Eric Gagne:
Checking Andy's picture...............so anything under 36x24 is crop ?
A cropped sensor is a very loose term. In daytime photography where Canon and Nikon and some other brands are concerned, a cropped sensor specifically refers to the APS-C size.
A little bit beyond that definition a cropped sensor can be anything smaller than 24x36mm
but we are setting ourselfs up for disaster with those definitions. In not too long we will be using sensor sizes way larger than full frame. And it will be very difficult to communicate in terms of full frame and cropped.
For astrophotography most of this doesn't really matter in the first place.
You need to make sure your image circel is big enough for te sensor. (this makes sense if you want to buy a big sensor)
The size of the pixel in combination with the focal length of the telescope determine the pixel scale and thus the level of detail.
The size of the sensor in combination with the focal length of the telescope determine the field of view. (How much of the sky will be in the frame)
The QE (quantum efficiëncy) indicates how light sensitive the camera is.
Good luck with your decisions!
