In article <3310532B.20EE@lagoon.ultranet.foo.com>, pnelson@lagoon.ultranet.foo.com says...

>I've been doing some wide-field astrophotography
>with a Nikkor 180mm f/2.8 and a Sigma 400 f/5.6.
>The Nikkor is marvelous but the Sigma is soft,
>especially wide open. Still, I need something
>longer than 180 for some of these shots. But
>I can't afford the $7K -$8K it would take to
>get a fast Nikkor. Even used, ~$4K, those
>lenses are out of my range.
>So what's my next best option? There are numerous
>indie-brand 300mm f/4 lenses on the market -
>Tokina, Sigma, etc. Are any of these half decent
>wide open? How do I tell which is best wide
>open?

How about a used 300mm f4 Nikkor? It is an excellent
lens wide-open, and likely better than the still-expensive
Tokina and Sigma alternatives.
Hope This Helps