Gratings: so many variables
)(
)4/1(cos
2/
cossin2
206265/#
/#
)cos
sinsin(
2,
,
cam
o
mbo
o
o
omb
telpix
pixcam
pixB
camprojcam
telBsproj
fldd
mmm
ADm
Dd
f
s
fGf
DRG
β
λλββ
λβσ
β
θδβδθα
θ
θδσλ
θ
θθ
αβα
θ
Δ=Δ
Δ=Δ
−=Δ
−=+=
=
=
=
=
=
+=
Variables to work with
δ Blaze Angle
σ Groove Spacing
m Order(s)
AD turn angle
Grating Orientation
Scientific Reqts Give
Rs Slit limited resolution
θB Slit size on sky
Dtel Telescope Dia
Dpix Detector Pixel Size
s pixels/slit width = sampling
Gratings: so many variables
Lessons Learned
Increase blaze angle to control projected beam size on grating (keep sizes reasonable)
Increase groove spacing to keep order width from exceeding width of detector
ααθ
tan1
tan2∝= telBs
projDRG When α = β
)4/1(cos 2,
−=Δ
o
mbo
o
o
mmm λ
βσβ
Schroeder, ‘Astronomical Optics’
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)
Gratings: so many variables
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’
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
Gratings: Anamorphic Magnification
Peak Efficiency drops, Δβdecreases when go off littrow in normal to camera case (α > β)
Schroeder, ‘Astronomical Optics’
Gratings: Anamorphic Magnification
Schweizer 1979, PASP
Gratings: Anamorphic Magnification
d1
d2
d1
d2
r = d1 / d2 = 71.81 / 91.13 = 0.79
Example: Triplespec
Gratings: Anamorphic Magnification
Slit Width (dispersion direction)
2.8 pix / arcsec
Slit Length (x-dispersion direction)
3.5 pix / arcsec
r = 2.8 / 3.5 = 0.8
Example: Triplespec
Scattering from optics
Surface Roughness Dust particles
Why do we care?
• Throughput losses
• Scattered light
Bennett & Mattson
Total Integrated Scatter
2)/)cos(4( ))cos(4(12
λθπδλθπδ o
o
d oeRRTIS ≈−== −
Where Rd = diffuse reflectance, Rs = diffuse reflectance, δ is rmssurface roughness, and cos θ is the incidence angle
TIS for surface roughness and dust
Bennett & Mattson
Hard Contact Lens with Scratches
Bennett & Mattson
Partially Polished Optic
Bennett & Mattson
Polished Silicon Wafer
Bennett & Mattson
Diamond Turned Optic
Bennett & Mattson
Dust-covered Optic
Bennett & Mattson
Cleaning Residue
Bennett & Mattson
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
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 (ft3)
Air Class 0.3 µm 0.5 µm 5 µm
100 300 100 -
1000 - 1000 7
10,000 - 10000 70
100,000 - 100000 700
Filtered Air
Supply air to enclosure is filtered with HEPA (High Efficiency Particle Air; 99.97 % 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.
Filters, Airflow & Pressurization
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 / ft2.
What determines Surface Cleanliness?
1. Air cleanliness
2. Time of exposure
3. Part orientation
Air cleanliness Surface cleanliness
Figures from Tribble 2000
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.)
How clean do we need Triplespec?
Scatter and surface roughness
Figure from Optics Handbook, Ch 38
Diffracted energy into wings of PSF
How clean do we need Triplespec?
Wavelength 2400 NM
MM
Wavelength 2400 NM
MM
Figure 1 Irradiance Distribution in Image of slit at wavelength 2400 nm and diffraction order 3.
How clean do we need Triplespec?
Wavelength 2400 NM
MM
Wavelength 2400 NM
MM
Figure 2 Stray light distribution for 2400 nm wavelength with slit image removed.
Rule of thumb per G. Peterson:
Diffraction dominates to approx 1 mm radial distance from psf center.
How clean do we need Triplespec?
Irradiance for 2400 nm wavelength
PowerMax
Irradiance
Watts Watts/mm2
Image of slit - diffraction order 3 2.10E-04 5.25E-03
- Refraction through flat mounting surfaces diffraction order 2 1.90E-08 1.58E-09
Scatter from lenses – diffraction order 3 3.44E-08 1.39E-08
Scatter from lenses – diffraction order 2 4.26E-12 1.66E-12
Scatter from mechanical surfaces 2.65E-09 5.68E-11
Total Power in all stray light paths 5.60E-08Approx 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.
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.
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.
Cleaning Unmounted Optics
Drag Wiping Method for tiny particles
** Don’t use acetone – it evaporates too quickly and can leave a white film
Bennett & Mattson
Blow off particles ≥ 10 microns w/ GN2, aerosol can
Cleaning Mounted Optics
Form loose swab from lens tissue, moisten with methanol or ethanol, gently move over optic. Repeat going in a different direction
“Poor cleaning is much worse than no cleaning”
Bennett & Mattson
Bennett & Mattson
Cleaning Gratings – Don’t
Send it back to the manufacturer for cleaning
Bennett & Mattson
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