KDP as a thermal blocking filter -- Deep near IR observations with a warm narrow band filter

Ground-based astronomy suffers from strong atmospheric line- and thermal continuum emission, at the near infrared (NIR, 0.7-1.1$\mu$m), and short-wave infrared (SWIR, 1.1-2.5$\mu$m) wavelengths. The thermal continuum emission increases exponentially towards the red sensitivity cutoff of the state-of-the-art 2.5$\mu$m cutoff SWIR detectors. Given availability of an optical quality shortpass filter material with strong blocking density in the SWIR, lower cost instrumentation, and higher performance filters could be designed. We demonstrate monopotassium dihydrogen phosphate (KDP, chemical formula KH$_2$PO$_4$) as a strong candidate for this purpose. KDP is fully transparent at wavelengths from ultraviolet to 1.3$\mu$m, but becomes highly opaque at wavelengths >2$\mu$m. We demonstrate on-sky use of KDP by improving performance of a cryogenic broadband filter with known off-band thermal leak, and using a non-cryogenic narrow band filter for deep observation. KDP reduces the sky background by 4.5 magnitudes in the leaky Z-band filter we use. Our 4nm wide, central wavelength 1.191$\mu$m narrowband filter in combination with KDP reduces the sky surface brightness by three magnitudes compared to a J broadband, although the effect of KDP is minor due to high blocking density of the broadband filter. We find a sky surface brightness of 18.5 mag arcsec$^{-2}$ in our bandpass at 1.191$\mu$m. KDP is an excellent thermal blocker, when its temperature is maintained above its Curie point at 123K. Below Curie point, KDP transforms its crystal structure, degrading its otherwise good imaging properties.