Astrophysics

Observation Strategy

Calibration Frames

Bias frames: Zero level is very stable. The averge value is 21 ADU. Therefore, 5 to 7 of these frames at the beginning and at the end of the night are enough, they can be combined into a median bias frame by using imcombine.

Dark frames: The dark current on all the chips is negligible. During cloudy nights it was tested with exposures up to 4 hours, the dark level of the frames was 21, hence dark frames are not usually taken.

It is possible to take comparison exposure at each new position in order to avoid shifts due to flexions of the instruments with the built-in comparison lamps (He,Ar, Xe). If you are satisfied with shifts of a pixel or two, then a single comparison exposure in the beginning of the night is good enough. There is another comparison source available by using the screen on the dome. The lamps available are called azul and roja.

Flat-field calibration: Flat-field calibration is needed to remove the pixel-to-pixel sensitivity variations on the chip. LFOSC has another built-in halogen lamp in order to do it. You can take a flat-field image after each exposure or use the screen on the dome and take several images at the beginning of the night.

Calibration to flux (and removal of atmospheric and instrumental wavelength effects) can be readily achieved by observing one or more spectrophotometric standards during the night. We recommend using the ``Kitt Peak Spectrophotometric Standards". These are 25 mostly line-free stars calibrated at 50Å intervals from 3200Å to 8100Å (Massey et al. 1988 ApJ 328, 315); 11 of these have been calibrated out to 1 micron (Massey et al. 1990, ApJ, 358, 344). Finding charts can be found in each of the domes, as well in the 1988 paper cited above. The spectrophotometric extinction curve for Cananea is not available, but considering its proximity to Kitt Peak, we recommend you to use the relatively well-known curves for Kitt Peak available in IRAF. It is desirable that you observe spectrophotometric standards at airmasses similar to that of your objects.

Holemasks: If you are using holemasks, perpendicular to the direction of dispersion, i.e. in N-S direction, it is necessary a minimum separation of the holes of at least 5pixels in order to avoid overlapping spectra.

• Flux Calibration
• Wavelength calibration