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Rigs William Optics Fluorostar 120

William Optics Fluorostar 120 with ZWO ASI183MM Pro

0.63″ per pixel across a 0.97° × 0.65° field, at 780 mm and f/6.5. 12 of the catalogue's 29 equatorial mounts carry the 10.48 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.

Focal length780mmnative; the corrector is 1.0x
Focal ratiof/6.5120 mm aperture
Image scale0.63″/px2.4 µm pixels
Field of view0.97 × 0.65°58′ × 39′
Sensor13.2 × 8.8mm15.86 mm diagonal; 43 mm circle stated
Each configuration of the William Optics Fluorostar 120 with the ZWO ASI183MM Pro
ConfigurationFocal lengthRatioScaleField
William Optics FLAT 68III (1.0x)780mmf/6.50.63″/px0.97° × 0.65°
William Optics FLAT 7A (0.8x), with the 45 mm Back Extender624mmf/5.20.79″/px1.21° × 0.81°

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 3.94 pixels on this sensor. Oversampled: lower per-pixel signal and tighter guiding; binning recovers most of it. Binned 2 × 2 it spans 1.97.

Star FWHM in pixels at three seeing conditions, for each configuration
Seeing1.0x · 780 mm0.8x · 624 mm
1.5″2.36pxpractical1.89pxpractical
2.5″3.94pxoversampled3.15pxpractical
3.5″5.51pxheavily oversampled4.41pxoversampled
  • 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 387 % of the frame and M27 Dumbbell Nebula 17 %. Of the 13, 5 fill the frame, 1 is a tight crop, 6 need a mosaic, 1 is small in the frame.

0.97° × 0.65°306 % · needs a mosaic
Target drawn at true angular size against this rig’s sensor, at native focal length. The glow and stars are synthetic, not survey imagery; the size is OpenNGC’s.

Open in the framing tool, at any camera angle

The 13 targets against the William Optics Fluorostar 120 with the ZWO ASI183MM Pro: size, frame filled and fit
TargetSize1.0x · 780 mm0.8x · 624 mm
M31 Andromeda Galaxy178′ × 70′306%needs a mosaic245%needs a mosaic
M45 Pleiades150′ × 150′387%needs a mosaic309%needs a mosaic
NGC 7000 North America Nebula120′ × 30′206%needs a mosaic165%needs a mosaic
NGC 1499 California Nebula160′ × 40′275%needs a mosaic220%needs a mosaic
IC 1805 Heart Nebula60′ × 60′155%needs a mosaic124%tight crop
M42 Orion Nebula90′ × 60′155%needs a mosaic124%tight crop
NGC 2237 Rosette Nebula80′ × 50′138%tight crop110%tight crop
M101 Pinwheel Galaxy24′ × 23′59%fills the frame48%fills the frame
M27 Dumbbell Nebula6.7′ × 6.7′17%small in the frame14%small in the frame
M51 Whirlpool Galaxy13.7′ × 11.7′30%fills the frame24%small in the frame
M81 Bode's Galaxy22′ × 11.3′37%fills the frame30%fills the frame
M13 Hercules Globular Cluster16.5′ × 16.5′43%fills the frame34%fills the frame
NGC 4565 Needle Galaxy16.8′ × 2.9′29%fills the frame23%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 15.86 mm

    Solve the spacer stack with this cameraSource

  • 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 15.86 mm

    Solve the spacer stack with this cameraSource

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 10.48 kg, 2.25 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 58 %. 17 more would run it over 60 %.

Mounts that carry the William Optics Fluorostar 120 with the ZWO ASI183MM Pro, smallest rated payload first
MountRatedUsedBand
Celestron CGEM II18kg58%Workable; more sensitive to wind, little room to grow
iOptron CEM40 (with tripod)18kg58%Workable; more sensitive to wind, little room to grow
iOptron HEM4420kg52%Workable; more sensitive to wind, little room to grow
Pegasus Astro NYX-10120kg52%Workable; more sensitive to wind, little room to grow
Sky-Watcher EQ6-R Pro20kg52%Workable; more sensitive to wind, little room to grow
WarpAstron WD-2020kg52%Workable; more sensitive to wind, little room to grow
The other 6 mounts that carry it
The remaining mounts that carry this train, by rated payload
MountRatedUsedBand
ZWO AM720kg52%Workable; more sensitive to wind, little room to grow
Losmandy GM811G (LW tripod)22.7kg46%Comfortable
Celestron CGX25kg42%Comfortable
iOptron CEM70 (iPolar)31.8kg33%Headroom for a future upgrade
Sky-Watcher CQ350 Pro (with tripod)35kg30%Headroom for a future upgrade
Sky-Watcher EQ8-R (head with counterweights)50kg21%Headroom for a future upgrade
What the 10.48 kg is made of
The imaging payload, component by component
ComponentMassBasis
OTA: William Optics Fluorostar 1207.10kgmaker’s figure
Rings and dovetail0.90kgassumption
Corrector: William Optics FLAT 68III (1.0x)0.72kgmaker’s figure
Camera: ZWO ASI183MM Pro0.41kgmaker’s figure
Filter drawer and filter0.20kgassumption
Guide scope and guide camera0.80kgassumption
Cables, dew strap, focuser motor0.35kgassumption
Total10.48kg

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 ASI183MM 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 computedThe planner is missing the sensor's quantum efficiency for this camera.

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.

Dated prices for the William Optics Fluorostar 120 and the ZWO ASI183MM Pro
PartPriceRead
Telescope: William Optics Fluorostar 120Provisional$3,298manufacturer, 2026-09-27OTA (Red/Gold/Space Grey); body mentions saddle handle and Losmandy-type plate
Camera: ZWO ASI183MM ProProvisional$999manufacturer, 2026-09-27variant ASI183MM-P
$999retailer, 2026-09-27SKU ZWO-ASI183MM-P
Corrector: William Optics FLAT 68III (1.0x)Provisional$656manufacturer, 2026-09-27WO store, 'Tax excluded.'
Corrector: William Optics FLAT 7A (0.8x), with the 45 mm Back ExtenderProvisional$691manufacturer, 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