Oct 17, 2001 SALT PFIS PDR - Structure 1

Structure

• Interface/ constraints• Loads• Structure design rationale• Truss• Weight and CG• Finite Element Analysis/ Image motion• Risks and concerns

Kenneth Nordsieck

University of Wisconsin

Oct 17, 2001 SALT PFIS PDR - Structure 2

Interface

• Straight-thru focus position

• 1.5m long x 3m diam cylinder nominal envelope

• Weight budget: 375 kg (50% of Prime Focus Platform)

• On the stage

Oct 17, 2001 SALT PFIS PDR - Structure 3

Loads

• Tracker carries optic axis anywhere within 6° radius cone centered on 37° zenith distance 360°

stage has 240° range. Loads modeled over full

360°cone

37

Oct 17, 2001 SALT PFIS PDR - Structure 4

Design Rationale

• Open truss: maximum stiffness/ wt; minimize wind loading

• Invar to minimize thermal effects• Square box beam (SBB) available commercially• All welded; annealed; machined• Collimator, Camera optics contained in invar-skin

tubes• All other structure Aluminum• Designed and costed by Swales Aerospace

Oct 17, 2001 SALT PFIS PDR - Structure 5

Visible camera rail supporting I beam

Mirror Mechanism structure

Near IR Beam Rails support with Curved I Beam

Etalon and VPH Grating Mechanisms Structure

IR Mirror with structure

Bottom SBB Ring with pads

Collimator Tube with top & bottom flange

Articulation bearing housing

Bottom SBB beams connecting to Slit mask invar lens holder

Truss Rings

• Bottom Ring with 12 mount pads

• Collimator tube mounted through slitmask assembly

• Articulation I-Beams coupled to bearings

Oct 17, 2001 SALT PFIS PDR - Structure 6

Truss Web

• Mount ring coupled to articulation rails

• Mount ring coupled to articulation bearings

• Rails coupled

• Collimator coupled

• Total weight 138 kg

Oct 17, 2001 SALT PFIS PDR - Structure 7

Weight and CG

• Mass is 40 kg high (visible beam); 95 kg high (2 beam)

• CG is close, erring in "good" direction

Oct 17, 2001 SALT PFIS PDR - Structure 8

Flexure - Image Motion• Finite element analysis

of optics nodes + optical sensitivity analysis:

• Differential image motion parallel and perpendicular to dispersion, relative to optical axis at telescope focal plane

• Rotation is largest effect

Center and four points on 6 deg coneAs a function of rotation. Truss only

mic

rons

Oct 17, 2001 SALT PFIS PDR - Structure 9

Image Motion during Track

• Angle of tracker at beginning and end of track, plus rotation evaluated for 6 declinations

• Evaluate worst-case flexure/ hour for two structure models– Truss (optics hard coupled)

– First cut optical attachment

• Specification– 0.1 arcsec (12 )/hr along

disp

– 0.15 arcsec/hr perp to disp

Oct 17, 2001 SALT PFIS PDR - Structure 10

Image Motion/ Future work

• Flexure-induce image motion within spec for truss-only model; worst case exceeds spec parallel to dispersion for current optics attachment structure

• Future work to fix this– Strengthen beam attaching rear of

camera to articulation bearing

– Strengthen IR fold structure

Oct 17, 2001 SALT PFIS PDR - Structure 11

Risks

• Weight (Low? High if ICD cannot be relieved)– Maximum 40 kg may be removed by weight savings. If

375 kg weight limit in ICD cannot be relieved, and NIR beam upgrade is to be retained, need to descope

– Possible weight descope: reduce beam 150 -> 116 mm etalons: -40 kg; optics and structure: ~-40 kg

• Flexure/ image motion - (Low - Medium)– Truss is well within stiffness specification

– Problematic attachment structure has been identified

Oct 17, 2001 SALT PFIS PDR - Structure 12

Risks - Continued

• Cost and Schedule (Low - Medium)– An inexpensive structure concept using commercially

available materials has been identified

– A complete bottom-up evaluation of materials and labor costs has been developed by an independent firm, and incorporated into project budget and schedule