Eyepieces: Planetary

All focal lengths are now available: the 2.5, 3.2, 4, 5, 6, 7, 8, and 9mm Planetary. The 4mm was bench tested by the designer, Thomas Back, who wrote in the BurgessRefractor Yahoo group on October 21, 2005:

Check out our construction and features:

• Designed by Thomas Back of TMB Optical
• Fully internally baffled
• Every Air to Glass surface has Multi Broadband coatings!
• Internal spacers have flat anodizing and micro-baffless to reduce reflections.
• Twist-up eyecups with flexible eyecups on top.

Planetary Eyepiece Slideshow

EP Planetary family 2.5 mm Planetary Down 2.5 mm Planetary Up 4 mm Planetary 4 mm Planetary Up - Down 5 mm Planetary Up - Down 6 mm Planetary Up - Down 7 mm Planetary 9 mm Planetary Up - Down

The designer, Thomas Back, has posted a message to the Yahoo BurgessRefractors group describing the path he has taken designing these eyepieces. Read that message here.

Planetary Eyepieces sold before December 16, 2005, were shipped with a retaining ring with a smooth inside bore. The retaining ring secured the relay lens group in the eyepiece barrel. The smooth retaining ring caused a slight flaring to be visible when viewing bright objects. Burgess Optical offered a free replacement retaining ring to those who purchased the eyepiece with the smooth ring. Click here for full details.

Originally, the first production run of the 6mm Garage Sale Planetary edition eyepiece was sold as a special offer. New production runs of the 6mm Planetary are now being sold and the 6mm Planetary eyepieces have the same specifications as all the other Planetary eyepieces.

What makes an eyepiece a planetary EP?

The typical eyepiece today is designed for wide-field views of deep space objects (DSO). They offer a portal to the sky, where the DSO of interest is nicely framed. To achieve these nice, wide field of views, design choices have to be made, such as using a large number of elements and groupings of elements. These design choices sometimes result in lateral color. Lateral color is an abberation where different colors intercept the focal plane at different points and can be seen when viewing bright objects off-axis. When observing dim DSO, lateral color in the typical eyepiece is rarely seen. Also, these design choices sometimes result in light scatter, seen as a halo or glow around bright objects. Again, light scatter is rarely seen when observing dim DSO. If there is a bright star near the edge of the field, then lateral color and light scatter might be seen when observing dim DSOs. But, the lateral color and light scatter would be associated with the bright star, not the DSO.

The design choices for typical wide-field eyepieces are different than the design choices for a planetary eyepiece. The typical eyepiece is optimized for wide-field views of DSO and suffers when viewing very bright objects, particularly off-axis.

A planetary eyepiece is optimized to view bright objects while showing maximum detail and definition. It is an eyepiece with very high light transmission, very high contrast, minimal lateral color, and minimal light scatter. For Dobsonian users and others without tracking drives, lateral color and other abberations are minimized to allow the planet to drift across the field of view while still being sharp and having high contrast. Also, because lunar and planetary observers seek to see as much detail as possible, planetary eyepieces typically have a short focal length for high magnification.