Engineering: NAHUAL Ireland

1
Engineering NAHUAL Ireland

• Acquisition Camera, Focal Plane Mechanisms and
  Layout
• Tully Peacocke, National University of Ireland
  Maynooth
• Carlos del Burgo, Dublin Institute for Advanced
  Studies
• Niall Murphy, Dublin Institute of Technology

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

3
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.

4
Focal plane layout 2

• 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.

5
Focal plane layout 3

• 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 same as in previous diagram.)
Here the optics are folded out of plane of the
optical bench (TBC).
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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 gt 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 gt smaller
  cryostat.

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Focal plane layout 5

• 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.

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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)

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Acquisition camera 2
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Acquisition camera 3
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Acquisition camera 4
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Acquisition camera 5
Bounding square is one pixel.
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Acquisition camera 6

• 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?

14
Acquisition camera 7

• 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.

15
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.

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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.

17
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.

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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-mechanic
  s) 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.

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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.

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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.

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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 gt 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.