Flat

Overview

This file describes the data model for the Flat. It is a series of images starting from the combination of all input pixelflat frames.

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

Inspecting

PypeIt provides the pypeit_chk_flats script to inspect the key Flat outputs.

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

$ pypeit_chk_flats -h
usage: pypeit_chk_flats [-h] [--type TYPE] [--try_old] file

Display flat images in Ginga viewer

positional arguments:
  file         PypeIt Flat file [e.g. Flat_A_1_DET01.fits]

options:
  -h, --help   show this help message and exit
  --type TYPE  Which flats to display. Must be one of: pixel, illum, all
               (default: all)
  --try_old    Attempt to load old datamodel versions. A crash may ensue..
               (default: False)

pypeit_chk_flats

This script takes a Flat as input and displays a series of images in a ginga viewer, each in a separate tab.

Here is a typical call:

pypeit_chk_flats Calibrations/Flat_A_1_DET01.fits

Below we describe the standard products. There is enough variation from spectrograph to spectrograph that we have not included example screen-shots.

Raw Flat

This is the processed and combined pixelflat image. Despite the name, it is not completely raw.

This image should look like any one of your input pixelflat images.

Pixel Flat

This is the normalized to unity image which is used to correct for pixel-to-pixel variations across the detector.

It should look mainly like a salt-and-pepper, random-noise image fluctuating very close around values of 1.

For many detectors there may be ‘pock’ marks, columns, and even funny patterns.

It is also typical for the extreme spectral portions (top/bottom) to have more structure or pattern noise. This is especially true if there is limited flux at these ends (e.g. the data goes below the atmospheric cutoff).

Illumination Flat

This image should also have most values near unity, but there will be vertical coherence. And the edges (left/right) may fall off well below unity.

Flat Model

This image should largely resemble the Raw Flat.

pypeit_show_pixflat

In addition to pypeit_chk_flats, if a custom pixel flat is provided by the user, another script pypeit_show_pixflat is available to inspect it. The script is called as follows:

pypeit_show_pixflat PYPEIT_LRISb_pixflat_B600_2x2_17sep2009_specflip.fits.gz

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

$ pypeit_show_pixflat -h
usage: pypeit_show_pixflat [-h] [--det DET [DET ...]] file

Show an archived Pixel Flat image in a ginga window.

positional arguments:
  file                 Pixel Flat filename, e.g.
                       pixelflat_keck_lris_blue.fits.gz

options:
  -h, --help           show this help message and exit
  --det DET [DET ...]  Detector(s) to show. If more than one, list the detectors
                       as, e.g. --det 1 2 to show detectors 1 and 2. If not
                       provided, all detectors will be shown. (default: None)

Troubleshooting

If one or more of your image appears to be in err, here are the things to consider:

Is one or more of your flat frames junk?

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

Did you saturate portions of the flat?

Current FlatImages Data Model

Internally, the image is held in FlatImages which subclasses from pypeit.datamodel.DataContainer.

The datamodel written to disk is:

Version 1.1.2

HDU Name	HDU Type	Data Type	Description
`PRIMARY`	astropy.io.fits.PrimaryHDU	…	Empty data HDU. Contains basic header information.
`PIXELFLAT_RAW`	astropy.io.fits.ImageHDU	float64	Processed, combined pixel flats
`PIXELFLAT_NORM`	astropy.io.fits.ImageHDU	float64	Normalized pixel flat
`PIXELFLAT_MODEL`	astropy.io.fits.ImageHDU	float64	Model flat
`PIXELFLAT_SPAT_ID-?-BSPLINE`	astropy.io.fits.BinTableHDU	…	bspline model of the pixelflat for `spat_id=?`; see `bspline`
…	…	…	…
`PIXELFLAT_SPAT_ID-?-FINECORR`	astropy.io.fits.BinTableHDU	…	2D polynomial fits to the fine correction of the spatial illumination profile of the pixelflat for `slit_id=?`; see `PypeItFit`
…	…	…	…
`PIXELFLAT_BPM`	astropy.io.fits.ImageHDU	int16	Mirrors SlitTraceSet mask for flat-specific flags
`PIXELFLAT_SPEC_ILLUM`	astropy.io.fits.ImageHDU	float64	Relative spectral illumination
`PIXELFLAT_WAVEIMG`	astropy.io.fits.ImageHDU	float64	Waveimage for pixel flat
`ILLUMFLAT_RAW`	astropy.io.fits.ImageHDU	float64	Processed, combined illum flats
`ILLUMFLAT_SPAT_ID-?-BSPLINE`	astropy.io.fits.BinTableHDU	…	bspline model of the illumflat for `spat_id=?`; see `bspline`
…	…	…	…
`ILLUMFLAT_SPAT_ID-?-FINECORR`	astropy.io.fits.BinTableHDU	…	2D polynomial fits to the fine correction of the spatial illumination profile of the illumflat for `slit_id=?`; see `PypeItFit`
…	…	…	…
`ILLUMFLAT_BPM`	astropy.io.fits.ImageHDU	int16	Mirrors SlitTraceSet mask for flat-specific flags
`SPAT_ID`	astropy.io.fits.ImageHDU	int64	Slit spat_id