VLBI: Visible Light Broadband ImagerInstrument Conceptual Design Presentation

Tom BergerLockheed Martin Solar and Astrophysics Lab

VLBI Science Requirements

• WBFI Mission statement: record the highest spatial and temporal resolution movies from the ATST across the visible spectrum from 330 nm to 1.1 m.

• Spatial resolution: 0.03” at 330 nm• Temporal resolution: 5 – 10 seconds • Field-of-view: 3’ x 3’ minimum, 5’ x 5’ goal • Spectral resolution: 1 – 10 Å• Polarimetric requirements: none

VLBI Basic Concept

• Simple transfer/beam conditioning optics• Thin film interference filters

– 2 or 3 cavity glass subtrate– > 60% transmission– 1 – 10 Å bandpasses

• Large format, fast readout CCD or CMOS detectors• Dedicated multi-TByte storage array

VLBI Preliminary Wavelengths

Filter Wavelength

Å

Bandpass

ÅCN Violet Bandhead 3883 10

Ca II H & K Lines 3933 & 3968 1

CH G-band 4305 10

Blue Continuum 4504 5

Green Continuum 5550 5

H-alpha 6563 1

Red Continuum 6684 5

TiO Sunspot Bands 7057 10

VLBI Instrument Requirements

• Spatial resolution– Plate scale: 2 pixels per Airy disk at a given – Optical quality: < /4 per 2 cm on all elements incl. filters– Phase diversity: wavefront measurement and image restoration

capability

• Temporal resolution– Filter change and settle: 2 seconds – Camera readout: 3 seconds

• Filter Characteristics– TFI filter diameter: 10 cm (3’ x 3’ F/20 telescope beam)

• Optical– Flat focal plane over full 3’ x 3’ FOV– Telecentric beam: probably not required

VLBI Plate Scales & F/#s

Wavelength

Å

Focal Length

m

Plate Scale

arcsec/mm

F/# Field-of-View

arcmin

3933 183.1 1.13 46 0.69

4305 167.2 1.23 42 0.76

4505 159.8 1.29 40 0.79

5550 129.7 1.59 32 0.98

6563 109.7 1.88 27 1.16

6684 107.7 1.91 27 1.18

7057 102.0 2.02 26 1.26

Requirement: 2 pixels per central Airy disk of a 4-m aperture at each Hypothetical detector: 9 m pixels, 4096 x 4096 format

VLBI Design Limitations

• Detector technology– 0.015” pixels: 6K x 6K format to cover 1.5’ x 1.5’ – QE > 90%– Exposure ~10 msec, Readout < 3 seconds– Such cameras do not exist– Unknown pixel size means plate scale cannot be specified

• Utility and science must be balanced– Must be first-light instrument - cannot be delivered late– Must have immediate science potential - publicity quality images

must be produced

• Cost

VLBI Design Philosophy

• Modular construction– Provide subsystems that can be used individually, in sets, or in a

turn-key “black box” – Major subsystems

• Filterwheels

• Detectors

• Feed optics/focal plane conditioners

• Mountable at Gregorian or Coudé focal planes• Conform to ATST controls interface • Easily accommodate detector upgrades• Room temperature operation

VLBI Modular Subsystems

• Detector subsystem– Two detectors mounted on x-y-z translation stages– Cube beamsplitter feeds cameras simultaneously. CBS is easily

removable.– Allows small format detectors to “roam” over the ATST focal plane – Allows dual-camera phase diversity at any wavelength via

adjustable defocus of one camera

C1: Camera 1

XYZ Stage 1

SH1: Shutter 1

C2: Camera 2

XYZ Stage 2

SH2: Shutter 2

DCS:Detector Control System

C2

C1

CBS

F2

F1

SH2

SH

1

VLBI Modular Subsystems

• Filterwheel subsystem– Four aperture wheel - three 10 cm filters, one blank – Brushless DC motor drive - TRACE/EPIC heritage– Optical encoder positional readout– Edge gear/worm drive mechanism– 3 wheels in series for up to 9 wavelengths– Beam height above table ~15 cm for wheel clearance

VLBI Design Concepts

• VLBI Level 1– Dual detector package– Integrated single filter holder – No transfer/conditioning optics

Beam directlyfrom ATST Detector subsystem

Single Filter C2

C1

CBS

F2

F1

SH2

SH

1

VLBI Design Concepts

• VLBI Level 2– Dual detector subsystem– Filterwheel subsystem– Fixed focal ratio optics. L1 removable/replaceable for F/#

change.

C2

C1

CBS

F2

F1

FW1

CLFL

AS

F0

SH2

SH

1

L1

VLBI Design Concepts

• VLBI Level 3, Option A– Dual detector subsystem– Multiple filterwheel subsystem– Two channel (Red/Blue) transfer/conditioning optics: F/25 & F/45

C2

C1

CBS

F2

F1

FW1

CLFL

AS

F0

FW3

FM1 L1

L2

FM2 FM3

FM4 SH2

SH

1

FW2ATST Focal Plane

VLBI Design Concepts

• VLBI Level 3, Option B– Detector subsystem– Multiple filterwheel subsystem– Varifocal zoom lens transfer/conditioning optics: F/20 F/60

Zoom LensSystem

C2

C1

CBS

F2

F1

FW1

CLFL

AS

F0FW2

FW3SH2

SH

1

ATST Focal Plane

VLBI Concept Summary

• Modular instrument– Will be designed to Level 3 configuration– Level 3 option TBD– Built as subsystems that can be assembled/disassembled easily

to create Level 2 and Level 1 instruments