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align.rst

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Alignment

Overview

This file describes the data model for the Alignment image. It is generally a simple combination of all input alignment frames, which are used to trace constant spatial position on a slit. These frames are useful in slit-based IFUs (e.g. cbar frames for KCWI).

The image is written to disk as a multi-extension FITS file prefixed by Alignment in the Calibrations/ folder. See :ref:`calib-naming` for the naming convention.

Inspecting

The first extension is the combined image. You can view it with any standard image viewer, e.g.:

ginga Calibrations/Alignment_A_1_DET01.fits

The image will also be a trimmed portion of the raw image and also re-oriented so that vertical is the spectral dimension with blue at the bottom, following the PypeIt :ref:`pypeit-orientation`.

Here is an screen shot of a ginga view for an example from the keck_kcwi spectrograph.

../figures/align_image.png

The alignment traces are color-coded by the slit number (i.e. the color alternates between orange and purple for each slit). In the example above, each slit contains five contbars, which are used for the spatial alignment.

Trouble Shooting

If your image appears to be in error, here are the things to consider:

  • Is one or more of your input alignment frames junk?
  • Check each of your input alignment frames. Some might be mislabeled.
  • If some alignments are apparently missing, you might want to check that the slits are correctly traced.

Current Alignments Data Model

The datamodel written to disk is:

The `astropy.io.fits.BinTableHDU`_ contains all the data from the underlying :class:`~pypeit.alignframe.Alignments` class. For details see its datamodel.