ZWO planetary and guide cameras: pixel sizes, tiny fields and the Mini series

ZWO’s uncooled line in our data has twelve cameras, and none of their sensors is wider than 13.2 mm. Their pixels span a fourfold range, from 1.45 µm on the ASI715MC to 5.86 µm on the ASI174MM and ASI174MM Mini. For planetary work the pixel size is the number to start with, because these cameras usually sit behind long focal lengths where the field is tiny no matter which body you pick.

Pixel size against a long focal length

Take an 8-inch Schmidt-Cassegrain from our telescope data: 203.2 mm aperture and 2032 mm focal length. Its Dawes limit, 116 ÷ 203.2, is 0.57″. The table compares that with the image scale each ZWO pixel size gives at 2032 mm.

image scale (″/px) = 206.265 × pixel size (µm) ÷ focal length (mm)

Pixel size	ZWO uncooled bodies	Scale at 2032 mm	Pixels across 0.57″
1.45 µm	ASI715MC	0.15″	3.9
2.0 µm	ASI678MC	0.20″	2.8
2.4 µm	ASI183MC	0.24″	2.3
2.9 µm	ASI462MM, ASI585MC, ASI662MC, ASI664MC	0.29″	1.9
3.75 µm	ASI224MC, ASI120MM Mini	0.38″	1.5
4.0 µm	ASI220MM Mini	0.41″	1.4
5.86 µm	ASI174MM, ASI174MM Mini	0.60″	1.0

The last column is just the Dawes figure divided by the scale. It shows why the smallest pixels need no Barlow at this focal length, while the 2.9 µm group reaches the same 0.15″/px only at 4064 mm, which is a 2× Barlow on the same tube. How many pixels you want across the resolution limit is a choice that depends on seeing and on the processing you plan; telescope resolution limits walks through the Dawes and Rayleigh figures behind it.

Small sensors make the Moon a mosaic

Field shrinks with the sensor, not the pixel. At 2000 mm the ASI715MC and the ASI462MM both cover about 9.6′ × 5.5′, because 3864 × 1.45 µm and 1936 × 2.9 µm come to nearly the same width. NASA’s Moon fact sheet gives a mean apparent diameter of 1896″, or 31.6′. With the mosaic planner’s default 15 % overlap, a 31.6′ square around the disc needs 28 panels from the ASI715MC at 2000 mm, 8 from the ASI585MC and 6 from the ASI183MC. Halve the focal length to 1000 mm and the ASI183MC’s 45.3′ × 30.3′ field needs only two.

The Mini guide cameras

The ASI120MM Mini, ASI174MM Mini and ASI220MM Mini are ZWO’s dedicated guide cameras. The 2026 guide-camera manual gives all three an 8.5 mm back focus and a USB 2.0 Type-C port, and the Mini cameras page says the 120 Mini reaches focus at the same position as an eyepiece. On a 240 mm guide scope their scales are 3.22″/px (3.75 µm), 3.44″/px (4 µm) and 5.04″/px (5.86 µm). The guide cameras comparison sets these against other brands.

The ASI174MM Mini carries a naming conflict. Its product description and the manual’s spec table say IMX249; the product page’s key-spec table, ZWO’s compare page and a descriptive paragraph inside the same manual say IMX174. Our table follows the description and the spec table. The same key-spec table also lists USB 3.0 and 164 fps, figures the manual contradicts with USB 2.0 and 18 fps at 12 bits. For field-of-view purposes the conflict is harmless: every source gives 1936 × 1216 pixels at 5.86 µm, so its rows below match the planetary ASI174MM exactly.

Back focus is not uniform

The Minis sit at 8.5 mm. Most of the small planetary bodies (ASI462MM, ASI678MC, ASI715MC, ASI662MC, ASI664MC, ASI224MC) are listed at 12.5 mm. The ASI174MM, ASI585MC and ASI183MC share the larger-body figure of 17.5 mm, which ZWO’s compare page writes as 6.5/17.5 mm; the ASI174MM page says 6.5 mm applies after removing the 11 mm T2 ring. Keep that in mind when a Barlow’s amplification depends on spacing.