The bottom line is "yes"... you can use the 60Da as a normal camera, but read on to understand the nuances. If you KNOW you want to get into astrophotography then teh 60Da is the best DSLR on the market for it. If what you really want is primarily a terrestrial use camera and only "might" want to get into astrophotography... then I'd probably suggest you buy a normal camera.
I own a 60Da which I use for astrophotography. But since I *also* own a couple of 5D bodies (II & III) I don't use the 60Da for normal terrestrial use. But in every way except for the modified filter, it's a normal 60D.
A "normal" camera's IR filter actually slowly ramps up the blocking level of IR beginning around 550nm (the visible spectrum runs from roughly 400nm to 700nm). Blocking reds at this point (gradually... and ramping up the amount of block until they reach 700nm) range means a normal camera is actually cutting out quite a bit of visible light. Cameras do this because it turns out human eyes are most sensitive to the greens (smack in the middle of the visible spectrum) and less sensitive to violets & blues on the short wavelength end and the reds on the long wavelength end of the visible spectrum. So a "normal" camera is a bit de-tuned to mimmick the look that a human eye expects.
But that's bad for astrophotography where there's an enormous amount of light in the Hydrogen alpha wavelength (656nm) because about 90% of all "normal" matter in the universe is hydrogen -- so not suprisingly most deep space nebulae are packed with the stuff and glowing at that wavelength.
The difference between the normal 60D (or most any camera's IR block filter) and the filter in the 60Da is HUGE. A normal camera is blocking about 80% of the light by the time they get to the Ha (656.28nm) wavelength. The 60Da actually lets most all of the Ha through.
I bought my 60Da specifically because a club member (in my astronomy club) shot a deep space photo through a scope and then I tried to use my 5D II to get the same shot of the same object through the same scope and after I realized I already had to TRIPLE his exposure time and still was nowhere near looking as good as what the 60Da was doing... I bought a 60Da the following week.
To use a 60Da for normal photography things will look a bit "warm" but it turns out the 60Da does still have an IR filter... it's just that it's filter doesn't block IR until you actually get into the IR zone (it doesn't block visible reds on the way to IR). Basically the 60Da has a more abrupt IR cutoff rather than the slow-ramp cutoff of a normal camera. So the pictures aren't super red (like a camera with no IR blocking whatsoever), they're just a bit on the warm side. A modified DSLR for astrophotography which has no filter at all will look extremely red.
There are actually numerous examples of photographs comparing the 60Da to normal cameras and actually the images out of the 60Da look pretty good. I tried using an Astronomik OWB "EOS-Clip" filter, but found it was over-correcting and images looked too blue (even Astronomik agreed with me that the filter was over-correcting for a 60Da and was really intended for cameras with no IR filter at all.) I've recently written to them to see if they've changed the OWB because I noticed they now refer to it as "OWB Type 3". Perhaps they have a new variant which is suitable. (Astronomik is a maker of high-end filters with an extremely good reputation in the astronomy community. They are among the best on the market... sort of like Schneider / B+W brand for regular camera filters.)
In any event... while the "straight out of the camera" photos from a 60Da will look a bit warm, they are not obnoxiously warm and easily corrected by uisng doing a white balance correction on the computer. I own a gray card and use that for accurate white balance -- but honestly I'm always using one of my 5D bodies for normal photography and the 60Da for astro. So while I think you would find you needed to white balance all your images, it's certainly cheaper than buying two cameras and there's no way you could get a normal DSLR to perform like a 60Da when it comes to taking astro photos (not without doing surgery on the camera which would certaily void any warranty it might have had.)
Tim Campbell
5D III, 5D IV, 60Da
