Dual Filter Wheel

Highlights

• Symmetrical weight distribution
• FEA optimized weight and rigidity
• Innovative high-repeatability drive mechanism
• Sky & Telescope Magazine Hot Product

Multi-Wavelength Acquisition

Astronomical observations in visible wavelengths are facilitated using sensitive, scientific-grade CCD cameras. Optimal collection efficiency is achieved by sensors without color filter arrays that record the intensity of the incoming light at each pixel to form a greyscale image. In order to complement the spatial data with color information, color filters for each individual wavelength of interest are placed in front of the camera sequentially, a process which is automated using a motorized filter wheels.

Weight and Size

Large-area detectors (full-frame and medium format are commonly used) require filters sizes of 50mm × 50mm or even larger, which translates to a large size and weight of the filter wheel and in some designs, a large off-center load that disrupts the balance of the payload attached to the telescope mount and negatively impacts the performance of the overall system.

Repeatability

Long exposures along with data calibration and averaging are employed to extract useful information from the low light intensities involved in astronomical observations. In multi-band imaging, calibration is performed independently for each channel.

In order for the calibration procedure to produce meaningful output, the configuration and state of the whole image train must be identical during the collection of the calibration data as well as the science data. System parameters that are adjustable but must be consistent for proper calibration include position of each optical element, temperature of the detector, and integration time. Since calibration and science data is typically acquired during different imaging sessions, all moving components of the system must very precisely return to their nominal position in order for the image train to remain identical and for the calibration data to be applicable to the science data.

Mechanical Design Optimization

The instrument is designed around a dual-wheel center-aperture arrangement with symmetrical weight distribution. The stacking of two wheels in tandem along the optical path required careful optimization of the geometry in order for the system to remain suitable for optical trains with limited back focus. Finite element analysis was utilized to optimize the weight and rigidity of the symmetrical clam shell housing and wheels. A low-profile implementation of a novel adapter attachment mechanism was designed with  the length of the mated features of only 5mm.

Novel drive mechanism

Manufacturing processes limit the accuracy of typical mechanical drive components (such as motors, pulleys, belts, and idlers) far below what is necessary for accurate imaging system calibration and maximizing the sensitivity of the overall setup.

An elegant drive mechanism was devised for maximizing positioning repeatability that overcomes the manufacturing limitations of standard motion components. By designing the geometry of the custom machined parts such that they complement a collection of components with specially selected parameters, it was possible to achieve positioning repeatability that would otherwise require a much more complicated and costly solution.

Specifications

Filter Count	5 × 2 wheels
Filter Size	50mm or 2″ (square)
Filter Thickness	1mm | 2mm | 3mm | 4mm | 5mm
Clear Aperture	48mm (square)
Size	346mm × 280mm × 25mm
Weight	2kg