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Document: VST_180798_optic1.doc Release: 1 First rel.date: 10.07.98 Last rel. date: 18.07.98 Page 1 of 10

A NEW POSSIBLE OPTICAL SOLUTION FOR THE VST

Authors: D. Ferruzzi, G. Marra Supervised by: D. Mancini Checked by: D. Mancini

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Introduction

In this document, a new optical solution for the VST is reported. This solution comes out from the Kickoff meeting held in Garching on 24 - 25 June 1998, and the telephone discussion with ESO of 15/07/98. It has a removable ADC and a curve dewar window. It is provided with a corrector made of two lenses from U to I bands (0.320 Â1.014 µm). From V to I bands (0.365 Â 1.014 µm), this corrector can be replaced by another one with one different lens and an ADC with curve entrance and exit surfaces. The aspherical surface was removed from the second lens of the corrector and the curvature of the exit surface of the ADC was increased and normalized to DIN tables. All Kick-off meeting requirements are satisfied and the rays of curvatures were normalized to DIN table 58166. The ADC type chosen is constituted of two identical couples of prisms, made of UBK7 and LLF6, which must be suitably counter rotated, to correct the atmospherical dispersion at the different observation angles, respect to zenith. The study of optical quality was done at zenith angle and at the z angle corresponding to the maximum dispersion of ADC.

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Optical layout and encircled energy values

This configuration has a curve dewar window and the optical quality meets requirements. The field corrector is made of one lens and an ADC with curved entrance and exit surfaces. The aspherical surface was eliminated from the second lens as requested. In Figures 2-1 and in 2-2, the complete optical layout of the telescope with one lens and the ADC and the zoom of the corrector are respectively shown. In Figures 23, 2-4 the complete optical layout of the telescope with the two lenses and the zoom of the corrector are reported. In Tables 2-1 and 2-2 the diffraction encircled energy values for the two configurations in which ADC is respectively inserted ( B, V, R, I band) and removed (U Â I), are reported for all fields of view, at zenith angle z=0° and at the zenith angle corresponding to the maximum dispersion of ADC. In Figure 2-5 the fraction of polichromatic diffraction encircled energy values curves are shown.

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Figure 2-1 VST complete optical layout of telescope with one lens and the, with a cuve dewar window

Figure 2-2 VST zoom of the optical layout of the corrector with one lens and the ADC and a curve dewar window

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Figure 2-3 VST optical layout of telescope with two lenses (U Â I bands)

Figure 2-4 VST zoom of the optical layout of the two lenses (U Â I bands)

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FRACTION (%) OF DIFFRACTION ENCIRCLED ENERGY VALUES FOR THE DIFFERENT FIELDS OF VIEW, AT Z=0° AND AT Z=66°
1 LENS +ADC CORRECTOR Field radius (deg) Zenith (z=0° ) 1.76 pxl (26.4 µm) 0 0.3535 0.5 0.707 0.735 94 95 92 81 80.4 z = 66° 2.6 pxl (39 µm) 98 97 91 81 80.6 z=0° 1pxl (15 µm) 96 89 80 86 88 z=66° 1pxl z=0° z=66° z=0° z=66° z=0° z=66° B Â I bands (365 Â 1014 nm) B band (420 Â 520 nm) V band (560 Â 650 nm) R band (650 Â 740 nm) I band (775 Â 900 nm)

1.16 pxl 1.25 pxl 1.46 pxl 1.5 pxl

1.76 pxl 1.76 pxl

(15 µm) (17.4µm) (18.8 µm) (22 µm) (22.5 µm) (26.4µm) (26.4µm) 95 93 87 85 85 89 96 89 80 82.5 91 98 91 81 83 81.4 96 95 86 87 80 96 95 85 85 93 98 95 82 81 91 98 95 80 80

Table 2-1 Percentage diffraction encircled energy values, from B to I bands and for each band, for the configuration of one lens and ADC FRACTION (%) OF DIFFRACTION ENCIRCLED ENERGY TWO LENSES Field radius (deg) U ÂI bands (0.320 Â 1.014 µm) 1.5 pxl (22.5 µm) 0 0.3535 0.5 0.707 0.735 86 93 93 83 80

Table 2-2 Diffraction encircled energy values for the configuration with two lenses

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VST optical data and performance

In Table 3-1 VST main optical data are reported. In Tables 3-2 and 3-3 the optical data, with the ADC inserted and with the ADC removed are respectively shown. In Tables 3-4, 3-5 the optical performance for the two configurations are reported.

VST MAIN OPTICAL CHARACTERISTICS Main optical data for the full system Optical configuration Pupil diameter Angular field of view F# Equivalent focal lenght Image scale Overall lenght Distance between M1 and M2 Spectral Range Distance M1 vertex to first corrector lens in B, V, R, I bands Distance M1 vertex to first corrector lens in UÂ I bands Distance M1 vertex to CCD plane Footprint diameter of light beams in M1 centre hole Distance between last corrector element and the image plane Modified Ritchey Chretien 2600 mm 1.47° 5.5 14381.04 mm (two lenses) 14484.9 mm (one lens +ADC) 0.21 arcsec/pixel 4447.11 mm (fixed) 3377.11mm (fixed) U Â I bands 420. 3 mm (>min. req) 502.8 mm (>min. req) 1070mm ( min. req) 259.1mm for two lenses (> min. req) One lens +ADC with curved entrance and exit surfaces. Two double prisms made of UBK7 and LLF6 Two lenses 16 k x 16 k 15µm x 15µm 2600 mm 720 -9746.3mm -1.151891 1.87 893.4 mm -4511.32 mm -5.778442
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Image plane corrector in B, V, R, I bands Atmospheric Dispersion Corrector (ADC) Image plane corrector in UÂ I bands Focal Plane CCd mosaic Ccd pixel size Primary Mirror parameters Outer Diameter Inner Diameter Ray of curvature Conic constant K1 f/number Secondary Mirror parameters Diameter Ray of curvature Conic constant K2


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Table 3-1 VST main optical data

OPTICAL DATA FOR ONE LENS AND ADC CORRECTOR, FILTER AND DEWAR WINDOW Element Prism angle R1 R2 Material Diameter Thickness Air thickness L1 0° 957.75 mm 1412.5mm Silica 441 mm 60mm 285.05mm 429.5 mm ADC S1 0° -1496.2 UBK7 380.6mm 20 mm 0 mm (First prism) ADC S2 2.8° Infinity LLF6 380.5mm 15 mm 3 mm (First prism) ADC S3 0° Infinity UBK7 377.1mm 15 mm 0 mm (Second prism) ADC S4 2.8° 8912.5 LLF6 377.1mm 20 mm 121.63mm (Second prism) Filter (B band) Filter (V band) Filter (R band) Filter (I band) Dewar window Infinity Infinity Infinity Infinity -7943.3 Infinity Infinity Infinity Infinity 8912.5 Silica Silica Silica Silica Silica 369 mm 369 mm 369 mm 369 mm 372 mm 372 mm 369 mm 369 mm 367.6mm 367.4 mm 15 mm 14.95 mm 14.92 mm 14.98 mm 20 mm 50 mm 50 mm 50 mm 50 mm 25 mm (req.)

Table 3-2 VST optical data for camera, ADC, filter and dewar window in, B, V, R, I bands

Element L1 L2 Filter Dewar window

OPTICAL DATA FOR TWO LENSES CONFIGURATION R1 R2 Material Diameter Thickness 794.33 mm 1044.1 mm Silica 430.3 mm 58.45mm 417.6mm -1453.8 8912.5mm Silica 384.1 mm 35 mm 381.9 mm Infinity Infinity Silica 375.4 mm 15 mm 375 mm Infinity Infinity Silica 372.8 mm 20 mm 372.5 mm

Air thickness 214.65mm 149.1 mm 50 mm 25 mm

Table 3-3 VST optical data for corrector filter and dewar window in U Â I bands

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VST OPTICAL PERFORMANCE (WORST CASE) FOR THE CONFIGURATION WITH 1 LENS +ADC B BAND Diffraction encircled energy Tangential maximum field of curvature Sagittal maximum field of curvature Maximum distortion Glass transmission V BAND Diffraction encircled energy Tangential maximum field of curvature Sagittal maximum field of curvature Maximum distortion Glass transmission R BAND Diffraction encircled energy Tangential maximum field of curvature Sagittal maximum field of curvature Maximum distortion Glass transmission I BAND Diffraction encircled energy Tangential maximum field of curvature Sagittal maximum field of curvature Maximum distortion Glass transmission Z=0°: 81 % in 1.76 pixel Z=66°: 80% in 1.76 pixel z=66°: 0.35 mm at =1.014 µm on axis z=66°: 0.33 mm at =1.014 µm on axis z=66°: 0.19% (< 0.3% goal) at =0.775 µm at the edge of the field 99 % at =0.810 µm Z=0°: 81.4 % in 1.46 pixel Z=66°: 80% in 1.5 pixel z=66°: 0.29 mm at =0.74 µm on axis z=66°: 0.32 mm at =0.74 µm on axis z=66°: 0.19% (< 0.3% goal) at =0.65 µm at the edge of the field 99% at =0.660 µm Z=0°: 80 % in 1.16 pixel Z=66°: 81% in 1.25 pixel z=66°: 0.27 mm at =0.65 µm on axis z=66°: 0.25 mm at =0.65 µm on axis z=66°: 0.19% (< 0.3% goal) at =0.56 µm at the edge of the field 99% at =0.580 µm Z=0°: 80 % in 1 pixel Z=66°: 85% in 1 pixel z=66°: 0.19 mm at =0.52 µm z=66°: 0.16 mm at =0.52 µm z=66°: 0.19% (< 0.3% goal), at =0.42 µm at the edge of the field 95% at =0.365 µm

Table 3-4 Optical performance for the configuration with ADC inserted

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OPTICAL OPERFORMANCE FOR THE CONFIGURATION WITH TWO LENSES (WORST CASE) U ÂI bands (0.320 Â 1.014 µm) 80 % in 1.5 pixel 0.34 mm at = 1.014 µm on axis 0.34 mm at = 1.014 µm on axis 0.18 % at the edge of the field at = 0.32µm 99 %

Diffraction encircled energy Maximum tangential field of curvature Maximum sagittal field of curvature Maximum distortion Glass transmission

Table 3-5 Optical performance for the configuration with two lenses

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Field curvature and distortion curves

In Figures 4-1, 4-2 the field curvature and distortion curves for the configurations with one lens and ADC and with two lenses are respectively reported.

Figure 4-1 Field curvature and distortion curves for the configuration with one lens and ADC at z angle of 63°

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Figure 4-2 Field curvature and distortion curves for the configuration with two lenses

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Spot diagram

In Figures 5-1, 5-2 the spot diagrams for one lens and ADC configuration at zenith and 66° are reported from B to I bands and in Figure 5-3 the spot diagrams for two lenses configuration are shown.

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Figure 5-1 Spot diagram for one lens and ADC at zenith from B to I

Figure 5-2 Spot diagrams for one lens and ADC at z=66°

Figure 5-3 Spot diagram for two lenses from U to I bands

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