A real eye-opener was the article out of 1955 in Vistas in Astronomy. Before I did actually receive the article of Jan Cuypers , Peter Abrahams of the ATM e-mail list did send me a summary of it, saying there were foucaultgrams in it. The article itself was more or less a translation of former articles. Otherwise glass does not come so quickly to thermal equilibrium as bronze did . So a new problem appeared and up to now it was only partially solved by better kinds of glass, partial isolation of the mirror and forced ventilation.
All methods being passive, the heating method is an active one and can be easily tuned.
The presence of the supporting points at the edge of the back-side prevent the heating element being applicated up to the edge, causing a narrow turned down edge. The second when 3W of heat is applied showing good overall correction and the third is showing the over-correction by applying 25W , comparable in size to that of a spherical mirror. It was this observation that made Couder think of using heat for figuring parabolic or more general aspheric surfaces. Natural variations are much slower than the effects of articial heating and adjustment is easy, excellent correction can be obtained in about a quarter of an hour. The 120cm has been fitted with such a heating arrangement, but also a wire around the edge where there's a groove.