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Hi Jo. I use a Canon 200mm f2.8L USM II, prime lens. Reported to be very sharp. But there are other factors that determine image sharpness in AP. With an APS-C sensor, it looks like 320mm. I find this provides a field not so wide that deep sky objects loose too much detail.
The lens needs stopping down. The best resolution is obtained at f5.6 - a completely flat field at f6.3. This is nice at iso400 under light polluted skies with exposure/integration times of 3 - 3.5 minutes with a 1000D.
I think it meets your criteria at f4, but you will get slightly eggy stars in the corners. Price second hand, might make it?
Iris diffraction spikes are very prominent on larger stars. Alternatively you can use an apeture ring on the front of the lens. Set at f2,.8, or tape over the contacts on the back of the lens.
You can of course image wide open or at any other aperture, but the edges will need cropping.
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