Imaging

The Z-filter we have has a severe red-leak. The optics supervisor at NDC Infrared Engineering (who made the Z-filter for us) suggested KG4 and BK7 as two possible ways to reduce the red leak. The 5mm BK7 reduces the thermal leak by only $\sim$25%, but has a 90% throughput in the Z-band. The 1mm KG4 we got was, however, out of specs with only 5% transmission at 830-930 nm, while it was supposed to have $\sim$70% transmission. The provider thought this was probably due to old age, and since they do not produce it any longer, they suggested to send us at no cost a 0.5mm KG2 filter as replacement.

The KG2 filter has now been installed. It is very thin and difficult to handle. Dome-flats with lamps ON and OFF were made to check throughput and thermal leak, scaling them to the K-band. Preliminary results show that the throughput of KG2 at 830-930 nm is much better than for KG4, but this needs to be properly quantified. The thermal blocking seems to be no better than the 25% reduction we get with BK7, but this also needs to be properly checked.

The best way to obtain imaging flats with NOTCam is differential twilight flats. The differential mode cancels both the thermal component and the dark. The flat field stability has been checked with a few master flats taken with the WF camera and the JHKs filters. Investigation of ratio flats in periods where NOTCam was mounted several days in a row suggest that stray light is a problem. There seems to be a larger difference between consecutive nights due to pointing on the sky than if the same blank field is repeated. This points to scattered light and different light gradients. We decided to re-install the WF-camera imaging mask, a square mask to be put in the aperture wheel to limit stray light in the system. Because the aperture wheel has only 4 large slots (+ 8 small) we decided to remove the 64 micron slit in order to find space for this mask. The mask was tested once in 2003, but not for twilight flats. On the dark sky it made little improvement and was removed to make space for slits. Full tests will now need to be made to see if this mask reduces stray light. Also, more extensive tests will be made to check the stability of the flat fields.