6.3. findApertures

6.3.1. Purpose

This primitive will take an existing polyfit model for the aperture locations in a frame, and fit the model to the observed data.

Note that this primitive is only run on prepared flat fields (i.e. images that have been run through the makeProcessedFlatG recipe). The primitive will abort if passed any other type of file.

6.3.2. Inputs and Outputs

findApertures takes no particular configuration inputs.

6.3.3. Algorithm

The RecipeSystem will automatically determine the appropriate initial polyfit model, based on the arm, resolution and date of the observation.

Note

Date selection of polyfit models has yet to be implemented.

The primitive will instantiate an Arm object from the polyfit.ghost module, which is included as a part of astrodata_GHOST. This Arm contains all the functions for fitting the aperture positions of the data.

The Arm object then makes the following three steps:

  • An initial model of the spectrograph is constructed based on the parameters read-on from the lookup system;
  • The reduced flat-field is convolved with the slit profile of the instrument, determined from the slit viewing camera
  • The initial model is fitted to the result of the convolved flat field, which represents the location of the middle of each order. The result of this fit is then used in the spectra extraction as the basis for the location of the orders.

A polyfit model FITS file is then written out to the calibration system.

6.3.4. Issues and Limitations

There is a placeholder retrieval function for getting back a fitted polyfit model, but there are no primitives yet for applying this model to data.

Future versions of this fitting procedure will include an extra polynomial describing the change in the spatial scale of the fiber images as a function of order location on the CCD. The current lack of such feature currently results in a slight innaccuracy of the fit on the edge of orders which are close together. This is due to the fact that the convolution with a fixed profile results is slight overlap at those locations.