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Page 1: VLT OPTICS - ESO · concave ones will be large off-axis ellipsoidal mirrors. If the manufacturing and the sensitivity for adjustments are too critical, there is an alternative solution
Page 2: VLT OPTICS - ESO · concave ones will be large off-axis ellipsoidal mirrors. If the manufacturing and the sensitivity for adjustments are too critical, there is an alternative solution
Page 3: VLT OPTICS - ESO · concave ones will be large off-axis ellipsoidal mirrors. If the manufacturing and the sensitivity for adjustments are too critical, there is an alternative solution

VLT OPTICS

DESIGN OF COUDE OPTICS

WP: T170

VLT Report No. 61

Status: October 1990

Prepared by: G . Avila B. Delabre D. Enard F. Merkle

Contents 1 Introduction

2 Coude Optics

3 Foci and Pupils Location 3.1 Foci location 3.2 Pupil location ......

4 Optical Quality of the Coude Focus

5 Mirrors 5.1 M4 and M6 Flat Mirrors . 5.2 M5 Elliptical Mirror ... 5.3 M7 Elliptical Mirror ... 5.4 Mirror M8 (Deformable Mirror for Adaptive Optics)

6 Alternative solution with toroidal mirrors

7 Sensitivity of the Coude Image

3

4

6 6 6

7

9 10 10 10 11

12

15

Page 4: VLT OPTICS - ESO · concave ones will be large off-axis ellipsoidal mirrors. If the manufacturing and the sensitivity for adjustments are too critical, there is an alternative solution
Page 5: VLT OPTICS - ESO · concave ones will be large off-axis ellipsoidal mirrors. If the manufacturing and the sensitivity for adjustments are too critical, there is an alternative solution

~V~L~T~R~E~P~O~R=T~N~o~61~ __________________________________ 3

1 Introduction Each unit telescope of the VLT will be equipped with a coude focus. There are two symetrical optical trains: one for the visible and one for the infrared wavelength range. Both trains will be fed via the Nasmyth beams. A flat mirror will be placed infront the Nasmyth focus in order to deflect the beam into the coude train.

Each coude train consists of 5 mirrors: three flats and two concaves. The concave ones will be large off-axis ellipsoidal mirrors. If the manufacturing and the sensitivity for adjustments are too critical, there is an alternative solution with on-axis toroidal mirrors at expenses of a slight degradation of the image quality.

The last mirror in the train will be the deformable mirror for adaptive optics for real-time seeing compensation.

The light beams will be protected by metal tubes for shielding and baffelling. They are designed to withstand vacuum in case air turbulences would degrade seriously the optical transmission inside the tubes.

Some of the mirrors of the train will be equipped with translation and tilt actuators for alignment. Some of these movements may be remotely controlled.

Because of the large number of mirrors of the coude trains, high efficiency coatings are mandatory.

The incoherent and coherent combined foci of the VLT will be fed via these coude beams by introducing a flat dichroic mirror and deflecting the beam towards the combinnig optics trains.

Page 6: VLT OPTICS - ESO · concave ones will be large off-axis ellipsoidal mirrors. If the manufacturing and the sensitivity for adjustments are too critical, there is an alternative solution

4 COUDE OPTICS

2 Coude Optics The optics of the coude trains is shown in the Figure 1. Only one of the two symetrical beams per telescope (visible and infrared) is represented. The second is symetrical with respect to the optical axis of the telescope. It is switched from one train to the other by rotation of the Nasmyth mirror M3 of the telescope.

In the coude configuration, the Nasmyth focus is situated at 700 mm below of the M4 flat mirror. The M5 concave mirror will produce an intermediate image between M6 and M7 (Figure 1). This image shows a high degree of astigmatism and it will be accessible only for alignments purposes.

Table 1 shows the main specifications for the coude focus.

Table 1: SPECIFICATIONS FOR THE COUDE FOCUS

F/# F/49.9 Scale 1936 JLm / arcsec Field of view 2 arcmin

232.3mm Focal length 399m

Page 7: VLT OPTICS - ESO · concave ones will be large off-axis ellipsoidal mirrors. If the manufacturing and the sensitivity for adjustments are too critical, there is an alternative solution

~V~L~T~R~E~P~O~R~T~N=o~6~1 ________________________________ . _____ 5

1

pupil for adaptive optics

M7 II

Coude focus

11iiit====BF====1Y" M4

M6

F/49.9

Nasmyth focus

N M5

_2m---l

Figure 1: Optical lay-out of the coude train

Page 8: VLT OPTICS - ESO · concave ones will be large off-axis ellipsoidal mirrors. If the manufacturing and the sensitivity for adjustments are too critical, there is an alternative solution

6 COUDE OPTICS

3 Foci and Pupils Location

3.1 Foci location Table 2 gives the locations and characteristics of the Nasmyth, intermediate and coude foci.

Table 2: FOCI LOCATION AND SPECIFICATIONS

FOCUS LOCATION F/# SCALE PROJECTED FOV (JLm I arcsec) (2 arcmin)

Nasmyth 700mm below M4 F/15 582 69.84 Intermediate 7057mm from M6 F /19.5 757 91

5556mm from M8 Coude and 13060 below F/49.9 1936 232.3

the center of M1

3.2 Pupil location The pupil is defined by the secondary mirror of the telescope, its intermediate image due to M5 is located between M6 and M7 at 2162 mm from M6 (Figure 1) and its diameter is 252 mm. The same as the intermediate foci, this intermediate pupil will be accessible only for alignment purposes. The exit pupil is located 011

M8 (the deformable mirror for adaptive optics) with a diameter of 110 mm.

Page 9: VLT OPTICS - ESO · concave ones will be large off-axis ellipsoidal mirrors. If the manufacturing and the sensitivity for adjustments are too critical, there is an alternative solution

~V~L~T~R~E~P~O~R=T~N=o~6~1 _____________________________________ 7

4 Optical Quality of the Coude Focus The optical quality of the image at the coude focus is given by the spot diagrams shown in the Figure 2 for a flat field. The astigmatism is pretty good corrected and only remains a small amount of residual coma.

Page 10: VLT OPTICS - ESO · concave ones will be large off-axis ellipsoidal mirrors. If the manufacturing and the sensitivity for adjustments are too critical, there is an alternative solution

8 ______________________________________ ~C~O~U~D~E~'~O~P~T~TC~S

COUDE FOCUS (F /50) "

Scale: + == 0.5 afcsec (968 ILm)

FIELD

I ANGLE Or. = 0 Or = 0.5 Br = 1.0 (."lfcmin)

Oy = + 1.0 ffi ffi r:~ ~ ... ;::. 1/1

By = + 0.5 EB EB ffi By = 0 E9 E9 ffi

EB ffi ® By = - 0.5 ", t\~! .~

I

® @~ Efj .~~i-O~ = -- 1.0 ::::::::: , I

Figure 2: Spot diagrams at the coude focus. M5 and M7 are on-axis elliptical nurrors.

Page 11: VLT OPTICS - ESO · concave ones will be large off-axis ellipsoidal mirrors. If the manufacturing and the sensitivity for adjustments are too critical, there is an alternative solution

~V~L~T~R=E~P~O~R=T~N=o~6=1 _____________________________________ 9

5 Mirrors

Tables 3 and 4 summarizes general characteristics and specifications of the mirrors.

Table 3: OPTICAL DESIGN CHARACTERISTICS OF THE MIRRORS

MIR- SURFACE SIZE* SHAPE THICK. MATE- MASS ROR PROFILE (mm) (mm) RIAL (kg **)

M4 Flat 164 X 115 Elliptical 20 Zerodur 0.75 M5 Ellipsoidal 640 Circular 91 Zerodur 74

or Toroidal M6 Flat 450 X 405 Elliptical 61 Zerodur 22 M7 Ellipsoidal 350 Circular 50 Zerodur 12

or Toroidal M8 Flat 118 X 115 Elliptical ~1 Silica(?) 21g

(Adaptive) *Useful areas **Diameter-thickness ratio = 7:1

Table 4: COORDINATES AND TILTS OF THE MIRRORS

MIRROR Z* Y* Tilt Beam Incidence Distance center-center (mm) (mm) (deg) Angle (deg) between two mirrors

M1 0 0 0 0 M4 6100 2500 -135 45 M3-M46100 M5 6100 -6097 -3.99 3.99 M4-M58597 M6 5654 -2928 -163 25.0 M5-M63200 M7 -1446 -10815 32.8 9.21 M6-M7 10612 M8 0 -7503 -168.2 11.8 M7-M83614 Focus 0 -13060 - 0 M8-CF 5556 *The coordinate system is the same defined for the Telescope and described

in §10 of the Design of Telescope Optics report, VLT Report No. 58 June 1989.

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10 COUDE OPTICS

5.1 M4 and M6 Flat Mirrors The M4 mirror will be retractable: it will stay in park position out of the Nasmyth field-of-view when the Nasmyth focus is in operation. When the coude focus is required, the mirror will be put in position on the optical axis to fold the beam 90° towards the coude train. M4 is a small flat mirror which will be integrated in the adaptor at the Nasmyth focus.

M6 is a large flat mirror (450 x 405 mm useful dimensions). Because the mirror stays very close to the beam coming from M4, a special support system has been foreseen in order to avoid vignetting.

5.2 M5 Elliptical Mirror Table 5 summarizes the main characteristics and specifications of the mirror.

Table 5: CHARACTERISTICS OF THE M5 ELLIPTICAL MIRROR

Useful diameter 640 mm Radius of curvature (primary) 8880 mm Conic constant (primary) -0.0216934

5.3 M7 Elliptical Mirror Table 6 summarizes the main Characteristics and specifications of the mirror.

Table 6: CHARACTERISTICS OF THE M7 ELLIPTICAL :MIRROR

Useful diameter 350 mm Radius of curvature (primary) -4990 mm Conic constant (primary) -0.214687

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~V~L~T~R~E~P~O~R~T~N~0~6=1~ _____________________________________ 11

5.4 Mirror M8 (Deformable Mirror for Adaptive Optics) This mirror together with a wave sensor located at the coude focus and a control unit, will provide compensation for atmospheric turbulence on the incoming wave-front. There will be also equivalent flat mirrors for both coude trains when the adaptive mirror is not in operation.

Table 7 summarizes the main characteristics and specifications of the mirror.

Table 7: OPTICAL DESIGN CHARACTERISTICS OF THE M8 MIRROR

Useful diameter 118 x 115 mm Number of actuators ~ 250 Stroke per actuator ~ ±5J.Lm

Page 14: VLT OPTICS - ESO · concave ones will be large off-axis ellipsoidal mirrors. If the manufacturing and the sensitivity for adjustments are too critical, there is an alternative solution

12 COUDE OPTICS

6 Alternative solution with toroidal mirrors There exists an alternative design with toroidal mirrors which can replace the elliptical ones in case of major manufacturing difficulties.

The optical quality of the image at the Coude focus is illustrated by the spot diagrams of Figure 3. The astigmatism is only partially compensated. Neverthe-less, it is interesting to note that the image quality can much improve if the image plane is tilted with respect to the optical axis of 26° (Figure 4). This tilt angle is the sum of all off-axis angles of the beam.

Tables 8 and 9 summarizes the main characteristics and specifications of the M5 and M7 toroidal mirrors.

Table 8: CHARACTERISTICS OF THE M5 TOROIDAL MIRROR

Useful diameter 640 mm Radius of curvature Ry * 8900 mm Radius of curvature R:r; ** 8943.5 mm * Ry on the figure plan ** R:r; on the perpendicular plan

Table 9: CHARACTERISTICS OF THE M7 TOROIDAL MIRROR

Useful diameter 350 mm Radius of curvature Ry * -5050.00 mm Radius of curvature Rx ** -.51 R2.45 mm * Ry on the figure plan ** R:r; on the perpendicular plan

Page 15: VLT OPTICS - ESO · concave ones will be large off-axis ellipsoidal mirrors. If the manufacturing and the sensitivity for adjustments are too critical, there is an alternative solution

~V~L~T~R~E~P~O~R~T~N~o~6~1 ____________________________________ 13

FIELD ANGLE ( arcmiu)

8y = + 1.0

8y = + 0.5

8y = - 0.5

8y = - 1.0

COUDE FOCUS (F /50) Scale: + == 0.5 arcsec (968 lim)

8" = 0

fffi'ffi\' , ...... ........ . •.•.....•. •••.....•..

~ .... j) .... ......... . ........ . .. . ......

~ W

8.r = 0.5 8" = 1.0

e ... : ....... : • "': I,'

I. I.

EBEBEB

., "

~... e··J

·:: ... :: •• ........ .... .... . ...... . . ::::::::::. .:::: :::: ::. I'::.:: : :: ............ ......... . ..... ~ .......... .......... . ....... . .......... .......... . ....... . ........ . ... ,.. . ... ,,-.. ,

Figure 3: Spot diagrams at the coude focus. M5 and M7 are on-axis toroidal mirrors. The image plane is perpendicular with respect to the optical axis.

Page 16: VLT OPTICS - ESO · concave ones will be large off-axis ellipsoidal mirrors. If the manufacturing and the sensitivity for adjustments are too critical, there is an alternative solution

14 ______________________________________ ~C=O~U~D~E~'~O~P~T~TC~S

COUOE FOCUS (F /50) Scale: + == 0.5 arcsec (9G8 J.lI1/)

FIELD ANGLE Or =:: 0 Or = 0.5 BT = 1.0 (arCllliIl)

Oy = + 1.0 EB EB ffi Oy = + 0.5 EB EB ffi

Oil = 0 EB EB E9 Oy = - 0.5 EB EB E9 By = - 1.0 EB EB EB

Figure 4: Spot diagrams at the coude focus. M5 and M7 are on-axis toroidal mirrors. The image plane is tilted of 26° with respect to the optical axis.

Page 17: VLT OPTICS - ESO · concave ones will be large off-axis ellipsoidal mirrors. If the manufacturing and the sensitivity for adjustments are too critical, there is an alternative solution

~V~L~T~R~E~P~O~R~T~N=o~6=1 _____________________________________ 15

7 Sensitivity of the Coude Image The sensitivity of the optical system with respect to tilts and displacements of the individual components is given in Table 10. The same coordinate system as for the telescopes will be applied. da, df3 and d, are the mirror tilts (in armin) around the x, y and z axis respectively, dx, dy and dz the mirror displacement (in mm). The coordinate system is indicated in Figure 1.

Page 18: VLT OPTICS - ESO · concave ones will be large off-axis ellipsoidal mirrors. If the manufacturing and the sensitivity for adjustments are too critical, there is an alternative solution

16, ______________________________________ ~C~O~U~D=E~'~O=P~T~T~CS

Table 10: SENSITIVITY OF THE COUDE IMAGE

MIRROR nlAGE Tilt Displacement Displacement Defocusing

(arcmin) (mm) (mm) (mm) do d(J d"( dy dx dz dz dx dy +1 -233

+1 -233 Ml +1 +765

+1 -27.7 +1 -27.7

+1 +32.4 +1 +32.4

M2 +1 -777 +1 -24.4

+1 +24.4 +1 -13.2

+1 +6.6 M3 +1 -6.5

+1 +3.33 +11.1 +1 +3.33 +11.1

+1 -1.35 +1 -0.66

M4 +1 +0.65 +1 -3.34 +12.80

+1 +3.44 -12.74 +1 -15.7

+1 +1.1 M5 +1 +15.6

+1 -0.14 -17.76 +1 -5.92

+1 -5.90 +2.51 +1 +10.8

+1 +5.3 M6 +1 +8.2

+1 -1.83 +10.8 +1 +1.18 -6.96

+1 +5.43 +1 +4.18

+1 -3.36 M7 +1 -2.59 -4.81

+1 +3.58 +1 +2.41 -5.98

+1 -3.3 +1 -0.67

M8 +1 1+3.,6

+1 -0.4 +1.92 +1 -0.4 +0.08

Page 19: VLT OPTICS - ESO · concave ones will be large off-axis ellipsoidal mirrors. If the manufacturing and the sensitivity for adjustments are too critical, there is an alternative solution

LIST OF FIGURES 17

List of Figures 1 Optical lay-out of the coude train .................. 5 2 Spot diagrams at the coude focus with elliptical mirrors . . . . .. 8 3 Spot diagrams with toroidal mirrors. The image plane is perpen-

dicular to the axis . . . . . . . . . . . . . . . . . . . . . . . . . .. 13 4 Spot diagrams with toroidal mirrors. The image plane is tilted of 26° 14

List of Tables 1 Coude focus specifications 4 2 Foci location and specifications 6 3 Characteristics of the mirrors 9 4 Mirrors coordinates and tilts 9 5 M5 elliptical mirror specifications . 10 6 M7 elliptical mirror specificatins 10 7 M8 mirror specifications . . . . . 11 8 M5 toroidal mirror specifications 12 9 M7 toroidal mirror specifications 12 10 Sensitivity of the coude image .. 16