Sky-Watcher Esprit 120EDX with ZWO ASI585MM Air
0.71″ per pixel across a 0.76° × 0.43° field, at 840 mm and f/7.0. 5 of the catalogue's 29 equatorial mounts carry the 12.77 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 840 mm, with the Sky-Watcher Esprit 120EDX Flattener (S20218).
Its maker publishes no factor for the Sky-Watcher Esprit 120EDX Flattener (S20218), so these figures are at the native focal length.
Image circle: 44 mm, the telescope's stated figure (Sky-Watcher Esprit 120EDX). 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 3.51 pixels on this sensor. Oversampled: lower per-pixel signal and tighter guiding; binning recovers most of it. Binned 2 × 2 it spans 1.76.
| Seeing | FWHM | Band |
|---|---|---|
| 1.5″ | 2.11px | practical |
| 2.5″ | 3.51px | oversampled |
| 3.5″ | 4.92px | heavily oversampled |
- Practical band for typical seeing.
- Oversampled: lower per-pixel signal and tighter guiding; binning recovers most of it.
- Heavily oversampled: the atmosphere limits resolution long before the pixel does.
Framing
The 13 catalogue targets against this field, by the fraction of the frame each fills along its tighter axis.
M45 Pleiades fills 582 % of the frame and M27 Dumbbell Nebula 26 %. Of the 13, 6 fill the frame, 7 need a mosaic.
| Target | Size | Frame filled |
|---|---|---|
| M31 Andromeda Galaxy | 178′ × 70′ | 388%needs a mosaic |
| M45 Pleiades | 150′ × 150′ | 582%needs a mosaic |
| NGC 7000 North America Nebula | 120′ × 30′ | 262%needs a mosaic |
| NGC 1499 California Nebula | 160′ × 40′ | 349%needs a mosaic |
| IC 1805 Heart Nebula | 60′ × 60′ | 233%needs a mosaic |
| M42 Orion Nebula | 90′ × 60′ | 233%needs a mosaic |
| NGC 2237 Rosette Nebula | 80′ × 50′ | 194%needs a mosaic |
| M101 Pinwheel Galaxy | 24′ × 23′ | 89%fills the frame |
| M27 Dumbbell Nebula | 6.7′ × 6.7′ | 26%fills the frame |
| M51 Whirlpool Galaxy | 13.7′ × 11.7′ | 45%fills the frame |
| M81 Bode's Galaxy | 22′ × 11.3′ | 47%fills the frame |
| M13 Hercules Globular Cluster | 16.5′ × 16.5′ | 64%fills the frame |
| NGC 4565 Needle Galaxy | 16.8′ × 2.9′ | 37%fills 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: “The field flattener needs to be purchased separately by the user.” Source
Sky-Watcher Esprit 120EDX Flattener (S20218)
- Backfocus
- 72mm ±1 mm stated
- Measured from
- edge marking the start of the male M62x1 thread on the flattener's exit (Esprit 120EDX manual)
- Focal length
- factor not published
- Image circle
- 44 mm; this sensor is 12.85 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 12.77 kg, 1.45 kg of it assumed. 5 of the 29 equatorial mounts in the catalogue carry it within the derate; the smallest rated is the Losmandy GM811G (LW tripod), at 56 %. 24 more would run it over 60 %.
| Mount | Rated | Used | Band |
|---|---|---|---|
| Losmandy GM811G (LW tripod) | 22.7kg | 56% | Workable; more sensitive to wind, little room to grow |
| Celestron CGX | 25kg | 51% | Workable; more sensitive to wind, little room to grow |
| iOptron CEM70 (iPolar) | 31.8kg | 40% | Comfortable |
| Sky-Watcher CQ350 Pro (with tripod) | 35kg | 36% | Headroom for a future upgrade |
| Sky-Watcher EQ8-R (head with counterweights) | 50kg | 26% | Headroom for a future upgrade |
What the 12.77 kg is made of
| Component | Mass | Basis |
|---|---|---|
| OTA: Sky-Watcher Esprit 120EDX | 10.70kg | maker’s figure |
| Rings and dovetail | 0.90kg | assumption |
| Camera: ZWO ASI585MM Air | 0.62kg | 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 | 12.77kg |
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 ASI585MM 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: Sky-Watcher Esprit 120EDXProvisional | $3,295 | manufacturer, 2026-09-27OTA with rings, D dovetail, camera adapter, hard case. Flattener NOT included. |
| Camera: ZWO ASI585MM AirProvisional | $1,299 | manufacturer, 2026-10-10variant ASI585MM Air (SKU ZWO ASI585MM-Air) |
| Corrector: Sky-Watcher Esprit 120EDX Flattener (S20218)Provisional | $320 | manufacturer, 2026-09-27S20218 |
| $320 | retailer, 2026-09-27SKU SKY-S20218 |
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 Sky-Watcher Esprit 120EDX