« on: November 28, 2012, 10:31:03 PM »
Hi Don, no personal experience either unfortunately and I'm hardly competent. But this year I've been playing around with a lot of longer cheapish lens combinations. You'd obviously know that the lenses aren't weatherproof. I don't know how badly water entering the lens will affect your sensor. I can't imagine this will be good for it. Also, I'd assume you'd be putting lenses caps on during daylight hours, but I'd also suspect that these lenses will be more susceptible to glues failing, lens coating separation issues, more dust and dirt entering the lens, fungal attacks etc which might be an issue if high image quality is critical. But I doubt other options will stand up long-term in the conditions you've described, anyway. Although a better quality weather sealed lens should stay in better condition longer. My other reservation is that this sounds very astronomy-like. I'd check that F/16 in your shooting conditions gives a shutter speed of less than 5 secs if that's an essential requirement. In this case, you'd be so much better off wth the $10,000 primes....
As for image quality, provided you've got a good mounting, a stationary subject/smooth tracking and the 650-1300 is at least "OK", my guess would be that a good 400mm lens would be significantly better than the 650mm end. But the 1300mm end will probably be better than a cropped 400mm lens. If not, and if it is important, you should be able to work with the image in post production to get a similar/ better outcome but at a much cheaper price. Overall, for what you've just described and if you know the limitations, I'd say go for it.