H-alpha all-sky survey. C. Stubbs May 25 2015
As was pointed out to me long ago by Greg Bothun, for resolved objects the surface brightness on the sensor depends only on f-number, not on aperture. So as long as we can Nyquist sample faint galaxies, a fast lens will do as well as a big telescope. Zeiss makes a nice 85mm f/1.4 lens that works out to 1 micron with excellent image quality, and covers a 35mm focal plane with a flat field.
At 85mm focal length one arcsecond spans 0.5 microns, and FOV is 28.6 x 24 degrees. A 10 micron pixel subtends 20 arcseconds, so we need arcminute-size galaxies with decent surface brightness. Assume the goal is to get out to redshift of 0.1, at which 1 kpc is 0.5 arcsec. Actually at 6500 A we don't need the fancy IR version, and 3-4 filters get us from z=0 to z=0.1, at 20 nm each. But the key is getting really small pixels and low dark current. So what's the smallest-pixel back-illuminated 35mm sensor? Say we want 5 micron pixels and 35mm diagonal, so 25mm on a side, which means 5K x 5K, TE cooled.
An LSST sensor would be OK, at 10 microns per pixel and 4 k x 4k.