dewcap

dewcap

[′dü‚kap]
(optics)
An open tube attached to the end of a refracting telescope to prevent moisture from condensing on the objective.
References in periodicals archive ?
The Meade astrograph comes with a compact metal case which can store the telescope with tube rings attached provided they are spaced close together to allow the dewcap to fully retract, as it is here.
Dew is less likely to condense on a reflector's mirrors in the night air, an annoyance with the other designs unless they are provided with a long dewcap. But a reflector does require occasional adjustment, such as tweaking the tilt of the mirror to bring it into proper alignment.
Any telescope in bright local lights should also have a long dewcap to keep stray light out of the tube.
The generously long dewcap is secured with two locking screws, so it won't slide back down the tube with a bang when aimed straight up.
But just 15 inches (38 cm) long with the dewcap retracted, the TMB tube is as short as some 80-mm apo refractors.
The retractable dewcap provides a generous 4.4 inches of extension ahead of the objective for warding off dew and stray light, but it slides back to minimize the scope's length for storage.
This happens only with the SD because its foot is reversed relative to that on the other scopes to allow the dewcap to fully retract (the SD has the shortest tube).
A sliding dewcap extends almost 2 inches beyond the objective.
The latter's 7.9-pound (3.6-kilogram) tube measures 16 3/4 inches (42 1/2 centimeters) long with the dewcap and focuser retracted.
The felt-lined dewcap retracts completely for storage but slides forward to add 312 inches of extension to keep dew off the front element.
But at 28 inches (71 cm) the tube is longer than it needs to be for storage or transport because the 6 1/2-inch-long dewcap does not retract as with most other high-end refractors.
With a tube only 14 1/2 inches (37 cm) long when the dewcap is collapsed and weighing only 4 pounds (1.8 kilograms) with mounting rings but without diagonal, the Megrez 80 is one of the most diminutive high-performance scopes on the market.