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

TL;DR

Ground-based NIR/SWIR observations are constrained by atmospheric and instrumental thermal emission and detector cutoffs. The authors propose and test potassium dihydrogen phosphate (KDP) as a high-OD thermal blocker that remains transparent up to ~$1.3~\mathrm{\mu m}$, enabling ambient-temperature optics to feed a $2.5~\mathrm{\mu m}$ cutoff detector. On-sky demonstrations with a warm narrowband filter at $1.191~\mathrm{\mu m}$ and a $0.5~\mathrm{mm}$ KDP window show a substantial sky-background reduction, up to $4.5$ magnitudes in the Z band and about $3$ magnitudes in the narrowband relative to a J-band reference, without measurable thermal emission from the warm filter. The study discusses practical filter-design implications, potential alternative materials like KDP isomorphs, and caveats including birefringence and radiation concerns, arguing that KDP can enable cost-effective SWIR instrumentation and flexible, non-cryogenic filter concepts with significant performance gains.

Abstract

Ground-based astronomy suffers from strong atmospheric line- and thermal continuum emission, at the near infrared (NIR, 0.7-1.1$μ$m), and short-wave infrared (SWIR, 1.1-2.5$μ$m) wavelengths. The thermal continuum emission increases exponentially towards the red sensitivity cutoff of the state-of-the-art 2.5$μ$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$μ$m, but becomes highly opaque at wavelengths >2$μ$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$μ$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$μ$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.

Figures (13)

• Figure 1: Internal transmission (a) and optical density (b) for unpolarized light through 1 mm crystal. Gray dashed lines indicate radiance from a $T=288$ K blackbody.
• Figure 2: The effect of $T_{\mathrm{C}}$ on the KDP photometric performance. (a) $T_{\mathrm{KDP}}>T_{\mathrm{C}}$ no structure present, (b) $T_{\mathrm{KDP}}\sim T_{\mathrm{C}}$ partial transmission effects, and (c) $T_{\mathrm{KDP}}<T_{\mathrm{C}}$ strong variation in transmission and PSF distortions. The panels (a) and (b) show a 0.5 mm thick KDP, while the (c) shows a 4 mm thick KDP.
• Figure 3: Curie temperatures $T_{\mathrm{C}}$ and cut-off wavelengths $\lambda_{\rm SP}$ of KDP and its isomorphisms
• Figure 4: BP1191-4 filter characteristics. Transmission and sky radiance (a), and off-band optical density (b). The bandpass has a 4 nm full-width-at-half-maximum, and it is centered at 1.191 $\mathrm{\mu m}$.
• Figure 5: Table of observations