Optics to correct focal distance across a plane I have a laser beam which is focused to a point at a certain distance.  I'm then going to use a galvanometer to scan that beam across a plane.  Obviously, as the beam scans across the plane, the distance between the beam source and the plane will vary and so the beam will only be focused on some of the plane (a circle equidistant from the central axis of the galvo).  I want to minimize this effect.
Obviously the further away the galvo is from the plane where I want the beam focused, the less significant this effect will be.  However, that makes for an awkwardly large machine.
Is there some sort of optics I can use to correct this so that the beam will be correctly focused across the target plane?
Edit I wanted to add this as a comment but I don't think I can put images in the comments.  What about this arrangement:

This relies on being able to form a collimated beam from the laser - in practice I think it will have some noticeable beam divergence but I'm not sure how bad it would be.  It focuses the beam on the target plane whatever steering is given from the galvo:

Obviously I'll have to correct for the deflection introduced by the lens, and it's a bit of a pain because it increases the mirror deflection required for a given beam deflection.  But it's a lot easier than eg a moving laser or a custom-printed lens.
Comments?
 A: If the angle of deflection is at all large I think the corrector plate will be very thick at the centre, possibly a large fraction of the focal distance.  If the beam was parallel rather than focussed before the mirror, the corrector would become a large lens which focusses the beam on a flat field. 
A: The problem is to deliver properly focused light to a planar surface from a lens with a fixed location.  Any method which keeps the optical path length constant, while not refracting the beam, would work.
Therefore introduce a carefully designed transparent phase plate, where the optical thickness at each point is intended to compensate for the shorter path; thus the plate would be thicker in the center of the field, and thinner towards the edges.  The minimum thickness is determined by structural requirements, so that it can be held rigidly in place. 
Ideally the beam would enter and leave the material normal to the surface, so that there are no focusing effects; the calculated distances for each of the galvo angles, and the location where the beams would intersect this plate provide most of the information required.
This is a passive element; no more moving parts; it just has be kept clean. 
A: Depending upon the overall size of the layout, you might want a field flattener lens. I don't know how large your screen is, but for Petzval curvature (the natural field curvature for all optical systems given by P = Sum(poweri/ni), where power is 1/(focal length) and n is the refractive index of the glass. But this is not field curvature. I'm just saying that to correct for curvature you can place a lens on the screen. For a curved field that is concave you would use a concave/plano lens. For a convex curved field, you would need a positive power lens. You want to make the focal length longer as a function of field angle so the light focuses on the flat screen instead of in front of it on the sphere. Thus, you would need a concave/plano lens on your screen. Probably not applicable in your situation. 
Depending up the size of everything you might consider an F/theta lens, or a telecentric setup. Place the galvo in the front focal plane of your lens. The collimated light is focused by the lens and will focus a distance f away (same distance from the lens as the galvo). But since the galvo is in the front focal plane, that means your aperture stop is effectively in the front focal plane as well and all your converging beams will be parallel. The spot will always be focused on the screen. You might consider using an off-axis parabola instead of a lens if the element is large. Effectively, your imaging element (lens or mirror) will be the same size as your screen. But it works very well. I have a simple Power Point drawing but cannot figure out how to attach it here.
