Filters

The NOTCam Z-band filter (0.83-0.93 micron) red leak was determined to be mainly at wavelengths red-ward of the K filter. A proper blocking filter was searched for - it should have a $>$95% transparency in the Z-window and block long-ward of 2.4 microns.

As suggested by our contact at NDC Infrared Engineering (who made the Z-band filter but the company is closed now), one cheap way of reducing the red leak would be to add another 5 mm of BK7 glass, or potentially 1 mm of KG4 glass. We discussed these alternatives with Customs Scientific and got a quote (444 euros plus freight) for both which was accepted. Both filters were installed in NOTCam in March and tested in April. When crossing BK7 with the Z-band the sky background relative to Ks, a measurement of the thermal leak, went from 0.88 to 0.61. The background is still 10 times higher than the Y-band background. The measured throughput in the Z-band went down by $\sim$10% The KG4 filter blocks the red leak better but has a very low throughput in Z. These measurements need confirmation, as the data was taken in very bad conditions with very high backgrounds, but preliminary data shows that the leak can only be partially reduced, and in hindsight we should probably have gone for the expensive Barr Z filter in the first place.