Sky-Watcher PDS and Quattro imaging Newtonians: f/5 vs f/4, correctors and obstruction

Same focal length, different speed

The PDS tubes (BK130PDS, BK150PDS, BK200PDS on skywatcher.com) all work at f/5, from 650 mm to 1000 mm. The Quattros all work at f/4, from 600 mm (150P) to 1200 mm (300P, listed only by Sky-Watcher USA).

Two pairs show what the focal ratio does. The BK200PDS and the Quattro 250 share 1000 mm, so with an ASI2600MC Pro both frame 1.35° × 0.90° at 0.78″ per pixel. The difference is aperture: 254 mm against 200 mm gives (254 ÷ 200)² = 1.61 times the collecting area, before subtracting the secondary. At equal focal length, the faster tube puts more light on every pixel because it gathers more.

The BK150PDS and the Quattro 150P share a 150 mm aperture instead. Both collect the same light from a nebula; the Quattro’s 600 mm spreads it over a smaller image. Scale goes from 1.03″ to 1.29″ per pixel, and each pixel receives (750 ÷ 600)² = 1.56 times the signal per second.

per-pixel signal rate ∝ 1 ÷ N², at fixed pixel size

Coma correctors at 1.0x

Sky-Watcher describes its F5 coma corrector as optimized for all its f/5 Newtonians and says magnification remains unaltered; it gives no back focus or image circle. The F4 coma corrector for the Quattros uses the same wording and adds the numbers that matter for a camera: 55 mm back focus and a 28 mm image height, from a four-element design with one FPL51 ED element.

Because the maker treats both as 1.0x, the FOV table below lists only native focal lengths for these tubes. The 28 mm figure lines up almost exactly with an APS-C sensor such as the IMX571 (28.26 mm diagonal from its pixel grid). A 1-inch-class square sensor like the ASI533’s (16.0 mm) sits well inside; a 43.3 mm full-frame sensor does not.

The Quattro 150P’s bundled reducer

The smallest Quattro is the one exception. Sky-Watcher USA’s 150P page says its included coma corrector/reducer gives 518 mm at f/3.45 and calls it ideal for APS-C sensors and smaller. No factor is stated; 518 ÷ 600 works out to 0.86. With an ASI2600MC Pro that configuration frames 2.60° × 1.74° at 1.50″ per pixel.

The published focal ratio is muddled. skywatcher.com gives f/4 in its summary but f/3.45 in its headline and spec table next to “600mm”, and the US spec table says f/3.9. This site records f/4 for the bare tube and 518 mm with the corrector.

The same US page says the Quattro is 6 times faster than an average 8-inch f/10 SCT, or 8.4 times with the reducer. Those are (10 ÷ 4)² = 6.25 and (10 ÷ 3.45)² = 8.40: per-pixel rates with the same camera. They leave out scale. A C8 at 2032 mm samples 0.38″ per pixel with 3.76 µm pixels, against 1.29″ for the Quattro 150P at 600 mm, so the two are not imaging the same thing.

Secondary obstruction

The spec tables on the US Quattro pages list secondary diameters: 64 mm (150P), 70 mm (200P), 89 mm (250P) and 102 mm (300P). Sky-Watcher does not publish the PDS secondaries.

clear area ∝ D² − d²; equivalent unobstructed aperture = √(D² − d²)

Tube	Aperture	Secondary	Blocked by area	Equivalent clear aperture
Quattro 150P	150 mm	64 mm	18%	136 mm
Quattro 200	205 mm	70 mm	12%	193 mm
Quattro 250	254 mm	89 mm	12%	238 mm
Quattro 300P	305 mm	102 mm	11%	287 mm

The 150P stands out at 43% of the aperture by diameter. Sky-Watcher USA explains the oversized secondary as a way to illuminate full-frame sensors, which trades some collecting area for a wider illuminated field.