Reference Data

Reference data has its history in IRAF STSDAS SYNPHOT:

synphot> lpar refdata
(area = 45238.93416) Telescope area in cm^2
(grtbl = "mtab$*_tmg.fits") Instrument graph table
(cmptbl = "mtab$*_tmc.fits") Instrument component table
(mode = "a")

In pysynphot, reference data is expanded to manage the following:

The current settings can be displayed with showref(). To use custom settings or reset any changes back to software default, you can use setref(). Meanwhile, getref() also returns the current settings, but as a Python dict instead of printing to screen. For example:

>>> S.showref()
thermtable: /my/local/dir/trds/mtab/tae17277m_tmt.fits
   waveset: Min: 500, Max: 26000, Num: 10000.0, Delta: None, Log: True
 comptable: /my/local/dir/trds/mtab/yah1742rm_tmc.fits
graphtable: /my/local/dir/trds/mtab/yah1742qm_tmg.fits
      area: 45238.93416
>>> S.setref(graphtable='new_tmg.fits', comptable='new_tmc.fits', area=1)
>>> S.showref()
thermtable: /my/local/dir/trds/mtab/tae17277m_tmt.fits
   waveset: Min: 500, Max: 26000, Num: 10000.0, Delta: None, Log: True
 comptable: new_tmc.fits
graphtable: new_tmg.fits
      area: 1
>>> S.setref()  # Reset to software default
>>> S.showref()
thermtable: /my/local/dir/trds/mtab/tae17277m_tmt.fits
   waveset: Min: 500, Max: 26000, Num: 10000.0, Delta: None, Log: True
 comptable: /my/local/dir/trds/mtab/yah1742rm_tmc.fits
graphtable: /my/local/dir/trds/mtab/yah1742qm_tmg.fits
      area: 45238.93416
>>> S.refs.getref()
{'area': 45238.93416,
 'comptable': '/my/local/dir/trds/mtab/yah1742rm_tmc.fits',
 'graphtable': '/my/local/dir/trds/mtab/yah1742qm_tmg.fits',
 'thermtable': '/my/local/dir/trds/mtab/tae17277m_tmt.fits',
 'waveset': 'Min: 500, Max: 26000, Num: 10000.0, Delta: None, Log: True'}

Changing the default tables is not recommended unless you know what you are doing because pysynphot always uses the most up-to-date version in your $PYSYN_CDBS/mtab/ directory.

The HST bandpass for available observation modes are defined by graphtable and comptable. In addition, for IR instruments, thermal component is defined by thermtable. These files are described in detail in Appendix C. It is also possible to provide your own tables and telescope area for other telescopes.

The tables decide which throughput files will be used for a particular observation mode. They can be displayed using showfiles(). A bandpass that does not rely on the tables does not have this feature. For example:

>>> bp_hst = S.ObsBandpass('wfc3,ir,f105w')
>>> bp_hst.showfiles()
>>> bp_nonhst = S.ObsBandpass('johnson,v')
>>> bp_nonhst.showfiles()
AttributeError: 'TabularSpectralElement' object has no attribute 'showfiles'

Every HST observation mode has an optimal binned wavelength set (binset) for constructing an Observation. The binset is set according to a pre-defined wavelength catalog in pysynphot.locations.wavecat and can be accessed via bandWave(). The default waveset shown above is used for when such a binset is not available. The example below illustrate both situations:

>>> S.locations.wavecat
>>> bp_hst.obsmode.bandWave()
array([  7000.,   7001.,   7002., ...,  17998.,  17999.,  18000.])
>>> bp_nonhst.obsmode.bandWave()
AttributeError: 'TabularSpectralElement' object has no attribute 'obsmode'


Some calculations require the telescope collecting area in \(\mathrm{cm}^{2}\). For example, flux conversion involving counts or obmag, and bandpass unit response calculation.

When an area is required, a spectrum object first looks in its primary_area class attribute. If it is undefined, the object then takes the value from pysynphot.refs.PRIMARY_AREA, which defaults to the area of the HST primary mirror but can be changed with setref() (see Reference Data).

For non-HST calculations, you can set the primary area to the value of your telescope right after you import pysynphot and just leave it at that for the rest of the session. For HST calculations, you do not have to do anything because it is the default value. When in doubt, check the primary_area class attributes of your spectrum objects.

Composite spectra (CompositeSourceSpectrum and CompositeSpectralElement) inherit their primary_area values from either of the input spectra, if defined. If both input spectra have defined but different values, then an error is raised.

Bandpass object constructed from observation mode string (ObsModeBandpass) inherits its primary_area value from GraphTable, which in turn read its value from PRIMAREA keyword in the table primary header of the given *_tmg.fits file.

Observation inherits its primary_area from the input bandpass.

Wavelength Table

The wavelength table is a feature inherited from IRAF STSDAS SYNPHOT, in which it is known as wavetab. It is used to specify the name of a file containing a list of wavelength values that determine the wavelength grid to be used in the calculations and plotting. In pysynphot, this is equivalent to waveset, binwave, or binset, depending on the type of spectral objects that you are working with.

The default waveset can be changed using setref(). This is used when a spectral object has no instrument-specific (see below) or custom wavelength set (e.g., a Gaussian source has its own values that tightly sample the peak). The default grid consists of 10000 points covering 500 to 26000 Angstroms (sufficient for most HST calculations), spaced logarithmically with numpy.logspace() such that:

\[\log \lambda = \log \lambda_{min} + (\log \lambda_{max} - \log \lambda_{min}) \frac{i}{N}\]


  • \(N\) is the number of data points
  • \(i\) is the index value, starting from 0
  • \(\lambda_{min}\) and \(\lambda_{max}\) are the wavelength limits

Instrument-specific wavelength sets (binwave) are stored in a data file defined by pysynphot.locations.wavecat, which is “wavecat.dat” that comes with the software by default; The wavelength grid contains optimal coverage and resolution that is appropriate for each HST instrument.

Instead of modifying wavecat, which requires the knowledge of how wavetable works, it is easier to just provide your own binset. You can generate wavelength values using numpy.arange() (also accessible as pysynphot.Waveset()). If you wish to save the values in a file, follow the instructions in File I/O but ignore the second column (for flux or throughput). The wavelength values must be monotonically increasing or decreasing. See Tutorial 6: Custom Wavelength Table for a working example.