September 26, 2008

In Today's One Minute Astronomer...

Planetary Eyepieces

As you've discovered, wide-field eyepieces give wonderful views of star fields and faint deep-sky objects. But because of their design, they are not the best choice for viewing bright objects like the moon and planets. So here's how to select one or two good eyepieces that work well for seeing the bright wonders of our solar system.

The Basics

• Here's the problem: wide-field eyepieces have 6-8 lenses, often grouped in pairs, to achieve a wide and flat field of view. But these lenses often bring light to focus at different points across the field of view. Not a big deal for faint objects. But this becomes a problem for brighter objects.

• Another problem: the multiple lens surfaces scatter light which causes distracting “halos” around bright objects and reduced contrast of bright and dark features.

• Since the moon and planets reveal the most detail under higher magnification, a good planetary eyepiece has a short focal length, typically 3-9 mm depending on your telescope.

• And the good news… with fewer lenses and a narrower field of view, planetary eyepieces often cost less than wide-field eyepieces.

A Deeper Look

Some good choices for planetary eyepieces:

• Plossls. Your telescope may have come with one or two of these. Perfectly acceptable for planets and the moon, with great contrast. Not a bad all-around eyepiece. But the eye relief is not very good at short focal lengths, so beware if you must wear glasses at the telescope. Plossls are made by dozens of manufacturers. Orion, Meade, Celestron, and Televue are all recommended. You'll find good short-focal length Plossls for less than $100.

• Orthoscopics. An older design, but still works well. Orthos have better eye relief than Plossls, but a narrower field of view. So these are best for the moon and planets only. Only $60 each.

• Radians. A premium eyepiece from Televue, Radians give long eye relief, great contrast, and a fairly wide field. Expensive… around $250.

• Specialty Eyepieces. Companies like Williams Optics and Burgess Optical have their own special designs optimized for the moon and planets. Good value for less than $150 or so.

Good To Know

So what focal length do you need? If you have a telescope with long focal ratio, like an f/10 Schmidt-Cassegrain, then don't go any shorter than 5-7 mm for an eyepiece. That gives you plenty of magnification; any higher and you'll see diminishing returns. With an f/5 or f/6 reflector or refractor, you can go as low as 3 mm, but you'll need to get your eye up close.

Personal View

I've got a couple of $100 Plossl's which work just fine on planets with an 8” SCT. If you have a high-quality scope and near-perfect skies, you might consider getting a Radian. Otherwise, you needn't pay more than $100-150 for a good short-focal length planetary eyepiece.

--- Highly Recommended ---

The short focal length and long eye relief of these premium Televue eyepieces give you razor-sharp images of Jupiter's Moons and Saturn's rings, as well as spaceship-views of the moon's surface. Learn more...

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