FOVMath

Small refractors from 178 to 408 mm: field, focal ratio and image circle compared

Between 178 mm and 408 mm of focal length, an APS-C sensor's field shrinks from 7.55° × 5.05° to 3.30° × 2.20°. That range decides which large targets fit in one frame: the whole Veil loop only below about 337 mm, M31 throughout. Focal ratio (f/3.2 to f/5.9) and image circle sort the rest; none of this ranks one telescope above another.

Published October 15, 2026 . Sources are listed at the end of the page.

Small refractors under 400 mm: published specifications
Model Design Aperture (mm) Focal length (mm) Focal ratio Image circle (mm) Back focus (mm) Weight (kg) Official reducer or flattener
William Optics MiniCat 51 WIFD 6-element Petzval (2 Super ED) 51 178 f/3.5 43.2 55 1.67 none listed on the product page
William Optics Mighty-Cat 71 (30th Anniversary Limited Edition) 7-element EVO Petzval (includes 3 Super ED) 71 230 f/3.2 43.2 55 3.38 none listed on the product page
William Optics Cat 51 WIFD (current RedCat 51) 4-element Petzval 51 250 f/4.9 48 55 1.77 none listed on the product page
Askar SQA55 Quintuplet air-spaced Petzval APO (one SD glass, one ED glass) 55 264 f/4.8 44 55 1.84 none listed on the product page
William Optics Ultra-Cat 56 5-element advanced Petzval (2 Super ED, 1 ED, 1 lanthanum, 1 'X-element') 56 269 f/4.8 50 55 2.56 none listed on the product page
William Optics Pleiades 68 7-element astrograph (3 Super ED) 68 260 f/3.8 48 not published 2.98 none listed on the product page
Askar SQA60 pro Sextuplet air-spaced Petzval APO (2 SD glass elements) 60 276 f/4.6 44 55 2.79 none listed on the product page
Askar FRA300 Pro Quintuplet fully air-spaced Petzval APO (one ED glass) 60 300 f/5 44 not published 2.26 none listed on the product page
William Optics Cat 61 WIFD (current RedCat 61) 4-element Petzval 61 300 f/4.9 46 55 2.47 none listed on the product page
Askar SQA70 Quintuplet air-spaced Petzval APO (2 ED glass elements) 70 336 f/4.8 44 55 2.68 none listed on the product page
William Optics Cat 71 WIFD 4-element Petzval 71 350 f/4.9 48 55 3.22 none listed on the product page
William Optics Zenithstar 61 II Doublet APO 61 360 f/5.9 45 not published 2.2 FLAT 61A: 360 mm, f/5.9; FLAT 61R (0.8x): 288 mm, f/4.7
William Optics Ultra-Cat 76 5-element advanced Petzval (2 Super ED, 1 ED, 1 lanthanum, 1 'X-element') 76 365 f/4.8 50 55 3.81 none listed on the product page
Askar FRA400 Quintuplet dual-ED air-spaced APO flat-field refractive astrograph 72 400 f/5.6 44 140 2.56 F3.9 Full Frame Reducer for FRA400/FRA500: 280 mm, f/3.9
Askar SQA85 Quintuplet air-spaced Petzval APO (2 SD glass elements) 85 408 f/4.8 not published 55 3.64 none listed on the product page
Sky-Watcher Esprit 80EDX ED triplet super-apochromatic refractor (one Ohara FPL-53 element) 80 400 f/5 not published not published 3.97 Optional matched field flattener (Esprit 80EDX)

Focal length sets the frame

With an APS-C sensor such as the ASI2600MC Pro (23.49 × 15.70 mm imaging area), these sixteen telescopes fall into six bands:

Focal lengthTelescopesAPS-C fieldScale (3.76 µm)
178 mmMiniCat 517.55° × 5.05°4.36″/px
230 mmMighty-Cat 715.85° × 3.91°3.37″/px
250–276 mmCat 51, Pleiades 68, SQA55, Ultra-Cat 56, SQA60 pro5.38° × 3.60° at 250 mm3.10″/px
300 mmFRA300 Pro, Cat 614.48° × 3.00°2.59″/px
336–365 mmSQA70, Cat 71, Zenithstar 61 II, Ultra-Cat 763.84° × 2.57° at 350 mm2.22″/px
400–408 mmFRA400, Esprit 80EDX, SQA853.36° × 2.25° at 400 mm1.94″/px

Going from 250 mm to 400 mm shrinks each side of the field by 1.6 times and its area by 2.56 times. The target framing calculator checks a specific object against a specific setup.

What changes between 250 and 400 mm

Three large targets show where the line falls.

The whole Veil Nebula, the Cygnus Loop, measures 230′ × 160′ in SIMBAD. The APS-C short side reaches 160′ only up to about 337 mm, so the loop fits with room at 250 mm (323′ × 216′), fits with no margin at the SQA70’s 336 mm, and needs a mosaic at 400 mm (202′ × 135′). The Veil page shows the framing, and the mosaic planner counts panels.

The Andromeda Galaxy, 178′ × 63′ in the SEDS catalogue, fits across the long side everywhere in this range. It fills 55% of the frame width at 250 mm and 88% at 400 mm, which leaves little room for its outer extent at the long end.

The Pleiades, 110′ across per SEDS, fill 82% of the APS-C short side at 400 mm and about half of it at 250 mm.

Focal ratio at similar focal length

Focal ratio does not change the field; focal length does. What a lower f-number means at a fixed focal length is a larger objective. The Pleiades 68 (68 mm, f/3.8) and Cat 51 WIFD (51 mm, f/4.9) work at 260 and 250 mm, nearly the same scale (2.98″ and 3.10″ per pixel), and the Pleiades collects (68 ÷ 51)² = 1.78 times as much light from each object. The same comparison between the Ultra-Cat 76 (f/4.8) and Zenithstar 61 II (f/5.9), both near 360 mm, gives (76 ÷ 61)² = 1.55.

The MiniCat (f/3.5) and Mighty-Cat (f/3.2) get their speed the other way, from short focal length on a modest aperture, so they also deliver the widest fields and coarsest scales in the group.

Image circle against sensor diagonal

An APS-C diagonal is about 28.3 mm and a full-frame diagonal 43.3 mm. Every published circle here covers APS-C. For full frame:

  • 44–50 mm, with margin: SQA55, SQA60 pro, SQA70, FRA300 Pro, FRA400, Cat 51, 61 and 71 WIFD, Pleiades 68, Ultra-Cat 56 and 76.
  • 43.2 mm, at the diagonal: MiniCat 51 and Mighty-Cat 71.
  • Zenithstar 61 II: the tube lists 45 mm, but its manual rates the image circle at 43 mm with either of its flatteners, the FLAT 61A or FLAT 61R.
  • Esprit 80EDX: Sky-Watcher quotes 32 mm with its optional flattener, an APS-C match.
  • SQA85: described as full frame, with no millimeter figure.

An APS-C sensor such as the IMX571 sits inside every published circle here, so coverage only becomes a question at full frame.

Field of view with common sensors

Width × height of the field and image scale for ZWO ASI2600MC Pro (IMX571), ZWO ASI533MC Pro (IMX533), ZWO ASI585MC (IMX585), ZWO ASI6200MC Pro (IMX455), computed from each camera's published pixel count and pixel size. Reducer focal lengths are the manufacturer's figure where published, otherwise native focal length × the published factor.

Telescope and configuration Focal length ASI2600MC ProASI533MC ProASI585MCASI6200MC Pro
William Optics MiniCat 51 WIFD, native 178 mm 7.55° × 5.05° 4.36″/px 3.64° × 3.64° 4.36″/px 3.58° × 2.02° 3.36″/px 11.55° × 7.72° 4.36″/px
William Optics Mighty-Cat 71 (30th Anniversary Limited Edition), native 230 mm 5.85° × 3.91° 3.37″/px 2.82° × 2.82° 3.37″/px 2.77° × 1.56° 2.60″/px 8.95° × 5.98° 3.37″/px
William Optics Cat 51 WIFD (current RedCat 51), native 250 mm 5.38° × 3.60° 3.10″/px 2.59° × 2.59° 3.10″/px 2.55° × 1.44° 2.39″/px 8.24° × 5.50° 3.10″/px
Askar SQA55, native 264 mm 5.10° × 3.41° 2.94″/px 2.45° × 2.45° 2.94″/px 2.42° × 1.36° 2.27″/px 7.80° × 5.21° 2.94″/px
William Optics Ultra-Cat 56, native 269 mm 5.00° × 3.34° 2.88″/px 2.41° × 2.41° 2.88″/px 2.37° × 1.33° 2.22″/px 7.66° × 5.11° 2.88″/px
William Optics Pleiades 68, native 260 mm 5.17° × 3.46° 2.98″/px 2.49° × 2.49° 2.98″/px 2.45° × 1.38° 2.30″/px 7.92° × 5.29° 2.98″/px
Askar SQA60 pro, native 276 mm 4.87° × 3.26° 2.81″/px 2.35° × 2.35° 2.81″/px 2.31° × 1.30° 2.17″/px 7.46° × 4.98° 2.81″/px
Askar FRA300 Pro, native 300 mm 4.48° × 3.00° 2.59″/px 2.16° × 2.16° 2.59″/px 2.13° × 1.20° 1.99″/px 6.87° × 4.58° 2.59″/px
William Optics Cat 61 WIFD (current RedCat 61), native 300 mm 4.48° × 3.00° 2.59″/px 2.16° × 2.16° 2.59″/px 2.13° × 1.20° 1.99″/px 6.87° × 4.58° 2.59″/px
Askar SQA70, native 336 mm 4.00° × 2.68° 2.31″/px 116′ × 116′ 2.31″/px 114′ × 64′ 1.78″/px 6.13° × 4.09° 2.31″/px
William Optics Cat 71 WIFD, native 350 mm 3.84° × 2.57° 2.22″/px 111′ × 111′ 2.22″/px 109′ × 62′ 1.71″/px 5.89° × 3.93° 2.22″/px
William Optics Zenithstar 61 II, native 360 mm 3.74° × 2.50° 2.15″/px 108′ × 108′ 2.15″/px 106′ × 60′ 1.66″/px 5.73° × 3.82° 2.15″/px
William Optics Zenithstar 61 II, FLAT 61R 288 mm 4.67° × 3.12° 2.69″/px 2.25° × 2.25° 2.69″/px 2.22° × 1.25° 2.08″/px 7.15° × 4.78° 2.69″/px
William Optics Ultra-Cat 76, native 365 mm 3.69° × 2.46° 2.12″/px 107′ × 107′ 2.12″/px 105′ × 59′ 1.64″/px 5.65° × 3.77° 2.12″/px
Askar FRA400, native 400 mm 3.36° × 2.25° 1.94″/px 97′ × 97′ 1.94″/px 96′ × 54′ 1.50″/px 5.15° × 3.44° 1.94″/px
Askar FRA400, F3.9 Full Frame Reducer for FRA400/FRA500 280 mm 4.80° × 3.21° 2.77″/px 2.31° × 2.31° 2.77″/px 2.28° × 1.28° 2.14″/px 7.36° × 4.91° 2.77″/px
Askar SQA85, native 408 mm 3.30° × 2.20° 1.90″/px 95′ × 95′ 1.90″/px 94′ × 53′ 1.47″/px 5.05° × 3.37° 1.90″/px
Sky-Watcher Esprit 80EDX, native 400 mm 3.36° × 2.25° 1.94″/px 97′ × 97′ 1.94″/px 96′ × 54′ 1.50″/px 5.15° × 3.44° 1.94″/px

Try another camera in the field of view calculator, or check star sampling for your sky with the sampling calculator.

Questions

Will the whole Veil Nebula fit in one APS-C frame?
SIMBAD lists the Cygnus Loop at 230′ × 160′. An APS-C sensor (23.49 × 15.70 mm) covers 160′ on its short side up to about 337 mm, so the loop fits without margin at the SQA70's 336 mm and with room at 250–300 mm. At 400 mm it needs a mosaic.
Do these telescopes cover a full-frame sensor?
Most publish circles of 44–50 mm, enough for the 43.3 mm full-frame diagonal. The MiniCat and Mighty-Cat list 43.2 mm, the Zenithstar 61 II gives 43 mm with its flatteners, and the Esprit 80EDX flattener covers 32 mm. The SQA85 is described as full frame without a number.
  • William Optics Cat refractors William Optics' Cat Petzval astrographs: the RedCat to Cat WIFD rename, MiniCat, Mighty-Cat and Ultra-Cat, image circles, and the 55 ± 3 mm spacing WO recommends.
  • Askar SQA, FRA and PHQ Askar SQA, FRA and PHQ astrographs: how the three lines differ, 0.7x–0.76x reducers with published focal lengths, and back focus measured from different threads.
  • Target framing Check whether a galaxy or nebula fits your sensor, with catalogue sizes and position angles, a star-chart preview and the focal length for a chosen fill.
  • Mosaic planner Count mosaic panels for a nebula or galaxy from your sensor, focal length and overlap, with both camera orientations tried and a panel layout chart.
  • IMX571 APS-C cameras Eleven cameras on Sony's IMX571: the same 3.76 µm pixels and near-identical fields, with differences in published cooling, back focus, full well and read noise.

Sources

References for the text first, then the manufacturer specification pages for the telescopes and the reference cameras.

  1. Cygnus Loop (NAME Cygnus Loop): angular size SIMBAD, CDS Strasbourg. Accessed October 12, 2026.
  2. Messier 31: Andromeda Galaxy SEDS Messier catalogue. Accessed October 12, 2026.
  3. Messier 45: The Pleiades SEDS Messier catalogue. Accessed October 12, 2026.
  4. Zenithstar 61 II product manual William Optics Support. Accessed October 12, 2026.
  5. William Optics MiniCat 51 WIFD specifications William Optics. Accessed October 12, 2026.
  6. William Optics Mighty-Cat 71 (30th Anniversary Limited Edition) specifications William Optics. Accessed October 12, 2026.
  7. William Optics Cat 51 WIFD (current RedCat 51) specifications William Optics. Accessed October 12, 2026.
  8. Askar SQA55 specifications Askar. Accessed October 12, 2026.
  9. William Optics Ultra-Cat 56 specifications William Optics. Accessed October 12, 2026.
  10. William Optics Pleiades 68 specifications William Optics. Accessed October 12, 2026.
  11. Askar SQA60 pro specifications Askar. Accessed October 12, 2026.
  12. Askar FRA300 Pro specifications Askar. Accessed October 12, 2026.
  13. William Optics Cat 61 WIFD (current RedCat 61) specifications William Optics. Accessed October 12, 2026.
  14. Askar SQA70 specifications Askar. Accessed October 12, 2026.
  15. William Optics Cat 71 WIFD specifications William Optics. Accessed October 12, 2026.
  16. William Optics Zenithstar 61 II specifications William Optics. Accessed October 12, 2026.
  17. William Optics Ultra-Cat 76 specifications William Optics. Accessed October 12, 2026.
  18. Askar FRA400 specifications Askar. Accessed October 12, 2026.
  19. Askar SQA85 specifications Askar. Accessed October 12, 2026.
  20. Sky-Watcher Esprit 80EDX specifications Sky-Watcher. Accessed October 12, 2026.
  21. ZWO ASI2600MC Pro specifications ZWO. Accessed October 12, 2026.
  22. ZWO ASI533MC Pro specifications ZWO. Accessed October 12, 2026.
  23. ZWO ASI585MC specifications ZWO. Accessed October 12, 2026.
  24. ZWO ASI6200MC Pro specifications ZWO. Accessed October 12, 2026.