Sky-Watcher Skymax Maksutovs and Dobsonians: magnification ranges and planetary scale

Focal ratio sets the eyepiece, aperture sets the limits

Magnification is telescope focal length ÷ eyepiece focal length, and exit pupil is aperture ÷ magnification. Put together, the aperture cancels out:

exit pupil (mm) = eyepiece focal length (mm) ÷ focal ratio

The eyepiece calculator brackets useful magnification between a 7 mm exit pupil (its default eye pupil) and about 2× the aperture in millimeters. That upper bound always corresponds to a 0.5 mm exit pupil, because D ÷ 2D = 0.5. The exit pupil guide covers where both limits come from. Applied to these tubes:

Tube	Focal ratio	7 mm exit pupil	0.5 mm exit pupil
Skymax 102	f/12.7	15×, 89 mm eyepiece	204×, 6.4 mm
Skymax 127	f/12.1	18×, 85 mm	254×, 6.1 mm
Skymax 150	f/12	21×, 84 mm	300×, 6.0 mm
Skymax 180	f/15	26×, 105 mm	360×, 7.5 mm
Classic 150P	f/7.9	22×, 55 mm	304×, 4.0 mm
Classic 200P	f/5.9	29×, 41 mm	406×, 3.0 mm
Classic 250P	f/4.7	36×, 33 mm	508×, 2.4 mm
Flextube 300P	f/4.9	44×, 34 mm	610×, 2.5 mm

The two families split cleanly. The Dobsonians reach their widest exit pupil with eyepieces in the 33–55 mm range and need 2.4–4 mm eyepieces at the top. The Maksutovs reach high power with 6–7.5 mm eyepieces, but their low-power end sits at eyepiece focal lengths of 84 mm and up, so in practice the lowest magnification you can reach is set by the longest eyepiece that fits. Sky-Watcher USA’s spec tables print nearly the same ranges, for example 18× to 250× for the Skymax 127.

Maksutovs with planetary cameras

Long focal length at modest aperture is what small-pixel cameras want for planets. With 2.9 µm pixels (ASI585MC, ASI462MM), scale runs from 0.46″ per pixel on the Skymax 102 to 0.22″ on the Skymax 180; the 2.0 µm ASI678MC goes from 0.32″ to 0.15″.

NASA gives Jupiter’s apparent diameter as 30.5″ to 50.1″ (fact sheet). At its largest, Jupiter spans 129 pixels on a 2.9 µm sensor behind the Skymax 127 and 226 pixels behind the Skymax 180. Saturn’s globe peaks at 19.9″ (NASA), or 51 pixels at 1540 mm, and Mars at 25.6″ (NASA), or 66 pixels.

For a sense of resolution, Dawes’ 116 ÷ D gives 0.91″ for the 127 mm and 0.64″ for the 180 mm. At 0.39″ per pixel the Skymax 127 places about 2.3 pixels across that distance; whether that is enough depends on seeing, covered in the resolution limits guide.

The fields are small. The ASI462MM covers 12.5′ × 7.1′ at 1540 mm and 7.1′ × 4.0′ at 2700 mm, plenty for a 50″ disk but tight for finding it. An APS-C chip at 1540 mm covers 52.4′ × 35.1′, enough for the whole Moon, whose mean apparent diameter at opposition NASA lists as 1896″, or 31.6′ (Moon fact sheet).

Sky-Watcher USA lists Maksutov secondaries of 31, 39, 47 and 41 mm, about 30–31% of the aperture on the three smaller tubes and 23% on the 180.

Skymax 127: 1540 or 1500 mm

Sky-Watcher USA gives the Skymax 127 as 127 mm, 1540 mm, f/12.1. The global BK MAK127 OTAW page gives 127 mm, 1500 mm, f/12 and a 5 kg tube. This site uses the US figure.

The 2.7% gap is small in practice. An ASI585MC frames 24.9′ × 14.0′ at 1540 mm and 25.5′ × 14.4′ at 1500 mm, and a 25 mm eyepiece gives 62× or 60×.

Dobsonian secondaries

The US spec tables list 34, 48, 64 and 70 mm secondaries for the Classic 150P, 200P, 250P and Flextube 300P, or 22–25% of the aperture by diameter. One printed percentage is off: the Classic 150P table says 16%, but 34 ÷ 152 is 22%.