Gratings: so many variables

Transcription

1 Gratings: so many variables Scientific Reqts Give R s Slit limited resolution θ B Slit size on sky D tel Telescope Dia D pix Detector Pixel Size s pixels/slit width = sampling Variables to work with δ Blaze Angle σ Groove Spacing m Order(s) AD turn angle Grating Orientation G f θ f λ cam B proj /# b, m θ = Rsθ BDtel = sinα + sin β ( ) cosα = G f /# = sθ cam proj pix AD / 2 α = θ + δ β = δ θ dβ Δβ = Δλ dλ Δl = Δβ ( f ) cam pix mo Δβ = σ cos β o o cam d = θ D tel m ( m pix 2σ sinδ cosθ = m o 2 o λ b, m 1/ 4)

2 Gratings: so many variables Lessons Learned Schroeder, Astronomical Optics Increase blaze angle to control projected beam size on grating (keep sizes reasonable) G proj Rsθ BD 2 tanα = tel 1 tan α When α = β Increase groove spacing to keep order width from exceeding width of detector mo Δβ = σ cos β o m ( m o 2 o λ b, m 1/ 4)

3 Gratings: so many variables Considerations: Efficiency of grating based on blaze angle Spot Size on Grating (size of collimator optics, size of grating, availability of grating) Stock v Custom Grating ($) Number of Orders necessary for wavelength coverage (affects amount of cross dispersion necessary, order blocking filters if single order) Width of FSR (effective use of detector) Blaze Peak per Order relative to Atmospheric Windows (keep efficiency in center of e.g. JHK)

4 Gratings: so many variables

5 Gratings: Anamorphic Magnification Anamorphic Magnification = Different plate scales in the slit width and slit length directions f = f/# D so focal length for a given camera (f/#) will be different for each direction Schroeder, Astronomical Optics

6 Gratings: Anamorphic Magnification Both normal-to-camera or normal-to-collimator orientation satisfy grating equation. But choice influences efficiency, order format, resolution and scattered light Shadowing by this ledge Reflection back towards collimator because of this ledge Allington-Smith 2002

7 Gratings: Anamorphic Magnification Schroeder, Astronomical Optics Peak Efficiency drops, Δβ decreases when go off littrow in normal to camera case (α > β)

8 Gratings: Anamorphic Magnification Schweizer 1979, PASP

9 Gratings: Anamorphic Magnification Example: Triplespec d1 d1 d2 d2 r = d1 / d2 = / = 0.79

10 Gratings: Anamorphic Magnification Example: Triplespec Slit Width (dispersion direction) 2.8 pix / arcsec Slit Length (x-dispersion direction) 3.5 pix / arcsec r = 2.8 / 3.5 = 0.8

11 Scattering from optics Surface Roughness Dust particles Bennett & Mattson Why do we care? Throughput losses Scattered light

12 Total Integrated Scatter TIS = R R d o = 1 e (4πδ cos( θ )/ λ ) o 2 4πδ cos( θo) ( ) λ 2 Where R d = diffuse reflectance, R s = diffuse reflectance, δ is rms surface roughness, and cos θ is the incidence angle TIS for surface roughness and dust Bennett & Mattson

13 Hard Contact Lens with Scratches Bennett & Mattson

14 Partially Polished Optic Bennett & Mattson

15 Polished Silicon Wafer Bennett & Mattson

16 Diamond Turned Optic Bennett & Mattson

17 Dust-covered Optic Bennett & Mattson

18 Cleaning Residue Bennett & Mattson

19 Purpose of a clean room: keep dust off optics Reduce particulate (micron sized dust ) contamination on optical surfaces during assembly. Particulates adversely affect instrument performance through the following means: 1. Surface Obscuration remove energy from beam since most particles are good absorbers. Most deleterious for small optics near the focal plane. 2. Scattering potential to scatter light into unwanted places. Creates background of scattered light onto image. 3. Symbolic let s people know we are taking reasonable means to keep instrument clean

20 How are clean room classes defined? By the number of particles greater than or equal to a certain size per cubic foot (FED-STD-209E): Class Limits (ft 3 ) Air Class 0.3 µm 0.5 µm 5 µm , ,

21 Filters, Airflow & Pressurization Filtered Air Supply air to enclosure is filtered with HEPA (High Efficiency Particle Air; % efficient at removing particles of 0.5 µm or larger) or even more efficient filters. Increased Airflow In a conventional room this helps dilute contaminated air; In a room with laminar flow the airflow keeps contaminates from settling and helps reestablish laminar flow after disruption e.g. from personnel movement. Room Pressurization Keep clean positively pressurized with respect to adjacent spaces to prevent particulates from entering.

22 Air cleanliness Surface cleanliness Surface cleanliness defined by MIL STD 1246C: numerical value is interpreted as size in microns of the particle which has a surface distribution of 1 / ft 2. What determines Surface Cleanliness? 1. Air cleanliness 2. Time of exposure 3. Part orientation

23 Air cleanliness Surface cleanliness Figures from Tribble 2000

24 How clean do we need Triplespec? Breault Research conducted a stray light analysis for Triplespec in 2004: The second assumption is that the optical surfaces have a particulate (dust) contamination of level 300 (253 parts per million). The scatter of the optical surfaces is dominated by the level 300 contamination. (Second sentence means that scatter dominates over surface roughness per G. Peterson, Breault.)

25 How clean do we need Triplespec? Scatter and surface roughness Diffracted energy into wings of PSF Figure from Optics Handbook, Ch 38

26 How clean do we need Triplespec? Wavelength 2400 NM MM Figure 1 Irradiance Distribution in Image of slit at wavelength 2400 nm and diffraction order 3.

27 How clean do we need Triplespec? Wavelength 2400 NM Rule of thumb per G. Peterson: Diffraction dominates to approx 1 mm radial distance from psf center. MM Figure 2 Stray light distribution for 2400 nm wavelength with slit image removed.

28 How clean do we need Triplespec? Irradiance for 2400 nm wavelength Power Max Irradiance Watts Watts/mm 2 Image of slit - diffraction order E E-03 - Refraction through flat mounting surfaces diffraction order E E-09 Scatter from lenses diffraction order E E-08 Scatter from lenses diffraction order E E-12 Scatter from mechanical surfaces 2.65E E-11 Total Power in all stray light paths 5.60E-08 Approx 5 orders of magnitude weaker irradiance due to scattered light assuming class 300 surfaces. Gary Peterson (Breault) in hindsight said Class 300 very difficult to achieve. Class 500 more realistic. Class 300 -> 500 is approx factor 10 increase in scatter irradiance.

29 How clean is the camera (already assembled)? 1. Assembled by Axsys IR Systems in laminar flow bench (usually class 100), so internal optics should meet Breault s assumption. 2. When not in use kept in cabinet in room that is probably as clean as our main lab.

30 Can t we assemble the instrument anywhere and then clean the optics when we re done? 1. Optics are expensive with fragile coatings in some cases. Cleaning them with anything other than GN2 is risky. 2. Blowing particles off optics when already inside the instrument may simply redistribute many of the particles within the instrument it s not like vacuuming out the particles. 3. Blowing particles off optics is not always effective they can be held to the surfaces by EM forces, not just gravity.

31 Cleaning Unmounted Optics Blow off particles 10 microns w/ GN2, aerosol can Drag Wiping Method for tiny particles ** Don t use acetone it evaporates too quickly and can leave a white film Bennett & Mattson

32 Cleaning Mounted Optics Form loose swab from lens tissue, moisten with methanol or ethanol, gently move over optic. Repeat going in a different direction

33 Poor cleaning is much worse than no cleaning Bennett & Mattson Bennett & Mattson

34 Cleaning Gratings Don t Send it back to the manufacturer for cleaning Bennett & Mattson