I found this in a forum talking about ET in 70mm. It’s all I’ve found so far for how these blow ups were created:
Regarding the image quality of the blow-ups, keep in mind that there may have been two grades of 70mm prints for “E.T.” The film’s cinematographer, Allen Daviau, ASC, informed me a few years ago when I was pestering him for some info that he believes they struck a few 70mm “show prints” off the original negative, with the balance of the print run (more than 30) utilizing the commonly-used procedure (i.e. 35mm O-Neg --> 35mm Inter-Positive --> 65mm Inter-Negative --> 70mm Release Print).
What’s the internal temperature of a TaunTaun? Luke warm.