In optimal world I would buy a lens that is good in every way. But I'm on a budget and have to find a balance in quality and price. I'm looking for a lens for star photography, so in my case it will be a wide angle lens with good characteristics for capturing points of light in a dark sky. What would those characteristics be?
So far I've learned about coma, which seems like a thing to note when comparing different lenses for star photography. It would be quite disappointing if stars turned into long teardrops on the edges of a photograph even when every precaution was taken to make all stars sharp round points of light.
Vignetting would be an example of lens characteristic not so important in this use.
Many characteristics of a lens turn better when aperture is slightly stopped down, but I suppose I'd be wanting to use a lens with wide open aperture when trying to capture the stars in a dark night sky.
I'm not asking for lens recommendations. But, as an example of what kind of lenses I've been looking at, I name Samyang 14 mm f/2.8 UMC manual focus lens. Its price level is $350'ish in Amazon and seems to run with name Rokinon too. It seems to be reasonably sharp even when wide open - my camera is an APS-C sensor camera.
Answer
It is probably easier to talk about what qualities in a lens that often add significant cost that you don't need in order to do astrophotography.
The first is Auto Focus. Stars are such tiny points of light that the accuracy of most AF systems is not quite good enough to resolve them to the absolute sharpest capability of the lens. Most AF systems can't focus in the dim light of a starry night anyway. Even the moon, which most AF systems can attempt to focus sharply, needs a slightly different adjustment than the stars. The good news is that once you get your lens properly manually focused on the stars, it will stay properly focused as long as the temperature stays the same. If the temperature changes drastically, you may need to refocus as different materials in the lens will expand and contract at different rates.
The second is Image Stabilization, at least the electronic kind. To do any remotely serious astrophotography, you'll need your camera mounted on a stable mount such as a sturdy tripod. For some types of astrophotography a telescope mount with a sidereal clock drive that synchronizes the motion of the mount to the apparent motion of the stars as the Earth rotates on its axis beneath them is even better.
As you have indicated in your question, vignetting is an issue that can easily be dealt with in post processing, so a lens with little or no vignetting when wide open is not necessary for astrophotography.
The one attribute that is absolutely vital for doing astrophotography is acutance. This is the combination of sharpness and contrast that allows clear separation of the dark background and the points of the light of stars. Chromatic aberration can also affect acutance, and ideally a lens for astrophotography will demonstrate minimal CA. Chromatic aberration can be dealt with in post processing, but for it to be effective a detailed lens/camera profile needs to be applied. Most lenses with a comprehensive profile available for a tool such as Canon's Digital Lens Optimization contained in their Digital Photo Professional will be higher end lenses which also have most of the features listed above (and the additional cost) that you don't need for astrophotography. Like many thing related to photography, it is easier to get it right in camera than worry about chromatic aberration in post.
Assuming you want to do wide field astrophotography, you are on the right track with a lens such as the Samyang/Bower/Rokinon/Pro-Optic/Walimex/(and whatever else it is called this week) 14mm f/2.8. But be aware not all astrophotography is concerned with wider angles of view. It is, however, a good place to start if on a tight budget.
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