Astrophysics

Observing Strategies

Calibration Frames

The amount and type of calibration data you require depends critically on what you are doing. We'll begin by briefly reviewing the type of calibration data that may be required, along with our recommendations.

Bias frames: You may wish to take 10-15 of these at some convenient time when the dome is dark; they can be combined into a master bias frame by using zerocombine. In reality, none of the two chips have much bias structure (the hot columns cannot be effectively removed via biases), and so this is fairly pro forma.

Dark frames: You may wish to take three frames of the same length of time as your longest exposure to evaluate how much dark current is contributing if you are attempting to do long-slit spectrophotometry without local sky subtraction. In practice, the dark current on all the two chips is fairly minimal, and hence dark frames are not taken. If you are taking, be sure to make these exposures with the dome dark.

Flat-field calibration: Flat-field calibration is needed to remove pixel-to-pixel sensitivity variations on the chip.

Any line-free source will do to remove the first of these: either exposures of the large white spots on the side of the dome, or exposures with the twilight sky. For narrow bands, dome flats take too much time to get decent frames.

Transformation: Calibration to flux can be readily achieved by observing one or more photometric standards during the night. The average extinction curve for Cananea is available. But considering its proximity to Kitt Peak, such a curve for Kitt Peak can give equally good result.