Narrow Bandpass Filter Development
In 2011, Materion began work on a coating chamber that could handle large bandpass (BP) filters for newly developed telescopes and instruments. Filters are used to select specific wavelength bands of interest while rejecting unwanted starlight and are critical elements that enable scientific discovery. Filter development projects were planned around the impending construction of these new systems that included Subaru HSC, LSST, DECam and others, most of which called for large wide BP filters.
Materion’s Large Optics facility officially came on line in mid 2013 and is now fully operational. Large astronomy instruments, which up to now have employed mostly or only wide bandpass (WBP) filters are now able to utilize narrow bandpass (NBP) filters. There has always been a desire to select a very narrow chunk of the spectrum in order to enhance the science return and for earlier instruments; this was easily accomplished because the size was small. However, as instrumentation increased in size over the past 10-15 years, this posed a new problem: narrow filters could not be made in sizes large enough to satisfy the astronomers’ needs.
The situation has now been resolved as Materion can now produce narrow filters in large sizes. In fact, there is significant activity in this instrument upgrade arena. Since wide filters are still needed for some applications, we will continue to make both wide and narrow filters in very large sizes.
Results with Large Narrow Bandpass Filters
More recent Materion projects involve manufacturing and testing narrow filters with full width at half maximum (fwhm) bandwidths (BW) around 1 to 1.5 % and sizes in the 450 to 600 mm range. A majority of the work to date has been in support of instrument upgrades to provide narrow bandpass (NBP) capability to existing telescopes and instruments. This will improve the science return from the basic instrument and be cost-efficient, since filters are less expensive than new instruments. Materion expects to produce large NBP filters for the new generation of telescopes as soon as their requirements are established.
The filter band width (BW) typically dictates the filter center wavelength spatial uniformity. Narrower filters need to be more uniform in order to ensure the spectral feature of interest is always within the pass band. A rule-of-thumb is that we should strive to keep the wavelength variation across the filter to about 1/10 of the BW. So for a 1.5% BW, we have a goal of 0.15% wavelength uniformity - less than 1 nm PV everywhere on the filter at 600 nm. This is a serious challenge for filters a half-meter or more in size, but we are in fact achieving this kind of performance. These filters not only exhibit exceptional spatial uniformity of wavelength, they also retain their band shape and transmittance extremely well over the full area of the filter.
Figure 1: This medium band filter (32nm BW) was measured at many points from center to 280 mm radius in four azimuthal directions. Note the consistency of band shape and transmission. Central wavelength variation is about 0.2% (2 nm), or about 6% of BW. Target was less than 10% of BW. (tright)
Figure 2: This NBP filter was the first project in the Large Optics facility (February 2013). It is a 1.3% BW filter, which exhibits center wavelength variation of 1.3 nm (0.26%) over a 560 mm diameter – well within the requirement. (below)
Telescopes Utilizing Narrow Bandpass Filters
Astronomy instruments, which previously used wide bandpass filters, are now able to upgrade to NBP filters. Examples of equipment that switched from wide to narrow filters include Skymapper in Australia and Canada France Hawaii Telescope on Mauna Kea. Perhaps less well known but important projects requiring the use of NBP filters are KMTNet and other multi-telescope surveys.
Spectrophotometer Ensures Accurate Measuring
It is critical to accurately characterize these NBP filters. Relying on the measurement of witness pieces is completely inadequate. To ensure such accuracy, Materion has commissioned a custom spectrophotometer to measure transmittance and reflectance of filters as large as 750 mm diameter. We can currently measure filter spectral response over the CCD wavelength range with available upgrade paths able to extend the range into the IR as necessary. Sample positioning, wavelength scanning and data collection are all automated, making it simpler to map the entire surface of a large filter.
A large H-alpha NBP for Skymapper - Photo credit: RSSA, Australian National University
Materion Offers Range of Options
Astronomers now have a feasible NBP option in sizes up to at least 600 or 700 mm when planning either instrument upgrades or new astronomical imagers. Materion offers wavelength range that runs from less than 380 nm to greater than 2000 nm to meet your requirements. For more information on narrow band filters or the services of our Large Optics Coating Facility, contact Tom Mooney, Product Engineering Manager, Thomas.firstname.lastname@example.org