2x8.4m TELESCOPE
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LBT PROJECT 2x8.4m TELESCOPE |
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- INDEX :
0 - Introduction.
1 - Supply Description.
2 - List Of Enclosed Drawings.
3 - Acceptance Tests.
4 - Delivery.
The present specification covers the detail design, construction, machining, vacuum testing and delivery to the pre-erection workshop of the dummy cell for the M1 cells of the two 8,4 m LBT telescope.
The LBT telescope has been designed with the concept of performing the aluminizing of the primary mirrors on board of the Elevation Mount, using thus the mirror cell as the closing structure of the aluminizing equipment, positioning on top of it the upper bell jar complete with the cryo-pumps and aluminizing hardware. The complete operation shall be done with the mirror in vertical position, i.e. with the telescope horizon pointing.
The necessity of including in the telescope design a dummy cell is derived from the programmed completion of the telescope's optics; the first light of the telescope is foreseen with one only 8,4 m M1, so the telescope shall start its operation with installed the dummy cell in substitution of the second M1 with the relevant cell that shall be assembled on the telescope about two years later.
The Dummy Cell shall be identical in structural stiffness, mass and centre of gravity to the M1 cells of the present specification, of course including also the possibility of being completed with the dummy of the M1, the dummy of the instruments and of all the mechanical and electric control auxiliaries.
The Dummy Cell shall also serve as the closure for the vacuum bell jar for testing the aluminizing system off the telescope.
Completed the vacuum acceptance test, the dummy cell shall be transported to the telescope pre-erection shop and assembled on the EL structure in the position of both the R and L M1 cells, because the project can decide in every moment which will be the first M1 cell to be assembled on the telescope.
1.1- Shop Drawings, planning and technical documentation.
This is the first activity to be performed by the Manufacturer and, being the same for all the contracts for the LBT Telescope, its description and rules are included in the document 401a016/C dated Mar 24, 1997.
1.2- Structural requirements and Surfaces protection.
Construction of the steel structure consisting of plates and profiles of standard structural steel Fe 420 quality, welded with the proper procedures and accuracy to generate the minimum deformations of the cell shape and to obtain a vacuum tight assembly. Before machining, the structure must be stress relieved with normal thermal treatments or, not being available sufficiently large furnaces, with mechanical vibration systems.
The most constraining requirements of the cells structure are the geometrical accuracy for obtaining the machining of all the flanges within the demanded dimensions and tolerances, respecting at the same time the design thickness of all parts. As already evidenced in the M1 cells Technical Specification, of the same importance is the positioning of all the reinforcing ribs and beams of the top plate that must be sufficiently accurate to create no interference problems with the force actuators supporting the M1, having a very small design clearance.
For the above reasons, we recommend to evaluate carefully the possibility of making tests and gain of experience in building and machining the dummy cell before starting the two M1 cells, being the only way to verify, with some sample bores and a suitable template of the actuators, the ability of matching the structural tolerances(that for some actuators are smaller than 5 mm toward the ribs or their bottom plates).
It is evident from the included drawings that the dummy cell has less machining problems compared to the two M1 cells because are not required all the bores and slots in the top plate for the accessories supporting and ventilating the M1 and the machining of the rear of the cell for the bearings supporting the instruments, rotator and the cable chain.
Notwithstanding, it is mandatory for the Manufacturer specifying in the offer the construction procedure foreseen, specially the number of heat or vibration treatments to be carried on between the pre-machining and the final one, if any pre-machining shall be foreseen.
Another series of pieces of structure that must be positioned within tight tolerance (± 1 mm), are the interface plates with the relevant bolts holes between the M1 cell bottom and the Elevation structure (rolling sector inside and diagonal outside), as quoted on the interface drawing Dwg. 721a011/D ; it is clear that the outer plates must have dimensions larger and bores double than the necessary, because the dummy must substitute both the R and the L M1 cell.
Surfaces protection cycle. As during the M1 aluminizing the inside of the cell is exposed to 10E-2 Torr vacuum, the suggested cycle is as follows:
a) - assemble and weld the structure. To be decided by the Manufacturer if with intermediate steps of heat treatments and pre-machining or with one only step.
b) - Final stress relieving treatment.
c) - Machining of all flanges, surfaces and holes.
d) - Protection of all the machined surfaces for the following sand blasting and painting.
e) - Near-white metal blast cleaning according with the Steel Structure Painting Council Specification SP-10, or SA 2.5 ISO 8501 - 1:1988/SS055900:1988
f) - Apply Bostik Finch Cat - a - Lac 463-6-5, or approved equal, as a primer, 100 µ thickness.
g) -After the completion of the vacuum tests and the integration of the Cassegrain bearings and driving units, apply a top coat of Bostik Finch Cat - a - Lac 463-3-100, CA-118, or approved equal, in accordance with manufacturer's recommendations for surfaces to be exposed to vacuum, 125 µ thickness.
h) - For all the external surfaces, being the same the cycle but not having the exposure to vacuum, the manufacturer shall submit the type of paint agreed with the Supplier, taking into account that the demanded functional requirements are 25 years service life (two months
continuous salt exposure during shipping).
1.3 - Machining and Transport requirements.
1.3.1. - Machining requirements.
See DWG. 721a020/B --Dummy cell - Assembly.
After having considered the machining problems for the flanges and the top plate, that means a lathe with the rotating table able to accommodate a piece of about 9,5m diameter, one has to position the cell on a Numerically Controlled milling machine or an equivalent gantry and execute the boring and milling operations on the bottom plate concerning the fixation of all the bearings supporting instruments and the vacuum flanges, on the top plate only the ones (about 20) selected from the pattern of the force
The external flange for inspection and electrical utilities inlet must be carefully machined, bearing in mind that every opening shall be closed with a vacuum tight flange when preparing for the acceptance vacuum test.
As already mentioned in the M1 cells specification, it is important defining carefully machining procedure and achievable tolerances for the six lateral removable interface plates and consoles for the clamping of the bell jar.
1.3.2. - Transport -
For taking into account the needs of the transport of these large single pieces it has been included in their design the possibility of applying two lateral bearings to allow the rotation of the unit , presenting it in every angle between horizon and zenith, according to the road and trucks exigency. At the charge of the Manufacturer it is only demanded the construction and machining of the fixation flange for the bearing support.
1.4 - Supply Limits -
Last items being part of the supply of the Dummy cell are the temporary flanges for the vacuum test (one set - identical to the one of the M1 cells, excluding the one of the cooling air inlet - complete with the relevant O-ring and fixation bolts), according the dwg. 721a018A.
No dummies of M1 and the instruments/accessories shall be needed for the Dummy cell, because when assembled the dummy cell on the telescope, both at the shop pre-erection and on Site, shall be used the dummies ordered with the M1 cells.
2 - LIST OF ENCLOSED DRAWINGS.
Dwg. 721a011/D - Interface between Cell and EL structure -
Dwg. 721a020/B - Dummy Cell - Assembly -
Dwg. 721a021/A - Dummy Cell - Detail Sect. GG -
Dwg. 721a022/B - Dummy Cell - Detail Sect. CC - DD - View N P Q R -
Dwg. 721a023/A - Dummy Cell - Detail Sect. EE - FF - HH -
Dwg. 721a018/A - Dummy Cell - Details of vacuum flanges -
Dwg. 721a024/A - Dummy Cell - Detail bores for M1 dummy support -
Dwg.721a025/A - Dummy Cell - Detail bores for instrument dummy attachment
a) - Dimensional Test-
The cell shall be checked and dimensionally verified on the same numerically controlled milling machine where it shall be completed; at the same time, with a proper template supplied by the P.O., it shall be verified that, in the selected test positions, the force actuators shall be properly assembled in their machined position without interference with the top plate reinforcing ribs.
b) - Vacuum leakage test -
Being intention of the P.O. to tender for the supply of the aluminising bell jar at the same time of the M1 and dummy cells construction, it will be possible performing, in the cells Manufacturer workshop, the acceptance of each piece after a satisfactory vacuum test. The objectives of the tests are to determine that no external leaks exist at the level of 8x10-8 atmosphere per cubic centimetre /sec; and to verify that the entire volume can be evacuated to a pressure of 10 -5 Torr with a 6400 liter / second diffusion pump (or equivalent).
Being the bell jar and the three cells part of the same Request for Quotation, the Manufacturer shall specify in his offer if he has the necessary equipment and experience to perform the vacuum test, with the Representative of the P.O. only witnessing; in the opposite case shall be at the charge of the P.O. supplying the necessary equipment and organising the test, of course with all the necessary assistance of the Manufacturer.
It is demanded the delivery of the tested and accepted dummy cell ex works, loaded on truck, ready for transport, at the charge of the P.O., to the pre-erection workshop of the telescope.