corrector lens for the prime focus of the wht

IAC TECHNOLOGY DIVISION

DM/SR-WEA/023

AD1. Procurement technical

specifications for L4.doc

17 de junio de 2015

PROJECT / DESTINATION:

CORRECTOR LENS FOR THE PRIME FOCUS OF THE WHT

TITLE:

PROCUREMENT TECHNICAL SPECIFICATIONS FOR L4

PROCUREMENT TECHNICAL

SPECIFICATIONS FOR L4

Page: 2 de 13

Date: 17 de junio de 2015

Code: DM/SR-WEA/023 File: AD1. Procurement technical

specifications for L4.DOC

AUTHOR LIST

Name Function

José Alonso Burgal Mechanical Engineer

APPROVAL CONTROL

Control Name Function

Revised by: José Miguel Herreros Project Manager

Approved by: José Alfonso López Principal Investigator

Authorised by:

DOCUMENT CHANGE RECORD

Issue Date Change Description

1 16/06/2015 Final version



Page: 3 de 13




SUMMARY

This document includes the product description, technical specifications and deliverables for the

supply of the corrector lens L4 for the Prime Focus of the WHT.



Page: 4 de 13




TABLE OF CONTENTS

AUTHOR LIST ....................................................................................................................... 2

APPROVAL CONTROL ....................................................................................................... 2

DOCUMENT CHANGE RECORD ...................................................................................... 2

SUMMARY ............................................................................................................................. 3

1. LIST OF ACRONYMS AND ABBREVIATIONS ................................................................. 5

2. INTRODUCTION AND SUMMARY ................................................................................ 6

3. APPLICABLE DOCUMENTS ........................................................................................... 6

4. OPTICAL LAYOUT OF THE PRIME FOCUS CORRECTOR ................................................ 6

5. SPECIFICATIONS FOR L4 ............................................................................................... 7

6. OPTICAL TESTING REQUIREMENTS ........................................................................... 10

7. PACKAGING, HANDLING, STORAGE AND TRANSPORTATION .................................. 11

8. ACCEPTANCE .............................................................................................................. 12

9. DELIVERABLES ............................................................................................................ 12



Page: 5 de 13




1. LIST OF ACRONYMS AND ABBREVIATIONS

This section details the acronyms and abbreviations used in this document (sorted by alphabetic order):

ADC Atmospheric Dispersion Corrector

AOI Angle Of Incidence

Bi Blank number, i = 1 to 6

CC Concave

CGH Computer Generated Hologram

CoC Centre Of Curvature

CX Convex

FDR Final Design Review

FoV Field-of-View

IAC Instituto de Astrofísica de Canarias

IFU Integral Field Unit

Li Lens number, i = 1 to 6

LIFU Large Integral Field Unit

LOFAR Low Frequency Array

M1 WHT primary mirror

mIFU Mini Integral Field Unit

MOS Multi-Object Spectroscopy

PFC Prime Focus Corrector

PRI WEAVE Prime System

RMS Root Mean Square

RoC Radius Of Curvature

RTV Room Temperature Vulcanizing

WEAVE WHT Enhanced Area Velocity Explorer

WFE Wave Front Error

WHT William Hershel Telescope



Page: 6 de 13




2. INTRODUCTION AND SUMMARY

2.1. A new wide-field spectroscopy facility is proposed for the prime focus of the 4.2m William Herschel Telescope (located in the Canary Islands, Spain). The facility comprises a new 2-degree field-of-view Prime Focus Corrector with a 1000-multiplex fibre positioner (MOS), a small number of individually deployable Integral Field Units (mini IFU), and a large single Integral Field Unit (large IFU). The IFU and the MOS fibres can be used to feed a dual-beam spectrograph that will provide full coverage of the majority of the visible spectrum in a single exposure at a spectral resolution of ~5000 or modest wavelength coverage in both arms at a resolution ~20000. The instrument is expected to be on-sky by 2017 to provide spectroscopic sampling of the fainter end of the Gaia astrometric catalogue, chemical labelling of stars to V~17, and dedicated follow up of substantial numbers of sources from the medium deep LOFAR surveys.

2.2. The purpose of this document is to specify one of the six lenses, lens number 4 (L4), required for the construction of the Prime Focus Corrector:

3. APPLICABLE DOCUMENTS

3.1. Manufacturing drawings: the current set of specifications is summarised in these drawings ready to manufacture the lens L4.

4. OPTICAL LAYOUT OF THE PRIME FOCUS CORRECTOR

4.1. The purpose of the Prime Focus Corrector (PFC) is to correct for the optical aberrations in a 2-degree field-of-view and to compensate for atmospheric dispersion whilst the telescope is moving in elevation. The latter correction is performed by a pair of counter-rotating air-separated doublets known as the Atmospheric Dispersion Compensator (ADC).

4.2. The PFC consists of six lenses (see Figure 1) made of three different types of material (fused silica, N-BK7 and PBL1Y).



Page: 7 de 13




Figure 1: A schematic representation showing the optical layout of the 2-degree PFC. Not to be used for manufacturing. L4 is shown in the layout.

5. SPECIFICATIONS FOR L4

5.1. The Supplier will ensure that L4 conforms to the specifications detailed in Table 1.

5.2. Specifications for lens L4 are detailed in Table 1. The superscript numbers used after each specification refer to the clauses in the text, for example superscripts *1 and *2 refer to clauses 5.3.1 and 5.3.2 respectively.

Item Characteristics L4

1 Lens quantity*1 1

2 Shape of the lens*2 Surface 1 CX sphere

Surface 2 CC sphere

3 Radius of Curvature*3 Surface 1 1677.0 +/- 1.7 mm

Surface 2 1659.5 +/- 1.7 mm

4 Clear aperture*4 Surface 1 560 +2/-0 mm

Surface 2 556 +2/-0 mm

5 Overall diameter*5 602 +1.6/-0 mm

L1 (fused silica)

L5 (N-BK7)

Field plate

410mm 2˚ Field-of-view

F/2.78

L2 (N-BK7) L6 (fused silica)

11

00

mm

2326 mm

L3 (PBL1Y)

L4 (PBL1Y)

250mm

ADC 1

ADC 2



Page: 8 de 13




6 Centre thickness*6 65 +/- 0.3 mm

7 Surface tilt*7

(absolute value)

Surface 1 0.65 degree

Surface 2 No tilt applied – Datum surface

8 Edge Thickness Difference tolerance*8

(tolerance about nominal tilt) +/- 27 µm

9 Surface form irregularity*9

(total surface form errors)

Surface 1 80 nm RMS over 560 mm

Surface 2 50 nm RMS over 556 mm

10 Maximum transmitted WFE*10 197 nm RMS over 560 mm

Table 1: The specifications for lens L4 are detailed.

5.3. Description of the superscript numbers of Table 1:

5.3.1. Only one single blank will be delivered for polishing L4. Thus, no spare lenses have been specified.

5.3.2. The abbreviation “CX” stands for “CONVEX” whereas the abbreviation “CC” stands for “CONCAVE”.

5.3.2.1. Lens L4 is a classical spherical lens: their convex and concave surfaces are spherical.

5.3.3. The tolerance on the radius of curvature for L4 has been specified as a standard value: +/- 0.1 % of their nominal radius.

5.3.4. The vignetted beams were taken into account to calculate the clear aperture diameter of L4:

5.3.4.1. The clear aperture diameter of L4 is centred with respect to the optical axis of the WHT primary mirror (see note in the manufacturing drawings mentioned in clause 3.1).

5.3.4.2. The clear aperture radial tolerance is 1 mm.

5.3.5. The overall diameter of L4 was determined using the clear aperture of the convex surface of the lens:

5.3.5.1. On top of the clear aperture diameter, a certain margin was added for handling the lenses, as specified in the manufacturing drawings mentioned in clause 3.1.

5.3.5.2. The overall diameter of the lens is also the same as the diameter



Page: 9 de 13




of the finished blank. In that case, the outer edge of each blank doesn't have to be machined. This information supersedes the one in the manufacturing drawings mentioned in clause 3.1.

5.3.6. The values of centre thickness must be updated after the melt-fit re-optimisation has been carried out.

5.3.7. The surface irregularity contains the total surface form errors that can be described with the Zernike terms equal to or higher than the focus term (which is included).

5.3.7.1. The surface irregularity (surface form error) is specified in nm RMS, but upon request along with the response to this tender, this can also be specified in terms of fringes for a Newton’s ring type test.

5.3.7.2. The surface irregularity is specified over the full clear aperture.

5.3.7.3. The total surface form errors (also known as the surface figure or surface accuracy) can be divided into three spatial frequency domain ranges: low-, mid- and high-ranges (Table2).

Low frequencies

(z4 - z11)

Mid frequencies

(z12 - z211)

High frequencies

(> z212)

Clear aperture diameter

L4 Surf_1 75 nm RMS 25 nm RMS 15 nm RMS 560 mm

Surf_2 45 nm RMS 20 nm RMS 10 nm RMS 556 mm

Table2: The total surface form errors divided into low-, mid- and high-frequency domain ranges.

5.3.8. The maximum allowed transmitted Wave Front Errors take into account:

5.3.8.1. The characteristics of the blank (homogeneity, stress birefringence and striae) are specified in the drawings mentioned in clause 3.1.

5.3.8.2. The total surface form errors of each lens surface (quadratic sum), as mentioned in Table2.

5.3.8.3. Furthermore, the maximum allowed transmitted Wave Front Error can be divided the into three spatial frequency domain ranges: low-, mid- and high-ranges, as mentioned in Table 3.



Page: 10 de 13




Low frequencies

(z4 - z11)

Mid frequencies

(z12 - z211)

High frequencies

(> z212)

Transmitted WFE

L4 176 nm RMS 78 nm RMS 39 nm RMS 197 nm RMS

Table 3: The total transmitted Wave Front Errors divided into low-, mid- and high- frequency domain ranges.

5.3.9. Lens will be manufactured with a flat on the concave surface. This provides a surface for improved handling during the polishing and testing phases. The flat is also required to allow for the assembly of each lens in their cells and subsequent alignment of all the cells in the PFC housing.

5.3.9.1. The flat is perpendicular to the lens edge, within a tolerance given in the manufacturing drawings mentioned in clause 3.1.

5.3.9.2. The radial dimensions of the flat are related to the maximum thickness of the lens and dependant on which lens surface is tilted. For each lens, the dimensions of the flat are specified in the manufacturing drawings mentioned in clause 3.1.

5.3.9.3. Depending on which lens surface is tilted and on how the edge of the lens has been designed for assembly, the radial length of the flat could vary, but should have a minimum radial length of at least 10mm and be located 1mm beyond the clear aperture of the lens.

5.3.10. A standard protective chamfer will be applied to all the sharp edges of each lens, in order to protect their fragile corners.

5.4. The manufacturing of the lenses should ensure the Prime Focus Corrector to be operable under the following environmental conditions:

5.4.1. Temperature: from -5˚C to +25˚C.

5.4.2. Humidity: from 5% to 95%.

5.4.3. Telescope elevation: from zenith (0 degree) to 65 degrees.

5.4.4. Telescope altitude: 2400m above sea level.

6. OPTICAL TESTING REQUIREMENTS

6.1. The Supplier will carry out the following tests and produce test reports which



Page: 11 de 13




will be used by the IAC to determine whether the Goods can be accepted:

6.1.1. Testing of the concave surfaces (spherical and aspherical surfaces) will consist of measuring the total surface form errors by a sub-aperture or full-aperture interferometry measurement in reflection on the concave surface.

6.1.2. Testing of the spherical convex surfaces (all spherical) will consist of measuring the total surface form errors and/or the maximum transmitted WFE (combined with the glass inhomogeneity) by a sub-aperture or full-aperture interferometry measurement in reflection on either the convex surface (internal double-pass) or a return spherical mirror (through-lens double-pass).

6.1.3. In case the optical testing set-up required the use of test plates, the Supplier will provide the IAC with the optical characteristics (e.g. interferometric measurements) of all the individual test plates.

6.1.4. The total surface form errors and maximum transmitted Wave Front Error have been divided into three spatial frequency domain ranges (low, mid and high, as mentioned in Table2 and Table 3).

6.1.5. The total surface form errors and the maximum transmitted Wave Front Error are specified over the full clear aperture of the surface considered. In case of sub-aperture interferometric measurements, the total surface form errors and the maximum transmitted Wave Front Error can easily be calculated from the main specifications using a simple proportional law.

6.1.6. All measurements will be carried out under an environment where temperature, humidity and dust level are controlled. These values will be recorded for each measurement and provided to the IAC along with the report of compliance of each lens.

7. PACKAGING, HANDLING, STORAGE AND TRANSPORTATION

7.1. The packaging and packing material will be procured by the Supplier.

7.2. The Supplier will ensure that L4 is appropriately packaged to protect it from exposure to dust and moisture. Furthermore, L4 will be appropriately wrapped to protect them from scratches.

7.3. The Supplier will ensure that each crate will be equipped with a desiccant bag to prevent from dew.

7.4. The Supplier will ensure that L4 is appropriately secured in the crate so that:

7.4.1. Unintended movement of the lens is minimised.

7.4.2. The packing material offers some protection to L4 in the event that the



Page: 12 de 13




crate is mechanically shocked.

7.5. The Supplier will provide the shipping crate which will be used to package the finished lens for secure delivery to the site of integration.

7.6. Handling of the crates will be performed with a forklift from the underside of the crate or through the use of safety lifting strops which will be secured under the crate. The underside of the crate will incorporate fixed spreader beams to separate the box from the ground and to allow for mechanical handling equipment and lifting strops to be deployed beneath the crate. The crate shall have guides to correctly locate and fix the lifting strops. The guides may be created from another set of fixed spreader beams which create a channel. These will be correctly spaced to allow forks from mechanical handing equipment to be deployed.

7.7. Once L4 is polished, the Supplier will take care to store it in an environment that is conducive to maintaining the integrity of the lens.

8. ACCEPTANCE

8.1. IAC shall visit the Supplier, in order to carry out an acceptance test, as well as to inspect the lenses before L4 is packaged, packed and shipped.

8.2. If for any reasons clause 8.1 could not be met, then the Supplier shall deliver to IAC before shipping the acceptance test results with photographs and record notes taken during the final visual inspection of L4, as well as during the packaging and packing of L4 in its crate. A live video link may be required (e.g. Skype) during these steps.

9. DELIVERABLES

The following deliverables will be provided by the Supplier:

9.1. Lens L4, clean and identified.

9.2. The shipping crate, clearly and permanently identified.

9.3. Report of compliance for the specifications detailed in section 5, consisting of the measurement data that correspond to the specified parameters.

9.4. Additional deliverables:

9.4.1. Information about weight and location of the centre of mass.

9.4.2. Information about weight of the crate.

9.4.3. Recommended handling procedure.

9.4.4. Recommended cleaning procedure.



Page: 13 de 13




9.4.5. Optical characteristics of the individual test plates, if required.

END OF THE DOCUMENT

corrector lens for the prime focus of the wht

Documents