README.md
This repository contains a set of pre-calculated photon-limited velocity uncertainties for the Keck Planet Finder project, as well as a simple Python lookup function for quickly estimating photon-limited RV precision for a user-specified target observation.
In addition to a photon noise estimator (etc.kpf_photon_noise_estimate), there are also functions that calculate the required exposure time to reach a desired photon-limited RV uncertainty (etc.kpf_etc_rv), and to reach a desired spectral signal-to-noise value at a specific wavelength (etc.kpf_etc_rv).
To install, simply download the package, change directories into the downloaded folder, and run:
pip install .
Example uses below:
from kpf_etc.etc import kpf_photon_noise_estimate, kpf_etc_rv, kpf_etc_snr
# calculate photon noise estimate for a known exposure length
exp_time = 46. # s
vmag = 8.
teff = 5000. # K
sigma_rv_val, wvl_arr, snr_ord, dv_ord = kpf_photon_noise_estimate(teff,vmag,exp_time)
# exposure time estimate to reach a desired RV precision
sigma_rv_desired = 0.5 # m/s
exposure_time_sigma_rv = kpf_etc_rv(teff, vmag, sigma_rv_desired)
# exposure time estimate to reach a desired spectral SNR
snr_desired = 500.
wavelength_desired = 550. # nm
exposure_time_snr = kpf_etc_snr(teff, vmag, snr_desired, wavelength_desired)
The model grid descriptions (located in kpf_etc/grids) are as follows:
'dv_uncertainty_master_order.fits': order-by-order velocity uncertainites [m/s] (4D grid, Teff x Vmag x Exposure time x Echelle order)
'dv_uncertainty_master.fits': integrated KPF velocity uncertainty values [m/s] (3D grid, Teff x Vmag x Exposure time)
'snr_master_order.fits': mean order-by-order SNR values (4d grid, Teff x Vmag x Exposure time x Echelle order)
The basis arrays used to generate the grids are contained in these files:
'order_wvl_centers.fits': mean wavelengths of echelle orders [nm]
'photon_grid_teff.fits': stellar effective temperature array [K]
'photon_grid_vmag.fits': stellar V magnitude
'photon_grid_exptime.fits': exposure time array [s]
The precomputed grids were calculated assuming parameter bounds of:
- Teff: 2700 - 6600 Kelvin
- Vmag: 2 - 19
- Texp: 10 - 3600 seconds
Interpolating beyond these bounds will result in errors in the returned RV values.
All calculations are based on BT-Settl synthetic stellar spectra, available at: http://phoenix.astro.physik.uni-goettingen.de/?page_id=15
The current uncertainty grid uses the latest KPF PDR throughput model (30 micron STA detector, green & red camera), a median WMKO atmospheric transmission curve (Buton et al 2012), and excludes regions of the spectrum with 1% or deeper telluric features (based on telfit model). A vsini value of 2 km/s is assumed for all stellar targets, and the observation airmass is fixed at 1.2.
Written by Sam Halverson (sphalverson@gmail.com) and Sean Mills