The Secondary Mirror, M2 Unit

Introduction

The M2 Unit is an optomechanical system constituted by a convex hyperbolic Beryllium mirror ("M2 mirror"), by the electromechanism used to support the mirror and to control its position, and by the related electronics. The position of the M2 mirror can be seen in VLT optical layout. The picture shows a CAD drawing of the M2 Unit. Also, you can see the front and back sides of the first M2 Beryllium mirror after machining at Loral, as of March 6, 1996. You can access the picture by clicking.
 [Secondary mirrow, front side]  [Secondary mirrow, back side]  [Secondary mirrow, CAD drawing]

Design

The M2-Unit is an optomechanical system constituted by the M2 mirror and by the mechanism used to control its position along 5 degrees of freedom. The M2 mirror is a convex hyperbolic mirror with an external diameter slightly in excess of 1 meter. By modifying its position and orientation it is possible to correct some optical aberration of the telescope (defocus and decentring coma) and to change the pointing. Physically the M2-Unit is comprised of the M2 mirror assembly, the electromechanical assembly, and an external electronic cabinet.

The M2 mirror assembly comprises the following functional assemblies:

  • The M2 Mirror, a lightweight hyperbolic mirror made of Beryllium in order to minimise its inertia and its mass. The central obstruction area of the mirror (central part which is not optically used) is equipped with an interface used for the mounting of optomechanical auxiliary equipment used for particular observations and for optical alignment.
  • The Mirror Support System

Electromechanical Assembly

The Electromechanical Assembly comprises the following functional assemblies:
  • A rigid Mechanical Structure connected to the telescope spiders
  • A Focusing Stage, which is used for focusing of the telescope and consisting of a servo controlled motor actuator generating a movement of the M2 mirror along the telescope optical axis.
  • A Centering Stage, used to keep the lateral alignment of the M2 mirror with respect to the primary mirror.
  • A Tilt/chopping Stage. This stage tilts the mirror along an axis contained in the plane perpendicular to the M2 mirror optical axis and ideally positioned at the mirror vertex. This is used to correct errors in telescope tracking and to perform chopping for infrared observation.
  • The Control System consists of the Local Control Unit (LCU) and of all the electrical and electronic hardware (including power supply) used to control the operation of all the systems inside the M2 Unit.
  • The Sky Baffle, which is used to obstruct an annular region of the sky immediately around the M2 Unit for particular observations. The sky baffle is deployed or retracted by means of remote control.
  • The Thermal Control System, which maintains the external surface temperature within a +/-3 deg C range with respect to ambient

Focusing

Focusing of the telescope is affected by disturbances like gravity induced deflections and thermal expansion which cause a modification of the distance between the telescope optical elements. This effect is recovered by the M2 Unit. Centring is used to laterally align the M2 mirror with respect to the primary mirror. A lateral misalignment of the two mirrors, mainly caused by structural flexibility or by residual optical adjustment errors, produces an optical aberration called decentring coma.

The pointing of the telescope is influenced by the tilt of the M2 mirror. This property is used both for correcting the tracking error of the telescope and for chopping for infrared observation. The tracking error sources of the telescope are the control errors of the telescope alt azimuth control system, the deflection of the telescope structure induced by gravity and wind and the atmospheric image motion. All these disturbances do not have a specified direction and vary continuously. A star tracker or autoguider will monitor a reference star and compute the angular error which has to be corrected by the M2-Unit.

To correct these fast disturbances the mechanics of the M2 Unit generates moderate magnitude tilt of the M2 mirror. To observe in the infrared range, the strong background noise of the sky is subtracted from the image to detect otherwise faint sources. This is achieved by a chopping system which is located inside the M2 Unit, and which, by a rapid oscillation of the M2 mirror causes a detector in the focal plane to see alternately two different, angularly close regions in the sky, one containing the object and the other not. The signal of the two regions is then subtracted to eliminate the background radiation which may be several orders of magnitude stronger than that of the astronomical object.

M2 Specifications

Drives

Focusing:
Range -39.0 mm to +8 mm Nominal Cassegrain position at zero
Velocity 0.5 mm/sec
Absolute accuracy ±50e-3 mm Max.
Differential accuracy ±1e-3 mm Max.
Shift of telescope focus due to focusing movement of secondary
Centering (around center of curvature):
Range ±6 arcmin Max.
Absolute accuracy ±5 arcsec Max.
Differential accuracy ±0.5arcsec Max.
Field stabilization:
Bandwidth 10 HzMin.
Amplitude spectral envelope of tilt input signal
Angular accuracy 0.05 arcsec rms 0 Hz < f < 6 Hz
0.1 arcsec rms 0 Hz < f < 10 Hz
Chopping:
Frequency 0.1 Hz to 5 Hz Square wave
Duty cycle 0.67 to 1.5
Amplitude 2 arcmin, peak to peak Max.
DC Offset ±1 arcmin Max.
Settling time 20 ms Max.
Accuracy 0.1 arcsec rms Throw < 40 arcsec ptp
Accuracy 0.1 * throw/40 Throw > 40 arcsec ptp
Drift 2 arcsec/hMax.

Secondary mirror

Optical useful diameter 1116 mm Outer
50 mm Inner
Radius of curvature -4553.57±10 mm
Conic constant -1.66926±0.003
Material Beryllium
Coating Al
Mass 51 kg mirror assembly 1800 kg total M2 unit
All surface slope errors 0.7 arcsec rms Max., after removal of defocus and 3. order spherical aberration of complete telescope, disregarding other errors
High spatial frequency surface errors CIR > 0.98
Micro roughness 2 nm rmsMax.