William Optics Zenithstar 61 II with ZWO ASI533MM Pro
2.15″ per pixel across a 1.80° × 1.80° field, at 360 mm and f/5.9. 23 of the catalogue's 29 equatorial mounts carry the 5.13 kg train within the imaging derate.
Every figure is computed from catalogue rows that link their maker's page. Rows marked provisional have not yet had a second, human check.
Optics
At 360 mm, with the William Optics FLAT 61A (1.0x).
Image circle: 43 mm, the corrector's stated figure (William Optics FLAT 61A (1.0x)). Source
Sampling
Seeing divided by image scale: how many pixels a star's core spans. Each band is a tradeoff, and wide-field work is undersampled on purpose.
At 2.5″ seeing a star’s core spans 1.16 pixels on this sensor. Undersampled; the standard, deliberate wide-field trade.
| Seeing | FWHM | Band |
|---|---|---|
| 1.5″ | 0.70px | badly undersampled |
| 2.5″ | 1.16px | undersampled |
| 3.5″ | 1.62px | practical |
- Badly undersampled: the pixel is wider than the seeing disc, so stars land as blocky squares.
- Undersampled; the standard, deliberate wide-field trade.
- Practical band for typical seeing.
Framing
The 13 catalogue targets against this field, by the fraction of the frame each fills along its tighter axis.
M31 Andromeda Galaxy fills 165 % of the frame and M27 Dumbbell Nebula 6 %. Of the 13, 3 fill the frame, 2 are a tight crop, 2 need a mosaic, 6 are small in the frame.
| Target | Size | Frame filled |
|---|---|---|
| M31 Andromeda Galaxy | 178′ × 70′ | 165%needs a mosaic |
| M45 Pleiades | 150′ × 150′ | 139%tight crop |
| NGC 7000 North America Nebula | 120′ × 30′ | 111%tight crop |
| NGC 1499 California Nebula | 160′ × 40′ | 148%needs a mosaic |
| IC 1805 Heart Nebula | 60′ × 60′ | 56%fills the frame |
| M42 Orion Nebula | 90′ × 60′ | 83%fills the frame |
| NGC 2237 Rosette Nebula | 80′ × 50′ | 74%fills the frame |
| M101 Pinwheel Galaxy | 24′ × 23′ | 22%small in the frame |
| M27 Dumbbell Nebula | 6.7′ × 6.7′ | 6%small in the frame |
| M51 Whirlpool Galaxy | 13.7′ × 11.7′ | 13%small in the frame |
| M81 Bode's Galaxy | 22′ × 11.3′ | 20%small in the frame |
| M13 Hercules Globular Cluster | 16.5′ × 16.5′ | 15%small in the frame |
| NGC 4565 Needle Galaxy | 16.8′ × 2.9′ | 16%small in the frame |
Target sizes: OpenNGC by Mattia Verga (tag v20260501), CC BY-SA 4.0. This table is shared under the same license. Source and license
Spacing
The corrector rows this telescope links in the Backfocus Solver's catalogue, with the face each figure is measured from.
This telescope needs a corrector. The maker: “Optional Flattener: FLAT 61A 1.0x | FLAT 61R 0.8x” Source
William Optics FLAT 61A (1.0x)
- Backfocus
- 67.7mm
- Measured from
- rear thread surface (WO: "All William Optics back focus measurements are referenced from the rear thread surface.")
- Focal length
- 1.0x, 360 mm
- Image circle
- 43 mm; this sensor is 15.99 mm
Mounts
The catalogue's equatorial mounts that carry this train within 60 % of their rated payload, the imaging derate. The sum is the Rig Reality Check's.
On the saddle this train weighs 5.13 kg, 2.25 kg of it assumed. 23 of the 29 equatorial mounts in the catalogue carry it within the derate; the smallest rated is the Sky-Watcher Wave 100i, at 51 %. 6 more would run it over 60 %.
| Mount | Rated | Used | Band |
|---|---|---|---|
| Sky-Watcher Wave 100i | 10kg | 51% | Workable; more sensitive to wind, little room to grow |
| iOptron CEM26 (AccuAlign, 1.5" tripod) | 12kg | 43% | Comfortable |
| iOptron GEM28 (AccuAlign, 1.5" tripod) | 12.7kg | 40% | Comfortable |
| iOptron HEM27 | 13.5kg | 38% | Headroom for a future upgrade |
| Rainbow Astro RST-135E | 13.5kg | 38% | Headroom for a future upgrade |
| WarpAstron WD-17 | 13.5kg | 38% | Headroom for a future upgrade |
The other 17 mounts that carry it
| Mount | Rated | Used | Band |
|---|---|---|---|
| Celestron Advanced VX (AVX) | 13.6kg | 38% | Headroom for a future upgrade |
| Pegasus Astro NYX-88 | 14kg | 37% | Headroom for a future upgrade |
| Sky-Watcher HEQ5-R Pro | 15kg | 34% | Headroom for a future upgrade |
| Sky-Watcher Wave 150i | 15kg | 34% | Headroom for a future upgrade |
| ZWO AM5N | 15kg | 34% | Headroom for a future upgrade |
| Celestron CGEM II | 18kg | 29% | Headroom for a future upgrade |
| iOptron CEM40 (with tripod) | 18kg | 29% | Headroom for a future upgrade |
| iOptron HEM44 | 20kg | 26% | Headroom for a future upgrade |
| Pegasus Astro NYX-101 | 20kg | 26% | Headroom for a future upgrade |
| Sky-Watcher EQ6-R Pro | 20kg | 26% | Headroom for a future upgrade |
| WarpAstron WD-20 | 20kg | 26% | Headroom for a future upgrade |
| ZWO AM7 | 20kg | 26% | Headroom for a future upgrade |
| Losmandy GM811G (LW tripod) | 22.7kg | 23% | Headroom for a future upgrade |
| Celestron CGX | 25kg | 21% | Headroom for a future upgrade |
| iOptron CEM70 (iPolar) | 31.8kg | 16% | Headroom for a future upgrade |
| Sky-Watcher CQ350 Pro (with tripod) | 35kg | 15% | Headroom for a future upgrade |
| Sky-Watcher EQ8-R (head with counterweights) | 50kg | 10% | Headroom for a future upgrade |
What the 5.13 kg is made of
| Component | Mass | Basis |
|---|---|---|
| OTA: William Optics Zenithstar 61 II | 2.20kg | maker’s figure |
| Rings and dovetail | 0.90kg | assumption |
| Corrector: William Optics FLAT 61A (1.0x) | 0.27kg | maker’s figure |
| Camera: ZWO ASI533MM Pro | 0.41kg | maker’s figure |
| Filter drawer and filter | 0.20kg | assumption |
| Guide scope and guide camera | 0.80kg | assumption |
| Cables, dew strap, focuser motor | 0.35kg | assumption |
| Total | 5.13kg |
Assumptions are the Rig Reality Check’s round placeholders, not measurements; replace them with your own figures there.
Run the Rig Reality Check with the ZWO ASI533MM Pro as yours, then give it your budget, sky and targets.
Sub length
The shortest sub at which read noise adds no more than 5 % to the stack's noise, from the Exposure Planner. Each figure opens the planner with the same inputs.
At gain 100 (high conversion gain on, 1.5 e⁻ read noise), 61 mm at 360 mm. Under a Bortle 6 sky it is 8.1 s with no filter and 146 s through a 7 nm dual-band filter.
| Sky | No filter | 7 nm dual-band |
|---|---|---|
| Bortle 421.00 mag/arcsec² | 51 s46 s to 56 s | 15 min14 min to 17 min |
| Bortle 619.00 mag/arcsec² | 8.1 s7.3 s to 8.8 sbelow the 30 s floor | 146 s133 s to 160 s |
| Bortle 817.90 mag/arcsec² | 2.9 s2.7 s to 3.2 sbelow the 30 s floor | 53 s48 s to 58 s |
- The read noise is read off the maker’s chart; the small figures give the range its reading uncertainty allows.
- Under 30 s, download and dither time dominate, so the planner raises shorter results to 30 s. The dual-band column uses the reference’s approximation: Dual narrowband 7 nm (Ha + OIII).
Prices
Dated readings from the maker's store or a major US retailer, with the page each came from. A first-year total needs a mount and the essentials, which the Rig Reality Check adds.
| Part | Price | Read |
|---|---|---|
| Telescope: William Optics Zenithstar 61 IIProvisional | $498 | manufacturer, 2026-09-27OTA (Red/Gold/Space Grey); body says it includes Vixen dovetail, handle bar, Bahtinov Mask cap and soft case |
| Camera: ZWO ASI533MM ProProvisional | $999 | manufacturer, 2026-09-27camera only (variant ASI533MM-P); the page's 'Sold out' applies to the filter-kit bundles |
| $999 | retailer, 2026-09-27 | |
| Corrector: William Optics FLAT 61A (1.0x)Provisional | $229 | manufacturer, 2026-09-27 |
| $229 | retailer, 2026-09-27 |
Provisional rows have not yet had a second, human check against their source. The Rig Reality Check prices whole builds, essentials included.
How this is computed
scale["/px] = 206.265 × pixel[µm] / focal length[mm]
field[°] = 2 × atan(sensor side[mm] / (2 × focal length[mm])) × 180/π
sampling = seeing FWHM["] / scale["/px]
frame filled = max(major axis / field width, minor axis / field height)
utilization = Σ imaging mass[kg] / rated payload[kg]
shortest sub = R² / (e_sky × ((1 + p)² − 1)), p = 5 %Focal length is the telescope’s, times a corrector’s factor only where its maker states one. The image scale and field use the exact forms; the worked example shows the arithmetic. The sampling bands are the physics reference’s presentation choice, and every band is printed so the tradeoff can be read in full.
The payload sums every part on the saddle, the way the Rig Reality Check does, and the bands encode the 50 to 60 % imaging derate, which is forum practice rather than a maker’s figure. Spacing figures are the Backfocus Solver’s rows; sub lengths are the Exposure Planner’s.
Source: docs/03-physics-reference.md §§ 1, 3 and 4. Implemented in packages/physics (optics.ts, payload.ts, exposure.ts, scoring.ts), each with the reference's worked examples as unit tests.
Last updated Computed from the catalogue when the site is builtEvery camera on the William Optics Zenithstar 61 II