DAZLE design concept

 

Introduction

DAZLE, the Dark Ages 'Z' Lyman Explorer, was funded from the PPARC Opportunity Scheme, which was set up to encourage truly innovative research, demonstrating originality and creativity. DAZLE is being designed by the AAO for the University of Cambridge - Institute of Astronomy.

DAZLE is intended to be the first imaging instrument optimised to detect faint emission lines in between the intense lines of the OH airglow spectrum which hinders current ground-based observations. In conjunction with the improved image quality now demonstrated with new 6.5-10 m telescopes, DAZLE will be well-suited for searching for early star-forming systems located beyond the redshift range probed by current instruments. The predicted flux limit for detecting unreddened Lyman-a emission at redshifts of 10 corresponds to a star formation rate of only 1 solar mass per year and the available large format detector makes it ideal for wide field survey work.


Functional Concept

The fundamental concept of DAZLE is to use the camera from CIRPASS in conjunction with very carefully chosen and constructed narrowband filters to detect high-z galaxies by differential imaging of redshifted Lyman-a emission at a specific wavelength, selected to be free of atmospheric emission. The CIRPASS camera has particularly low noise, high sensitivity and good imaging characteristics that make it especially suitable for this application.

DAZLE is intended to operate as a VLT visitor instrument, fitted to the Nasmyth platform of VLT UT3. Such a concept immediately requires certain system components:

  • An optical system is required to allow imaging of the Nasmyth focal plane by the CIRPASS camera.
  • Carefully specified filters are used for differential imaging. The technology to construct filters suitable for the purposes of DAZLE has only relatively recently been demonstrated.
  • A filter exchange mechanism is required to apply the narrowband filters as required for differential imaging.

Design constraints on the system are further imposed by operational and design features of the CIRPASS camera, and constraints imposed by ESO on visitor instruments fitted to the VLT. The CIRPASS camera can only operate viewing vertically downwards, and so cannot be mounted directly to the VLT Nasmyth rotator to track the rotating sky image during exposures. This constraint implies that a fold mirror must be used to turn the horizontal axis of the Nasmyth focus vertically so it may be imaged into the camera, and the camera rotated to track the sky by a mechanism that duplicates the action of the VLT Nasmyth rotator, but with a vertical axis.

VLT constraints on visitor instruments limit the following aspects of the system:

  • Size and weight
  • Vibration transmitted to the VLT structure
  • Heat transfer to or from the telescope enclosure
  • Earthquake survivability
  • Health and safety requirements

A sensitivity analysis further reveals that the DAZLE system needs to be maintained at a low temperature (-40C) to achieve the required performance, and so a refrigerated enclosure will be needed to house the system on the Nasmyth platform.


Mechanical Concept Design

The mechanical design presented in the AAO document 'DAZLE Mechanical Design' presented at the concept design review in March 2002 (DAZLE-AAO-Doc_0016) is shown in Figure 1 below. In this concept, the DAZLE instrument is housed in a refrigerated enclosure raised to the height of the Nasmyth focus on a subframe.

Conceptual design of DAZLE, mounted on the VLT UT3 Nasmyth platform
Figure 1: Conceptual design of DAZLE, mounted on the VLT UT3 Nasmyth platform

A collimator aligned with the Nasmyth focus collimates the input (f/15) beam and presents it to a fold mirror beneath the CIRPASS camera. The CIRPASS camera is supported in a structure that provides rotation capability for tracking the sky image. A filter deployment mechanism is mounted immediately beneath the camera, using filter wheels to present the different narrowband filters for differential imaging.

The overall assembly is described in the following substructures:

Support Structure

Figure 2 shows the support structures, identifying the separate substructures. The instrument is constructed around a main carrying plate, comprising a stainless steel optical bench. To allow the instrument to be housed in a refrigerated enclosure, the optical bench is supported by four insulating legs that will penetrate the floor of the enclosure. These legs are in turn supported on a mounting frame allowing the instrument to be transported in the refrigerated enclosure, separate from the lower support frame that raises the instrument to the correct height. Smaller structures are mounted on the optical bench to support the camera and the wheels assembly. The diagram also shows a conceptual mount for the collimator, although discussions with the IoA at the Concept Review have led to the intention of mounting the collimator components directly and independently to the bench.

DAZLE structural elements
Figure 2: DAZLE structural elements

Camera Assembly

The camera assembly shown in Figure 3 is that component of DAZLE that supports the CIRPASS camera, providing for rotation suitable to track the sky image during an exposure. Such rotation is an exacting requirement, and the bearing selected to enable this needs to be exceedingly accurate. Described in the AAO document 'DAZLE Camera Rotator Bearing Stability Investigation' (DAZLE-AAO-Doc_0017), the AAO has conducted tests to verify that the axial runout specification can be met by a commercially available precision bearing. Further prototyping work will form part of the preliminary design to demonstrate a suitable bearing support. Although it is expected that only one DC servomotor will be necessary to provide the required rotation torque, the design allows for the addition of a second motor should that become necessary.

Camera rotation assembly
Figure 3: Camera rotation assembly

Collimator

The optical train that collimates the light entering the instrument is being designed by the IoA, and the AAO's involvement at the CoDR stage is limited to noting certain design constraints and highlighting areas to which particular attention should be paid. The decision was made to mount the collimator components directly to the optical bench, taking advantage of the standard mounting screw holes in the bench surface and eliminating te need for an additional support structure. This approach additionally minimises the height of the overall structure.

Irrespective of the design of collimator is the need to avoid condensation on the outer surface of the first collimator lens, since the refrigerated enclosure keeps the instrument at a temperature of -40C, and the outer lens is exposed to ambient air. The suggested approach is to heat the air contained between the first and second lenses, although this leads to high thermal gradient across the second collimator lens, and the first lens at ambient temperature rather than chilled.

Filter and Mask Wheel Assembly

Figure 4 shows the assembly housing the two filter wheels used to deploy the narrowband filters and focussing masks.

Wheels assembly
Figure 4: Wheels assembly

This design shows electrically operated mechanical detents for holding the wheels in position, although the electrical design has now settled on the use of servomotors, rendering such detents unnecessary. The wheels assembly, mounted in the collimated beam in between the fold mirror and the camera, also provides support for a system cold stop, required to minimise contamination of the images by thermal radiation.

Insulating Enclosure

Scope of AAO involvement in DAZLE does not include detailed design of the refrigerated enclosure, being limited to specification of gross geometry. Discussions at the concept review led to a view by the IoA that the enclosure should take the form of a box which is lifted off the top of the instrument, with a floor or lower section that remains in place (penetrated by the legs of the optical bench).

There will be no penetrations in the removable section of the enclosure, all cables, LN2 feeds and the collimator penetrations being in the lower, fixed section.

As the camera rotates inside the enclosure, the cables and LN2 feed hoses connected to it must not be stressed beyond their limits. Several options for accomodating this were presented, and the preferred solution is to use a horizontal arm mounted to the camera rotation mechanism, supporting the cables. Cable movement is accomodated with extra length inside the enclosure, utilising free space above the collimator, with the penetrations low in the enclosure wall. IoA noted that the camera's CCD controller may be mounted immediately alongside the camera inside the enclosure, to minimise electrical noise associated with the long cables needed for the rotation relief arrangement.

Not shown on the concept diagrams is the evaporator associated with the refrigeration system. This is a large component that requires mounting inside the enclosure, and it is proposed to construct a support structure built up from the optical bench that straddles the collimator. The evaporator then sits near the top of the enclosure, above the front of the collimator, where it is mounted using standard anti-vibration mounts.

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