Achromats and Apochromats
Update Note: Some of the telescopes referred-to in this article have been superceded by newer models. However, they are often available in the used marketplace.
Refractors of about 4-inches and up have historically been one of the most expensive telescope types, but something unexpected happened a few years ago. It became possible to buy a relatively good-sized refractor for under $500. What made this possible was that the Chinese optical firm Synta began selling its products through various suppliers, undercutting just about everyone else's prices. Celestron used to sell the GP-C102 for $1300 - it was a 4-inch achromat made by the Japanese firm Vixen. Celestron now has its 4-inch made by Synta and it's priced at only $399 as the model C4R. That price even includes a German equatorial mounting!
The current models look a lot like the old ones, though the optics and mechanics are not as good as the Vixen-made scopes. But for that price how bad could they be? Despite inconsistent quality control early on, the Chinese products are improving. This can be an inexpensive way to satisfy a craving for a refractor.You may find it curious that someone would shell out well over $2000 (without a mounting) for an apochromatic refractor of the same size as one of the Chinese achromats. Are the images that much better to warrant the much higher cost? A correspondent from an Internet discussion group actually did make the move from the Chinese-made C102-HD (predecessor of the C4R) to a Tele Vue 102, which costs more than five times as much. The following are comments from the new TV-102 owner.
"I've been looking at M13 for a couple of months now through my cheapo C102-HD. From my notes, while it looked okay at 125x (which is best), I was always disappointed mainly because of the description regarding it being the most spectacular globular cluster in the northern sky. Until, now... The view in my new TV-102 of M13 is stunning! It just kept getting better and better until I reached the maximum of 220x (4mm TV Radian). I can't believe it's still bright with countless little pin-point salt-and-pepper stars resolved in the core. (I could only resolve a handful in the core in my C102-HD and the view was unacceptably dim after 125x.) The TV-102 seemed to ask for more magnification. Why is it that my TV-102 is able to maintain brightness, contrast and resolution at high magnification while my cheapo C102-HD could not? The difference is not small, but stunning!"
One answer came from Matt Tarlach, who is a former employee of Orion Telescopes: "Less chromatic aberration and probably better optical figure means more light from each star is concentrated in the Airy disk...the smallest theoretical size of a stellar image in a given scope. With less light scattered and smeared across the core of M13, each star is able to stand out much more clearly. Also, the TV's lenses may be polished better, and baffling more effective, both of which reduce scattered light and increase contrast. If the TV's coatings (and diagonal) are better, the image may be slightly brighter, but with scopes of the same size it's usually the contrast that makes the difference in visual impact. The brain may interpret the image as brighter but if you put a photometer on it they'd probably be pretty close in terms of actual brightness."
Exotic types of glass and unique lens configurations give apochromats their superior images in comparison to traditional achromats. But labels can be misleading; just as there are better quality achromats, not all apochromats are created equal. Some are not totally apochromatic in that they show varying levels of chromatic aberration, though less than in a simple achromat.
An insight on one type of specialized optical glass (often referred-to as "ED") came from Thomas Back, himself a maker of apochromats: "ED glass is a group of glasses that have low dispersion (Abbe number of greater than ~80). The lowest cost true ED glass is Ohara FPL-51, which is used in many of the lower cost ED doublets (such as the Meade ED refractors). SD glass (which is still an ED glass) stands for Super low Dispersion. These glasses have an Abbe number of 90 or greater, and allow the designer to make lenses that not only have better color correction, but lower monochromatic aberrations."
There are several ways to make a refractor objective lens. Some examples:
Simple Achromat - two lenses, one of crown glass the other flint. This design has been in use for a couple hundred years. There is chromatic aberration on bright objects and in fast achromats (f/6 and lower) image quality is poor at high magnification. Slow achromats (around f/10 and higher) show lesser amounts of aberration and take high magnification very well. In addition to the C4R, the Vixen 102M is an achromat. Except for the color, the Vixen is basically identical to the old Celestron GP-C102, and arguably one of the best of this type on the market.
- Fluorite - a single doublet with man-made fluorite monocrystal and SD glass (Takahashi FC/FSseries, former Vixen F series)
- Petzval - doublet objective lens with another doublet near the eyepiece end (Takahashi FSQ-106 Tele Vue Genesis and NP-101). Vixen also used the Petzval design in its "Neo-Achromats", though the color correction is not considered as good as in the Takahashi and Tele Vue.
- Oil-Spaced Triplets - Astro-Physics scopes use the oil-spaced Christen triplet with FPL-53 glass in the f/6 and f/7. There are no rear elements unless using a field corrector for photography.
- Air-Spaced Triplets - three air-spaced lenses using a variety of very low dispersion glasses (all TMB brand scopes and some other manufacturers)
- Air-Spaced Doublets - similar to the basic achromat, but with one element of ED, SD, or CaF2 glass. These are considered "semi-APOs" in that the color correction is not quite as good as in other APO designs, but this varies depending on the type of glass used and the quality of the optical work. The former Meade ED series was of this design, but was plagued with optical quality problems. On the other hand, the Orion 80 and 100mm ED refractors use high quality optics of FPL-53 glass. Tele Vue also uses this design.
Small, short-focus refractors in the range of 60mm to 100mm have become popular as convenient "grab-n-go" scopes. Many of these are apochromatic or semi-APO and provide surprisingly high quality images in a compact package. Of course, a reflecting-type telescope (mirror-based system), such as a Newtonian or SCT, is also "apochromatic" by its very nature since the image is free of extraneous color.
Other considerations center on the quality of the mechanics. For example, Tele Vue has one of the smoother focusing mechanisms, plus overall excellent fit and finish. Some owners regard the Chinese mechanicals as "crude". But not to be outdone, the Chinese are now making some very commendable small apochromats (Orion ED80 and ED100).
While it may not have as much finesse as the higher priced variants, the fact that you can get a workable 4-inch refractor complete for only $399 makes the Chinese achromat one heck of a bargain. It's often noted that you have to pay a lot more money for just a little increase in image quality. How much is a better achromat or the step up to an apochromat worth? Your call...Published in the July 2006 issue of the NightTimes