E2 Aperture Positions for Better Fringe Flats

In 1999, the E1 aperture positions were introduced to allow first order CCD spectra to be positioned at row 900 near the CCD readout amplifier, thereby reducing the effects of Charge Transfer Inefficiency (CTI). This works well, however, for G750L and G750M spectra taken near row 900, the fringe flats have to be done using the 52X0.1 aperture rather than the 0.3X0.09 aperture, which is usually used for fringe flats near the center of the detector (see Section 11.2.3 for a more detailed discussion of IR fringe flats). Unfortunately, the 52X0.1 slit is shifted by about one pixel in the dispersion direction from the centers of the wider long slits. This misalignment reduces the accuracy of fringe subtraction.

To address this, we have defined three new aperture locations: 52X0.2E2, 52X0.5E2, and 52X2E2. When these apertures are specified, the target is placed off-center in the slit, at a position coincident with the 52X0.1E1 aperture. This improves the match between the fringes in the target and lamp spectra. Be aware, however, that the 52X0.2E2 aperture position is offset sufficiently from the physical center of the aperture that there will be noticeable changes in the aperture throughput.

These E2 aperture positions should only be used for ACCUM exposures with the G750L or G750M gratings when fringe flats with the 52X0.1 aperture are also being done. If a peakup is desired before using the E2 apertures, the peakup should be done using the 52X0.1E1 aperture.