Deputy Instrument Scientist:
The operation of KPF involves calibration exposures (CCD calibration, broadband flat lamp, ThAr lamp, etalon, and laser frequency comb) and science exposures of astronomical objects. The measurement precision of the science exposures is inextricably tied to the regularity and consistent quality of the calibration exposures. While the detector calibration and flat lamp frames help correct for CCD imperfections, trace the echelle orders, and measure the blaze function; the final sources in the above list are key for measurement of instrumental variation (i.e. the instrument precision). The calibration strategy is thus based on a tiered scheme to ensure both absolute wavelength calibration accuracy and dense sampling of the spectral bandpass to provide excellent constraint on stellar line positions. The ThAr lamp provides the initial absolute wavelength calibration across all orders, and allows labeling of etalon lines. The etalon then provides a high-SNR, uniform, and densely sampled wavelength solution across the bandpass.
Daily calibration exposures of ThAr (during daytime) are used to validate the long-term stability of individual etalon lines. This removes a concern about varying line positions in the etalon, since the constancy in dispersion far from the Rb line on > 12 hour timescales is currently unproven (although expected to be stable for our purposes). Subsequently, the operation of KPF itself will provide long-term validation of the etalon stability due to the daily cross-checking against ThAr.
At night, hourly calibration exposures from the etalon feeding both fibers simultaneously will demonstrate fiber-fiber drifts throughout the night. In mature operation, if KPF proves stable enough, these hourly calibration exposures may be dropped.
In addition, simultaneous calibration with the etalon during science exposures will track bulk instrument drift during the time of observation. The drift measured on the calibration fiber will then be translated to the science fiber by the Data Reduction Pipeline, and corrected from the final RV measurement.
Lastly, there are known issues with ThAr lamp aging and causing variation in line positions (Fischer et al. 2016). This effect will be monitored through monthly/biannual cross-checks of the “daily” ThAr lamp against a pristine “gold” ThAr lamp, and the daily lamp replaced with a spare if it shows too much degradation.
KPF runs automated calibrations multiple times per day. As a result, most observers should not need to take their own afternoon calibrations as they might have with other Keck instruments. If your program has special requirements and you need afternoon calibrations, please contact the Staff Astronomer supporting your night well in advance to coordinate this.
During the night, we recommend that observers take "simultaneous calibrations" which illuminate a fiber in the spectrograph's pseudo slit with light from the etalon.
In order to preserve the life of consumables in the calibration system and to maximize consistency for tracking instrument stability, the observatory will choose which light source is used for simultaneous cals (and for slew cals, see below). This is expected to be the etalon in almost all cases.
In addition, we recommend that on roughly hourly timescales, observers take a calibration exposure in which the etalon source is used to illuminate the science and sky fibers. This is done by routing light from the cal bench up through the FIU and injecting it in to the science and sky fibers via a fold mirror inserted in to the beam. Because this blocks light from the telescope, we recommend this be done during long slews and we refer to this as a "slew cal". There is an option in the KPF GUI (details TBD) to insert this in to an observer's OB such that the calibration is performed automatically during the slewing phase and should thus result is minimal (or zero during long slews) lost science time.
As with simultaneous calibrations above, the observatory will choose which light source is used during slew cals. This is expected to be the etalon in almost all cases.
KPF automatically runs the above set of standard calibrations every day. In addition, the solar calibrator is used to feed solar photos to the instrument for further calibrations during the middle of the day. This occupies the instrument essentially continuously from about 7am HST until after 5pm HST (exact time TBC). The standard calibrations are automatically ingested and used by the KPF DRP which also runs automatically.
Because high precision radial velocities depend on constant and consistent calibrations, we expect the vast majority of users will obtain optimal results by using the standard scheduled calibrations. As a result, there is no time budgeted for individual users to obtain custom calibrations. Furthermore interrupting standard calibrations will result in a damaging gap in the data which is used to calibrate out instrumental drift.
Thus, if your science program requires unique calibrations, please contact the Staff Astronomer supporting your run well ahead of your observing time to see if your calibrations can be scheduled within the constraints resulting from the daily operations of the instrument. Please note that it may not be possible to accommodate custom calibrations in all cases.