Askar APO triplets, F-series refractors and N Newtonians: focal lengths and sensor coverage

APO triplets: the accessory defines the imaging setup

The six APO tubes are air-spaced triplets with one ED element, from the 103APO (700.4 mm, f/6.8) to the 203APO (1421 mm, f/7). Askar publishes no native image circle or back focus for them. Those numbers come with the matched accessories: a 1x full-frame flattener and a 0.8x full-frame reducer for each tube from the 103 to the 185, both working at 55 mm back focus.

Askar states the reduced focal lengths to the hundredth of a millimeter, and they are exact products of the factor: 560.32 mm for the 103APO at 0.8x is 700.4 × 0.8. The 103APO also gets a 0.6x reducer to 420.24 mm at f/4.08. The 203APO page says its own reducer and flattener will be launched later, so its row in the FOV table shows the native 1421 mm only.

A 103APO at 0.6x against an FRA400

That 0.6x option lands the 103APO next to Askar’s own FRA400 (72 mm, 400 mm, f/5.6). With an ASI2600MC Pro the two frame 3.20° × 2.14° and 3.36° × 2.25°, nearly the same sky at 1.85″ and 1.94″ per pixel.

What the faster focal ratio means here is aperture at a fixed scale: (103 ÷ 72)² = 2.05, so the reduced 103APO collects about twice the light from each nebula and spreads it over nearly the same pixels. It also weighs 4.75 kg against 2.56 kg for the FRA400, and it puts a reducer in the path where the FRA400 needs nothing. The FRA line is covered on the SQA, FRA and PHQ page.

F-series: self-flattened, with reducers that round differently

The 60F, 71F, 91F, 111F and 131F are quadruplets with one ED element, 408 mm to 983 mm at f/6.7–f/7.5. The 60F page says the built-in flattener removes the need for a separate one and for back-focus calculations, and all five list a 44 mm circle.

Each has a dedicated reducer, and here the published focal lengths do not quite equal native × factor:

Telescope	Factor	Published	Native × factor
60F	0.75x	311 mm, f/5.1	306 mm
71F	0.75x	369 mm, f/5.2	368 mm
91F	0.75x	455 mm, f/5	457 mm
111F	0.74x	566 mm, f/5.1	567 mm
131F	0.74x	733.6 mm, f/5.6	727 mm

The 60F reducer and 131F reducer differ most, implying factors of 0.762 and 0.746. The tables on this page use Askar’s published focal lengths. As the reducer guide explains, a reducer’s effective factor moves with its spacing, so a plate solve is the final word.

N160 and N210: fast, with a 28 mm circle

The two carbon-tube Newtonians run at f/3.6 natively. Their included 0.95x triplet corrector takes the N160 from 576 mm to 544 mm and the N210 from 756 mm to 714 mm, both f/3.4, and Askar specifies the format as APS-C, 28 mm.

That figure decides the camera. An APS-C sensor such as the IMX571 has a 28.26 mm diagonal, so its corners sit right at the edge of the rated circle; the ASI294MC Pro (23.2 mm) and ASI533MC Pro (16.0 mm) sit inside it; a 43.3 mm full-frame sensor does not. With the APS-C camera the N160 frames 2.47° × 1.65°.

Askar publishes the secondary mirror’s short axis: 61 mm on the N160 (38% of the aperture, 15% of the area) and 71 mm on the N210 (34%, 11%). By clear area those equal unobstructed apertures of 148 mm and 198 mm.