Innovative Multiband Filters

Filters for astronomical imaging traditionally have a simple bandpass that admits (more or less equally) all the photons within some bandwith !! around some central wavelength !0. However, there are situations where not all photons are equally desirable. Filters with more complex bandpasses can provide substantial benefits under such circumstances. We propose to develop multiband filters for practical astronomical application. A multiband filter is a bandpass filter whose transmission dips to zero at select, undesired wavelength ranges. A closely related concept is a notch filter, where the gaps in filter transmission are narrow, typically encompassing a single emission line or doublet. We will pursue three distinct scientific applications of such filters. First, because the near-infrared night sky is dominated by bands of atmospheric OH emission, we will develop a multiband pseudo-J filter that omits the wavelengths with the brightest sky emission. Such filters have been discussed in the literature for some years (most notably by Offer & Bland-Hawthorn 1998). However, they were not practical when first proposed, and have not yet been implemented for astronomical applications. Recently, the technology to build such filters to the required level of complexity and precision has become available, and qualitatively similar filter designs are in widespread use in other scientific fields (notably fluorescence microscopy). We will design and implement a 5- band OH suppressing J filter, which should reach a target sensitivity level five times faster than a conventional J band filter. This gain is comparable to the gain produced by doubling the primary mirror diameter of a telescope used for J band imaging, or quadrupling the detectors pixel count, at a modest fraction of the cost. Such a filter will also reduce time variability in the sky background, which is an important source of systematic errors. Second, multiband filters can be used to increase the bandpass complement of instruments that allow multiple filters in series. This can be done through combinations of multi-band filters and blocking filters. It is especially valuable when physical filter changes are difficult, as when the filters are in a sealed cryostat (or, for that matter, in orbit). We will demonstrate this application with a triple narrowband filter (Paschen " 1282 nm, Fe II 1644nm, and Brackett # 2166 nm) to be used in series with broad band blockers, providing three narrowbands in a single filter slot.