FOVMath

William Optics Zenithstar, Gran Turismo, Pleiades and FluoroStar: flatteners by factor

Outside the Cat line, William Optics sells Zenithstar doublets, Gran Turismo and FluoroStar triplets, and seven-element Pleiades astrographs that need no flattener. For the doublets and triplets WO publishes each flattener's power (1.0x to 0.72x) and a back focus per telescope, but never the resulting focal length, so this page and its tables compute it.

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

WO Zenithstar, GT, Pleiades, FluoroStar: published specifications
Model Design Aperture (mm) Focal length (mm) Focal ratio Image circle (mm) Back focus (mm) Weight (kg) Official reducer or flattener
Pleiades 68 7-element astrograph (3 Super ED) 68 260 f/3.8 48 not published 2.98 none listed on the product page
Pleiades 111 7-element astrograph (3 Super ED) 111 528 f/4.8 45 not published 7.95 none listed on the product page
Pleiades 181 7-element astrograph (3 Super ED) 181 998 f/5.5 55 not published 15 none listed on the product page
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
Zenithstar 73 III (discontinued) Doublet APO 73 430 f/5.9 45 not published 2.83 FLAT 73A: 430 mm, f/5.9; FLAT 73R (0.8x Reducer): 344 mm, f/4.7
Zenithstar 81 WIFD Doublet APO 81 559 f/6.9 45 not published 3.55 FLAT 6AIII (0.8x Reducer): 447 mm, f/5.5
Zenithstar 103 Doublet 103 711 f/6.9 not published not published 4.36 FLAT 6AIII (0.8x Reducer): 569 mm, f/5.5
Gran-Turismo 71 II Triplet APO 71 419 f/5.9 44 not published 2.95 FLAT GT: 419 mm, f/5.9; FLAT 6AIII (0.8x Reducer): 335 mm, f/4.7
Gran-Turismo 81 IV Triplet APO 81 478 f/5.9 45 not published 3.9 FLAT GT: 478 mm, f/5.9; FLAT 6AIII (0.8x Reducer): 382 mm, f/4.7
Gran-Turismo 81 WIFD Triplet APO 81 478 f/5.9 45 not published 3.58 FLAT GT (requires adapter): 478 mm, f/5.9; FLAT 6AIII (0.8x Reducer): 382 mm, f/4.7; FLAT 68III (requires adapter): 478 mm, f/5.9
Gran Turismo 102 Triplet 102 703 f/6.9 not published not published 5 FLAT 68III: 703 mm, f/6.9; FLAT 7A (0.8x Reducer): 562 mm, f/5.5; FLAT 8 (0.72x Reducer): 506 mm, f/5
Gran-Turismo 153 Triplet APO 153 1188 f/7.8 43.2 not published 13.64 FLAT 68III: 1188 mm, f/7.8; FLAT 7A (0.8x Reducer): 950 mm, f/6.2; FLAT 8 (0.72x Reducer): 855 mm, f/5.6
FluoroStar 91 Triplet APO 91 540 f/5.9 43 not published 4.16 FLAT 68III (requires adapter): 540 mm, f/5.9; FLAT 6AIII (0.8x Reducer, requires adapter): 432 mm, f/4.7; Ultra Flat 91 (0.79x Reducer): 427 mm, f/4.7
FluoroStar 120 Triplet APO 120 780 f/6.5 43 not published 7.1 FLAT 68III: 780 mm, f/6.5; FLAT 7A (0.8x Reducer, requires adapter): 624 mm, f/5.2
FluoroStar 132 Triplet APO 132 910 f/6.9 45 not published 9 FLAT 68III: 910 mm, f/6.9; FLAT 7A (0.8x Reducer): 728 mm, f/5.5; FLAT 8 (0.72x Reducer): 655 mm, f/5; Ultra Flat 132 (0.79x Reducer): 719 mm, f/5.4
FluoroStar 156 Triplet APO 156 1217 f/7.8 43 not published 17 FLAT 68III: 1217 mm, f/7.8; FLAT 7A (0.8x Reducer): 974 mm, f/6.2; FLAT 8 (0.72x Reducer): 876 mm, f/5.6; Ultra Flat 156 (0.79x Reducer): 961 mm, f/6.2

Doublets, triplets and astrographs

The Zenithstars are doublets: the 61 II (360 mm, f/5.9) and 81 WIFD (559 mm, f/6.9) on sale, the 73 III discontinued, and the 103 documented only in a support manual. The Gran Turismo and FluoroStar tubes are triplets, from the GT71 II at 419 mm to the FluoroStar 156 at 1217 mm. All of them list optional flatteners in their spec tables.

The Pleiades 68, 111 and 181 are a different design: seven elements including three Super ED, with “No Flattener Required” in the spec table. They run faster than the triplets of similar size, f/3.8 at 260 mm on the Pleiades 68, and WO publishes no back focus for them. On a full-frame ASI6200MC Pro the Pleiades 68 frames 7.92° × 5.29° at 2.98″ per pixel, inside its 48 mm circle.

Flattener power, computed focal length

WO’s flattener pages give a “Flattener Power” rather than a focal length: 0.8x for the FLAT 7A and FLAT 6AIII, 0.72x for the FLAT 8, 0.79x for the Ultra Flat series and 1.0x for the FLAT 61A, FLAT GT and FLAT 68III. The FOV table below multiplies native focal length by that power and labels the row with the flattener’s name. Three tubes show the spread:

Tube and flattenerPowerComputed focal lengthFocal ratio
FluoroStar 132, FLAT 7A0.8x728 mmf/5.5
FluoroStar 132, Ultra Flat 1320.79x719 mmf/5.4
FluoroStar 132, FLAT 80.72x655 mmf/5.0
Gran Turismo 153, FLAT 80.72x855 mmf/5.6
Zenithstar 81 WIFD, FLAT 6AIII0.8x447 mmf/5.5

Treat these as nominal. A reducer’s factor depends on its distance to the sensor; the thin-lens model in the reducer calculator shows how a few millimeters shift it. WO tunes that distance per telescope, which is the next point.

Back focus belongs to the pair, not the flattener

WO’s support manuals publish a back focus for each telescope and flattener combination, and the same flattener lands at different distances on different tubes. The FLAT 7A needs 65.3 mm on the FluoroStar 156 but 69.24 mm on the FluoroStar 132. The Ultra Flat reducers are the exception: 55 mm on both, with no adjustment. On the Zenithstar 61 II, the FLAT 61A needs 67.7 mm and the FLAT 61R 56 mm.

The practical consequence: look up the figure for your exact pairing, set it, and confirm with a star test or plate solve. The back focus guide shows how to build up spacers to a target.

The FluoroStar 156 image circle

The store page lists 43 mm; the support manual lists 44 mm. The difference sits right at the size of a full-frame diagonal (43.3 mm), so it decides on paper whether a full-frame sensor is covered at the native 1217 mm, where an ASI6200MC Pro would span 1.70° × 1.13°.

With a flattener the manual is more specific: 44.4 mm with the FLAT 68III, FLAT 7A or FLAT 8, and 43.2 mm with the Ultra Flat 156. The FluoroStar 91 and 120 store pages list 43 mm as well, the 132 lists 45 mm. Where coverage of a 43.3 mm sensor depends on a millimeter, treat full frame as marginal and judge the corners on your own frames.

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
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
Pleiades 111, native 528 mm 2.55° × 1.70° 1.47″/px 74′ × 74′ 1.47″/px 73′ × 41′ 1.13″/px 3.91° × 2.61° 1.47″/px
Pleiades 181, native 998 mm 81′ × 54′ 0.78″/px 39′ × 39′ 0.78″/px 38′ × 22′ 0.60″/px 2.07° × 1.38° 0.78″/px
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
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
Zenithstar 73 III, native 430 mm 3.13° × 2.09° 1.80″/px 90′ × 90′ 1.80″/px 89′ × 50′ 1.39″/px 4.79° × 3.20° 1.80″/px
Zenithstar 73 III, FLAT 73R 344 mm 3.91° × 2.61° 2.25″/px 113′ × 113′ 2.25″/px 111′ × 63′ 1.74″/px 5.99° × 4.00° 2.25″/px
Zenithstar 81 WIFD, native 559 mm 2.41° × 1.61° 1.39″/px 70′ × 70′ 1.39″/px 68′ × 39′ 1.07″/px 3.69° × 2.46° 1.39″/px
Zenithstar 81 WIFD, FLAT 6AIII 447 mm 3.01° × 2.01° 1.74″/px 87′ × 87′ 1.74″/px 86′ × 48′ 1.34″/px 4.61° × 3.08° 1.74″/px
Zenithstar 103, native 711 mm 114′ × 76′ 1.09″/px 55′ × 55′ 1.09″/px 54′ × 30′ 0.84″/px 2.90° × 1.94° 1.09″/px
Zenithstar 103, FLAT 6AIII 569 mm 2.37° × 1.58° 1.36″/px 68′ × 68′ 1.36″/px 67′ × 38′ 1.05″/px 3.62° × 2.42° 1.36″/px
Gran-Turismo 71 II, native 419 mm 3.21° × 2.15° 1.85″/px 93′ × 93′ 1.85″/px 91′ × 51′ 1.43″/px 4.92° × 3.28° 1.85″/px
Gran-Turismo 71 II, FLAT 6AIII 335 mm 4.02° × 2.69° 2.32″/px 116′ × 116′ 2.32″/px 114′ × 64′ 1.79″/px 6.15° × 4.11° 2.32″/px
Gran-Turismo 81 IV, native 478 mm 2.82° × 1.88° 1.62″/px 81′ × 81′ 1.62″/px 80′ × 45′ 1.25″/px 4.31° × 2.88° 1.62″/px
Gran-Turismo 81 IV, FLAT 6AIII 382 mm 3.52° × 2.35° 2.03″/px 102′ × 102′ 2.03″/px 100′ × 56′ 1.57″/px 5.40° × 3.60° 2.03″/px
Gran-Turismo 81 WIFD, native 478 mm 2.82° × 1.88° 1.62″/px 81′ × 81′ 1.62″/px 80′ × 45′ 1.25″/px 4.31° × 2.88° 1.62″/px
Gran-Turismo 81 WIFD, FLAT 6AIII 382 mm 3.52° × 2.35° 2.03″/px 102′ × 102′ 2.03″/px 100′ × 56′ 1.57″/px 5.40° × 3.60° 2.03″/px
Gran Turismo 102, native 703 mm 115′ × 77′ 1.10″/px 55′ × 55′ 1.10″/px 54′ × 31′ 0.85″/px 2.93° × 1.96° 1.10″/px
Gran Turismo 102, FLAT 7A 562 mm 2.39° × 1.60° 1.38″/px 69′ × 69′ 1.38″/px 68′ × 38′ 1.06″/px 3.67° × 2.45° 1.38″/px
Gran Turismo 102, FLAT 8 506 mm 2.66° × 1.78° 1.53″/px 77′ × 77′ 1.53″/px 76′ × 43′ 1.18″/px 4.08° × 2.72° 1.53″/px
Gran-Turismo 153, native 1188 mm 68′ × 45′ 0.65″/px 33′ × 33′ 0.65″/px 32′ × 18′ 0.50″/px 104′ × 70′ 0.65″/px
Gran-Turismo 153, FLAT 7A 950 mm 85′ × 57′ 0.82″/px 41′ × 41′ 0.82″/px 40′ × 23′ 0.63″/px 2.17° × 1.45° 0.82″/px
Gran-Turismo 153, FLAT 8 855 mm 94′ × 63′ 0.91″/px 45′ × 45′ 0.91″/px 45′ × 25′ 0.70″/px 2.41° × 1.61° 0.91″/px
FluoroStar 91, native 540 mm 2.49° × 1.67° 1.44″/px 72′ × 72′ 1.44″/px 71′ × 40′ 1.11″/px 3.82° × 2.55° 1.44″/px
FluoroStar 91, FLAT 6AIII 432 mm 3.12° × 2.08° 1.80″/px 90′ × 90′ 1.80″/px 89′ × 50′ 1.38″/px 4.77° × 3.18° 1.80″/px
FluoroStar 91, Ultra Flat 91 427 mm 3.15° × 2.11° 1.82″/px 91′ × 91′ 1.82″/px 90′ × 50′ 1.40″/px 4.83° × 3.22° 1.82″/px
FluoroStar 120, native 780 mm 104′ × 69′ 0.99″/px 50′ × 50′ 0.99″/px 49′ × 28′ 0.77″/px 2.64° × 1.76° 0.99″/px
FluoroStar 120, FLAT 7A 624 mm 2.16° × 1.44° 1.24″/px 62′ × 62′ 1.24″/px 61′ × 35′ 0.96″/px 3.31° × 2.21° 1.24″/px
FluoroStar 132, native 910 mm 89′ × 59′ 0.85″/px 43′ × 43′ 0.85″/px 42′ × 24′ 0.66″/px 2.27° × 1.51° 0.85″/px
FluoroStar 132, FLAT 7A 728 mm 111′ × 74′ 1.07″/px 53′ × 53′ 1.07″/px 53′ × 30′ 0.82″/px 2.83° × 1.89° 1.07″/px
FluoroStar 132, FLAT 8 655 mm 2.05° × 1.37° 1.18″/px 59′ × 59′ 1.18″/px 58′ × 33′ 0.91″/px 3.15° × 2.10° 1.18″/px
FluoroStar 132, Ultra Flat 132 719 mm 112′ × 75′ 1.08″/px 54′ × 54′ 1.08″/px 53′ × 30′ 0.83″/px 2.87° × 1.91° 1.08″/px
FluoroStar 156, native 1217 mm 66′ × 44′ 0.64″/px 32′ × 32′ 0.64″/px 31′ × 18′ 0.49″/px 102′ × 68′ 0.64″/px
FluoroStar 156, FLAT 7A 974 mm 83′ × 55′ 0.80″/px 40′ × 40′ 0.80″/px 39′ × 22′ 0.61″/px 2.12° × 1.41° 0.80″/px
FluoroStar 156, FLAT 8 876 mm 92′ × 62′ 0.89″/px 44′ × 44′ 0.89″/px 44′ × 25′ 0.68″/px 2.35° × 1.57° 0.89″/px
FluoroStar 156, Ultra Flat 156 961 mm 84′ × 56′ 0.81″/px 40′ × 40′ 0.81″/px 40′ × 22′ 0.62″/px 2.15° × 1.43° 0.81″/px

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

Questions

What focal length does a FluoroStar 132 have with the FLAT 7A?
William Optics publishes only the 0.8x power, so 910 × 0.8 = 728 mm at about f/5.5. The real value depends on spacing; a plate solve gives the exact figure for your setup.
Do the Pleiades astrographs need a flattener?
No. Their spec tables say no flattener is required. William Optics does not publish a back focus value for them.
  • 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.
  • Reducer and Barlow Effective focal length, focal ratio, image scale and field with a reducer or Barlow, from its nominal factor or from lens focal length and spacing.
  • Reducers, Barlows and spacing Thin-lens formulas for reducers (R = 1 − d/f) and Barlows (M = 1 + d/|f|), why spacing changes the factor, designed back focus, and what a faster f-ratio buys.
  • Back focus and spacing What back focus is, why reducers and flatteners need an exact sensor distance, published values for real telescopes, and how to add up a spacer chain.
  • IMX455 full-frame cameras Seven IMX455 full-frame cameras: the 43.3 mm image circle the sensor needs, 122 MB per 16-bit FITS frame, and how the ZWO, QHY, Player One and ToupTek bodies differ.

Sources

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

  1. FLAT 7A (0.8x Reducer) William Optics. Accessed October 12, 2026.
  2. FLAT 8 (0.72x Reducer) William Optics. Accessed October 12, 2026.
  3. Ultra Flat 156 (0.79x Reducer) William Optics. Accessed October 12, 2026.
  4. FluoroStar 156 product manual William Optics Support. Accessed October 12, 2026.
  5. FluoroStar 132 product manual William Optics Support. Accessed October 12, 2026.
  6. Zenithstar 61 II product manual William Optics Support. Accessed October 12, 2026.
  7. William Optics Pleiades 68 specifications William Optics. Accessed October 12, 2026.
  8. William Optics Pleiades 111 specifications William Optics. Accessed October 12, 2026.
  9. William Optics Pleiades 181 specifications William Optics. Accessed October 12, 2026.
  10. William Optics Zenithstar 61 II specifications William Optics. Accessed October 12, 2026.
  11. William Optics Zenithstar 73 III specifications William Optics. Accessed October 12, 2026.
  12. William Optics Zenithstar 81 WIFD specifications William Optics. Accessed October 12, 2026.
  13. William Optics Zenithstar 103 specifications William Optics. Accessed October 12, 2026.
  14. William Optics Gran-Turismo 71 II specifications William Optics. Accessed October 12, 2026.
  15. William Optics Gran-Turismo 81 IV specifications William Optics. Accessed October 12, 2026.
  16. William Optics Gran-Turismo 81 WIFD specifications William Optics. Accessed October 12, 2026.
  17. William Optics Gran Turismo 102 specifications William Optics. Accessed October 12, 2026.
  18. William Optics Gran-Turismo 153 specifications William Optics. Accessed October 12, 2026.
  19. William Optics FluoroStar 91 specifications William Optics. Accessed October 12, 2026.
  20. William Optics FluoroStar 120 specifications William Optics. Accessed October 12, 2026.
  21. William Optics FluoroStar 132 specifications William Optics. Accessed October 12, 2026.
  22. William Optics FluoroStar 156 specifications William Optics. Accessed October 12, 2026.
  23. ZWO ASI2600MC Pro specifications ZWO. Accessed October 12, 2026.
  24. ZWO ASI533MC Pro specifications ZWO. Accessed October 12, 2026.
  25. ZWO ASI585MC specifications ZWO. Accessed October 12, 2026.
  26. ZWO ASI6200MC Pro specifications ZWO. Accessed October 12, 2026.