Getting Started

A basic SkyCalc query

SkyCalc_iPy is very basic. We start by making a SkyCalc object:

>>> import skycalc_ipy
>>> skycalc = skycalc_ipy.SkyCalc()

and then call the get_sky_spectrum() method to get a default data set from the ESO SkyCalc server:

>>> tbl = skycalc.get_sky_spectrum()
>>> print(tbl[:2])
 lam         trans                 flux
  um                      ph / (arcsec2 m2 s um)
----- ------------------- ----------------------
  0.3 0.03407993552308744     14.237968836733746
0.301 0.04638097547092783     19.640944348240403

If we were to plot up the columns trans and flux against lam we would have something like this:

(Source code, png, hires.png, pdf)

Returned FITS file

The FITS file returned from the ESO server is automatically saved in our working directory under the name skycalc_temp.fits. This can changed by passing a different name to .get_sky_spectrum(filename=).

Returning different data formats in Python

By default the data returned by the method is formatted as an astropy Table() object, and is a shortened version of the full FITS file. In order to have the full 18-column table returned, the parameter return_type="table-extended" should be passed (tab-ext also works). A number of formats can be returned by .get_sky_spectrum(return_type=...) including:

>>> tbl = skycalc.get_sky_spectrum(return_type="array")
>>> tbl = skycalc.get_sky_spectrum(return_type="synphot")
>>> tbl = skycalc.get_sky_spectrum(return_type="tab-ext")

Value	Shortcut	Returned
table	tab	an astropy.Table object with 3 columns
table-extended	tab-ext	an astropy.Table object with 18 columns
array	arr	3 arrays: (wavelength, transmission, flux)
synphot	syn	2 synphot spectral objects: transmission, flux
fits	fit	an astropy.HDUList object with a BinTableHDU
none	none	a None object

Editing parameters

When initiated a SkyCalc object contains all the default parameters for a SkyCalc query, as given on the SkyCalc CLI website. The parameter names can be listed by calling keys:

>>> # Print the first 5 keys
>>> skycalc.keys[:5]
['airmass', 'pwv_mode', 'season', 'time', 'pwv']

The current value held in the SkyCalc object can simply be seen by calling the SkyCalc object directly. Alternatively, one can look in the values attribute.:

>>> skycalc["airmass"]
1.0
>>> skycalc["airmass"] = 1.2
>>> print(skycalc["airmass"])
1.2

Some of the keywords are not very descriptive. An extended description for the keywords can be found in the comments attribute:

>>> skycalc.comments["wgrid_mode"]
"Wavelength grid mode ['fixed_spectral_resolution','fixed_wavelength_step']"

Similarly allowed values or ranges for a parameter are kept in the allowed attrribute:

>>> skycalc.allowed["observatory"]
['lasilla', 'paranal', 'armazones', '3060m', '5000m']

To check what the default value for a parameter was, use the defaults attribute:

>>> skycalc.defaults["incl_moon"]
>>> 'Y'

In summary, the SkyCalc object contains the following 5 list/dictionaries:

• keys

• values

• defaults

• comments

• allowed

Getting spectral data from the ESO Almanac

It is also possible to get model spectral data for a specific date and time based on the recorded atmospheric conditions using the ESO Almanac service:

>>> skycalc.get_almanac_data(ra=83.8221, dec=-5.3911,
                             date="2018-12-06T06:00:00")
{'airmass': 1.07729,
 'msolflux': -1,                # See WARNING below!
 'moon_sun_sep': 347.059,
 'moon_target_sep': 149.041,
 'moon_alt': -37.9918,
 'moon_earth_dist': 1.02626,
 'ecl_lon': -172.651,
 'ecl_lat': -28.6776,
 'observatory': 'paranal'}

By default the returned values DO NOT overwrite the current skycalc values. This is to give us the chance to review the data before adding it to our SkyCalc query. If we already know that we want these values, we can set the update_values flag to True:

>>> skycalc.get_almanac_data(ra=83.8221, dec=-5.3911,
                             date="2018-12-06T06:00:00",
                             update_values=True)
>>> skycalc["airmass"]
1.07729

Warning

The Almanac currently returns msolflux=-1 for dates after 2019-01-31.

This indicates an error on the Almanac side. The only way to deal with this (without being super hacky) is for the user to reset the average solar flux to something normal before proceeding:

>>> skycalc["msolflux"] = 130       # sfu

If we would like to review the almanac data (i.e. default update_values=False) and then decide to add them to our SkyCalc object, the easiest way is with the update() method:

>>> alm_data = skycalc.get_almanac_data(ra=83.8221, dec=-5.3911,
                                        date="2018-12-06T06:00:00",
                                        update_values=False)
>>> skycalc.update(alm_data)
>>> skycalc["airmass"]
1.07729

With the updated parameters we simply call the get_sky_spectrum() method again to get the spectral data that corresponds to the atmospheric conditions for our desired date and time:

>>> wave, trans, flux = skycalc.get_sky_spectrum(return_type="arrays")

For dates after 2019-01-31, we must manually reset the average solar flux before calling get_sky_spectrum method again:

>>> skycalc["msolflux"] = 130       # sfu
>>> wave, trans, flux = skycalc.get_sky_spectrum(return_type="arrays")

In full we have:

>>> import matplotlib.pyplot as plt
>>> from skycalc_ipy import SkyCalc
>>>
>>> skycalc = SkyCalc()
>>> skycalc.get_almanac_data(ra=83.8221, dec=-5.3911,
...                          date="2017-12-24T04:00:00",
...                          update_values=True)
>>> # skycalc["msolflux"] = 130       # [sfu] For dates after 2019-01-31
>>> tbl = skycalc.get_sky_spectrum()
>>>
>>> plt.plot(tbl["lam"], tbl["flux"])
>>> plt.xlabel("Wavelength " + str(tbl["lam"].unit))
>>> plt.ylabel("Flux " + str(tbl["flux"].unit))
>>> plt.semilogy()

(Source code, png, hires.png, pdf)

Customs lists of values

SkyCalc_iPy uses the pyyaml package to parse input.

More coming here