Engineering: NAHUAL Ireland Acquisition Camera, Focal Plane Mechanisms and Layout Tully Peacocke,...

Engineering: NAHUAL Ireland

Acquisition Camera, Focal Plane Mechanisms and Layout

Tully Peacocke, National University of Ireland MaynoothCarlos del Burgo, Dublin Institute for Advanced Studies

Niall Murphy, Dublin Institute of Technology

NAHUAL Consortium Meeting, Fuerteventura, Sept. 2008 2

Overview

• What is the focal plane layout to be?– Advantages and disadvantages of two concepts.– Packaging, cost, baffles, risk, ….

• The Acquisition camera• Focal plane slit deployment• Time scale for funding and effort• Funding and effort available in Ireland• Delivery schedule


Focal plane layout 1

• Ideally need as small an entrance window as possible– Must admit 10 arc second field.– Optimal heat rejection: reflecting prolate spheroidal

baffle.

• Stable alignment of feed from telescope to the slit in all modes.

• Stable alignment of acquisition camera to the science instrument.

• Manufacturing and maintenance cost and complexity.



• Current design has two paths to the slit.

• The four fold mirrors must pass the 10 second field in both directions.

• We lose 50mm of space between the telescope and the focal plane.

• Cannot fit the acquisition camera into the main cryostat: two cryostats.

• ADC in double pass to acquisition camera.

Illustration of the focal plane layout with both modes.



• Small acquisition camera within the main cryostat.

• Aligned to the science instrument once only.

• Can be used for alignment of the instrument to the telescope.

• Window clear aperture ø17mm admits full field of view.

• ADC used in single pass only.

Alternative: no fold mirrors between the window and the slit. (Optical path length between window and slit sameas in previous diagram.) Here the optics are folded out of plane of the optical bench (TBC).


Focal plane layout 4• Having the acquisition camera within the main

cryostat has the following additional advantages:– The gas cell is kept to a minimum diameter.– Higher throughput: 4 mirrors > ~5-7% loss.– Secondary mode stability is greater: both versions

have the same slit interchange requirements,– but no moving mirrors: this is critical; the mirrors have

pitch, roll and yaw as well as the problem of parking position. This risks degraded secondary mode.

– 50mm extra space between telescope and instrument and grating 100mm further forward => smaller cryostat.



– No secondary cryostat: cost and maintenance saving.– Alignment of camera to main optics is done once – at

assembly, and remains fixed.– ADC used in single pass: smaller prisms. Also the

acquisition camera sees the same aberration as slit.

• Potential risk to main mode needs to be considered – see later slide.– If secondary mode is unstable, both methods have

the same fall-back to main mode only operation.


Acquisition camera 1

• Provisional design based around NICMOS InGaTe detector– Operating temperature 70K– Detector size 256 x 256 pixels– Pixel size 40 microns– Plate scale 1.024 mm per arc second, pixel scale

0.039 arc seconds per pixel– Image space F# 13.14– Field of view 10” X 10”– Wavelength range 1.0 to 2.5 microns– Total mass of glass ~12 g (fold mirror not included)



Bounding squareis one pixel.



• The design is only provisional– If the location of the camera is to be outside the main

cryostat it will have to change completely.– If inside the main cryostat the packaging constraints

may force changes – work with the mechanical engineers and cryostat designers.

– A change in the choice of the detector will force a change in the design, but only in detail.

• Need to know if a filter wheel is required.– Filters would need to be of ø 10mm.– How many would be needed?



• Basic concept for the mounting is– Mount off the main optical bench, but on G10 truss.– Cold strap directly to the cold bath: no thermal contact

with the optical bench.

• Must have a fixed optical design before any mechanical design work is started– The exact configuration (folding, mounting etc) can

change, but not the optics or the detector.– Could design in a filter wheel that is never fitted, just

build in a compensation plate.


Focal plane slit deployment 1

• We are proposing to design, build and test a slit deployment wheel.

• Provisional specification:– Six slits and pinholes.– Repeatability ± 0.1µm spectral direction, 3 µm spatial.– Stability: spectral ~± 0.1 (1.0) µm, and for pinholes

needs to be ~± 0.1µm (?) spatial. (Compare this with 1/(plate scale) = 824 µm per arc second.)

– These are of the of the order • 1 milliradian at the axle.• 1/1000 (1/100) pixels in the spectral direction on the science

detector.


Focal plane slit deployment 2• How do you test a wheel for 0.1 µm tolerances?• Method 1

– Use capacitive sensors: commercially available with resolutions down to 2 nm. We need ~50 nm.

– Preliminary testing warm, then proof of conformance to specification in a test cryostat.

– Monitor long period stability (hours).

• Method 2– Direct observation for repeatability tests.

• Lambertian illumination of pinhole and microscope type setup, CCD and image processing.

• Both methods are non-contact.


Focal plane slit deployment 3

• Drive mechanism to be decided by test:– either a stepper motor with no holding current, ideally

no gearing. Requires a datum,– or modified Geneva mechanism. No datum required.

• Use the acquisition camera and calibration source to ensure that the correct slit is deployed– but would not have resolution required for check on

accuracy etc. Can use science detector for that.

• No holding current and minimal heat generation when deploying a slit is important.

• Must not require encoding.


Funding and available effort 1

• We have received €38K funding for a full-time postgraduate student to work on the mechanical engineering of the acquisition camera and focal plane mechanisms – subject to need.– Supervised by Tully Peacocke (optics/opto-

mechanics) Niall Murphy (mechanical engineering and testing) Carlos del Burgo (advisory supervisor).

– Supported by the design, manufacture and test facilities and staff of DIT.

– Starts October 2008 for two years.

• Optical design effort available – T. Peacocke.


Funding and available effort 2

• Need to decide what work is to be undertaken, and when.– Is the current layout fixed?– If so, we need to be told what is to be done because

none of our proposals are valid.• New design for the acquisition camera: must be done

quickly.• No design work on the focal plane layout and mechanisms.• Guidance, specification and interface control documentation

must be provided by the system engineer before we can do anything.


Time scale for funding and effort

• We have two years to design, build and deliver the acquisition camera opto-mechanics, and the slit deployment mechanism, if that is required:– Start date October 2008.– After Sept. 2010 we cannot be certain that there will

be funding to continue mechanical engineering work.

• We can start on generic lens mount design and cryogenic testing, and slit wheel design and testing, but by Jan. 2009 we will need to have started on the real system. Otherwise effort will be lost permanently.


Delivery schedule proposal• We have proposed that there is a two phase

approach to the delivery of the project:– Phase 1: main mode only

• The aim is to ensure that the time-line to main mode operation is as short as possible with minimum risk – no moving parts => no single point mechanical failure risk.

– Phase 2: Integration of secondary modes during a scheduled instrument service/warm-up.

• Install a thoroughly tested slit exchange mechanism, any prisms, etc, etc.

• Having obtained science grade data in Phase 1 we can observe any effects due to mechanisms introduced in Phase 2.

Engineering: NAHUAL Ireland Acquisition Camera, Focal Plane Mechanisms and Layout Tully Peacocke,...

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