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edges.rst

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Edges

Overview

This file describes the Edges object. It contains the core information required to describe the edges of each slit on a given detector (or orders for echelle).

See below for the Current EdgeTrace Data Model. This is written to disk as a multi-extension FITS file prefixed by Edges in the Calibrations/ folder. See the :ref:`calib-naming` docs for more.

We also describe how to view the slit edges using pypeit_chk_edges.

Viewing

The preferred way to view the slit edges identified by PypeIt is with the pypeit_chk_edges script.

The script usage can be displayed by calling the script with the -h option:

pypeit_chk_edges

There are currently 2 options for viewing the slit edges on the image used to construct them. Each uses the pypeit_chk_edges script.

ginga

This is the default mode when executing, e.g.:

pypeit_chk_edges Calibrations/Edges_A_1_DET01.fits.gz

Warning

These files can take an annoyingly long time to load because they tend to be large and require decompression.

Two images are shown, the trace image (typically a combination of dome flats) and the Sobel filtered image used to detect the slit edges.

Here is a zoom-in screen shot for the :ref:`keck-lris-red` spectrograph.

../figures/slit_edges_ginga.png

A few things to note from this good-performing example:

  • The slits run nearly vertically across the image
  • The left/right edge of each slit identified is a green/red (purple in more recent versions) line
  • There were 13 slits identified (0 indexing)
  • The brightest slit is an alignment box and was discarded by the code

matplotlib

To avoid ginga, use the --mpl flag:

pypeit_chk_edges Calibrations/Edges_A_1_DET01.fits.gz --mpl

The color scheme is distinct and the labeling now includes -1 or +1 for left/right edges.

SlitTrace

The Edges file allows one to fully reconstruct its underlying object (:class:`~pypeit.edgetrace.EdgeTraceSet`) when instantiated from the output file. Unfortunately, this means the PypeIt object and output file are a bit too unwieldy to pass through the remainder of the code just to access its primary product, the slit-edge pixel coordinates. For that reason, we also create a :ref:`slits` object; follow the link for more description of that object.

Edges Troubleshooting

See :doc:`slit_tracing` for a discussion of how to customize, debug, and finesse your slit tracing results.

Current EdgeTrace Data Model

Internally, the processed image is held in :class:`~pypeit.edgetrace.EdgeTraceSet`, which subclasses from :class:`~pypeit.datamodel.DataContainer`.

The datamodel written to disk is: