I found a 100mm spotting scope to have plenty of light for my APS-C chip. The issue was the aberrations on a cheap scope. The second issue was keeping it steady. That can be done. My SLT has no mirror to slap. I use cable release on a tripod.
I agree it has to be over 600mm fl which I can do with my camera lenses. I would not pay for the duplication. It has to be 800mm and up with a fast aperture.
The 500mm zooms are not great at 500. I have the tammy, it stays in the cupboard as not worth carrying when a top end prime is available. I'm sure the sig is the same. They can not keep up with a prime.
I agree it has to be over 600mm fl which I can do with my camera lenses. I would not pay for the duplication. It has to be 800mm and up with a fast aperture.
The 500mm zooms are not great at 500. I have the tammy, it stays in the cupboard as not worth carrying when a top end prime is available. I'm sure the sig is the same. They can not keep up with a prime.