ED glass cancels the color error of ordinary lenses
Every glass bends blue light more than red, so a simple lens focuses each color at a different distance. An achromat corrects two colors and leaves a violet halo on bright objects. An apochromat adds glass with unusually low dispersion, such as Ohara FPL-53, FPL-55 or fluorite crystal, to bring three colors to a common focus. Doublets use one ED element; triplets and quadruplets add elements to also flatten the field for cameras.
Many current imaging apos are Petzval designs, with a second lens group near the focuser that gives a flat field without a separate flattener: the William Optics RedCat, Askar FRA and Takahashi FSQ are examples.
Small apos give the widest true fields of any telescope
Short focal lengths make apos ideal for the Milky Way, large nebulae and comets. Their resolving power follows aperture, so a 4-inch apo splits stars as finely as a 4-inch of any other type, with better contrast than an obstructed telescope.
| Telescope | Aperture | Focal length | Ratio | Obstruction | Resolves | Widest field | Max power |
|---|---|---|---|---|---|---|---|
| RedCat 51 | 51 mm | 250 mm | f/4.9 | none | 2.3" | 10.5° | 102x |
| Esprit 70EDX | 70 mm | 455 mm | f/6.5 | none | 1.7" | 5.8° | 140x |
| ED80 class (80 mm f/6) | 80 mm | 480 mm | f/6.0 | none | 1.4" | 5.5° | 160x |
| Askar 91F | 91 mm | 609 mm | f/6.7 | none | 1.3" | 4.3° | 182x |
| Esprit 100EDX | 100 mm | 550 mm | f/5.5 | none | 1.2" | 4.8° | 200x |
| ED127 Essential | 127 mm | 952 mm | f/7.5 | none | 0.9" | 2.8° | 254x |
Obstruction figures are approximate, as a share of the aperture's diameter. Widest true field uses the largest eyepiece the focuser takes: a 46 mm field stop for 2-inch focusers, 27 mm for 1.25-inch. Highest useful power is 2x per millimeter of aperture, capped at 350x by the air.
Current apochromats, from China to Japan
Chinese makers (Sky-Watcher/Synta, Sharpstar and its Askar brand, William Optics of Taiwan, ZWO) now supply most apos under $3,000; Japan's Takahashi and America's Tele Vue hold the premium end.
| Model | Maker | Aperture | Ratio | US price | Notes |
|---|---|---|---|---|---|
| EvoGuide 50DX | Sky-Watcher | 50 mm | f/4.8 | $375 | Guide scope or tiny imager |
| RedCat 51 III | William Optics | 51 mm | f/4.9 | $844 to $888 | Petzval; the classic travel imaging apo |
| FF65 | ZWO | 65 mm | f/6.4 | about $1,300 | Quadruplet astrograph |
| Gran Turismo 71 | William Optics | 71 mm | f/5.9 | $933 to $1,098 | Triplet |
| Esprit 70EDX | Sky-Watcher | 70 mm | f/6.5 | $1,295 | Triplet with optional flattener |
| RedCat 61 | William Optics | 61 mm | f/4.9 | $1,328 to $1,398 | Petzval |
| 91F | Askar | 91 mm | f/6.7 | $999 | Flat-field quadruplet |
| ED127 Essential | Explore Scientific | 127 mm | f/7.5 | $1,200 to $1,500 | Triplet; visual and imaging |
| Esprit 100EDX | Sky-Watcher | 100 mm | f/5.5 | $2,095 | Triplet |
| FSQ-85EDX | Takahashi | 85 mm | f/5.3 | $4,280 | Four-element Petzval; premium |
Prices are from US dealers in November 2026; tariffs on Chinese-made optics raised most prices in 2025.
Best for astrophotography, travel and high-contrast planetary views
Apos need no collimation, cool quickly, and pack small: a RedCat 51 weighs about 3.5 lb. They are the default tube for imaging with a small tracking mount such as a ZWO AM3, Sky-Watcher Star Adventurer GTi or iOptron GEM28. Their limit is price per inch: a 4-inch apo costs about as much as a 12-inch Dobsonian, and apertures above 5 inches get heavy and very expensive. For galaxies by eye, aperture wins; for photographs and the planets, a good apo punches above its size.