Mkir nici stray light
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MKIR NICI Stray Light . Dennis Charles Evans Evans Engineering [email protected] (301)262-2230 April 2, 2002. Outline. Task Goal – suggest baffles and verify implementation Stray Light Evaluation Image Ghosts ZEMAX Prescription Names Cold Optics AutoCAD Orthographic and Perspective Views

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MKIR NICI Stray Light

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Mkir nici stray light

MKIR NICI Stray Light

Dennis Charles Evans

Evans Engineering

[email protected]

(301)262-2230

April 2, 2002


Outline

Outline

  • Task Goal – suggest baffles and verify implementation

  • Stray Light Evaluation

  • Image Ghosts

  • ZEMAX Prescription Names

  • Cold Optics AutoCAD Orthographic and Perspective Views

    • CAD conversion Issues

    • Red Channel

    • OAP Center

    • Blue Channel

    • From the TFP Mask

  • Sample Ghost Analysis

  • Summary


Task goal

Task Goal

  • Coronagraphs are concerned with very narrow angle stray light.

  • Stray Light Analysis Software and BRDF data are not accurate at narrow angles.

  • Theoretical Models of image formation show 4 to 6 orders-of-magnitude central image peaks.

  • Task Concern was to suggest baffling scheme and verify implementation, not perform stray light analysis.


Stray light evaluation

Stray Light Evaluation

  • NICI is a slow optical system: f/38

  • The input to the Cryo Assembly is well defined.

    • No intense off axis sources (Sun, Moon, Earth)

    • Simple baffling will be very effective

  • Baffles added to prevent glancing wall paths

    • There are no direct paths to walls, only second order diffuse surface scattering

    • Walls and edges parallel to optical axis should be painted black

    • Second order scattering reduces stray signal by many orders-of-magnitude

  • Critical concept

    • View from the detector

    • Final stage baffles dominate performance

    • Any off-axis scattering forward of final optical surface is less than direct signal


Mkir nici stray light

10 millirad = 0.5729 degree (34 arc-minutes)

Alan W. Greynolds, “A consistent theory of scatter from optical surfaces”, pp10-18.

In Robert P. Breault, (August 17-19, 1988) Stray Light and Contamination in Optical Systems, Proceedings of the SPIE, Volume 967


Image ghosts

Image Ghosts

  • Ghost Images not significant!

  • NICI Reflective Optics do not Create Ghosts

  • No Transmission Lenses

    • No diffuse ghosting from reimaged points!

  • Dichroics, Window, and Filters

    • Plane parallel surfaces aligned with optical axis

    • Centered ghost (lost in bright image)

      • f/38 (worst case) Ghost radius=0.080 mm; Airy radius=0.060 mm

      • Coronagraph mask to 5-10 Airy radii.

    • Wave front sensor dichroic in f/16 beam. Ghost is out of focus, offset, and less than 1E-6 of signal. (See next 2 slides)

  • Detector

    • Can be tilted to offset narcissistic ghosts


Dichroic ghost

Dichroic Ghost

1.000 (IR)

Main

Image

0.95 (IR)

0.01 (IR)

0.01 (IR)

0.000002 (IR)

Ghost

Image

0.02 (IR)

0.95 (IR)

IR Ghost=0.01 x 0.02 x 0.01 = 0.000002


Wfs dichroic ghost

WFS Dichroic Ghost

  • Path through 6 mm thick Dichroic is offset 4.8 mm

    • Angle equivalent is 7.8 arcseconds in Telescope Focal Plane

    • Position equivalent is 11.5 mm at Coronagraph Image Plane

      • 192 times first diffraction ring

      • May be far enough removed to not be a problem.

  • Dichroic Ghost Intensity Properties (IR reflective, Visible transmissive)

    • First Surface: 0.95 reflective; 0.01 IR transmissive (could be lower, out of band)

    • Second Surface: 0.02 IR reflective (not well optimized antireflection coated)

    • Repeat First Surface

    • Could make Dichroic out of band an IR absorber for even lower ghost signal

    • Ghost Signal = 0.01 x 0.02 x 0.01 = 0.000002 ( 14 stellar magnitudes)

      • This is a conservative upper limit. If an absorption dichroic backing is used the ghost signal could be reduced many more orders of magnitude. The 0.01 out of band transmission is probably high for an estimate.

  • Alternatives: Mask primary or secondary - create asymmetric diffraction

    • Observe in diffraction minima

    • Secondary Apodization; Square Aperture; Cat’s Eye; …. Other?


Cold optics autocad orthographic and perspective views

Cold Optics AutoCAD Orthographic and Perspective Views


Mkir nici stray light

Blue Channel

NICI Cold Assembly

Light

Baffles

Optics

OAP Camera Mirror 54

Fold 51

OAP Camera Mirror 62

OAP

Collimator

35

37

Filter

55

56

Filter

48 49

TFP

Mask

29 30

28 29

Flat

48 42

Dichroic

45

50 52

Fold

58

Window

30 31

31 32

Stop

4146

Red Channel

ZEMAX Surface Number BlueRed


Cad conversion issues

CAD Conversion Issues

  • IGES conversion is an art in actual use.

    • All Baffles converted as 3D solids. (Magic?)

      • No Problem!

    • Structures converted as multifaceted surfaces

      • Holes don’t “subtract” out of flat surfaces

      • Flat surfaces extend across holes.

      • Care (time) required in selecting proper facets

        • Many years experience with this traditional problem

        • Forces a very close look at the structural design

        • No problems seen in the NICI model


Zemax prescriptions

ZEMAX Prescriptions

  • Blue =

    • Gemini AO Coronagraph - rel_blue1.ZMX

  • Red =

    • Gemini AO Coronagraph - rel-redc1.ZMX


Red channel

Red Channel


View from oap center

View from OAP Center


Blue channel

Blue Channel


View from the tfp mask

View from the TFP Mask


Sample ghost analysis

Sample Ghost Analysis

  • Sample Telescope is f/10 Cassegrain

    • Lens set to improve off axis image quality

    • Dual filter wheel.

    • Filter and Lens surfaces were 0.05 reflective.

      • Ghost Ratio = 0.05 x 0.05 = 0.0025

    • NICI Filters anti-reflection coated, 0.02 reflective.

      • NICI Ghost Ratio = 0.02 x 0.02 = 0.004

      • NICI Filter ghosts a factor of 6 lower than sample.

      • Filter ghosts estimated about 1/10,000 of central signal.

      • NICI ghosts smaller than sample & centered on axial image.

  • ASAP (Advanced Systems Analysis Program)

    • “Picture” image flux display for point & extended sources


Mkir nici stray light

Floor at 1E-6

Near ghost between 1/1000 and

1/10,000.

General ghosting obvious.


Mkir nici stray light

Floor at 1E-4

Near ghost is about 1/600.

General ghosts between

1/600 to 1/1000.

All look like surface ghosts.


Summary

Summary

  • Suggested baffles have been implemented effectively

  • NICI performance should be dominated by diffraction point spread function, not instrumental stray light.

  • Only one Ghost Image is even possible

    • May not be a problem (too far off axis)

    • Intensity estimate is 2E-6 and it could be many orders of magnitude lower than that.


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