When observers want to use fast read mode, we should first make sure it is reasonable to use. Fast read mode is not compatible with cadenced observing – an RV taken by KPF in fast read mode will have an arbitrary offset to an RV taken of the same star in normal read mode. Thus, fast read mode really only makes sense for observations which are complete within a single night (or small number of nights) such as astroseismology or RM measurements.

See Fast Read Mode on the KPF Web Page to see what we’re telling observers.

Split Nights: Fast and Normal

Changing modes induces a thermal transient which affects subsequent RV measurements, so we should avoid this unless it is well motivated scientifically and we should minimize the number of switches.

Procedures

The SA for the night is responsible for making sure that the read mode gets switched to fast for the appropriate section of the night and for switching it back to normal when appropriate. Even if a night (or portion of a night) is fast read mode, we must take the normal set of daily calibrations in normal read mode to maintain our knowledge of the instrumental drift and wavelength solution.

In addition to the normal calibrations we need to switch in to fast read mode and take a set of fast read mode calibrations (primarily biases). These should be done when the mode is changed to fast for science, not in a separate toggle of the read mode.

Switching to Fast Read Mode

To change mode to fast run: kpfSetReadModeFast

Then run the cals using:

kpfRunCalOB -f /kroot/rel/default/data/obs/kpf/FastReadoutBiases.yaml

Run these cals during nighttime if needed. This only takes about 8 minutes and we need time for the system to thermally relax after the disturbance of changing the read mode.

Observers should plan on not taking data for 20 minutes after any read mode change while the system relaxes. Taking data is possible, but the RV quality is questionable, so we recommend not taking bad data as we don’t want anyone to use bad RV measurements from the archive in a time series.

If switching read modes during the night when another instrument is in operation (e.g. you’re doing the read mode switch during a first half while another instrument is observing before a second half with KPF) try to avoid switching during the automatic overnight calibrations. You’ll see the script running via the script keywords or the display on the KPF OB GUI.

The overnight cals (when no observing is happening) run at approximately the following times:

• Q1 Etalon starts at 08:11pm HST and runs for 56 min until 09:07pm HST

• Q2 Etalon+LFC starts at 10:30pm HST and runs for 66 min until 11:36pm HST

• Q3 Etalon starts at 01:00am HST and runs for 56 min until 01:57am HST

• Q4 Etalon starts at 03:30am HST and runs for 56 min until 04:27am HST

After Fast Read Mode Observations are Done

To change mode back to normal run: kpfSetReadModeNormal

This must be done before the normally scheduled cals begin at 7am HST.