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5.8 Flat Fielding - Dome Spot or Internal Quartz Lamp?

There are trade-offs that must be considered to answer this question. For working in the region 3500Å 4100Å, the dome spot has poor reflectivity and the internal lamp is the preferred choice. Figure 10 shows the output of the internal quartz lamp.

The internal quartz is much better color balanced over this limited region and will require less exposure time than the dome spot, although it still may be tens of minutes. Dome flats (using the high banks) are preferred for long slit work (not in the UV) due to a more uniform slit illumination. In the 8500Å region, the high dome lamps also have a dip in response caused by the dichroic reflector on the back of the lamp. The low dome lamps (low banks) can be tried but generally have too low an output for most spectroscopic setups. The low banks reflectors have been aluminized to eliminate the dip at 8500Å.

For true long slit work (>3 arc-minutes), the uniformity along the slit is better using the dome spot. For stellar objects requiring only a limited slit length, it will be more advantageous to use the internal lamp. The uniformity over the central part of the slit is nearly identical for the spot and internal lamp. It is, however, more convenient to use the internal lamp since the telescope does not have to be repositioned to the spot.

Another approach to the long slit problem is to do twilight sky exposures. Although these are uniform, they do change quickly in time and, one may encounter some problems with polarization in the twilight sky.

Bear in mind that the Gold Camera chip has 8-9 electrons noise and that, for 1% work, above bias are required. On a per resolution element basis, this total charge is really spread over a 3x3 pixel area, or above bias. At low dispersion in the violet, the spectra will not be flat, and you may have to take several long exposures to attain at least 400 ADU above bias (gain=2.8 electrons/ADU) per resolution element.



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kessel@noao.edu
Wed Aug 2 13:05:25 MST 1995