LWIR FPA Mirror Image Problem & Recovery

April 11, 2011Roy W. Esplin

Dave McLain

LWIR FPA Mirror Image Problem

A p e r t u r e F u l l X W i d t h : 1 7 . 0 0 0 0A p e r t u r e F u l l Y H e i g h t : 2 . 0 0 0 0

Scale: 2.2000 Millimeters

6 . 7 1 5 0 7 . 3 2 5 0 8 . 2 8 0 0 8 . 5 5 0 0 9 . 7 3 0 0 1 0 . 2 0 0 0 1 1 . 0 3 0 0 1 2 . 0 2 0 0 1 2 . 6 0 0 0 1 3 . 3 3 5 0 1 3 . 6 3 5 0 1 3 . 9 3 5 0

E M A S _ P o r t 4 _ V 5 g _ C o l d . Z M XC o n f i g u r a t i o n 1 o f 1

F o o t p r i n t D i a g r a mM A S M o d e l4 / 8 / 2 0 1 1S u r f a c e 5 0 : D E T E C T O RR a y X M i n = - 8 . 3 8 1 0 R a y X M a x = 8 . 3 4 9 1R a y Y M i n = - 0 . 2 8 7 5 R a y Y M a x = 0 . 6 3 8 0M a x R a d i u s = 8 . 3 8 1 2 W a v e l e n g t h = A l l

% r a y s t h r o u g h = 6 3 . 6 5 %

EMAS_Port4_V5g_Cold.ZMX ray trace

As-Built FPA

EMAS_Port4_V5g_Cold.ZMX ray trace footprint on detectors. View from back side of detectors

X

Y• FPA was rotated 180º to put wavelengths on correct side, but we overlooked the fact that this converted smile into a frown.

• Problem discovered April 4, 2011 during final checking of LWIR Detector/Lens Assembly Dwg. 155-0006.

2

A p e r t u r e F u l l X W i d t h : 1 7 . 0 0 0 0A p e r t u r e F u l l Y H e i g h t : 1 . 5 0 0 0

Scale: 1.5200 Millimeters

6 . 7 1 5 0 7 . 3 2 5 0 8 . 2 8 0 0 8 . 5 5 0 0 9 . 7 3 0 0 1 0 . 2 0 0 0 1 1 . 0 3 0 0 1 2 . 0 2 0 0 1 2 . 6 0 0 0 1 3 . 3 3 5 0 1 3 . 6 3 5 0 1 3 . 9 3 5 0

E M A S _ P o r t 4 _ V 6 a _ C o l d . Z M XC o n f i g u r a t i o n 1 o f 1

F o o t p r i n t D i a g r a mM A S M o d e l4 / 8 / 2 0 1 1S u r f a c e 5 3 : D E T E C T O RR a y X M i n = - 8 . 3 4 9 1 R a y X M a x = 8 . 3 8 1 0R a y Y M i n = - 0 . 2 8 7 5 R a y Y M a x = 0 . 6 3 8 0M a x R a d i u s = 8 . 3 8 1 2 W a v e l e n g t h = A l l

% r a y s t h r o u g h = 6 3 . 6 2 %

LWIR FPA Mirror Image Solution

As-Built FPA

EMAS_Port4_V6a_Cold.ZMX ray trace footprint on detectors.View from back side of detectors

Solution: Modify optical design to match “as-built” LWIR FPA.New optical design maintains same EMAS performance.

EMAS_Port4_V6a_Cold.ZMX ray trace

X

Y

3

Changes Required Optical prescription changed from EMAS_Port4_V5h_Cold to EMAS_Port4_V6a_Cold with the following changes

Changed diffraction order from +1 to -1

Reversed sign on alpha tilt angle of grating

Rotated grating 180 degrees to maintain blaze efficiency

One part of the grating mount has to be remade Rough machining and first thermal cycle have been completed. Some paint on cold bench has to be removed to mount this remade part.

The clocking angle of the FPA must be changed by 180 degreesRequires 3 new holes in LWIR lens cell. Model and drawing already modified.

Length of thermal strap must be increased approximately 3 inches . Work on strap had just begun, so only rework is fabrication of 2 copper blocks.

Requires 2 new holes and covers for clearance of screws bolting thermal strap to FPA

Part of a rib in the thermal shroud must be removed to provide clearance for thermal link. This is not a strength issue because partial rib is similar to other ribs in shroud.

Some paint on cold bench must be removed to mount thermal strap.

FPA connector must be rotated 180 degrees

No changes to LWIR FPA or EMAS performance4

Revised Optical-Mechanical Configuration

New grating mounting part(Shown in blue)

FPA rotated 180º from its previous orientation

Thermal strap ~3 in. longer.

FPA unchanged.5

Location of Spectral Band Images Relative to Lens and FPA for Revised Configuration

L1 Image

L12 Image6

Relative Orientation of FPA and Lens for Revised Configuration

7

L1 Band Rays Now aligned with FPA

8

Field-Stop Spectral Images Now Aligned with Detectors (Slide 1 of 2)

9

Field-Stop Spectral Images Now Aligned with Detectors (Slide 2 of 2)

10

Revised Thermal StrapThe FPA is now rotated 180º from its previous orientation, so the thermal strap connection is now on the left rather than on the right.

Location of FPA thermal strap connection for previous configuration

Thermal strap connection to FPA

Paint on cold bench under this part of strap needs to be removed

FPA electrical connector rotated l80º from previous orientation11

Revised Configuration Fits Within Existing Thermal Shroud

Thermal Shroud

12

Repositioned Grating and Thermal Link Fit Within the Existing Thermal Shroud

New Grating Mount

Thermal Shroud

Thermal LinkThermal Shroud

Part of shroud rib removed for clearance

Heads of screws mounting thermal strap to FPA. Holes cut in shroud to provide clearance.

Thermal Strap

13

Two Clearance Holes Cut in Thermal Shroud

Two holes cut in thermal shroud to provide clearance for Screws bolting thermal strap to FPA.This holes will have to be covered to prevent the cold bench from seeing the warm enclosure.

Location of holes Vacuum enclosureThermal shroud

14

Orientation of Grating in Revised Configuration

Grating spacing has been increased in this figure to make the grating facets visible15

Geometry Used To Verify 15.27º Blaze Angle for Revised Configuration (Slide 1 or 2)

Z-axis is the grating normal for revised configuration of EMAS_Port4_V6a_Cold

Incident Gut Ray

Diffracted Gut Ray for 9.730μm wavelength

The blue line is the bisector of the angle determined by the incident and diffracted rays. This blue line defines the direction of the normal to the grating facets for a grating blazed for maximum efficiency at a wavelength of 9.730µm because the incident ray is reflected from this facet in the direction of the diffracted ray.

Grating spacing has been increased in this figure to make the grating facets visible

This yellow plane is normal to blue bisector line

This yellow plane contains the incident and diffracted gut rays for 9.730mm wavelength

16

Geometry Used To Verify 15.27º Blaze Angle for Revised Configuration (Slide 2 or 2)

Incident Gut Ray

Diffracted Gut Ray for 9.730μm wavelength

This yellow plane is normal to blue bisector line

The blue line is the bisector of the angle determined by the incident and diffracted rays. This blue line defines the direction of the normal to the grating facets for a grating blazed for maximum efficiency at a wavelength of 9.730µm because the incident ray is reflected from this facet in the direction of the diffracted ray.

The angle between the incident and diffracted rays appears much smaller than it really is because of the viewing direction

17