Glass factsOptical glass varies considerably in its ability to expand and contract when experiencing a temperature change. Indeed the coefficient of thermal expansion of these glasses ranges from between 4 and 19 x 10-6/K. Dr. Juergen Schmoll, an astronomer and instrument scientist based at the Centre for Advanced Instrumentation, Netpark, Durham, UK, informed me that the thermal expansion of low dispersion glasses is significantly higher than either of those used in a classical achromat. Consider first, the Coefficients of Thermal Expansion (CTE) of tried and trusted crown and flint glasses:
F2: 8.2 * 10-6 /K
F5: 8.0 * 10-6 /K
N-BK7: 7.1 * 10-6 /K
N-BAK4: 6.99 * 10-6 /K
Now, compare these values to modern low dispersion glasses;
S-FPL51: 13.1 x 10-6 /K
S-FPL53: 14.5 x 10-6 /K
Fluorite: 18.9x 10-6/K
The higher the CTE, the more the glass is likely to change shape while acclimating which in turn affects the definition of the image. For example, a lens that morphs as it cools will be more difficult to focus accurately as it will introduce aberrations similar to spherical aberration into the optical train. As you can see, the new, synthetic fluorite glasses have CTEs that are ~ 1.75x to 2x higher than the old glasses, with fluorite itself exhibiting even higher values (~2.5x). This is the reason that oil spacing had been invented for lenses such as the as the legendary Zeiss APQ series (now sadly discontinued) and those more recently offered by TEC (USA) and CFF (Hungary).
This is a very significant revelation, as Plate glass is well known to change shape while cooling. Originally Plate glass was employed to make Newtonian mirrors but was gradually replaced by Pyrex owing to its lower CTE (4 x 10-6 /K compared to 9 x 10-6 /K for Plate glass). Fluorite, in contrast, has a CTE more than double that of Plate glass!
We can conclude, with absolute certainty, that modern low dispersion glasses will undergo significant changes in shape as they struggle to acclimate to the outside air, and indeed will continue to change shape as temperatures fall during a typical night’s observing. Curiously, the classical achromat, with its continued use of traditional glasses (crown & flint) fairs considerably better in this regard.
- szklo.jpg (11.08 KiB) Obejrzany 9345 razy
Image credit: Oldham Optical UK.
This goes quite some way to explaining reports made by observers (yours truly included). Indeed, in observations made with a long focus Zeiss refractor, the Czech particle physicist and keen amateur astronomer, Dr. Alexander Kupco, posted his findings comparing an older, f/15 Zeiss AS 80 and a modern Stellarvue SV80S f/6 triplet apochromat.
See
http://www.cloudynights.com/ubbthreads/ ... art/2/vc/1If this posting, Kupco reported that the long focus doublet Zeiss gave sharper, more stable images than his short tube triplet apochromat right from the beginning of his observing session and that despite having acclimated, the Zeiss always maintained an advantage in this regard.
Lens thickness and cooling ratesThe focal length of a simple lens can be determined from the lens maker’s formula:
1/f = (n−1)[1/R1-1/R2+(n−1)/dnR1R2]
where
f is the focal length of the lens,
n is the refractive index of the lens material,
R1 is the radius of curvature of the lens surface closest to the light source,
R2 is the radius of curvature of the lens surface farthest from the light source, and
d is the thickness of the lens (the distance along the lens axis between the two surfaces
You can see from the equation that the thickness of the lens d is inversely proportional to f, the focal length. Thus, lenses with long focal length can be made (and generally are) made more thinly than their shorter focal length counterparts. The equation also shows that the focal length scales directly as the radius of curvature of the lens, implying that as R increases so too does focal length.
Of course, this is first principle optics and it can be modified to accommodate two or three lens elements, but the broad result is the same. After all, an objective is designed so that all the elements behave as one, or as closely as possible anyway.
Działałby nie tylko przeciwko własnym interesom ile przeciwko samej idei handlu. 
