April 8, 2008

In Today's One Minute Astronomer...

Apochromatic Refractors: The Ultimate Telescopes?

An apochromatic refractor uses an objective lens of extra-low dispersion glass that gives remarkably crisp images without chromatic aberration. Many consider an “apo” to be the ultimate telescope for photography and planetary observing. Said one amateur astronomer, “The sight of Saturn through a good apochromat is enough to make a grown man weep”.

The Basics

• Most refractors use achromatic lenses, which bring only two colors (typically red and blue) into focus in the same plane. So when you look at bright objects like the moon or planets, you get an ugly purple halo that detracts from the observing experience.

• But apochromatic lenses use exotic fluorite or extra-low dispersion (ED) glasses that bring three colors (typically red, green, and blue) into focus in the same plane. That means as much as 10x less residual color than an achromat.

• The down side? Top-notch apochromats are shockingly expensive, up to $1000 per inch of aperture. But in the fast few years, many well-reviewed apos that use simpler lens designs and new materials have hit the market. For example, you can get a decent 4-inch ED optical tube assembly that gives you tack-sharp images of planets and wonderful contrast on binary stars for only $1000 or so.

A Deeper Look

• Many astrophotographers swear by apochromats to produce the sharpest possible images. That's because the optical sensitivity of typical CCD chips extend from the ultraviolet through the visible spectrum and into the near infrared wavelength range, and only apochromats focus all colors simultaneously.

• Whether an achromat or an apochromat, chromatic aberration increases as aperture increases and focal ratio decreases. So a 6” lens has more aberration than a 3” lens with the same focal ratio. And an f/5 lens has more false color than an f/10 lens of the same aperture. So beware low-cost 5”-6” f/6 achromats. Yes, they're inexpensive and give you a bright image. But the purple halo around bright objects is atrocious.

Tech Term

The “apo” in apochromat is a Greek suffix that means “an utter and complete lack of”. So an apochromat is a lens with an utter lack of color in the image.

Personal View

While the contrast and image sharpness of a good apo refractor are impressive, I've never caught the apo bug because no matter how good the lens, a 3 or 4-inch telescope is still too small to show much detail on faint deep sky objects. But if the moon, planets, and brighter star clusters are your primary interest, a good apo might be the scope for you.