Using 500mm lens for astrophotography

[arifmshaikh]

I have 170-500mm apo lens which I am thinking to use for wide field astro photography. I see that refractor needs field flattner. Do I also need ff with this lens. What are the pros and cons of using this lens vs refractor telescope.

[cometcatcher]

No you won't need a field flattener. Should make a reasonable lens for AP.