William Optics Fluorostar 120 with ZWO ASI2600MC Air
0.99″ per pixel across a 1.73° × 1.15° field, at 780 mm and f/6.5. 12 of the catalogue's 29 equatorial mounts carry the 10.04 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 780 mm, with the William Optics FLAT 68III (1.0x). The other configuration is in the table below.
| Configuration | Focal length | Ratio | Scale | Field |
|---|---|---|---|---|
| William Optics FLAT 68III (1.0x) | 780mm | f/6.5 | 0.99″/px | 1.73° × 1.15° |
| William Optics FLAT 7A (0.8x), with the 45 mm Back Extender | 624mm | f/5.2 | 1.24″/px | 2.16° × 1.44° |
Image circle: 43 mm, the telescope's stated figure (William Optics Fluorostar 120). 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 2.51 pixels on this sensor. Practical band for typical seeing.
| Seeing | 1.0x · 780 mm | 0.8x · 624 mm |
|---|---|---|
| 1.5″ | 1.51pxpractical | 1.21pxundersampled |
| 2.5″ | 2.51pxpractical | 2.01pxpractical |
| 3.5″ | 3.52pxoversampled | 2.82pxpractical |
- Practical band for typical seeing.
- Oversampled: lower per-pixel signal and tighter guiding; binning recovers most of it.
- Undersampled; the standard, deliberate wide-field trade.
Framing
The 13 catalogue targets against this field, by the fraction of the frame each fills along its tighter axis.
M45 Pleiades fills 217 % of the frame and M27 Dumbbell Nebula 10 %. Of the 13, 4 fill the frame, 1 is a tight crop, 3 need a mosaic, 5 are small in the frame.
| Target | Size | 1.0x · 780 mm | 0.8x · 624 mm |
|---|---|---|---|
| M31 Andromeda Galaxy | 178′ × 70′ | 172%needs a mosaic | 137%tight crop |
| M45 Pleiades | 150′ × 150′ | 217%needs a mosaic | 173%needs a mosaic |
| NGC 7000 North America Nebula | 120′ × 30′ | 116%tight crop | 93%tight crop |
| NGC 1499 California Nebula | 160′ × 40′ | 154%needs a mosaic | 124%tight crop |
| IC 1805 Heart Nebula | 60′ × 60′ | 87%fills the frame | 69%fills the frame |
| M42 Orion Nebula | 90′ × 60′ | 87%fills the frame | 70%fills the frame |
| NGC 2237 Rosette Nebula | 80′ × 50′ | 77%fills the frame | 62%fills the frame |
| M101 Pinwheel Galaxy | 24′ × 23′ | 33%fills the frame | 27%fills the frame |
| M27 Dumbbell Nebula | 6.7′ × 6.7′ | 10%small in the frame | 8%small in the frame |
| M51 Whirlpool Galaxy | 13.7′ × 11.7′ | 17%small in the frame | 13%small in the frame |
| M81 Bode's Galaxy | 22′ × 11.3′ | 21%small in the frame | 17%small in the frame |
| M13 Hercules Globular Cluster | 16.5′ × 16.5′ | 24%small in the frame | 19%small in the frame |
| NGC 4565 Needle Galaxy | 16.8′ × 2.9′ | 16%small in the frame | 13%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 68III 1.0x | FLAT 7A 0.8x (requires adapter)” Source
William Optics FLAT 68III (1.0x)
- Backfocus
- 65.7mm
- Measured from
- rear thread surface (WO: "All William Optics back focus measurements are referenced from the rear thread surface.")
- Focal length
- 1.0x, 780 mm
- Image circle
- 44.4 mm; this sensor is 28.26 mm
William Optics FLAT 7A (0.8x), with the 45 mm Back Extender
- Backfocus
- 56.4mm
- Measured from
- rear thread surface, with the 45 mm Flattener Back Extender fitted (WO datum statement)
- Focal length
- 0.8x, 624 mm
- Image circle
- 44.4 mm; this sensor is 28.26 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 10.04 kg, 1.45 kg of it assumed. 12 of the 29 equatorial mounts in the catalogue carry it within the derate; the smallest rated is the Celestron CGEM II, at 56 %. 17 more would run it over 60 %.
| Mount | Rated | Used | Band |
|---|---|---|---|
| Celestron CGEM II | 18kg | 56% | Workable; more sensitive to wind, little room to grow |
| iOptron CEM40 (with tripod) | 18kg | 56% | Workable; more sensitive to wind, little room to grow |
| iOptron HEM44 | 20kg | 50% | Workable; more sensitive to wind, little room to grow |
| Pegasus Astro NYX-101 | 20kg | 50% | Workable; more sensitive to wind, little room to grow |
| Sky-Watcher EQ6-R Pro | 20kg | 50% | Workable; more sensitive to wind, little room to grow |
| WarpAstron WD-20 | 20kg | 50% | Workable; more sensitive to wind, little room to grow |
The other 6 mounts that carry it
| Mount | Rated | Used | Band |
|---|---|---|---|
| ZWO AM7 | 20kg | 50% | Workable; more sensitive to wind, little room to grow |
| Losmandy GM811G (LW tripod) | 22.7kg | 44% | Comfortable |
| Celestron CGX | 25kg | 40% | Comfortable |
| iOptron CEM70 (iPolar) | 31.8kg | 32% | Headroom for a future upgrade |
| Sky-Watcher CQ350 Pro (with tripod) | 35kg | 29% | Headroom for a future upgrade |
| Sky-Watcher EQ8-R (head with counterweights) | 50kg | 20% | Headroom for a future upgrade |
What the 10.04 kg is made of
| Component | Mass | Basis |
|---|---|---|
| OTA: William Optics Fluorostar 120 | 7.10kg | maker’s figure |
| Rings and dovetail | 0.90kg | assumption |
| Corrector: William Optics FLAT 68III (1.0x) | 0.72kg | maker’s figure |
| Camera: ZWO ASI2600MC Air | 0.78kg | maker’s figure |
| Filter drawer and filter | 0.20kg | assumption |
| Guiding: built into the camera | 0.00kg | maker’s figure |
| Cables, dew strap, focuser motor | 0.35kg | assumption |
| Total | 10.04kg |
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 ASI2600MC Air 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.
Not computedThis camera has no sensor profile (read noise against gain) in the catalogue.
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 Fluorostar 120Provisional | $3,298 | manufacturer, 2026-09-27OTA (Red/Gold/Space Grey); body mentions saddle handle and Losmandy-type plate |
| Camera: ZWO ASI2600MC AirProvisional | $1,999 | manufacturer, 2026-10-10variant ASI2600MC Air (SKU ZWO ASI2600MC-Air) |
| Corrector: William Optics FLAT 68III (1.0x)Provisional | $656 | manufacturer, 2026-09-27WO store, 'Tax excluded.' |
| Corrector: William Optics FLAT 7A (0.8x), with the 45 mm Back ExtenderProvisional | $691 | manufacturer, 2026-09-27WO store, 'Tax excluded.'; the 45 mm Back Extender ($49) the FLT120 needs is NOT included |
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 Fluorostar 120