EUROPEAN SOUTHERN OBSERVATORY

La Silla Observatory

SCIENCE OPERATION DEPARTMENT

3.6m Telescope M2 Upgrade: Minutes of the Final Design Review Meeting LSO-MIN-ESO-90000-0003/1.0

Prepared	Olivier Hainaut	2003-06-04
Reviewed	Gerardo Ihle	2003-06-10
Released	Olivier Hainaut	2003-06-11

Revision History

Content

• 1.- Introduction
• 1.1- Purpose and Scope
• 1.2- Stylistic Conventions
• 1.3- Acronyms and Definitions
• 1.4- Documents and References
• 2.- Minutes
  • 2.1- Attendance
  • 2.2- Mechanics
  • 2.3- Electronics & control
  • 2.4- Simulations
  • 2.5- Variae & Summary of Actions

1.- INTRODUCTION

1.1- PURPOSE & SCOPE

This document contains the minutes of the 3.6m telescope f/8 M2 project Final Design Review (2003-06-03).

More than a formal FDR, this meeting was a progress report of the project, as many items are already completed or well into construction phase. This document also makes references to the "Delta Review" that took place on 2003-05-06.

1.2- STYLISTIC CONVENTION

1.3- ACRONYMS AND DEFINITIONS

SciOp: La Silla Science Operation Department
LED: La Silla Engineering Department

1.4- DOCUMENTS AND REFERENCES

• [1] 3P6-SPE-ESO-36022-1: f/8 top-end overhaul: user requirements, Sterzik
• [2] LSO-PLA-ESO-nn-mmm-zzz: 3p6 Telescope M2 electronics control, Macchino & Gojak
• [3] 3P6-TRE-ESO-80030-0003: Software Feasibility Report, Kastinen
• [4] 3P6-SFS-ESO-80030-0004/1.3: Software Functional Specification, Kastinen

2.-MINUTES

2.1- ATTENDANCE

The meeting took place at LSO and Vitacura on 2003-06-03, 08:30 till 10:30. It was attended by G.Andreoni, I.Kastinen, J.Alonso, W.Eckert, R.Parra, O.Lavin, N.Goncalvez at La Silla, and F.Franza, G.Ihle (Chair), G.LoCurto, A.Gilliotte and O.Hainaut in Vitacura.

2.2- MECHANICS

G.Ihle reported on the mechanical design of the upgraded M2 unit, and on the advancement of the work. This was done in detail to the benefit of F.Franza, guest from ESO/HQ and specialist in opto-mechanic design.

Focus Unit:

• the design is modified compared to that of the NTT. Care was taken not to have backlash, the rotation of the mirror is prevented by guiding sliders.
• The full range of motion is +/- 20mm, i.e. much larger than the +/-6mm in UsrReq, to ensure that the actual position of the focus will not be too close to one of the limits. The full encoder resolution is available over the whole range. FF notes that the init/limit switch(es) could be made adjustable, so that the whole range does not need to be scanned at each init. Nevertheless, IK point out that we have to make sure that the mirror is never actually beyond its init switch. GIH will investigate this.
• The system is balanced, although it is designed in such way that it will work with significative unbalance (this could potentially be used in case some weight has to be removed).

XY table:

• The design is directly inspired by that of FF for the NTT, with modification to work with modern motors.
• Most parts are manufactured, only the XY support flange in under machining afer its stress release by heat treatment process..

Pantograph:

• At previous meeting, there was some discussion about the best configuration for the Pantograph, i.e. parallel or conical. After investigation, it conical is much better, indeed it permits corrections without changing the pointing. A parallel pentograph would correct the coma in a similar way, but would introduce a collimation and depointing error.
• The legs of the pantograph should match the radius of curvature of M2. This is mechanically not possible bc of space constraints. Therefore, they are as long as mechanically possible. This will introduce a small depointing error, that shall be corrected by the pointing model, and refined on-the-fly by the auto-guider.

Spider:

• The current spider is operated with pre-stress. As this pre-stressing is performed using only the fixation bolts, it is inaccurate, resulting in an uncontrolable system. As the upgraded M2 will be installed on the current f/8 top ring, this cannot be completely solved. However, as the new system will permit control of the coma (either closed loop, or at least open-loop), this is not critical anymore.
• The thickness of the spider is 20mm, i.e. equal to the current one (10mm + 2x5mm reinforment plates).
• The design consider either "open" spider, or "filled". The filling could be performed by installed plates in the triangular opening (without causing additional light losses). The filled spider has slightly better rigidity, but the sensitivity to wind would be much higher. It is therefore decided NOT to install the filling plates and to work with open spider.
• GIh will design gutters to be installed on the top part of the spider to guide the cables and cooling fluid pipes. These gutters must be water-tigh, so that cooling fluid drips or water condensing on the cooling pipes will flow to the edge of the top ring and not fall on the mirror [question: when the telescope is inclined, some of these gutters will point toward the M2 unit, not toward the top ring. What do we do with water at that level??].

Inner body:

Construction under way - there were some delays caused by other projects interfering. GAn will take care of assigning priorities in order not to cause unacceptable delays.

Outer Body:

The outer body is designed with some stiffening rims, that can take the load of the current full preload of the spider while remaining within specs.

Cooling of the top control unit:

• A full LCU is located on the top of the M2 unit. As it uses standard VLT components, it dissipates significant heat, so that a cooling unit must be installed. This will be a standard VLT cooling unit, with liquid coolant. The total dissipation is ~500W, roughly distributed as following: cooling unit: 150W, fans: 90W, control electronics and drives: 200W. JAl remarks that a non-VLT compliant system could have been designed with very low power consumption, and that the VLT system is "an atomic bomb to kill ducks" for what we have to do, but this is the price for full VLT compliance.
• The presence of cooling fluid on the M2 unit is a source of concern. Some remarks:
  • the only joint will be at the Top Ring to Serrurier connection. These must be auto locking joints of high quality (only one droplet of fluid is lost when opening them).
  • the temperature of the fluid must be controlled. For instance, VLT uses a Delta-T of 6deg between the fluid and the air.
  • Instead of insulating the pipes (which is problematic considering the total budget of 20mm for the spider thickness), it may be worth to install the pipes in the gutters on top of the spider, the gutter taking care of evacuating the condensation water.

M2 Baffle:

This upgrade will re-install the current M2 baffle. A secondary project will take care of replacing it with an advanced baffle with air circulation (like NTT). Indeed, the improvement of the 3p6m Image Quality is likely to make the aberrations caused by the current baffle significant (while they are currently lost in the noise of the other sources).

2.3- CONTROL ELECTRONICS

Note: The presentation of the control electronic by JAl at this meeting was very short, essentially addressing the points that remained pending after the 2003-05-06 meeting. This section includes part of the discussion that took place at that previous meeting.

The control electronics is designed as fully VLT compliant, in a thermally neutral enclosure. It consists in a harmonic motor - encoder units, forming with the controller a closed velocity loop. The system includes precision limit switch, backed up by heavy duty safety limit switches.

As mentioned before, the total dissipation is about 500W, requiring a cooling system using cooling water. This, in turns, has some constraints on the mechanical design (see above), and implies that the water temperature must be controlled and adjusted wrt the dew point.

At the time of the previous meeting, the CPU, Maccon and amplifiers had not been purchased yet. This has been done by the time of this meeting.

The motors had originally been purchased equipped with brakes. However, the presence of these brakes does not make sense in the closed loop design used, therefore the motors had to be returned to the manufactor for removal. This has been done. GIh will track the motors and make sure they are not stalled in some remote storage.

2.4- SIMULATIONS

At the previous meeting, NGo presented detailed simulations of the full control system, in order to study the precision achieved in positioning the mirror. Indeed, as the position of the motors (as opposed to that of the M2) is measured, this is critical. It appears from these simulations that damping of the system is critical in order for M2 to reach its position in reasonible time and not to oscillate around that position.

2.5- VARIAE and SUMMARY OF ACTIONS

• GIh (with AGi) will prepare procedure to center the new M2 unit in the top ring while the top ring stands on the dome floor. This includes the preparation of required tools.
• Emilio Barrios (with help of IKa and AGi if needed) will optimize, streamline and automatize the aberration measurment procedure.
• GIh will communicate the final schedule to OHa before the end of June, so that this info can be communicated in due time to J.Breysacher for schedulling the telescope

Actions listed earlier:

• GIh will investigate the possibility of having adjustable init switch for the focus, to restrict the range available in operation.
• GIh will design water tight gutters for cables and water pipes, to be installed on the top edge of the spider legs.
• GAn will keep track of the work assigned to the workshop in order not to delay this project.
• GIh will trak the motors and make sure they return to la Silla in due time.