Section 2 - More on Cooling

Another aspect of the above mentioned cooldown problems is the fact, that a lens can perfectly cool down only in an environment with constant temperature. The problem here is that we use an astronomical telescope most often on nights when the sky is clear. When there are no clouds above the surface, nothing prevents Earth to radiate a lot of heat energy into space, and the temperature starts to drop (by usually about 1-2 degrees an hour). This way, after taking out the telescope, at first the scope starts to cool down quickly, but as the temperature of the optics are more similar to that of the surrounding air, the process slows down. After waitin an infinitely long time, we will see that the temperature difference between the optics and the air becomes a constant value, which depends on the rate of the temperature drop of the air at the observing location, and the thermal properties of the lens. The only thing we can do to improve the situation is to optimize the lens do it can quickly cool down, this way we can decrease the constant value that describes the temperature of the lens after the first, transient phase of the cooldown process.

Many amateur astronomers have noticed, that small aperture and very high quality lenses (e.g. Zeiss AS 80/1200) deliver excellent planetary images, that can be hardly rivalled by even todays modern scopes that use much more modern optical materials, thus produce even less false color and also in many cases produce lab measurement values closely matching those of the Zeiss lenses. Still, despite the better material and excellent manufacturing quality, most modern scopes usually can not achieve the image quality (sharpness, contrast and clarity) of those images produced by old Zeiss Semi-APO lenses. We believe the key of this contradiction is the fact that the old Zeiss lenses were smaller and thinner, so these could closely follow the temperature of the environment (with focal ratios of F/10-F/20). But todays optically equally perfect lenses have F/5 - F/7 focal ratios using much thicker glass materials and several air spaces that prevent them from cooling down in a reasonable time. This is why they can not compete with the old and legendary Zeiss planet killers in "field performance" while they easily beat the old Zeiss lenses in "lab performance".

<< Section 2 - Under the Stars! F-Ratio || PREVIOUS || NEXT || Section 2 - Market Trends >>