Is There a Case for an Achromatic Refractor?

The Celestron CR 150 on a vintage LXD600 mount.

With all of the high-quality Apochromatic and ED refractors, why would anyone buy an achromatic refractor? The achromats are often riddled with purple haze around bright objects and the resulting loss of clarity is frequently pointed out by the purists. Is there a case for purchasing and using one of these classicly designed telescopes in the 21st century?

In the early 1800’s Joseph Von Fraunhofer perfected a commercially available achromatic refractor. Though Jon Dolland was the first known inventor of an achromatic telescope, Fraunhofer was the first to commercialize the design. He was able to make high-quality Crown and Flint glass that when coupled optically brought 2 of the 3 colors into focus. A single lens can only bring one color into focus unless you have an extremely long focal length. Fraunhofer was able to get from slightly blue to red into a passible focus using his quality glass and design. This still left dark blue and purple out of focus and noticeable around bright objects.

By today’s standards, this sounds less than ideal. However, by the standards of that day, this new design was of incredible quality. Suddenly telescopes were of a more manageable size as well as giving better views. Also, the human eye is not as sensitive to deep blue/purple light as it is to green and red, so for visual observation this design reigned for over 100 years as the instrument of choice, especially among professional astronomers. In the late 1800’s the firm of Alvin Clark and Sons built 2 of the world’s largest telescopes, the 36″ Lick refractor and the 40″ Yerkes refractor. The Yerkes telescope, administered by the University of Chicago, was considered to be the 1st modern telescope ever built. It was designed for visual viewing and photographic imaging with a slate of add-on scientific instruments.

Still, telescopes for the amateur community were either very small or of a very high price. Your best bet before World War 2 was to grind your own mirror and mount it on a homemade mounting. Otherwise, you were limited to small refractors or very expensive large ones. This changed after the war. Post-war Japan began to manufacture fine optics for cameras and telescopes. Soon they were turning out very affordable achromatic refractors up to 4″ or more.  Though the larger scopes were still somewhat pricy they allowed the dedicated amateur to at least have a shot to obtain a quality scope with some size.

This is the Celestron CR-4 and a simple Alt-Az tripod. This is a grab-and-go scope since you can move it with one hand but it is quite steady even at high powers. There is a solar filter covering the objective.

In the 1980s larger scopes came out of Japan. The high-quality Celestron CR-4 revolutionized low-cost refractors. In the 1990s  China began to manufacture low-cost, large-aperture refractors. The crown jewel of this era was the Meade AR-6 and the Celestron CR 150. These were high-quality achromats of 6″ diameter objective with a focal ratio of F/8. These short focal length telescopes gave a lot of bang for the buck and made this size of refractor attainable to the average hobbyist. The trade-off was increased false color or purple haze around bright objects. The other issue with these telescopes was the inadequate focusers. If you were just doing visual observing using the eyepieces supplied with the telescope you were fine. If however, you used heavy premium eyepieces or a camera the focusers were inadequate.

Replacing the focuser with even a low-end GSO 2 speed focuser made this into a very high-quality instrument. With the upgrade you could image or fine focus for visual like a telescope costing much, much more. Otherwise, you had to get it focused and wrap a piece of blue tape around to tube to make sure the focus didn’t shift!

Today there is still the venerable Celestron CR 150 around with a large variety of other different focal length refractors available in many different diameters. Surprisingly enough, even with their inherent false color,  short tube F/5-F/7 achromats are very popular. With a few exceptions, the achromats are still about 3 times less expensive than the lowest-level Apochromat or ED scope.

This is an Explore Scientific 102mm F/6.5 achromatic scope used for photometry. Since you use filters false color is not an issue and you get lots of bang for the buck!

The main case for an Achromat is still budget. You can buy a good mount and a few accessories with the money you save on an achromat. If you buy an Apochromat you still need a mount and other accessories which quickly add to the cost. There are a few other things of note. Even though you have some haze around bright objects this is not seen inside the view of the moon and planets. Venus still looks as white as it does in another style of telescope (see below). So if you can learn to live with the purple like the professionals did 100 years ago you can have a good view with a larger aperture telescope.

Venus through the 12.25″ Clark Refractor at Morrison Observatory in Fayette, Missouri. Hand-held cell phone image in color.

There are some things you can do to enhance the maximize the view. A Wratten equivalent #8 filter makes a good haze filter but it does add a yellow tint to some objects. The trick is to buy a quality #8 filter. Some of the old Celestron and Meades are not very good and don’t work well for filtering out the haze. The old Celestron Minus-V filter works very well but it is no longer made. Baader has the Semi-Apo and Fringe Killer filters as well as quality color filters. Thousand Oaks has a Minus V filters as well.

Just adding a filter will not make it an instant Apochromat telescope! What it will do is bring you to about 90% of the performance of a premium scope costing 3 times as much. You have to decide if that 10% improvement is worth it to you!

In the age of Dobsonian telescopes, it may seem a bit silly to go with a smaller achromat. But in side-by-side comparison tests, you find that a 6″ refractor performs as good or better than an 8″ reflector.   The image is slightly brighter with the 8″ mirror but the contrast is better with the refractor, false color, and all.

All in all, there is still something wonderful about joining the achromat club. These telescopes have shown millions their first view of the universe. They are still well worth serious consideration and use today.

This was taken with the Celestron CR 150, ZWO120 MC camera and mounted on the Meade LXD600 mount. A Minus V filter was used.