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

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Partially Polished Optic

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Polished Silicon Wafer

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Diamond Turned Optic

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Dust-covered Optic

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Cleaning Residue

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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