>>3622857>What is the ideal lens for astro?I'd say the most important trait by far, is coma. You want as little coma as you can get. A fast aperture means very little if you have to stop down 3 stops to reduce coma.
>picrelated what coma looks likeAnd then you need fast aperture. So, the less comma, the better. The faster the aperture, the better. It's a delicate balance where you can't have it all. It's like
1. fast aperture
2. low comma
3. price
Choose 2.
Also, don't focus too much on lens aperture and spend all your money there. Your money is much better invested (in terms of results) in an equatorial mount (star tracker). This "tracks" the star by rotating the camera to exactly counteract the earth's rotation, so the stars don't move in relation to the camera. All professional astrophotography and deep space photography is done with those. You can get one for ~$250 or so, looks like this
>https://www.bhphotovideo.com/c/product/1092106-REG/sky_watcher_s20510_star_adventurer_motorized_mount.htmlThis allows you to use far longer exposures, without any trails. So fast aperture becomes quite irrelevant, since you can expose for as long as you want. You can even use normal and tele lenses for closer app photos of the various formations. Keep in mind, the longer the focal length, the more careful you need to be when aligning the tracker (polar alignment). Use an app and the scope on the tracker, and you can easily align it.
Keep in mind, with an equatorial mount, stars are perfectly still and without trails, but the foreground (anything on earth) becomes motion blurred. So if you need to include a sharp foreground too, after getting the star shot, turn the mount off and take a long exposure as normal, then combine the 2 in post with some masking.
You can also use stacking methods instead of one long exposure, but the latter is much more convenient usually.