This file describes the data model for the Arc image. It is generally a simple combination of all input arc frames.

The image is written to disk as a multi-extension FITS file prefixed by Arc in the Calibrations/ folder. See Calibration Frame Naming for the naming convention.


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

ginga Calibrations/Arc_A_1_01.fits

Most often you use only one arc frame and this appears very similar to the raw image. If you do stack several, the output could be quite different.

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 shane_kast_red spectrograph.


Yup, an arc.

Trouble Shooting

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

  • Is one or more of your input arc frames junk?

  • Is one or more of your input arc frames mis-labeled?

Current ArcImage Data Model

Internally, the image is held in ArcImage, which subclasses from PypeItImage and DataContainer.

The datamodel written to disk is:

Version 1.3.0

HDU Name

HDU Type

Data Type



Empty data HDU. Contains basic header information.



Primary image data


Image mask


The detector (see DetectorContainer) or mosaic (see Mosaic) parameters



If a detector mosaic, this image provides the detector that contributed to each pixel.