QHY5III planetary and guide cameras: pixel sizes, focal ratios and back focus

QHY’s QHY5III series is its uncooled line for planets, the Sun and Moon, and guiding. The eight models in our data use three sensor formats (1/2.8-inch, 1/1.8-inch and 1/1.2-inch) and a fourfold range of pixel size. Because QHY publishes no millimetre size for most of them, the spec table computes each sensor from pixels × pitch (marked †).

The focal ratio is what matches a pixel size

For planetary imaging, the question is how many pixels fall across the smallest detail the aperture can resolve. Using the Dawes limit (116″ ÷ aperture in mm) and the image-scale formula, the aperture cancels out and only the focal ratio N remains:

pixels across the Dawes limit = 0.562 × N ÷ pixel size (µm)

focal ratio for n pixels = 1.78 × n × pixel size (µm)

That is why the same camera behaves alike on an 8-inch and an 11-inch f/10 tube. If you want two pixels across the Dawes limit, for example, the focal ratios work out as follows:

Pixel size	QHY5III models	Focal ratio for 2 px
1.45 µm	QHY5III715C	f/5.2
2.0 µm	QHY5III678M, QHY5III678C	f/7.1
2.9 µm	QHY5III462C, QHY5III485C, QHY5III585C	f/10.3
4.0 µm	QHY5III200M	f/14.2
5.86 µm	QHY5III174M	f/20.8

On an f/10 tube, the 2.9 µm group lands close to that target without optics in the path, the 715C would be at almost four pixels, and the 174M would need about a 2× Barlow. How many pixels you aim for is a choice that depends on seeing; the resolution limits guide explains where 116″ ÷ D comes from and how it compares with the Rayleigh criterion.

Back focus with and without the adapter

The newer QHY5III bodies give two figures. The QHY5III585 page lists “17mm (with adapter); 8mm (without adapter)”, and the QHY5III678, QHY5III715C and QHY5III200M pages give the same pair. Our table records the with-adapter figure, 17 mm. The QHY5III462C, QHY5III485C and QHY5III174M list a single 12 mm. A Barlow’s amplification grows with the distance behind it, so the adapter you use changes the effective focal ratio slightly as well as the focus position; see back focus and spacing.

Guiding use

QHY’s comparison page files the whole line under planetary and guiding cameras, and the QHY5III462, 585, 174 and 200M pages list an ST-4 guide port. The QHY5III200M page says its 4 µm pixels make it suitable for guiding; on a 240 mm guide scope that is 3.44″/px over a 110′ × 62′ field. The QHY5III174M’s 5.86 µm pixels give 5.04″/px at the same focal length with a much larger 2.69° × 1.68° field. The guide camera comparison puts both next to ZWO, Player One and ToupTek guiders.

Where QHY’s pages disagree

Two conflicts are worth knowing. The QHY5III585 page lists the model as available in mono and color (QHY5III585M/C), while QHY’s Specifications Comparison table lists “QHY5III585C” as color only; the specifications are identical either way. The QHY5III174 page describes an electronic global shutter, while the comparison table gives “Electronic Rolling Shutter” for the same model. Shutter type matters for fast solar and lunar sequences, so check with QHY if it decides your purchase.

Smaller details: the QHY5III462C’s read noise is published as one value, 0.5 e⁻, not a range, and its page notes that, being uncooled, the camera’s thermal background rises with long exposures, so QHY suggests short exposures stacked in large numbers for deep-sky targets. The QHY5III485C and QHY5III585C share the 1/1.2-inch format and 2.9 µm pitch but are listed at very different full wells, 12 ke⁻ and 52 ke⁻.