July 18, 2008

In Today's One Minute Astronomer...

Grab-and-Go Refractors

A great big telescope is indispensable for seeing faint deep-sky fuzzies. But on nights when you don't have the time or energy to set up a big telescope, it's handy to have a smaller scope you can quickly take outside for a short observing session. Here's how to choose a “grab-and-go” refractor to help you get the most out of every clear night.

The Basics

• The Telescope Mount. Some will disagree, but no telescope with equatorial mount qualifies as a grab-and-go. If you've only got 20 minutes, you don't want to spend time polar-aligning your telescope. So a grab-and-go telescope should have a simple and sturdy altazimuth mount.

• Telescope Tube. A good grab-and-go refractor has an aperture between 60 mm and 100 mm and focal ratio of f/8 or less. This keeps the length of the tube to 40-60 cm and the weight to 2-5 kg or so.

• Accessories. For a quick session, 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. It's worth the effort.

A Deeper Look

• Optical quality. Here's a complication. A short-tube refractor with low focal ratio often gives a lot of false color if it uses a standard achromatic lens. That means you'll get distracting purple halos when you look at the moon, planets, and bright stars. The solution? Go with an apochromatic or extra-low dispersion (ED) lens that nearly eliminates false color. If you insist on going with an achromat for budget or technical reasons, stick with a focal ratio of f/8 or more to minimize false color.

• Aperture. Any aperture bigger than 100 mm will be too long and heavy for grab and go. Less than 70 mm will render objects dim and hard to resolve. It comes down to a trade-off between size and budget.

• Price. You can pick up a 90 mm achromatic with mount for $300. Or you can spend $7,000 on a high-end 100 mm apochromat and a high-end altazimuth mount. It's up to you and your budget.

Good To Know

There's a lot of choice out there for grab-and-go refractors. Here's an example of a good achromatic refractor with altazimuth mount. And here's a compact apochromatic refractor that will not disappoint (although you need to buy a mount separately).

Personal View

I have a rock-solid altazimuth mount from Universal Astronomics and a 102 mm apochromat from Stellarvue. I can be outside looking through the eyepiece in less than 5 minutes, and the views of the Milky Way, planets, binary stars, and star clusters are fantastic.

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