PFIS Home > Documents > Code 3120: Optics

Southern African Large Telescope

Prime Focus Imaging Spectrograph

Anti-Reflection Coatings Specification

SALT-3120AS0016

Eric Burgh

Modification Record
Version Date Comments
1.0 05-Mar-2003
1.1 10-Mar-2003 Change UV cutoff from 300 to 320 nm

1. Introduction

The optical system of the Prime Focus Imaging Spectrograph (PFIS) comprises a refracting collimator with internal reflecting fold mirror, refractive dispersers (in the form of five volume phase holographic gratings, one standard transmission grating, and two Fabry-Perot etalons), and a refracting camera.

Each air-glass interface, if left uncoated, would reflect approximately four percent of the incident light. With 20 air-glass interfaces in the optical system (with a grating in), these reflections would result in only a 44 percent throughput.

PFIS will use three types of anti-reflection coatings.

2. Collimator Optics

The following table provides information about each optic in the collimator and what type of coating is needed for each surface.
Group Element Material Diameter Center Thickness Edge Thickness Clear Aperture Radius of Curvature Wavelength range (nm) Coating type
D1 O-L1 F_quartz 130 10 29.9 120 94.1409 320-1700 MgF2
64.7932   None
O-L2 CaF2 124 51 11.8 113 64.7932   None
269.1785 320-1700 MgF2
S1 O-L3 F_quartz 132 10 46.8 115 -66.7515 320-1700 MgF2
198.8691 320-1700 Solgel/MgF2
S2 O-L4 F_quartz 154 30 10.9 142 -686.2812 320-1700 Solgel/MgF2
-139.4617 320-1700 Solgel/MgF2
T O-L5 CaF2 184 55 14.0 171 930.2364 320-1700 Solgel/MgF2
-134.3700   None
O-L6 NaCl 196 15 29.3 185 -134.3700   None
-228.0297   None
O-L7 CaF2 204 37 15.1 194 -228.0297   None
-140.1829 320-1700 MgF2
D2 O-L8 F_silica 202 15 43.2 187 Plano 320-900 Multi-layer
188.1678   None
O-L9 CaF2 202 53 14.2 187 188.1678   None
-544.9774 320-900 Multi-layer
All units mm (unless otherwise specified)

3. Camera Optics

Group Element Material Diameter Center Thickness Edge Thickness Clear Aperture Radius of Curvature Wavelength range (nm) Coating type
Q A-L1 F_silica 238 15 31.9 221 407.5475(asph) 320-900 Multi-Layer
225.6610   None
A-L2 CaF2 238 70 14.3 223 225.6610   None
-335.7677   None
A-L3 F_silica 244 15 65.1 228 -355.7677   None
268.3832   None
A-L4 CaF2 244 50 16.5 230 268.3832   None
-1777.484 320-900 Solgel/MgF2
S1 A-L5 CaF2 250 54 14.8 241 464.1057 320-900 Solgel/MgF2
-365.7249 320-900 Solgel/MgF2
T A-L6 F_silica 210 45 21.7 197 248.3816 320-900 Solgel/MgF2
Plano   None
A-L7 NaCl 194 15 50.2 183 Plano   None
120.1303   None
A-L8 F_silica 170 90 60.1 160 120.1303   None
488.2939 320-900 Multi-Layer
S2 A-L9 F_silica 90x123 8 23.9 72x105 -173.1466 320-900 H2Oresistant ML
539.5584 320-900 H2Oresistant ML

4. VPH gratings

We have ordered three grating substrates per grating. Two for the construction of the grating plus one spare. One side of each substrate needs to be coated.

The following table provides the dimensions, minimum and maximum angle of rotation, and wavelength coverage over which we need to minimize reflection for each VPH grating. These are specified to have multi-layer coatings.

Grating Substrate size (mm) Clear Aperture (mm) Minimum Angle (deg) Maximum Angle (deg) Wavelength Range (nm)
G0900 200x175x10 180x155 12 20 320-900
G1300 230x175x10 210x155 20 35 390-900
G1800 250x175x10 237x155 25 50 450-900
G2300 250x175x10 237x155 25 50 350-700
G3000 250x175x10 237x155 30 50 320-550

Dimensions in mm, angles in degrees, and wavelengths in nm

5. Other optics

Other optics will also need to be coated with anti-reflection coatings. These include: