Suggested Exposure Times For Calibration Spectra

Flats: Properly exposed flats over all echelle orders require a two-step process. Flats are taken both with and without a CuSO4 (aka "blue") filter in the beam. The "blue" flats are then combined in software with "red" flats in order to recover enough response in the blue orders for good overall flattening. Suggested exposures are 7 seconds ("red"; no filter) and 240 seconds ("blue", CuSO4 filter) yielding S/N values of ~480. The filter for "blue" flats is listed as "blue" in the Config window in Echelle control under Remark.

Th-Ar Comparisons: 30 seconds with the Th-Ar lamp on gives high (~150) S/N for most bright lines at the expense of some saturated lines in the reddest orders. Median combining 30 second Th-Ar exposures with those of shorter duration can be used to retrieve useful signal in these orders if they are crucial for good dispersion solutions in the far red. Shorter exposures alone will not result in enough signal for good global dispersion solutions.

Exposure times for objects can be derived from the following plot: