Flat Fielding

Overview

This doc describes the Approach used to perform flat-fielding in PypeIt, how one goes about Modifying the Default Approach using the PypeIt Reduction File, and how to guide Generating the Flat Field Images.

Note that Slit Tracing is frequently performed with flat-field images; refer to that doc for a full description.

Approach

Flat-fielding corrections to the detector response

There are two primary components to flat-fielding in PypeIt:

Pixel-level flat fielding
Illumination corrections

The first accounts for pixel-to-pixel variations in the detector, whereas the latter corrects for spatial variations along each slit and at its edges assuming the slit should be uniformly illuminated from edge to edge. PypeIt also provides routines to:

perform a fine correction to the spatial illumination profile; and

correct for the relative spectral illumination of multiple slits.

Application

The code can implement any, none, or all of the corrections described above. We have set a default approach for each of the Spectrographs based on our expertise and the data in the PypeIt Development Suite.

Of course, for the correction to be applied the user must supply the required images in the PypeIt Reduction File. We discuss that process in greater detail in Generating the Flat Field Images.

Modifying the Default Approach

Each spectrograph has a default approach to flat fielding. For most, the code will apply both the pixel-level and illumination corrections.

And for most if not all spectrographs, these are only applied to the science and standard Frame Types.

If you wish to modify the default either for a custom approach or because you lack the necessary calibration data, you will need to modify the PypeIt Reduction File and specifically the use_pixelflat and use_illumflat parameters in the ProcessImagesPar Keywords.

We now provide a few typical examples.

No Flat Fielding

If you wish to turn off flat fielding entirely during data reduction, add the following to the Parameter Block:

[baseprocess]
    use_pixelflat = False
    use_illumflat = False

Note that if you provide flat field images in the Data Block, then these will still be processed during reduction. But they will not be used.

No Illumination Flat

If you wish to turn off application of the illumination flat for all files, add the following to the Parameter Block:

[baseprocess]
    use_illumflat = False

Of course, you can do the same for pixel-level flat fielding. Or you can choose to make this choice for only a specific frametype. For example, to skip the illumination correction for standard frames include:

[calibrations]
    [[standard]]
        [[[process]]]
            use_illumflat = False

No Fine Correction to the Spatial Illumination

By default, a fine correction to the spatial illumination profile is performed. If you wish to turn off the fine correction to the spatial illumination profile (based on the appearance of the QA that is output in the PNG folder), add the following to the Parameter Block:

[calibrations]
    [[flatfield]]
        slit_illum_finecorr = False

Apply Illumination Flat

For an instrument where applying the illumination flat is not the default, you may turn this on with, e.g.:

[calibrations]
    [[standard]]
        [[[process]]]
            use_illumflat = True

Of course, you will need to provide one or more images labeled as illumflat Frame Types in your PypeIt Reduction File. See below for further details.

Lamps off Flats Subtraction

When flats taken with the lamps OFF are provided, PypeIt will subtract them from the flats taken with the lamps ON, before creating the Flat images. The lamp off flats are not automatically identified (except for Keck MOSFIRE), so the user should label those as lampoffflats Frame Types in the PypeIt Reduction File.

Note

It is responsibility of the user to ensure that the lampoffflats frames in the PypeIt file have the same exposure time as the trace, pixelflat, and illumflat frames. The lampoffflats frames are always subtracted from the trace, pixelflat, and illumflat frames. If distinct frames are desired for trace, pixelflat, and illumflat, we currently advise users to simply not use the lampofflats.

Apply Spectral Illumination Correction

Spectral illumination corrections are not applied by default. The main usage case at the moment is for correcting the relative spectral sensitivity of different slits/slices for IFU data. If you would like to calculate the relative spectral sensitivity, you can do so by including the following in your PypeIt Reduction File:

[calibrations]
    [[flatfield]]
        slit_illum_relative = True

To apply this correction to science frames, you need to make sure the following keyword argument is set as well:

[scienceframe]
    [[process]]
        use_specillum = True

You will need to provide one or more images labeled as pixelflat Frame Types in your PypeIt Reduction File. See below for further details.

Generating the Flat Field Images

Input files

If you wish to apply one or more of the Flat-fielding corrections to the detector response, you will need to provide the matching flat field images in your PypeIt Reduction File and specify them with the appropriate Frame Types.

In short, if use_pixelflat=True for any of your frame types, at least one of the data files in the PypeIt Reduction File Data Block must be labelled as pixelflat (unless you Feed a PixelFlat).

And, if use_illumflat=True for any of your frame types, at least one of the data files in the PypeIt Reduction File Data Block must be labelled as illumflat.

In some cases, it may be desirable to use a different set of frames for the pixel and illumination corrections. This is supported, but we recommend that you set the trace frames to be the same as the illumflat frames.

Feed a PixelFlat

If you have generated your own pixel flat (or were provided one) and it is trimmed and oriented following the expected Image Orientation, then you may feed this into PypeIt. This is the recommended approach at present for LRIS BLUE.

And you perform this by modifying the Parameter Block:

[calibrations]
    [[flatfield]]
        pixelflat_file = /Users/joe/python/PypeIt-development-suite/CALIBS/PYPEIT_LRISb_pixflat_B600_2x2_17sep2009.fits.gz

None of the frames in the Data Block should be labelled as pixelflat.

Algorithms

TBW

Tuning

If you wish to tune the algorithms used to generate the pixel flat and/or illumination flat, you will want to modify the FlatFieldPar Keywords in the PypeIt Reduction File.

Below we list common modifications.

Saturated Slits

Occasionally one or more slits are saturated (a common case is the Keck DEIMOS LVMCslitC mask) and the code exits during the flat-field construction. If you wish to continue on with the slits that are ok, add this to your PypeIt Reduction File:

[calibrations]
    [[flatfield]]
        saturated_slits = mask  # or continue

Using the mask option will preclude the slit from any further reduction. Alternatively, using continue will set the flat to unity and extraction will be attempted.

Ignoring Extrema

If you wish to set the pixelflat to unity below/above a user-specified wavelength, then use pixelflat_min_wave or pixelflat_max_wave, e.g.:

[calibrations]
    [[flatfield]]
        pixelflat_min_wave = 3750.

This will set the flat to be unity for pixels at wavelengths less than 3750 angstrom in every slit.