ZWO FF131 APO with ZWO ASI2600MC Pro
0.79″ per pixel across a 1.37° × 0.92° field, at 983 mm and f/7.5. 10 of the catalogue's 29 equatorial mounts carry the 10.85 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 the native focal length.
Image circle: 44 mm, the telescope's stated figure (ZWO FF131 APO). 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 3.17 pixels on this sensor. Practical band for typical seeing.
| Seeing | FWHM | Band |
|---|---|---|
| 1.5″ | 1.90px | practical |
| 2.5″ | 3.17px | practical |
| 3.5″ | 4.44px | oversampled |
- Practical band for typical seeing.
- Oversampled: lower per-pixel signal and tighter guiding; binning recovers most of it.
Framing
The 13 catalogue targets against this field, by the fraction of the frame each fills along its tighter axis.
M45 Pleiades fills 273 % of the frame and M27 Dumbbell Nebula 12 %. Of the 13, 3 fill the frame, 3 are a tight crop, 4 need a mosaic, 3 are small in the frame.
| Target | Size | Frame filled |
|---|---|---|
| M31 Andromeda Galaxy | 178′ × 70′ | 216%needs a mosaic |
| M45 Pleiades | 150′ × 150′ | 273%needs a mosaic |
| NGC 7000 North America Nebula | 120′ × 30′ | 146%needs a mosaic |
| NGC 1499 California Nebula | 160′ × 40′ | 195%needs a mosaic |
| IC 1805 Heart Nebula | 60′ × 60′ | 109%tight crop |
| M42 Orion Nebula | 90′ × 60′ | 110%tight crop |
| NGC 2237 Rosette Nebula | 80′ × 50′ | 97%tight crop |
| M101 Pinwheel Galaxy | 24′ × 23′ | 42%fills the frame |
| M27 Dumbbell Nebula | 6.7′ × 6.7′ | 12%small in the frame |
| M51 Whirlpool Galaxy | 13.7′ × 11.7′ | 21%small in the frame |
| M81 Bode's Galaxy | 22′ × 11.3′ | 26%fills the frame |
| M13 Hercules Globular Cluster | 16.5′ × 16.5′ | 30%fills the frame |
| NGC 4565 Needle Galaxy | 16.8′ × 2.9′ | 20%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.
No separate corrector is needed. The maker: “The FF131 APO features a native flat-field optical design, eliminating the need for an additional field flattener.” Source
ZWO FF131 APO (built-in flattener)
- Connection range
- 42 to 73mm
- Measured from
- base of the M48x0.75 male thread on the scope's M54-M48 adapter (connection range)
- Focal length
- the telescope's own
- Image circle
- 44 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 10.85 kg, 2.25 kg of it assumed. 10 of the 29 equatorial mounts in the catalogue carry it within the derate; the smallest rated is the iOptron HEM44, at 54 %. 19 more would run it over 60 %.
| Mount | Rated | Used | Band |
|---|---|---|---|
| iOptron HEM44 | 20kg | 54% | Workable; more sensitive to wind, little room to grow |
| Pegasus Astro NYX-101 | 20kg | 54% | Workable; more sensitive to wind, little room to grow |
| Sky-Watcher EQ6-R Pro | 20kg | 54% | Workable; more sensitive to wind, little room to grow |
| WarpAstron WD-20 | 20kg | 54% | Workable; more sensitive to wind, little room to grow |
| ZWO AM7 | 20kg | 54% | Workable; more sensitive to wind, little room to grow |
| Losmandy GM811G (LW tripod) | 22.7kg | 48% | Comfortable |
The other 4 mounts that carry it
| Mount | Rated | Used | Band |
|---|---|---|---|
| Celestron CGX | 25kg | 43% | Comfortable |
| iOptron CEM70 (iPolar) | 31.8kg | 34% | Headroom for a future upgrade |
| Sky-Watcher CQ350 Pro (with tripod) | 35kg | 31% | Headroom for a future upgrade |
| Sky-Watcher EQ8-R (head with counterweights) | 50kg | 22% | Headroom for a future upgrade |
What the 10.85 kg is made of
| Component | Mass | Basis |
|---|---|---|
| OTA: ZWO FF131 APO | 7.90kg | maker’s figure |
| Rings and dovetail | 0.90kg | assumption |
| Camera: ZWO ASI2600MC Pro | 0.70kg | 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 | 10.85kg |
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 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, 1.4 e⁻ read noise), 131 mm at 983 mm. Under a Bortle 6 sky it is 13 s with no filter and 235 s through a 7 nm dual-band filter.
| Sky | No filter | 7 nm dual-band |
|---|---|---|
| Bortle 421.00 mag/arcsec² | 81 s70 s to 93 s | 25 min21 min to 28 min |
| Bortle 619.00 mag/arcsec² | 13 s11 s to 15 sbelow the 30 s floor | 235 s202 s to 269 s |
| Bortle 817.90 mag/arcsec² | 4.7 s4.0 s to 5.4 sbelow the 30 s floor | 85 s73 s to 98 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: ZWO FF131 APOProvisional | $1,999 | manufacturer, 2026-09-27OTA-only variant ("FF131 APO"); in stock |
| Camera: ZWO ASI2600MC ProProvisional | $1,499 | manufacturer, 2026-09-27variant ASI2600MC-P25 (2025 version) |
| $1,499 | retailer, 2026-09-27ASI2600MC-P25 (2025 version) |
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 ZWO FF131 APO