Stressed out by astro-jargon and the astro-gadget learning curve? Astronomy Now'sequipment guru, Martin Mobberley, tries to soften the mental pain, This month helooks at cooling telescope optics as well as a reader's nifty mount for a Coronado PST.

摘要

At first sight atmosphericseeing and local thermalseeing (the latter causedby your telescope and its mainmirror being warm) seem to do thesame damage to stars and planetsat high magnifications. Stars cantwinkle rapidly and even movearound, bloating to a diameterof many arcseconds. Planets canappear to have rippling edges andthe fine features simply disappear.However, examine those defocusedstar images at high power and youmay spot something that is uniqueto the specific problem of a warmtelescope. If you defocus the stardoes its diffraction disc show anelongation at a particular angle,like smoke escaping from a distantbonfire? This is the sign of warmair seeking a way out. It is mostnoticeable in sealed tube systemswith heavy optics, like MaksutovsandSchmidt–Cassegrains. Tryleaving your telescope outdoors fora few hours before observing withthe dew caps on, but with the tubeexposed to the night air. You maywell be amazed at the improvement.To totally eliminate the effects ofa warm telescope you need thetemperature of the tube and opticsto be within a fraction of a degreeof one another. This is really trickyto achieve if the telescope has spenta warm day in the observatory.Indeed, the observatory itself maybe a veritable bread oven for storingtrapped heat if it has a brickworkconstruction