The Case for Achromatic Refractors

While working from my home office I noticed a worn and tattered paperback on the shelf. It was my 2nd copy of Tony Simons “The Search for Planet X.” By the time I was in college my original Scholastic copy purchased at a grade school book fair had fallen apart to the point where tape couldn’t hold it together any more. Taking this 2nd copy off of the shelf last night I was soon immersed in the quest to find Planet X or Pluto!

13″ Pluto Astrograph

The thing that jumps out at you is the search telescope Clyde Tombaugh used to find Planet X was a refractor. An achromatic refractor as well. It was 13″ in diameter and only had 2 elements in it’s objective. Modern, high quality, expensive refractors have 3 or more lenses. This means if you could look through the 13″ planet hunter the bright stars would have a purple fringe. Now this scope was an astrograph which means that it was dedicated to photography so it was not designed for visual use. In fact it was maximized to deliver good performance in blue light since that is where photographic plates were most sensitive at that time.

Once Clyde Tombaugh discovered Pluto, the 24″ refractor was one of the telescopes used for follow up observations, both visually and photographically. It was a “short” focal ratio refractor 24 inches in diameter that would have chromatic aberration, the scourge of short refractors. This “CA” is what gives achromatic refractors such a bad rap today.

The 24″ Clark Refractor at Lowell Observatory

At the time the 24″ was still a front line telescope being used for science every clear night. In fact Vesto Slipher used the 24″ telescope and a custom spectrograph to discover the redshift of galaxies. Edwin Hubble took the redshift concept to the next level but without Slipher’s groundbreaking work, Hubble’s expanding universe would have been delayed.

With this level of historical precedent why is the achromatic telescope so maligned today? Log onto the Cloudy Nights forum or just about any other amateur astronomy site and you find refractor proponents bad mouthing any but the most expensive “Apochromatic” telescopes. With 2 lens you always get false color because some of the wavelengths of light don’t actually focus properly. “Common wisdom” is that an achromat is almost worthless for observing.

Over the years I have looked through the 24″ Clark telescope at a variety of objects. The one I remember most was looking at the “Ring Nebula” or M 57 in the constellation of Lyra. A faint violet star floated in a ring of green! The image is sort of frozen in my mind and I can readily call it up for remembrance. Turns out the central star of the ring nebula is violet and through a large telescope the ring would appear green. I have also looked at Saturn, Jupiter and several other deep sky objects through this scope and never noted any false color in the images. The fact that I could see a 15th magnitude violet star says that a filter was not being used since it would have filtered out the violet color.

One thing that is driving amateur astronomy these days is imaging. Thousands of amateurs worldwide are using off the shelf components to take breathtaking images of the night sky. If you are using a one shot color camera to take images then you probably don’t want a lowly achromat. The chromatic aberration is hard to process out of color images. When you have violet fringes around all of the bright objects in your image then an apochromat or other type of telescope would be better. Thus, “old salts” in the imaging world pronounce any refractor but the most expensive apochromats to be “worthless.”

Are they really worthless? I have a number of achromats in my telescope stable. A Celestron 6″ achromatic refractor, a Explore Scientific AR102 mm and a stable of simple 60-90 mm fun scopes. Using a cheap Celestron Minus V filter takes away all but a small amount of false color around the image when observing with eyepieces. This cheap but no longer produced filter was made by Baader of Europe and does an excellent job visually. You do lose some of the wavelengths of light and it does not make your telescope perform like an expensive apochromat. Still an achromat is usually 3-6 times cheaper than a similar sized apochromat.

It has also been found that if you use a good quality color #8 Yellow filter it works just about as well. It’s negative is that it gives a yellow tint to white areas on a planet. Still a good quality filter can be purchased for a achromat and it is still less than a apochromat

What if you were using filters to image with? Then the CA (chromatic aberration) is no longer a factor. Filters were part and parcel of 20th century astronomy and are still being used today. The biggest negative of this system is that each filter comes to a slightly different focus. This becomes a non-issue if you use a motorized focuser like a moonlite or robo focus. With a computer the program will automatically set offsets for your filters and this allows smooth change up between filters without the need to refocus. Even one shot color cameras rely on a system of red, green and blue microfilters (one color on each pixel) to make a color image. A lot of the best imagers use red, green and blue filters to make color images. This puts the achromat back on an equal footing with it’s more expensive cousins.

The AR102, Sirius mount, ST-8E Camera and a laptop to control it all!

Temple 10 ( A mentor named all of my telescopes for me) is an achromatic 102mm refractor. It uses 3 different cameras depending on what the target is. For planets I use a ZWO ASI 120 MC color camera or the ATIK 414 monochrome camera. For deep space I use the ATIK 414 or the ST-8 camera. Though I have the hand controller for the Orion Sirius mount I use  a laptop to control it.

Though I occasionally use Temple 10 for planetary work it is used primarily for science. The ST-8 has a full range of Sloan photometric filters to measure the brightness of the stars. The ATIK 414 uses a Johnson V filter (green), a Johnson I filter (infrared) and a Sloan z (infrared). Since these filters have a narrow bandpass it negates the issue of Chromatic aberration. No rings around stars and it gives the achromats, more bang for the buck.

Approximately every 2 years I get out my 6″ Celestron refractor and put it in place of the AR102. Turns out the 6″ is a great scope to use for imaging planets and Mars comes close to the earth about every 2 years. Even with it’s chromatic aberration issues I really enjoy using this scope for imaging and visual astronomy. Since I bought it in 2008 for 350 dollars and it came with a mount, it has given me years of astro enjoyment.

However, I have used my 102mm Celestron achromat refractor to image planets in color. These images have come out good and CA does not seem to be much of a problem using a filter. The image below was taken with a Celestron Minus V filter and a ZWO 120 color camera. Recently I purchased a Ultra Violet-Infrared cutoff filter and use it with the Minus-V filter. This sharpened up the planet images since it blocks the infrared light leakage of the Minus V. For visual use the leakage doesn’t show but for imaging it can blur the definition a bit.

Here is Mars with Temple 10. AR102 refractor

Should you buy an Apochromat over a achromat? Absolutely! If you can afford it. If not then the above can help you make the most of a historic, low cost, high quality achromatic refractor. Happy observing!