THOClabs · THOClabs · Aug 6, 2026 · Aug 2, 2026 · Aug 2, 2026
diff --git a/design/mechanical/MECHANICAL_DESIGN.md b/design/mechanical/MECHANICAL_DESIGN.md
diff --git a/design/mechanical/cad/SECTION.md b/design/mechanical/cad/SECTION.md
@@ -0,0 +1,47 @@
+## Mechanical CAD (Phase D) — parametric geometry
+
+The repository shipped **zero usable CAD**: every `.step` reference in the build package is marked *Pending*. Phase D replaces that void with **text-based, parametric geometry driven entirely by the computed design** in `design/mechanical/calc/params.py`. Nothing is hand-dimensioned that already exists as a parameter — change a number in `params.py` and the models (and the preview) move with it.
+
+### The immediate visual
+
+![NIGHTWATCH GEM meridian elevation](cad/preview_assembly.svg)
+
+`cad/preview_assembly.svg` is a to-scale meridian-plane side elevation of the mount in its park pose — pier, RA housing on the polar axis inclined at the **38.9° site latitude**, DEC head, MN78 OTA pointing at the pole, and the counterweight shaft hanging on the down side. It is produced by a **pure-stdlib** Python script (no OpenSCAD, no third-party libraries) that imports `params.py` and writes the SVG with the real numbers annotated. This is the deliverable you can look at right now; the OpenSCAD files below are the manufacturable source.
+
+### The parts
+
+| File | Part | Headline dimensions (all traced to `params.py`) |
+|---|---|---|
+| `cad/ra_housing.scad` | RA (polar) axis housing | 203.2 × 203.2 × 76.2 mm block (`RA_HOUSING` 8″×8″×3″), 8 mm wall; stepped axial bore — 6008 seat (Ø68/Ø40, `BRG_RA_6008`) opening to the CSF‑32 Ø80 hollow‑drive register (`RA_DRIVE.bore_m`); flange bolt circle ≈ Ø104 |
+| `cad/dec_housing.scad` | DEC axis housing + **Losmandy‑D saddle** | 152.4 × 152.4 × 63.5 mm (`DEC_HOUSING` 6″×6″×2.5″); 6006 seat (Ø55/Ø30, `BRG_DEC_6006`) → CSF‑25 Ø64 register (`DEC_DRIVE.bore_m`); dovetail saddle 76.2 mm / 15° with side clamp |
+| `cad/pier_adapter.scad` | Pier adapter plate | 254 × 254 × 19.05 mm (10″×10″×0.75″) bridging the SOURCED 304.8 × 9.525 mm (12″×0.375″) `PIER` top plate to the RA housing corner pattern |
+| `cad/counterweight_shaft.scad` | Counterweight shaft + weights | Ø31.75 mm × 457.2 mm 303‑SS (`COUNTERWEIGHTS` 1.25″×18″), stud + safety knob, 5+5+2.5 kg discs (12.5 kg available) |
+| `cad/assembly.scad` | Full GEM assembly | Composes all four parts + the MN78 OTA tube (Ø210 × 1400 mm) on the pier, posed at latitude `SITE.latitude_deg` |
+| `cad/svg_preview.py` | Zero‑dep previewer | Imports `params`/`units`, emits `preview_assembly.svg` |
+
+### Parametric approach
+
+Each `.scad` opens with a **params block** whose every entry carries a `// = P.<...>` comment tracing it to `params.py` (SI metres in the calculator → millimetres in OpenSCAD, the CAD convention). Parts are pure `module`s; `assembly.scad` pulls them in with `use <...>` and places them with explicit transforms (`rotate([0, 90-lat, 0])` puts local *+z* on the celestial pole; a nested `rotate([90,0,0])` sets the DEC axis perpendicular to RA). The SVG previewer is the strictest link in the chain — it reads `params.py` **at runtime**, so it can never disagree with the proofs, and a test pins the committed SVG byte‑for‑byte to the renderer output.
+
+### How to render
+
+OpenSCAD is **not installed** in the design sandbox, so each part header documents the command to run locally:
+
+```
+openscad -o ra_housing.stl                      design/mechanical/cad/ra_housing.scad
+openscad -o assembly.stl                        design/mechanical/cad/assembly.scad
+openscad -o assembly.png --imgsize=1600,1200    design/mechanical/cad/assembly.scad
+```
+
+The SVG needs nothing but Python: `python3 design/mechanical/cad/svg_preview.py` regenerates `preview_assembly.svg` (re‑run it whenever `params.py` changes).
+
+### Honest assumptions (tagged ASSUMED/DERIVED in‑file and in `svg_preview`)
+
+- **Pier adapter (10×10×0.75″)** is an *added* part — the repo's `PIER` only specifies a 12×12×0.375″ top plate, so the adapter geometry is **ASSUMED**.
+- **Losmandy‑D dovetail** 76.2 mm / 15° is the **ASSUMED** industry standard; the repo never dimensions the saddle.
+- **Bearing widths** (6008 = 15 mm, 6006 = 13 mm) and **counterweight disc OD/thickness** are **ASSUMED** — `params` gives bore/OD and weight mass only.
+- **CSF flange bolt circle** is taken as **DERIVED** ≈ 1.3 × drive bore (`RA_DRIVE.bore_m` / `DEC_DRIVE.bore_m`), since datasheet PCDs are not in the repo.
+
+### Verification
+
+`design/mechanical/tests/test_cad.py` — **13 tests, all green** (and the full mechanical suite stays at 118 passed): the polar‑axis vector is a unit vector at the site latitude with the perpendiculars orthogonal to it; drawn dimensions equal `params.py` exactly; swapping MN76↔MN78 changes the geometry (proof it is parametric); the head sits above the pier top, the OTA clears the DEC head toward the pole, and the counterweight hangs on the opposite side of the inclined axis; the SVG is valid, dimensioned with the real numbers, and actually writes; and every `.scad` carries its render command and `params.py` traceability.
diff --git a/design/mechanical/cad/assembly.scad b/design/mechanical/cad/assembly.scad
@@ -0,0 +1,83 @@
+// ============================================================================
+// NIGHTWATCH GEM — full mount assembly
+// Composes the parametric parts into a German Equatorial Mount on its pier,
+// posed in the classic "counterweight-down / OTA-to-pole" park.
+// Parametric OpenSCAD model.  Units: MILLIMETRES.
+//
+// Render (OpenSCAD not installed here):
+//     openscad -o assembly.stl                     assembly.scad
+//     openscad -o assembly.png --imgsize=1600,1200 assembly.scad
+//
+// The immediate, dependency-free visual is design/mechanical/cad/preview_assembly.svg
+// (generated by svg_preview.py, which reads the SAME params.py numbers).
+//
+// Dimensions trace to design/mechanical/calc/params.py (SI metres -> mm).
+// ============================================================================
+
+use <ra_housing.scad>
+use <dec_housing.scad>
+use <pier_adapter.scad>
+use <counterweight_shaft.scad>
+
+$fn = 64;
+
+// ---- parameters (traced to params.py) --------------------------------------
+lat        = 38.9;     // P.SITE.latitude_deg  -> polar-axis altitude
+pier_h     = 914.4;    // P.PIER.height_above_m = u.inch(36)
+pier_d     = 304.8;    // P.PIER.diameter_m     = u.inch(12)
+tp_side    = 304.8;    // P.PIER.top_plate_side_m = u.inch(12)
+tp_thk     = 9.525;    // P.PIER.top_plate_thk_m  = u.inch(0.375)
+adapter_thk = 19.05;   // pier_adapter.scad (0.75 in), ASSUMED added part
+
+ra_depth   = 76.2;     // P.RA_HOUSING.depth_m  = u.inch(3)
+dec_face   = 152.4;    // P.DEC_HOUSING.outer_x_m = u.inch(6)
+dec_depth  = 63.5;     // P.DEC_HOUSING.depth_m  = u.inch(2.5)
+
+// Default OTA = MN78 (the params default across the torque/balance/pier proofs).
+ota_od     = 210.0;    // P.MN78.tube_od_m      = u.mm(210)
+ota_len    = 1400.0;   // P.MN78.tube_length_m  = u.mm(1400)
+saddle_stack = 50.0;   // stiffness.SADDLE_LEVER_M (assumed saddle offset)
+
+// Derived placements
+adapter_top = pier_h + tp_thk + adapter_thk;
+L_ra = ra_depth + dec_face/2 + 40;     // DEC-head seat along the polar axis (DERIVED)
+ota_offset = dec_face/2 + saddle_stack + ota_od/2;   // saddle-side radial offset
+
+// ---- assembly --------------------------------------------------------------
+module gem_assembly() {
+    // Concrete pier (exposed height only)
+    color([0.62, 0.62, 0.60]) cylinder(d = pier_d, h = pier_h);
+
+    // Steel top plate (SOURCED, P.PIER)
+    color([0.30, 0.30, 0.32])
+        translate([-tp_side/2, -tp_side/2, pier_h]) cube([tp_side, tp_side, tp_thk]);
+
+    // Pier adapter plate (10x10x0.75 in, ASSUMED added part)
+    color([0.80, 0.67, 0.24])
+        translate([0, 0, pier_h + tp_thk]) pier_adapter();
+
+    // ---- mount head, mounted on the polar (RA) axis ----
+    translate([0, 0, adapter_top])
+        rotate([0, 90 - lat, 0]) {          // local +z -> celestial pole (alt = lat)
+
+            color([0.75, 0.78, 0.80]) ra_housing();
+
+            // DEC head at the far end of the RA axis; DEC axis _|_ RA axis
+            translate([0, 0, L_ra]) {
+
+                rotate([90, 0, 0]) color([0.70, 0.73, 0.76]) dec_housing();
+
+                // Counterweight shaft on the "down" side (classical GEM option;
+                // balance.py recommends deleting it — drawn for completeness).
+                translate([0, dec_face/2, 0])
+                    rotate([-90, 0, 0]) color([0.20, 0.20, 0.22]) counterweight_shaft();
+
+                // OTA tube, parallel to the polar axis, on the saddle side.
+                // Saddle sits ~1/3 up the tube (matches OTA.cg_from_saddle model).
+                translate([0, -ota_offset, -ota_len/3])
+                    color([0.10, 0.10, 0.12]) cylinder(d = ota_od, h = ota_len);
+            }
+        }
+}
+
+gem_assembly();
diff --git a/design/mechanical/cad/counterweight_shaft.scad b/design/mechanical/cad/counterweight_shaft.scad
@@ -0,0 +1,56 @@
+// ============================================================================
+// NIGHTWATCH GEM — counterweight shaft + weights
+// Parametric OpenSCAD model.  Units: MILLIMETRES.
+//
+// Render (OpenSCAD not installed here):
+//     openscad -o counterweight_shaft.stl                     counterweight_shaft.scad
+//     openscad -o counterweight_shaft.png --imgsize=1200,900  counterweight_shaft.scad
+//
+// Dimensions trace to design/mechanical/calc/params.py (SI metres -> mm).
+// NOTE: the counterweight-FREE study (balance.py) recommends DELETING this shaft;
+// it is modelled so the classical GEM option remains fully drawable.
+// ============================================================================
+
+$fn = 96;
+
+// ---- parameters (traced to params.py) --------------------------------------
+shaft_d = 31.75;   // P.COUNTERWEIGHTS.shaft_dia_m = u.inch(1.25) -> 0.03175 m (303-SS)
+shaft_l = 457.2;   // P.COUNTERWEIGHTS.shaft_len_m = u.inch(18.0) -> 0.4572 m
+
+// Weights available = 12.5 kg (2 x 5 kg + 1 x 2.5 kg) = P.COUNTERWEIGHTS.weights_available_kg
+// Cast-iron disc geometry is ASSUMED (params gives mass only, not OD/thickness).
+disc_bore  = 33.0;   // slip fit over the shaft — ASSUMED
+disc5_od   = 127.0;  // 5.0 kg disc OD — ASSUMED
+disc5_t    = 40.0;   // 5.0 kg disc thickness — ASSUMED
+disc25_od  = 102.0;  // 2.5 kg disc OD — ASSUMED
+disc25_t   = 33.0;   // 2.5 kg disc thickness — ASSUMED
+
+// End fittings — ASSUMED.
+stud_d = 20.0; stud_l = 26.0;   // threaded stud into the DEC block
+knob_d = 46.0; knob_h = 16.0;   // safety stop knob at the far end
+
+module _disc(od, t) {
+    difference() {
+        cylinder(d = od, h = t);
+        translate([0, 0, -1]) cylinder(d = disc_bore, h = t + 2);
+        // knurl-substitute relief so the disc reads as a weight, not a plain ring
+        translate([0, 0, t/2]) rotate_extrude() translate([od/2 - 3, 0, 0]) square([3, t], center=true);
+    }
+}
+
+module counterweight_shaft() {
+    // threaded stud (mount end) at z<0
+    translate([0, 0, -stud_l]) cylinder(d = stud_d, h = stud_l);
+    // main 303-SS shaft
+    cylinder(d = shaft_d, h = shaft_l);
+    // safety stop knob at the far (top) end
+    translate([0, 0, shaft_l]) cylinder(d = knob_d, h = knob_h);
+
+    // Stacked weights near the far end (parked position): 5 + 5 + 2.5 kg.
+    z0 = shaft_l - 40;
+    translate([0, 0, z0 - disc5_t])                       _disc(disc5_od,  disc5_t);
+    translate([0, 0, z0 - disc5_t - disc5_t])             _disc(disc5_od,  disc5_t);
+    translate([0, 0, z0 - disc5_t - disc5_t - disc25_t])  _disc(disc25_od, disc25_t);
+}
+
+counterweight_shaft();   // standalone; ignored by `use <>` in assembly
diff --git a/design/mechanical/cad/dec_housing.scad b/design/mechanical/cad/dec_housing.scad
@@ -0,0 +1,90 @@
+// ============================================================================
+// NIGHTWATCH GEM — DEC axis housing + Losmandy-D saddle
+// Parametric OpenSCAD model.  Units: MILLIMETRES.
+//
+// Render (OpenSCAD not installed here):
+//     openscad -o dec_housing.stl                     dec_housing.scad
+//     openscad -o dec_housing.png --imgsize=1200,900  dec_housing.scad
+//
+// Dimensions trace to design/mechanical/calc/params.py (SI metres -> mm).
+// ============================================================================
+
+$fn = 96;
+
+// ---- parameters (traced to params.py) --------------------------------------
+dec_outer_x  = 152.4;  // P.DEC_HOUSING.outer_x_m = u.inch(6)   -> 0.1524 m
+dec_outer_y  = 152.4;  // P.DEC_HOUSING.outer_y_m = u.inch(6)
+dec_depth    = 63.5;   // P.DEC_HOUSING.depth_m   = u.inch(2.5) -> bearing separation
+dec_wall     = 8.0;    // P.DEC_HOUSING.wall_m    = u.mm(8)
+
+brg_od       = 55.0;   // P.BRG_DEC_6006.od_m     = u.mm(55)  (6006 output bearing)
+brg_bore     = 30.0;   // P.BRG_DEC_6006.bore_m   = u.mm(30)
+brg_w        = 13.0;   // 6006 nominal width — ASSUMED (not in params.py)
+
+drive_bore   = 64.0;   // P.DEC_DRIVE.bore_m      = u.mm(64)  (CSF-25-80 hollow bore)
+flange_bc    = 83.0;   // ~ P.DEC_DRIVE.bore_m ; CSF-25 flange PCD (DERIVED ~1.3*bore)
+flange_nbolt = 8;      // ASSUMED
+flange_bolt_d = 4.5;   // M4 clearance — ASSUMED
+
+// Losmandy-D dovetail (industry standard) — ASSUMED (repo does not dimension it).
+losmandy_plate_w = 76.2;   // 3.00 in nominal Losmandy-D plate width — ASSUMED std
+dovetail_angle   = 15.0;   // deg, standard dovetail flank — ASSUMED std
+saddle_height    = 24.0;   // jaw block height — ASSUMED
+saddle_slot_depth = 14.0;  // dovetail engagement depth — ASSUMED
+clamp_bolt_d     = 8.0;    // side clamp knob thread — ASSUMED
+
+// ---- dovetail channel (trapezoid: wide at base, narrow mouth) ---------------
+module _dovetail_channel(chlen, plate_w, ang, depth) {
+    mouth = plate_w - 2 * depth * tan(ang);      // narrower opening at the top
+    translate([0, chlen / 2, 0])
+        rotate([90, 0, 0])                        // extrude +Z -> run along -Y
+            linear_extrude(height = chlen)
+                polygon(points = [
+                    [-plate_w / 2, 0],            // base (inside), wide
+                    [ plate_w / 2, 0],
+                    [ mouth / 2,  depth],         // mouth (top), narrow -> jaws grip
+                    [-mouth / 2,  depth]
+                ]);
+}
+
+// ---- DEC bearing/drive housing ---------------------------------------------
+module dec_core() {
+    difference() {
+        translate([-dec_outer_x/2, -dec_outer_y/2, 0])
+            cube([dec_outer_x, dec_outer_y, dec_depth]);
+
+        // Stepped axial bore along +z (the DEC axis):
+        translate([0, 0, -1])      cylinder(d = brg_od,     h = brg_w + 1);
+        translate([0, 0, brg_w])   cylinder(d = drive_bore, h = dec_depth - brg_w + 1);
+        translate([0, 0, -1])      cylinder(d = brg_bore - 6, h = dec_depth + 2);
+
+        // CSF-25 flange bolt circle from the rear face.
+        for (i = [0 : flange_nbolt - 1])
+            rotate([0, 0, i * 360 / flange_nbolt])
+                translate([flange_bc / 2, 0, dec_depth - 14])
+                    cylinder(d = flange_bolt_d, h = 18);
+    }
+}
+
+// ---- Losmandy-D saddle jaw block (sits on the +Y face of the DEC output) -----
+module losmandy_saddle() {
+    blk_w = losmandy_plate_w + 44;     // jaw width straddling the plate — ASSUMED
+    blk_l = dec_outer_y;               // saddle length = DEC housing face
+    difference() {
+        translate([-blk_w/2, -blk_l/2, 0]) cube([blk_w, blk_l, saddle_height]);
+        // dovetail channel, mouth flush with the top surface
+        translate([0, 0, saddle_height - saddle_slot_depth])
+            _dovetail_channel(blk_l + 2, losmandy_plate_w, dovetail_angle, saddle_slot_depth + 2);
+        // side clamp knob cross-hole
+        translate([blk_w/2 - 8, 0, saddle_height/2])
+            rotate([0, 90, 0]) cylinder(d = clamp_bolt_d, h = 24, center = true);
+    }
+}
+
+module dec_housing() {
+    dec_core();
+    // Mount the saddle on the DEC output face (+z), lifted clear of the bore boss.
+    translate([0, 0, dec_depth]) losmandy_saddle();
+}
+
+dec_housing();   // executed standalone; ignored by `use <>` in assembly
diff --git a/design/mechanical/cad/pier_adapter.scad b/design/mechanical/cad/pier_adapter.scad
@@ -0,0 +1,59 @@
+// ============================================================================
+// NIGHTWATCH GEM — pier adapter plate
+// Bridges the concrete pier's steel top plate (12" x 0.375") to the RA housing.
+// Parametric OpenSCAD model.  Units: MILLIMETRES.
+//
+// Render (OpenSCAD not installed here):
+//     openscad -o pier_adapter.stl                     pier_adapter.scad
+//     openscad -o pier_adapter.png --imgsize=1200,900  pier_adapter.scad
+//
+// Dimensions trace to design/mechanical/calc/params.py (SI metres -> mm).
+// ============================================================================
+
+$fn = 96;
+
+// ---- parameters ------------------------------------------------------------
+// The adapter plate itself is 10" x 10" x 0.75" — an ADDED part (repo is silent
+// on an adapter; tagged ASSUMED).  It seats on the SOURCED pier top plate below.
+adapter_side = 254.0;   // 10 in  = u.inch(10)  -> ASSUMED (adapter geometry added)
+adapter_thk  = 19.05;   // 0.75 in = u.inch(0.75) -> ASSUMED
+
+// Below: the pier's steel top plate (SOURCED, params.py P.PIER).
+pier_plate_side = 304.8;   // P.PIER.top_plate_side_m = u.inch(12)
+pier_plate_thk  = 9.525;   // P.PIER.top_plate_thk_m  = u.inch(0.375)  (reference)
+pier_dia        = 304.8;   // P.PIER.diameter_m       = u.inch(12)     (cable-hole guide)
+
+// Above: the RA housing corner bolt pattern it must accept.
+ra_outer     = 203.2;   // P.RA_HOUSING.outer_x_m = u.inch(8)
+ra_corner_inset = 18.0; // must match ra_housing.scad corner_inset — DERIVED
+
+// Fastener geometry — ASSUMED.
+down_bolt_d  = 13.5;    // M12 anchor pattern into the pier top plate
+down_bc      = 228.6;   // bolt circle to the pier plate (9.0 in square) — ASSUMED
+up_bolt_d    = 7.0;     // M6 tapped-clearance for the RA housing corners
+cable_bore   = 60.0;    // central cable / drawbar pass-through — ASSUMED
+
+module pier_adapter() {
+    ra_bc = ra_outer - 2 * ra_corner_inset;   // RA housing corner square (mm)
+    difference() {
+        // 10" square plate with lightly chamfered footprint (via minkowski-free
+        // bevel: a plain square keeps the model unambiguous).
+        translate([-adapter_side/2, -adapter_side/2, 0])
+            cube([adapter_side, adapter_side, adapter_thk]);
+
+        // central cable / drawbar bore
+        translate([0, 0, -1]) cylinder(d = cable_bore, h = adapter_thk + 2);
+
+        // DOWN pattern: 4 bolts on a square to the pier top plate
+        for (sx = [-1, 1], sy = [-1, 1])
+            translate([sx * down_bc/2, sy * down_bc/2, -1])
+                cylinder(d = down_bolt_d, h = adapter_thk + 2);
+
+        // UP pattern: 4 tapped holes matching the RA housing corner bolts
+        for (sx = [-1, 1], sy = [-1, 1])
+            translate([sx * ra_bc/2, sy * ra_bc/2, -1])
+                cylinder(d = up_bolt_d, h = adapter_thk + 2);
+    }
+}
+
+pier_adapter();   // standalone; ignored by `use <>` in assembly