Filters

When commissioning the new broad band filters Z (0.83-0.93 micron) and Y (0.98 - 1.08 micron) last spring, it became evident that while the Y-band filter was performing according to expectations, the Z-band had a strong leak. Based on testing, the leak was suspected to be partly due to light leaking around the filter edge (owing to it having coating on only one side and being essentially transparent at the edges), and partly due to a red leak in the filter itself.

When NOTCam was opened due to the problems with the filter wheel (see above) we flipped the Z-filter around in its holder and added a cold stop ring on top of it. This lowered the background levels by $\sim$30% (normalised to the K band to correct for varying thermal backgrounds), but still a major red leak is going through the filter itself. Based on tests we know that this leak is mainly thermal emission from beyond 2.5 microns. A suitable blocking filter mounted in the other wheel would be needed to stop this leak. For details see the commissioning report on

 
  http://www.not.iac.es/instruments/notcam/staff/ZYfilters.html

Such a blocking filter should let through $>$95% in the Z-window and block long-wards of 2.4 microns. Quotes have been asked at several filter manufacturers, but apparently such a filter is difficult to make since the chemicals used to block from 1-3 microns also block at 0.8-1.0 microns. After a first estimate of $13000, we eventually got a quote: two pieces for $2400 (1713 euros) each. We are waiting for an estimate of the cost if we only order one filter. For comparison, the Z filter itself cost 2964 euros.

A cheaper work-around that will help somewhat was suggested by Marcus Wallace at NDC Infrared Engineering who made the Z-band filter. A 5mm thick BK7 glass installed in the other filter wheel should cut a lot more light beyond 2.5 microns while not influencing much the Z-band. Waiting for a quote on this. Another option is a 1 mm Schott KG4 glass, which however will reduce the peak in the Z-band to $\sim$70%.