ephem

Summary:

Cached ephemeris calculations for desisurvey.

Naming Convention:

ephem_YEAR-MM-DD_YEAR-MM-DD.fits, where YEAR-MM-DD are the first and last dates covered by this ephemeris cache.

Regex:

ephem_[0-9]{4}-[0-9]{2}-[0-9]{2}_[0-9]{4}-[0-9]{2}-[0-9]{2}\.fits

File Type:

FITS, 100 KB

Contents

Number	EXTNAME	Type	Contents
HDU0		IMAGE	Blank
HDU1	EPHEM	BINTABLE	Ephemeris table

FITS Header Units

HDU0

EXTNAME = (None)

Empty HDU.

HDU1

EXTNAME = EPHEM

Ephemeris table

Required Header Keywords

KEY	Example Value	Type	Comment
NAXIS1	2920	int	length of dimension 1
NAXIS2	31	int	length of dimension 2
NAME	Survey Ephemerides	str
START	2020-03-15	str	Calculated ephemerides start on the evening of this date.
STOP	2020-04-15	str	Calculated ephemerides stop on the morning of this date.

Required Data Table Columns

Name	Type	Units	Description
noon	float64		MJD of local noon before night
dusk	float64		MJD of dark/gray sunset
dawn	float64		MJD of dark/gray sunrise
brightdusk	float64		MJD of bright sunset
brightdawn	float64		MJD of bright sunrise
brightdusk_LST	float64	deg	Apparent LST at brightdawn
brightdawn_LST	float64	deg	Apparent LST at brightdusk
moonrise	float64		MJD of moonrise before/during night
moonset	float64		MJD of moonset after/during night
moon_illum_frac	float64		Illuminated fraction of moon surface
nearest_full_moon	float64		Nearest full moon - local midnight in days
programs	int16[4]		Program sequence between dusk and dawn (see notes)
changes	float64[3]		MJD of program changes between dusk and dawn
moon_ra	float64[25]	deg	RA of moon tabulated during the night (see notes)
moon_dec	float64[25]	deg	DEC of moon tabulated during the night (see notes)
venus_ra	float64[25]	deg	RA of venus
venus_dec	float64[25]	deg	DEC of venus
mars_ra	float64[25]	deg	RA of mars
mars_dec	float64[25]	deg	DEC of mars
jupiter_ra	float64[25]	deg	RA of jupiter
jupiter_dec	float64[25]	deg	DEC of jupiter
saturn_ra	float64[25]	deg	RA of saturn
saturn_dec	float64[25]	deg	DEC of saturn
neptune_ra	float64[25]	deg	RA of neptune
neptune_dec	float64[25]	deg	DEC of neptune
uranus_ra	float64[25]	deg	RA of uranus
uranus_dec	float64[25]	deg	DEC of uranus

Notes and Examples

The default date range is chosen large enough to cover commissioning, survey validation and the 5-year main survey, so should not normally need to be changed.

The desisurvey package includes a convenience wrapper that should normally be used to access this file:

import desisurvey.ephem
ephem = desisurvey.ephem.get_ephem()

This wrapper implements memory and disk caching, for efficiency, and returns an Ephemerides with useful methods for working with the table data. If you need direct access to the table use, for example:

print(ephem.table[:10])

The integer codes used for the nightly program sequence are DARK=0, GRAY=1, BRIGHT=2. Use program names where possible, instead of integer codes: the mapping between them is defined in the desisurvey.tiles module:

import desisurvey.tiles
assert deisurvey.tiles.Tiles.PROGRAM_INDEX['GRAY'] == 1

RA, DEC coordinates for the moon and planets are tabulated on an hourly grid spanning local noon - noon (inclusive, so 25 grid points) spanning each night. You will normally want to interpolate these values using the get_object_interpolator convenience method:

night_ephem = ephem.get_night('2022-12-25')
moon_altaz = desisurvey.ephem.get_object_interpolator(night_ephem, 'moon', altaz=True)
alt, az = moon_altaz(mjd)

Note that you can either interpolate in (RA,DEC) or (Alt,Az).