The original HTSB starfield is based on the Tycho/Hipparcos star catalogue visualization (version 2) that is or was available at
NASA's Scientific Visualization Studio / Goddard Space Flight Center pages, but at the moment the site seems to be down and I have no idea if that's just a momentary problem or if they have been brought down entirely.
The source file I used was, if I recall right, this:
https://svs.gsfc.nasa.gov/vis/a000000/a003500/a003572/TychoSkymapII.t5_16384x08192.tifIt is unavailable at the moment in that address but maybe you can find it elsewhere on the internet, it's the 16384x8192 resolution TIF version.EDIT: looks like the site has simply moved to
http://svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=3572, the high resolution version of the file is available directly at
http://svs.gsfc.nasa.gov/vis/a000000/a003500/a003572/TychoSkymapII.t5_16384x08192.tifHowever I did edit the levels, saturation etc. somewhat to produce the basic HTSB version that was delivered with the AoA Director's Cut, and was subsequently used as the basis for most Sol starfields including those with planets. My WIP files (Blender settings) for that have evaporated during a few HDD crashes, so I can't help with that more.
Newer version of the Earth skybox uses source textures from Space Engine, with multiple layers to get it looking how I wanted it to look. I couldn't get it to look the way I wanted with a single render, so each layer pretty much had to be rendered with custom settings, and it was a huge pain in the arse. While the end result was pretty good I don't know if I would want to use Space Engine to do it again any time soon.
The big advantage with Space Engine, though, is that you can set the time and place and you'll automatically get more or less correct placement of celestial objects - the Sun, the stars, the planets and their moons. But the process of switching through different settings, making sure the camera angle doesn't change, and rendering each element separately at high resolution, and then checking that it actually rendered correctly and loaded the texture properly instead of changing to lower MIP maps because of the field of view settings... yeah. Lots of potential in that program but really annoying to use for more complex work.