Filters

The new Z and Y filter which were purchased during the last period were installed in NOTCam in January 2011. It was noted that the Z-filter looks quite different compared to all the other NOTCam filters (non-transparent coating only on one side, a different edge fitting), but consultation with the provider assured it was completely vacuum grade proof and should not out-gas, and the direction of mounting should be arbitrary.

In February the filters were tested inside NOTCam on the sky for the first time. An internal report about the results where presented to the staff. The main results are that the zeropoints look good, the image quality looks good, but the background in the Z-band is very high. Tests indicate that part of the leak may be due to physical leaks around it and it is believed that this may improve by making a special mounting for this filter, potentially turn it around and installing a cold stop on top of it. However, some tests indicate there is a high percentage of thermal emission in this additional background. By crossing filters it is clear that there are no leaks in the J, H, and K wavelength regions, so it is thought that the thermal emission may originate from leaks at wavelengths longer than the K-band.

A main task will determining the specific cause of the additional background in the Z filter and looking at ways how to reduce it. Specifically, we are considering to open the instrument (in principle in conjunction with the replacement of the wheel bearings; see above) to improve the mounting and maybe add a long-wavelength blocking filter to reduce the background.