# Source object interface

## Input required for a Source object

A ScopeSim `Source` object is essentially a 2+1D (x, y, lambda) description of
an on-sky object. Hence to build one of these objects we need the following
data:

- A spatial description

  Either in table form (point sources) or in image/bitmap form (extended source)

- A spectral description

  A spectrum with wavelength and flux information

For example we could use two FITS images to describe the spatial flux
distribution of the young and old components of a spiral galaxy, and two spectra
from a starburst and an elliptical galaxy for the spectral description.

Internally this infor mation is stored in the following attributes:

- `<Source>.fields` - the spatial information \[Table or ImageHDU from astropy\]
- `<Source>.spectra` - the spectral information \[SourceSpectrum from synphot\]

More detailed information about the `scopesim.Source` object can be found in
the `scopesim` documentation.

Below is a description of how to initialise (create) a `Source` object

## Spatial description

### Series of arrays

The most basic method for creating point source `Source` objects is by
passing a series of arrays to the following argument keys:

- `x, y` : positions \[arcsec\] from the centre of the field of view
- `ref` : refers to the spectrum associated with a given point source

Optionally, if multiple point sources use the same spectrum, a scaling factor
can be applied to each of the degenerate point sources:

- `weight` : scaling factor

Code Example:

```python
src = Source(x=[1, 2], y=[-1, 5], ref=[0, 0], weight=[3.14, 0.5])
```

### Astropy Table

`astropy.table.Table` objects are a handy way to organize the information of the objects.
Each column can contain the spatial information and the reference to the spectra used in the table.

### FITS ImageHDU

Extended souces should provide an intensity map in the form of an
`fits.HDUImage` object. The header keywords should contain information
regarding position or the centre of the image (`CRPIXn`) relative to the
centre of the field of view (`CRVALn`) \[degrees\], and pixel scale
\[`CDELTn` | `CDn_m`\] \[degress / pixel\].

Each `ImageHDU` should also be
associated with an appropriate spectrum (`synphot.SourceSpectrum`) contained
in the list `<Source>.spectra`. The keyword `SPEC_REF` must be present and
refer to the list index of the corresponding spectrum in `<Source>.spectra`.

## Spectral description

### Series of arrays

- `wave`
- `flux`

### `synphot.SourceSpectrum`