectrum. Coupled with the high QE of the sensors, this produces very high S/N and high resolution in a short amount of time. This is proven to be an extremely efficient way of producing high quality color pictures, and DSLR just can't match this. Even if DSLRs have similar QE as mono CCDs, it will still take a Bayer-style DSLR 2x (or even 3x) more exposure time to achieve the same quality. In my opinion, the biggest advantage of mono CCDs is their flexible filter usage, which allows for the very efficient LRGB and other possibilities.
Many people also think cooled CCDs are good because they are cooled and they have low noise. Unfortunately, this is not correct. First, recent DSLRs produced by Canon and Nikon all have LOWER noise comparing to CCDs from Kodak and Sony. Here I am talking about readout noise. DSLRs' readout noise can be as low as 2-3 electrons rms per pixel (for example, 5D2 at ISO 1600 and 3200), but CCDs are still in the range of 7-10 (even > 10) electron per pixel. This is the same for dark current. Recent Canon and Nikon CMOS chips have dark current that's MANY TIMES LOWER than that of Kodak/SONY CCD chips under the same temperature. The low temperature produced by the cooling system in those cooled CCDs help to reduce their dark current from being miserable to better than DSLRs, but not much better. Plus, even when uncooled, recent DSLRs already have dark current that's comparable or even lower than sky photon rates in most astrophoto environments. So dark current is no longer a limiting factor for the performance of DSLRs on astrophoto, as long as one subtract dark carefully, which is not difficult and doesn't take much time.
So, be happy with the Canon DSLRs you have. They are great, maybe not up to the level of the best cooled CCDs, but not far behind too.