SALT-3120AS0016 version 1.0 (05 March 2003)

Specification for the anti-reflection coatings for the optics of the Prime Focus Imaging Spectrograph for the Southern African Large Telescope.

1. Introduction

The optical system of the Prime Focus Imaging Spectrograph (PFIS) comprises a refractiving collimator with internal reflecing 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. Multiplied by 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.

GroupElementMaterialDiameterCenter
Thickness
Edge
Thickness
Clear
Aperture
Radius of
Curvature
Wavelength
range (nm)
Coating type
D1O-L1F_quartz1301029.9 12094.1409300-1700MgF2
64.7932 None
O-L2CaF2124 51 11.8 113 64.7932  None
269.1785 300-1700 MgF2
S1O-L3 F_quartz 132 10 46.8 115 -66.7515 300-1700 MgF2
198.8691 300-1700 Solgel/MgF2
S2 O-L4 F_quartz 154 30 10.9 142 -686.2812300-1700 Solgel/MgF2
-139.4617 300-1700 Solgel/MgF2
T O-L5 CaF2 184 55 14.0 171 930.2364 300-1700 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 300-1700 MgF2
D2O-L8F_silica 202 15 43.2 187 Plano 300-900 Multi-layer
188.1678  None
O-L9CaF220253 14.2 187 188.1678  None
-544.9774 300-900 Multi-layer
All units mm (unless otherwise specified)

3. Camera Optics

GroupElementMaterialDiameterCenter
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) 300-900 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
-1777.484 300-900 Solgel/MgF2
S1 A-L5 CaF2 250 54 14.8 241 464.1057300-900Solgel/MgF2
-365.7249300-900Solgel/MgF2
T A-L6 F_silica 210 45 21.7 197 248.3816300-900Solgel/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
488.2939300-900Multi-Layer
S2 A-L9 F_silica 90x123 8 23.9 72x105 -173.1466300-900H2Oresistant ML
539.5584300-900H2Oresistant 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.

GratingSubstrate sizeClear ApertureMinimum
Angle
Maximum
Angle
Wavelength
Range (nm)
G0900 200x175x10 180x155 12 20 300-900
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 50 300-550
Dimensions in mm, Angles in degrees, Wavelengths in nm

5. Other optics

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