Pier height
NexDome's formula, from their own site:
P = A − B
A= 53″, the NexDome wall heightB= OTA center to bottom of mount base, tube horizontal, mount at site latitude
B must be measured on the assembled rig. It includes tube radius, so it changes
with every scope.
Source NexDome — How to calculate telescope pier height — https://www.nexdome.com/single-post/2018/08/21/how-to-calculate-telescope-pier-height
Full stack to the site reference:
concrete_top_above_floor = target_OTA_center − B − adapter_height
concrete_top_above_joists = concrete_top_above_floor + decking_thickness
Worked for this build: 47 − 12.5 − 4.875 = 29.625″ above floor,
+ 1.0 = 30.625″ above the joist tops.
Horizon lost to the dome
Dome inner radius 43.3″. Dropping the OTA center below the wall top costs low-altitude sky:
blocked_below_deg = atan(drop_below_wall_top / 43.3)
Roughly 1.3° per inch. At 6″ low the dome blocks below 7.9°.
Only worth paying for if your actual horizon is open — measure your treeline before optimizing against it.
Deck squaring
diagonal = sqrt(x² + y²)
120″ × 120″ → 169.71″ (169-11/16″).
Scaled 3-4-5: use 72″ / 96″ / 120″ rather than 36/48/60 — same triangle, roughly double the accuracy over a 10′ run.
Wall ring geometry
radius = circumference / 6.2832
chord(n_posts, k_apart) = 2 × radius × sin(k × π / n_posts)
For 7 posts at radius 44.32″:
- adjacent (k=1): 38.5″
- skip-one (k=2): 69.25″
Both sets must be internally consistent. Adjacent chords alone do not prove a circle — a polygon can have equal sides and still be squashed.
Deck block loading
total_load = (live_psf + dead_psf) × area_sqft + dome_weight
line_load_plf = tributary_width_ft × (live_psf + dead_psf)
worst_reaction = continuous_beam_coefficient × line_load × span
soil_pressure = reaction / block_face_area
For 100 sq ft at 40+10 psf plus 250 lb dome = 5,250 lb total. Eight blocks → 903 lb worst case, 50% of the 1,800 lb rating, 524 psf on soil.
Source CAMO Block polyolefin deck block, product listing — https://www.homedepot.com/p/CAMO-2-2-lb-Deck-Block-for-Joist-and-Post-Foundation-0420001/319396055
Concrete volume
cu_ft = π × (diameter/2)² × height / 1728 [inches]
bags_60lb = cu_ft / 0.45
12″ form to this height → ~5.4 cu ft → 12 bags.
Pier bending stiffness
I = π × d⁴ / 64
Diameter⁴ is why going 10″ → 12″ doubles stiffness (12⁴/10⁴ = 2.07) for two extra bags of concrete.
The same exponent is why four ½″ rods are not a substitute for a short gap: a ½″-13 rod's root diameter is 0.406″, giving I = 0.0013 in⁴.
Adapter compliance
Modeled as a short frame with the scope mass at height h above the mount face:
k_shear = 12 × E × (n × I_column) / gap³
k_rock = E × (A_rod × spacing²) / gap
deflection = F/k_shear + (F×h/k_rock)×h
Compliance goes as gap³, which is why shortening the gap or stiffening the columns dominates everything else.
Bay fit
Sweep the bay footprint through every angle and take the minimum slack to any deck edge:
for θ in 0..90:
center = dome_center + (ring_radius + bay_depth/2) × (cos θ, sin θ)
corners = rotate(bay_rect, θ) + center
slack(θ) = min(distance from any corner to any deck edge)
Negative slack means it overhangs. On a square deck the result is one workable angle; on a rectangle it is a range.