NFM-AD-02-1001 Filter Wheel Design Requirements

1.                   Introduction

The purpose of this note is to summarize requirements for the filter wheel assembly.

In the conceptual design, the filter wheel assembly holds two filter wheels located sequentially along the optical axis. They are positioned directly ahead of the Lyot stop. Each filter wheel is driven by a cold stepper motor and gear train. Individual filter positions are defined by mechanical detents.  Limit switches determine which filter is in the light path. Filters are retained individually in cells formed in the wheel. The entire assembly is removable as a unit for filter changing.

2. Basic Design Requirements

2.1   Operating environment

2.1.1          Filter wheels must operate in a vacuum, 10^-6 torr.

2.1.2.        Filter wheels must operate at a low temperature, 65 K.

2.1.3.        The assembly shall be oil- and grease-free.

2.1.4.         The filter wheel shall be designed so as to keep the filters cold both in and out of the beam.

Radiative and conductive couping to the active cold structure provides the necessary thermal paths. Filters must be sufficiently heat-sunk to the cold mass as to avoid any thermal “pulse” that affects their bandpass or the internal instrument background when moved into the cold beam.

2.2   Number of filter positions

2.2.1.        Each filter wheel shall have eight positions. One position in each wheel is to remain open, and seven positions are available for filters.

One filter position will have a cold dark slide for calibration purposes. A total of 13 different filters can be placed in the wheels.

2.3   Filter location along optical axis

2.3.1.        The front face of any filter in the rear wheel shall be no further than XX mm from the Lyot stop.

2.3.2.        The front face of any filter in the front wheel shall be no further than YY mm from the Lyot stop.

These distances accommodate the expanding beam from the Lyot stop, plus allowances for mounting, positional repeatability, and flexure, with the filter dimensions specified in 2.4 .

2.4               Filters

Three wideband filters have been procured through a consortium:

J Band filter           1.10 – 1.35 microns

H Band filter          1.50 – 1.80 microns

Ks band filter         2.00 – 2.30 microns

These filters are 117 mm diameter x 5 mm thick.

2.4.1.        Procured filters must be placed in the filter wheel closest to the Lyot stop.

2.4.2.        All future filters shall be 124mm diameter x 5mm thick, to allow for placement versatility.

2.5               Positioning repeatability

2.5.1.        Filter center with respect to the optical axis shall be repeatable and stable to ±1.0 mm. 

2.5.2.        Filter tilt shall be less than 0.1 degree across filter diameter.

The tilt requirement applies to the total tilt from all causes, e.g. fabrication, assembly, alignment of the filter subassembly when installed, and operational flexure.

2.6               Speed of operation

2.6.1.   The replacement of any given filter by any other shall take no longer than 20 seconds.

2.7               Electronics interfacing

2.7.1. Drive motors shall be located on the filter wheel assembly (cold and in vacuum).

2.7.2. Cabling and connector configurations shall be consistent with the relevant Electronics SDN’s.

2.7.3. Cabling shall employ cold stationing as necessary to comply with requirements of Thermal SDN’s.

2.7.4.        Filter wheel control and readout shall be through instrument control electronics that interface to the NEWFIRM system controller and user interface.

See appropriate Electronics and Thermal SDN’s for further requirements.

2.8               Servicing

2.8.1.        Filters shall be replaceable individually.

2.8.2.        Filter replacement shall not require realignment of other optical elements or the detector.

2.8.3.        Alignment of filters to the optical axis shall be automatically recovered by the assembly installation process.

2.8.4.        Mechanical or electromechanical parts shall not require service intervals of less than three years.

2.8.5.        Service and reassembly procedures shall avoid the possibility of small parts such as screws being dropped inside the dewar. 

A design goal is that the entire electromechanical assembly function without maintenance or parts replacement over the lifetime of the assembly.

2.9               Lifetime

2.9.1. The filter wheel assembly shall have a 10 year lifetime, for operation within performance requirements, for the motors, gear train, detent mechanism, and central hub assembly.

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