Instrument Configuration Files

Most of these parameters are the subject of considerable calibration efforts, and as such, should not be adjusted if the results are to be trusted. These files are obtained with the configuration module and are stored in $HOME/.slitlessutils/<VERSION>/instruments/

Instrument-Wide Settings

Table 5.3 Instrument-Wide Settings
Keyword	Unit	Description
Image units	$\mathrm{e}^-/\mathrm{s}$ or $\mathrm{e}^-$	The units of the images to be written.
File suffix	`flt` or `flc`	The file suffix in the HST parlance denoting files without and with the CTE corrections, respectively.
Path	`str`	The relative path from the `yaml` file where the files for this instrument are stored.
Focal-plane position	3-elements	The $(v_2,v_3)$ position of the reference point and $v_{3y}$ angle with respect the $v_3$-axis.

Instrument-Wide Grating/Blocking Parameters

Table 5.4 Instrument-Wide Grating/Blocking [1]
Keyword	Unit	Description
Global-Sky Image		The name of the global-sky image.
Tabulation Parameters	`dict`	This contains the starting wavelength (`wave0`), ending wavelength (`wave1`), sampling rate (`dwave`), units (usually `angstrom`), and `disptype` (which is always “grism”).
Extraction Parameters [2]	`dict`	This contains the starting wavelength (`wave0`), ending wavelength (`wave1`), sampling rate (`dwave`), units (usually `angstrom`), `disptype`, and if `alpha` for prisms.

Details on Extraction Settings

The extraction parameters for a grism and prism will be different, owing to the non-linear dispersion in a prism. Since a grism is linear, the formulae for the $i^\mathrm{th}$ extraction wavelength ($\lambda_i$) and the number of wavelength elements ($N$) are:

\[\begin{split}\begin{aligned} \lambda_i &=& \lambda_0 + \delta\lambda\,i\\ n &=& \frac{\lambda_1-\lambda_0}{\delta\lambda}+1\\ N &=& \lceil n \rceil \end{aligned}\end{split}\]

where $\lambda_0$, $\lambda_1$, and $\delta\lambda$ are the starting wavelength (wave0), ending wavelength (wave), and sampling rate (dwave), respectively. The notation $\lceil x \rceil$ refers to the ceiling function of x. For the prisms, which are highly nonlinear, the same two equations become:

\[\begin{split}\begin{aligned} \lambda_i &=& \lambda_0 + \delta\lambda\,\left(\frac{\alpha-1}{\alpha-i}\right)\,i\\ N &=& \left\lceil\frac{n-2+\alpha n}{n-2 + \alpha}\right\rceil \end{aligned}\end{split}\]

where $\Delta\lambda\!=\!(\lambda_1-\lambda_0)$ and $\alpha$ is a curvature parameter that adjust the degree of non-linearity. This form has several limiting forms worth mentioning. If $\alpha=0$, then there will be a single wavelength element ($N=1$), emulating the photometry from an imaging mode. Second, if $\alpha\gg n$, then the prism function approaches the linear form for a grism.

Note

In general, users are discouraged from adjusting these settings in the reference files, and are recommended to instead use the API (see sources). Extraordinary care should be taken regarding the curvature parameter.

Detector Parameters

Table 5.5 Detector Settings [3]
Keyword	Unit	Description
Focal-plane position	3-elements	$(v_2,v_3)$ position of the reference point $v_{3y}$ angle with respect the $v_3$-axis
Extension properties		`name`: the name of the extension (must be `str`) `ver`: the version of the extension (must be `int`) `dtype`: a valid `np.dtype`
Noise properties		dark current $D$ in $e^-/s$ readnoise $R$ in $e^-$
Detector dimensionality		`naxis`: 2-element list of size of detector (must be `int`) `crpix`: 2-element list for reference position (can be `float`) `scale`: 2-element list for pixel scale (can be `float`)
Distortion model		SIP coefficients should be a `dict`
Configuration files		The file name for each grating/blocking combination

Footnotes