I think there is something to this in theory - but perhaps it doesn’t make much difference in practice.
An example comparing optical glass properties of Ohara FPL-53 with BK7 (or the Ohara equivalent) shows the ED glass has about 2 times the thermal expansion coefficient of a “normal” crown glass. This means the shape of an ED lens element will be affected twice as much when undergoing a temperature change. Likewise the ED glass conducts heat only about 75% as well as the normal glass. This means it takes longer for the ED glass to reach thermal equilibrium.
When you expose the scope to a temperature change - some parts change temperature relatively quickly and other take more time. The outer surface of the objective feels the change quickly but the second surface, buried in the lens cell feels the change more slowly until the temperature change is conducted through the glass and the element reaches essentially a uniform temperature (and returns to its nominal shape).
The lens is distorted by the temperature change because not all regions of the glass change temperature at the same rate. Same goes for the second element. Complicating things further is how heat is conducted through the lens cell from or into the lens elements.
This all means the shape of an FPL-53 lens element will be affected more, and the lens will take longer to return to its nominal shape than a dimensionally similar “normal” glass lens in an otherwise similar telescope when exposed to the same temperature change.
BUT - life is not so simple. The optical prescription (curves, lens thickness, airspace gap, etc.) of an achromat doublet and an ED doublet of the same aperture and focal length will not be the same due to differences in optical properties of the glasses used and the lens designer’s choices in optimizing the prescription. e.g. the ED lens and the “normal glass” lens would not be “dimensionally similar”. Differences in lens cell design and fit, differences in tube baffles (affecting convection cells within the tube) and the thickness and material of the tube itself probably have some effect as well. Direct comparisons would be a challenge.
But in practice - based on the many field reports in this thread it seems any difference between an ED doublet and achromat doublet is small. How many would notice their ED scope took 21 minutes to settle down and their acromat only took 19?
I actually own two nearly identical Kunming 102mm F11 scopes, one a crown/flint achromat and the other an ED doublet. So I guess I could set up a small experiment. But there are differences between the two scopes that might mask any difference in time to reach equilibrium.
So, while it looks like there is a difference in theory, I think I’ll not worry too much about it and just observe.
Edited by Don Taylor, 16 October 2020 - 08:37 PM.