THE QUESTAR 3.5" TELESCOPE
Optical Design
The Questar 3.5" combines a Maksutov-Cassegrain telescope with a unique "Control Box" at the rear. The Maksutov Cassegrain design is a folded catadioptric optical tube that provides a nominal specification of a 1288mm focal length at f/14.4. Since focusing is controlled by moving the primary mirror at the base of the telescope, the actual focal length and f-ratio can vary slightly depending on the specific focus position.
One benefit of the moving mirror design is that the Questar 3.5" can be used as a telescope for viewing the planets and Earth's moon, as a spotting scope for wildlife, and even as a "macro lens" for indoor close ups of flowers or other objects with a minimum focus distance less than 10 feet.
The Control Box is an elegant ergonomic design feature that allows a single eyepiece to be used as a finder scope (low magnification), standard telescope (medium magnification), and high power magnification through the use of a barlow. A central axial port allows the use of a second eyepiece at a single magnification or the attachment of a camera for photography.
While the Maksutov-Cassegrain and similar variants can be found in many other telescope manufacturers today, the Questar stands apart from the competition with its precision manufacturing and assembly. Since 1959, the optics of the Questar product line have been developed by J.R. Cumberland, Inc. a specialty provider of custom optics based in Marlow Heights, Maryland.
Each optical element in a Questar telescope is manufactured to a target of 1/50th wave (λ) accuracy for a total-system performance of 1/8th λ or better at the focal plane. A premium upgrade to guaranteed 1/10th λ accuracy is also available.
Precision Manufacturing
Deviations from surface flatness or power, when it comes to curved optical surfaces, can be described in values of waves (λ), which are multiples of the wavelength of the testing source. If the wavelength being tested is 632.8 nm (as would be the case with a He-Ne red laser), an error of 1λ would be 632.8 nm and an error of 1/4λ would be 158.2 nm.
If the Questar's 1/8th λ accuracy for a 3.5" diameter telescope was scaled up to the width of the an American Football Field (160 ft), the deviation from specification would be less than the thickness of a piece of paper.
The image above is an actual copy of a test report on a premium 1/10th λ Quartz Questar 3.5 Standard whose optics were completed in 2019. As impressive as the smoothness of the mirror is the accuracy of the power at -0.010 λ, or 1/100th λ accuracy. This is the deviation from the target curve of the mirror and the actual finished mirror.
The specific numbers may be higher or lower for individual "guaranteed 1/10th" λ optics. The optics in the Questar 3.5 telescope are truly handmade and some variability is present. For smoothness, it is possible to grind down a mirror or lens further to take care of any small sleeks or micro-scratches, but then the actual intended curve or power is off. After all, telescope mirrors are not flat -- they are designed to bend light. J.R. Cumberland carefully balances the smoothness of the finished mirror with the target aspherical curvatures when manufacturing optics specifically for Questar.
Timeless Mid-Century Modern Design
Though the optical performance and precision of the Questar 3.5" telescope are noteworthy accomplishments, the instrument itself is an example of timeless mid-century design and craftsmanship. It offers an elegance that would be appropriate for a jeweler or watchmaker. Each component of the mount is handmade and many surfaces are polished to a mirror finish. The visual design and optical performance is also backed by robust mechanical performance with specimens from the 1960s continuing to be in perfect working condition today.
At the high magnifications used for observing the Moon and planets, the motion of the night sky both from planetary and lunar orbits as well as the rotation of the Earth can be seen. Planets will drift slowly out of the view of the telescope eyepiece. Consequently, it is important to have the telescope follow the motion of the skies through the night. The base of the Questar 3.5" standard and duplex models integrates a motor that allows the telescope to follow the motion of the rotating Earth so that the stars remain stable and centered in the eyepiece. The base model is designed to run of AC household current while an upgrade to a 9V battery-driven "PowerGuide" is also possible.
This integration of both telescope and mount in a small portable package is part of what makes the Questar 3.5" timeless. Mid Century Modern design remains just as popular in 2020s as it was in the 1950s because "it is not a style and is based on solid principles of design and human needs, along with functionalism, respect for the environment, and solving the basic human desires for livability.” (W. Krisel).
This thoughtfulness in form of function without embellishment simply for the sake of embellishment is one reason why the Questar 3.5" endures. It's all about the livability. Take for example, the star-map that has been silk screened onto the dewshield. At initial glance, this may simply seem to be an ornamental design. In fact, it's a very functional component of the Questar 3.5".
First, it serves to protect the front surface of telescope from wayward fingers as well as dew that can form during the evening. More importantly, the star chart is actually usable. A Questar owner need only position the telescope in an equatorial position, and sit down facing South. The chart can then be rotated so that the current month is displayed. The constellations on the chart will match the overhead stars at 9:00PM local time. Each rectangle on the grid represents 1 hour, and as the night grows later, the sky moves westward and the dew shield can be rotated accordingly.
When the dew shield is removed completely, a simplified map of the Moon is revealed. This helps novices learn some of the basic features of the Moon and is presented in a classic "inverted-view" that represents what would be seen with an eyepiece at the axial port alone rather than one with the inverting star diagonal or prism.