Two glasses that cancel most of the color error
John Dollond patented the achromatic lens in London in 1758. A positive element of crown glass and a negative element of flint glass bend light in opposite senses, so red and blue meet at one focus; green and violet still land slightly off. The leftover color, called secondary spectrum, grows with aperture and shrinks with focal ratio. A common rule says it stays invisible when the focal ratio exceeds about 0.12 times the aperture in millimeters, roughly f/12 for a 102 mm lens.
Short achromats give wide fields; long ones show less color
Fast achromats (f/5 to f/6.5) make fine wide-field and Milky Way scanners; slow ones (f/10 to f/15) make sharp planetary telescopes with little fringe, at the cost of a long tube.
| Telescope | Aperture | Focal length | Ratio | Obstruction | Resolves | Widest field | Max power |
|---|---|---|---|---|---|---|---|
| Travel Scope 70 DX | 70 mm | 400 mm | f/5.7 | none | 1.7" | 3.9° | 140x |
| StarSense Explorer LT 80AZ | 80 mm | 900 mm | f/11.2 | none | 1.4" | 1.7° | 160x |
| StarSense Explorer DX 102AZ | 102 mm | 660 mm | f/6.5 | none | 1.1" | 2.3° | 204x |
| 102 mm f/10 class | 102 mm | 1,000 mm | f/9.8 | none | 1.1" | 1.5° | 204x |
| 120 mm f/8.3 class | 120 mm | 1,000 mm | f/8.3 | none | 1.0" | 2.6° | 240x |
| 152 mm f/5.9 class | 152 mm | 900 mm | f/5.9 | none | 0.8" | 2.9° | 304x |
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 achromats for beginners
Achromats are now mostly beginner telescopes sold with a mount, eyepieces and a phone or red-dot finder.
| Model | Maker | Aperture | Ratio | US price | Notes |
|---|---|---|---|---|---|
| Travel Scope 70 DX | Celestron | 70 mm | f/5.7 | $130 | With backpack and tripod |
| StarSense Explorer LT 80AZ | Celestron | 80 mm | f/11 | $250 | Phone-guided; little color |
| StarSense Explorer DX 102AZ | Celestron | 102 mm | f/6.5 | $490 | Phone-guided; wide field |
| AstroMaster 102AZ | Celestron | 102 mm | f/6.5 | sold out | Basic alt-azimuth |
| FirstLight 102 | Explore Scientific | 102 mm | f/6.5 or f/9.8 | check dealers | Twilight Nano or EQ mount |
Prices are from US dealers in November 2026; tariffs on Chinese-made optics raised most prices in 2025.
Best for first telescopes, the Moon and sweeping the Milky Way
An achromat on a sturdy mount is a better first telescope than a toy reflector: it is ready in minutes, never needs alignment and shows the Moon, Saturn's rings, Jupiter's moons and the bright Messier objects well. A 102 mm reaches 108 of the 110 Messier objects from a dark site and 70 from the suburbs. The violet fringe on Jupiter and Venus is harmless, and a minus-violet filter reduces it. For imaging or high-power planetary work, an apochromat is worth the extra cost.