# GigaBIT M1 — Reference Frame Stiffness Characterization

**Client:** StarSpec
**Project:** P04-GigaBIT-M1
**Created:** 2026-03-04
**Status:** Active — Model Setup

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## Objective

Determine minimum stiffness requirements for StarSpec's reference frame at 5 mirror support interface points. Quantify WFE sensitivity to frame compliance so StarSpec can design their frame to spec.

## Approach

**Hybrid B+D method** (war-room validated):

1. **Method D (Sensitivity):** Prescribed unit displacements at each unique interface point, extract Zernike WFE sensitivity coefficients (nm/μm per DOF per point)
2. **Method B (Stiffness Sweep):** CBUSH spring-to-ground at 5 interface points, sweep K to find knee points
3. **Robustness:** One-soft-point, combined compliance, correlated displacement tests

## Key Numbers

| Metric | Value |
|--------|-------|
| Baseline WFE @ 40° | ~7.0 nm RMS (J5+ filtered) |
| Baseline WFE @ 60° | ~13.0 nm RMS (J5+ filtered) |
| ΔWFE budget (frame) | ≤ 1.0 nm RMS |
| System allocation | ≤ 22 nm RMS (contract) |
| Mechanical target | < 14 nm RMS |
| Total runs | ~42 |

## Interface Points (5 total)

| Label | Type | Symmetry |
|-------|------|----------|
| V_top | Vertical support | Unique (on Y-Z symmetry axis) |
| V_left | Vertical support | = V_right mirrored |
| V_right | Vertical support | = V_left mirrored |
| L_left | Lateral support | = L_right mirrored |
| L_right | Lateral support | = L_left mirrored |

**Unique points to test:** 3 (V_top, V_side, L_side)

## Model

- **Solver:** NX Nastran SOL 101 (linear static)
- **Coordinate system:** Global (Z = optical axis)
- **Zernike extractor:** Annular OPD, 50 modes, J5+ filtered RMS
- **Reference subcase:** 20° elevation
- **Test subcases:** 40° and 60° elevation
- **Inner radius:** 135.75 mm (central hole)
- **Outer radius:** 500.0 mm

## Project Structure

```
gigabit-m1-frame-stiffness/
├── README.md                   ← You are here
├── models/                     # NX model files (drop .prt and .sim here)
├── studies/
│   ├── 01_sensitivity_method_d/  # Unit displacement sensitivity (9+3 runs)
│   ├── 02_stiffness_sweep_method_b/  # K sweep campaigns (20 runs)
│   └── 03_robustness/           # One-soft-point, combined, correlated (8 runs)
├── deliverables/               # StarSpec requirement document (Rev A)
├── kb/                         # Knowledge base notes
└── images/                     # Plots, screenshots
```

## PKM Reference

Full documentation: `obsidian-vault/2-Projects/P04-GigaBIT-M1/03-studies/Reference-Frame-Stiffness-Characterization/`