MKIR NICI Stray Light

1. MKIR NICI Stray Light Dennis Charles Evans Evans Engineering ee@erols.com (301)262-2230 April 2, 2002

2. 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

3. 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.

4. 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

5. 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

6. 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

7. 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

8. 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?

9. Cold Optics AutoCAD Orthographic and Perspective Views

11. 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

15. 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

19. ZEMAX Prescriptions • Blue = • Gemini AO Coronagraph - rel_blue1.ZMX • Red = • Gemini AO Coronagraph - rel-redc1.ZMX

20. Red Channel

24. View from OAP Center

26. Blue Channel

31. View from the TFP Mask

35. 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

36. Floor at 1E-6 Near ghost between 1/1000 and 1/10,000. General ghosting obvious.

37. Floor at 1E-4 Near ghost is about 1/600. General ghosts between 1/600 to 1/1000. All look like surface ghosts.

38. 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.