This really depends on what type of astrophotography you want to do
Milky-Way Nightscapes?
Solar/Lunar?
Planetary?
Deep Sky Objects?
etc.
Are you planning to use a telescope...or a camera lens? Do you plan to do very long exposure (with a tracking mount)? et.c.
Also, without the right type of telescope, it's not a good idea to use a "full frame" camera. Many (possibly most) scopes can't maintain field flatness across the 44mm diagonal size of the sensor. A camera with an APS-C crop-frame sensor is more more typical.
I own a 60Da (no longer in production) which was a special edition of the 60D which was modified by Canon for astrophotography.
The most abundant element in the universe is Hydrogen. If hydrogen is ionized, it glows in specific wavelengths (the Balmer series) and the most prominent of these is the Hydrogen alpha wavelength (this is the "red" color you see is so many emission nebulae).
Trouble is... human eyes are not equally sensitive to all wavelengths in the visible spectrum. We are most sensive to wavelengths in the center of the spectrum (where you find the greens) and less sensitive the edges (where you find blues and reds). At the hydrogen alpha wavelength, our eyes are only maybe 20-25% sensitive to the amount of red that is REALLY present.
So traditional cameras are not designed to be senstive to everything equally... they're designed to try to immitate the sensitivity of human eyes. That way the image you capture resembles what you remembered seeing with your eye. This means traditional cameras have filters that reduce the amount of light that can pass in these parts of the spectrum where our eyes are less senstive. For H-alpha... the filter is blocking about 75-80% of the light.
But for astrophotography where so many things are based on this particular wavelength, that would translate into needing to record images (already many minutes long) by a factor of 4-5x to make up for the loss.
This is WHY people modify cameras for astrophotography.
I have several cameas. My 60Da is used heavily for astrophotography. I also own a few special cooled and unfiltered CCD imaging cameras (once upon a time, CCD cameras were much better at extremely long exposure imaging, and CMOS cameras were much better at short exposures. Today those lines have really blurred and you'll find many "CCD" cameras are really CMOS cameras.) Which camera I use depends on what I'm trying to image.
For both telescopes and cameras, there is no "best". There's just a "what's best for the type of object".
Tim Campbell
5D III, 5D IV, 60Da
