The NOTCam Z-band filter (0.83-0.93 micron) red leak is located to be mainly at wavelengths long-wards of the K filter. We have looked for a proper blocking filter - it should let through >95% in the Z-window and block long-wards of 2.4 microns - but they are expensive. A cheap option of reducing the red leak would be to add another 5 mm of BK7 glass, or potentially 1 mm of KG4 glass. We got both of these installed in March and tested them in April. The KG4 filter blocks the red leak better but has a poor throughput in Z. We verified the performance of the glass filters in better sky conditions than used for the preliminary testing reported previously. This was then reported to the supplier who were surprised about the low throughput in the Z-band of the KG4 filter and explained it as the material having deteriorated with age. They offered to send a new filter (free of charge) that should give better throughput, since the actual KG4 has gone out of production. This filter arrived in July but we are still waiting to install it as we have to wait for the next time NOTCam will be warmed-up.
Thomas Augusteijn 2013-05-10