Sky Talk September 2010: Astroscan FAQs

There are a couple of questions about the Edmund Scientifics’ immensely popular Astroscan wide-field telescope that both potential buyers and existing user’s often ask me, since I was a project consultant on the original model and more recently on the upgraded Astroscan-Plus one as well. Readers will hopefully find the questions themselves — and my response to them — informative!

Q: Why is the Astroscan red?

A: This is the most-oftened asked question. There are actually two reasons for its distinctive color. One was for appearance; red was chosen to set this unique instrument apart as different from all other telescopes — which it certainly has done! The other reason is the more important one of the two. A glossy white telescope tube may look attractive, but for reflecting telescopes where the observer is looking into the tube surface at the eyepiece it’s a bad choice. A white surface reflects any stray light hitting the tube in the area around the focuser onto the observer’s face, thus reducing the dark adaptation of the eyes. As is well known, red illumination retains the eye’s dark adaptation (thus the standard practice of using a red light to read star maps and make notes at the telescope). A white tube is not a problem for refractors and compound telescopes since the eyepiece is located at the bottom of the tube and the observer isn’t looking directly into its surface.

Q: Why is the Astroscan shaped like a big bowling ball?

A: The spherical body of the telescope (which houses the parabolic primary mirror) acts as a huge “ball bearing” when placed on its tabletop mounting base. This allows the scope to easily turn in any direction with rock-steady stability. It’s like having a standard mounting with a shaft some 10” in diameter! The result is that you can actually bump or hit the Astroscan and any vibration damps out instantly. No telescope jitters or shaky mounting here!

Q: I want to use the Astroscan for nature study as well as for stargazing. But the image is upside down. My other two telescopes (one a refractor and the other a Schmidt-Cassegrain) give upright images. Why the difference?

A: All basic forms of telescope invert the image they produce — including your other two instruments. But refractors and compound scopes are normally equipped with a star diagonal for viewing the sky without straining the back and neck. Although they do provide an upright view, the image itself is mirror-reversed — as trying to read a distant sign or license plate will show! (Fully erecting diagonals are available as an accessory from most telescope companies.) In the case of the Astroscan, two solutions are available to users. One is to purchase an image erector designed for this scope and available from Edmund as an accessory. But the other alternative is easier and involves no cost. Simply look into the eyepiece with your back turned to what you’re viewing (standing slightly to the side so as not to block the light path) and the image will appear upright. (This technique is nicely illustrated in the Astroscan User’s Guide itself.

–James Mullaney
Former assistant editor at Sky & Telescope magazine and author of eight books on stargazing.