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 Calibration Frame 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 Image Orientation.
Here is an screen shot of a ginga view for an example from
the keck_kcwi spectrograph.
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:
Version 1.1.0
HDU Name |
HDU Type |
Data Type |
Description |
|---|---|---|---|
|
… |
Empty data HDU. Contains basic header information. |
|
|
… |
Spatial alignment data; see |
The astropy.io.fits.BinTableHDU contains all the data from the underlying
Alignments class. For details see its datamodel.