The 1.68-3.06 μm segment of the new CVF has a thermal leak near its lower edge. Images taken with this segment have backgrounds which are 10-20 times higher in the lowest 5-8" of the slit at wavelengths less than about 2.3μm. The 2.4-3.0μm region is less affected. Fortunately most of this background is rejected when the grating is used and the resulting spectra are only marginally affected. The contamination from this leak is only a few percent.
The leak can be completely eliminated by inserting the short-pass blocker in the filter wheel (select CVF/blocker from the filter menu). Unfortunately, this can produce fringing at certain wavelengths. Therefore DO NOT USE THE BLOCKER FOR MOST OBSERVATIONS.
The only time to use the blocker is when you are observing sources which completely fill the slit AND when you are very concerned about absolute fluxes or absolute equivalent widths.
LAST UPDATE: February 1, 2010 (format only)