Quote:
Originally Posted by Nikolas
just for once I'd like to see it explained in layman's terms
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OK...
For a lens to perform as designed (to the diffraction limit) the thicknesses and spacings of the lens elements are quite critical and must conform to tight tolerances for the lens to resolve fine detail at its best (ie at the diffraction limit).
The author realised that the ED and fluoro glass types used in many modern objectives have very high coefficients of expansion - as much as 18X the coefficients of the glass types used in older achromatic designs.
When a modern lens is at thermal equilibrium - at the temperature where the lens elements were measured (ie when they were made) - all well and good - the thickness of each lens element will be as expected and it will perform as expected.
The first issue that arises is that a lens element may swell or shrink well beyond the thickness tolerance to remain diffraction-limited. Lens manufacturers don't state what nominal temperature the lens was manufactured for, however it is almost certainly somewhere in the range 15-25 degrees C. However, in normal use refractors are exposed to the ambient air which could be anything from -10 to +40 degrees celsius, or a variation of as much as +/- 20 degrees from the ambient temps when it was manufactured. This means that in cold/hot conditions the lens elements will expand or contract accordingly, and this will degrading the sharpness of the image.
The second issue is that each lens element is not uniformly thick - they are either thick in the middle with thin edges (positive element) or thin in the middle with thick edges (negative element). This means that while the lens is cooling or warming, the degree of expansion/contraction is not uniform across the lens and the curvature of the lens surfaces will be deformed noticeably from the original radius of curvature. This also degrades the performance of the lens.
So... if you are wondering why images from your ED refractor in extreme hot/cold don't seem as sharp as they could be, this is one possible reason. The old achromatic refractors don't suffer from these effects as the glass types used have far smaller coefficients of expansion, to the extent their performance is not affected by the typical range of temperatures you might encounter.
This expansion/contraction occurs in all directions - including across the diameter of the lens, as well as its thickness. Given the coefficients of expansion of these ED and fluoro glass types - and the fact that they are soft and brittle compared to crown or flint glass - they may fracture or chip if they are tightly restrained in the lens cell. It is important that they be slightly loose.
Lastly, this also means you should NOT use an ED eyepiece for eyepiece projection of the sun - such as the Vixen LV/LVW, Televue Panoptics, Naglers or Radians. There's a good chance the localised heating and expansion will fracture the ED elements.
