November 25, 2008

In Today's One Minute Astronomer...

Grab-and-Go Reflectors

We've covered portable grab-and-go refractors that are ideal for a quick observing session when time is tight. But refractors are expensive relative to their aperture. And some have bothersome chromatic aberration which ruins the view of bright objects like the moon and planets. So if you're considering a grab-and-go telescope, a reflector might be the choice if you prefer brighter images with truer color at a reasonable price.

The Basics

• As I wrote in the last article, a true grab-and-go telescope should have a manual altazimuth mount. No polar alignment, no assembly, no messing with GPS or alignment stars. Just set up the scope on a flat surface and start observing.

• A small Dobsonian telescope with 6" aperture or less might be ideal as a grab-and-go if you can leave it assembled in your house or garage and simple carry it outside. A 6" Dob weighs 25-30 pounds or so. Manageable, but not feather-light.

• There's another problem with small Dobs: they tend to have the eyepiece located closer to the ground. That means you need to crouch down or sit on a small stool for objects closer to the horizon. Not for everyone. And unless you have a solid table at your regular disposal, you should probably skip Dobs with table-top mounts.

• Schmidt-Cassegrains and Maksutov-Cassegrains also make great grab-and-go scopes. The tube assemblies themselves weigh less than 5-12 pounds for 5" to 8" aperture. And if the tube assembly has a dovetail rail, you can mount the telescope on a light altazimuth mount and tripod that weigh only 10-20 pounds. Universal Astronomics or Astronomy Technologies have solid grab-and-go altazimuths well worth a look. Here's a picture of what I mean:

An 8-inch SCT on a Universal Astronomics altazimuth

A Deeper Look

• You can also slide the optical tube and dovetail bar onto a heavy-duty equatorial mount for more serious observing when you have more time. So you can have one optical tube and two mounts. Very versatile. There are two types of dovetail adapters: Vixen/GG-5 and Losmandy/CGE. Make sure your dovetail and mount are compatible.

• For the ultimate in portability, you can get small Mak-Cassegrains from Meade and Orion with 80-90 mm aperture. Some are so light, you can mount them on a sturdy camera tripod. Total weight: less than 10 pounds. Total cost: as little as $300.

• Explore Scientific has just released a new Maksutov-Newtonian reflector suitable for grab-and-go. It's made for hunting comets and designed in part by David Levy. I have not tried one yet, nor have I seen the price.

Good To Know

For accessories, take along two eyepieces, one for low power and one for high-power views. And you can take a Barlow lens to double the power of each eyepiece. If you're looking at nebulae and you live under light-polluted skies, take an OIII filter or UHC filter for the best views.

Personal View

Remember, keep it light, keep it simple, and get as much aperture as you can carry, and you'll have a great grab-and-go reflector suitable for quick oberving sessions on busy nights.