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While this set presents plenty of very interesting NASA produced documentaries, its image quality leaves much to be desired. It is understandable that most videos are more than 30 years old, so one can't expect DV like resolutions. However, the image quality in the first 4 discs is at best comparable to a typical VHS tape.
The 5th disc -- which covers the ISS and recent shuttle flights such as John Glenn's and Columbia's disaster -- has better video, but it's still similar to a PAL video broadcast. So while the content is unique to DVD series, be aware that the transfer leaves much to be desired. If you're hoping to find high resolution footage of the space vehicles in all their glory (as I was), this shortcoming may be quite disappointing.
As an unrelated note, my set had no region encoding. This is reasonable since NASA material is released to the public domain and shouldn't be published in a restricted format.