Sharpstar 15028HNT-AL with ZWO ASI991MM Pro
2.46″ per pixel across a 0.45° × 0.35° field, at 420 mm and f/2.8. 21 of the catalogue's 29 equatorial mounts carry the 7.55 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 (Sharpstar 15028HNT-AL). 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 1.02 pixels on this sensor. Undersampled; the standard, deliberate wide-field trade.
| Seeing | FWHM | Band |
|---|---|---|
| 1.5″ | 0.61px | badly undersampled |
| 2.5″ | 1.02px | undersampled |
| 3.5″ | 1.43px | 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.
M45 Pleiades fills 705 % of the frame and M27 Dumbbell Nebula 31 %. Of the 13, 5 fill the frame, 1 is a tight crop, 7 need a mosaic.
| Target | Size | Frame filled |
|---|---|---|
| M31 Andromeda Galaxy | 178′ × 70′ | 662%needs a mosaic |
| M45 Pleiades | 150′ × 150′ | 705%needs a mosaic |
| NGC 7000 North America Nebula | 120′ × 30′ | 447%needs a mosaic |
| NGC 1499 California Nebula | 160′ × 40′ | 596%needs a mosaic |
| IC 1805 Heart Nebula | 60′ × 60′ | 282%needs a mosaic |
| M42 Orion Nebula | 90′ × 60′ | 335%needs a mosaic |
| NGC 2237 Rosette Nebula | 80′ × 50′ | 298%needs a mosaic |
| M101 Pinwheel Galaxy | 24′ × 23′ | 108%tight crop |
| M27 Dumbbell Nebula | 6.7′ × 6.7′ | 31%fills the frame |
| M51 Whirlpool Galaxy | 13.7′ × 11.7′ | 55%fills the frame |
| M81 Bode's Galaxy | 22′ × 11.3′ | 81%fills the frame |
| M13 Hercules Globular Cluster | 16.5′ × 16.5′ | 78%fills the frame |
| NGC 4565 Needle Galaxy | 16.8′ × 2.9′ | 62%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.
No separate corrector is needed. The maker: “the focuser has a built-in triplet corrector ... supports a 44mm full-frame image circle” Source
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 7.55 kg, 2.75 kg of it assumed. 21 of the 29 equatorial mounts in the catalogue carry it within the derate; the smallest rated is the iOptron GEM28 (AccuAlign, 1.5" tripod), at 59 %. 8 more would run it over 60 %.
| Mount | Rated | Used | Band |
|---|---|---|---|
| iOptron GEM28 (AccuAlign, 1.5" tripod) | 12.7kg | 59% | Workable; more sensitive to wind, little room to grow |
| iOptron HEM27 | 13.5kg | 56% | Workable; more sensitive to wind, little room to grow |
| Rainbow Astro RST-135E | 13.5kg | 56% | Workable; more sensitive to wind, little room to grow |
| WarpAstron WD-17 | 13.5kg | 56% | Workable; more sensitive to wind, little room to grow |
| Celestron Advanced VX (AVX) | 13.6kg | 56% | Workable; more sensitive to wind, little room to grow |
| Pegasus Astro NYX-88 | 14kg | 54% | Workable; more sensitive to wind, little room to grow |
The other 15 mounts that carry it
| Mount | Rated | Used | Band |
|---|---|---|---|
| Sky-Watcher HEQ5-R Pro | 15kg | 50% | Workable; more sensitive to wind, little room to grow |
| Sky-Watcher Wave 150i | 15kg | 50% | Workable; more sensitive to wind, little room to grow |
| ZWO AM5N | 15kg | 50% | Workable; more sensitive to wind, little room to grow |
| Celestron CGEM II | 18kg | 42% | Comfortable |
| iOptron CEM40 (with tripod) | 18kg | 42% | Comfortable |
| iOptron HEM44 | 20kg | 38% | Headroom for a future upgrade |
| Pegasus Astro NYX-101 | 20kg | 38% | Headroom for a future upgrade |
| Sky-Watcher EQ6-R Pro | 20kg | 38% | Headroom for a future upgrade |
| WarpAstron WD-20 | 20kg | 38% | Headroom for a future upgrade |
| ZWO AM7 | 20kg | 38% | Headroom for a future upgrade |
| Losmandy GM811G (LW tripod) | 22.7kg | 33% | Headroom for a future upgrade |
| Celestron CGX | 25kg | 30% | Headroom for a future upgrade |
| iOptron CEM70 (iPolar) | 31.8kg | 24% | Headroom for a future upgrade |
| Sky-Watcher CQ350 Pro (with tripod) | 35kg | 22% | Headroom for a future upgrade |
| Sky-Watcher EQ8-R (head with counterweights) | 50kg | 15% | Headroom for a future upgrade |
What the 7.55 kg is made of
| Component | Mass | Basis |
|---|---|---|
| OTA: Sharpstar 15028HNT-AL | 4.80kg | maker’s figure |
| Rings and dovetail | 0.90kg | assumption |
| Camera and filter drawer | 0.70kg | assumption |
| Guide scope and guide camera | 0.80kg | assumption |
| Cables, dew strap, focuser motor | 0.35kg | assumption |
| Total | 7.55kg |
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 ASI991MM 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.
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: Sharpstar 15028HNT-ALProvisional | $1,975 | retailer, 2026-09-27OTA, listing named 'Sharpstar 15028HNT Hyperbolic Newtonian Reflector - Aluminum' (i.e. the -AL) |
| Camera: ZWO ASI991MM ProProvisional | no dated price |
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 Sharpstar 15028HNT-AL