$K_s$-band photometry of the Extreme T Subdwarf CWISE J221706.28$-$145437.6

We present deep $K_s$-band imaging of the extreme T subdwarf CWISE J221706.28$-$145437.6. Using the new photometry, we construct its spectral energy distribution and find this object exhibits exceptionally strong collision-induced absorption in the $H$ and $K$ bands. The comparison with the nearest benchmark extreme T subdwarf WISEA J181006.18$-$101000.5 suggests the object would be cooler and more metal-poor than the benchmark.