I'm not sure what you mean by "stability". You're dealing here with two issues.

(1) Smaller apertures have more "stable" images because of the* size of atmospheric thermal cells*. The Schief is a 200 mm aperture and the cells are usually between 3 and 4 inches (about 75 to 100 mm to round it off). The bigger the aperture the more "dancing" the image will exhibit.

(2) Depth of focus simply means that an image will remain within the 1/4 wave Rayleigh limit over a certain range of focal shift. The definition of the depth of focus (DoF) is the amount of focal aberration corresponding to 1/4 OPD =**± λ/(2***N*sin<sup>2</sup>*U**<sub>m</sub>*), where λ = wavelength (usually 0.00055 mm) , *N* is the ref. index of the medium (in this case the air *N*= 1.0), and sin<sup>2</sup>*U**<sub>m</sub>*is the sine of the angle subtended by the marginal (peripheral) ray and the paraxial focal distance. The expression can be re-written in terms of the*focal ratio, F#,*as **OPD =****±2λ(F#)<sup>2</sup>**. Doubling the focal ratio quadruples the DoF distance. If you go from f/10 to f/20, doubling the focal ratio the DoF tolerance will increase 2<sup>2</sup> or four-fold. In case of an *f*/10, the depth of focus OPD tolerance =±0.11 mm. For an*f*/20 it's 0.44 mm.

Clearly, the slower the system the more it's going to stay "in focus", thereby appearing more stable. If this is coupled with an aperture not exceeding 4 inches or so the image will also be more stable because of the thermal cell size of small air pockets, so the performance will be judged "superior" to that of a bigger, and/or faster system.

Mladen