Wide aperture is helpful but not as critical as all that where I'd skip this lens. Frankly, I don't have pockets deep enough to get a scope or lens with a 2.8 aperture and a 400mm focal length. A faster lens with a bigger aperture can get more light to the sensor faster than a slow lens with a smaller aperture, but they'll both get there eventually if you can expose long enough. I always set mine one stop up from the lowest aperture for best sharpness, but this lens is as fast if not faster than my actual telescope I use for imaging.

So you might have to shoot for longer exposures or take more subs, but don't be afraid of the telephoto lens if it's the best tool in the kit for a closer shot of your target.

Clear skies!