APF-Levy HOWTO

Overview

This document provides a guide to using the APF-Levy spectrograph with PypeIt. It covers the specific configurations, calibration procedures, and data reduction steps necessary.

The final extractions cover 55 orders in total, with the first order, physical order 122, located at 3800 Angstroms. The reddest order, 68, is located at 6850 Angstroms.

Setup

Organize the data

PypeIt reduces all 3” and 8” long slits separately. The reductions share, however, the WideFlat frames which are acquired using the 8” long slit with a 2” width. These files are used to estimate the pixel flat images. Therefore, there are additional, manual steps that the user must take.

It is best to separate science and ThAr frames taken with the 3” deckers from data taken with the 8” deckers, this is not required, but it does make the reduction process easier. NarrowFlat images, flat field images taken with the 3” long slit, are used only for reducing the 3” long slit science data.

Run pypeit_setup

After running pypeit_setup with a specific configuration - the -c A flag, there will be a pypeit file created in the appropriate directory for that configuration. This file will have the name of the configuration, e.g., apf_levy_A.pypeit. If you mix your 3” and 8” data, you will also have apf_levy_B.pypeit if you type -c B. The number of configurations can be determined by reading the file setup_files/apf_levy.sorted.

Note

If you mix the data, you will get an error when running pypeit_setup.

For 8” data, the WideFlat images should be used for trace frames as well as pixflat frames, no editing is required.

For the 3” data, you need to edit the pypeit file. By default the WideFlat images will be listed as pixflat,trace and the labeled must be updated to just pixflat. As there are 100 WideFlat images taken per night, editing by hand can be time consuming. Use a search and replace function or a script to automate the process. Below are two examples of how to do this:

sed -i 's/pixflat,trace/pixflat/' apf_levy_A.pypeit
perl -pi -e 's/pixflat,trace/pixflat/' apf_levy_A.pypeit

For the 3” data, use the NarrowFlat images for trace frames. These will be identified automatically.

Non-standard binning

The pipeline has not been tested for non-standard (non 1x1) binning. Please contact the developers if there are issues.

Main Run

Once the PypeIt Reduction File is ready, the main call is simply:

run_pypeit apf_levy_A.pypeit -o

The -o indicates that any existing output files should be overwritten. As there are none, it is superfluous but we recommend (almost) always using it.

The PypeIt’s Core Data Reduction Executable and Workflow doc describes the process in some more detail.

This will take a while, as there are 55 orders per exposure which will be traced, wavelength calibrated, flat fielded, and extracted.

By default the 3” data has no local sky subtraction, but this can be enabled. The 8” data has local sky subtraction enabled by default. To change this for the 3” data add to the PypeIt file:

[reduction]
    [[skysub]]
        no_local_sky = False

The other setting that is unusual for the 3” slit is that the optimal extraction always uses a Gaussian profile instead of modeling the profile with a B-spline. To restore normal behavior, add to the PypeIt file:

[reduction]
    [[extraction]]
        sn_gauss = 4

The narrow length slit means that the model covers the full lenth of the slit, so local sky subtraction is a challenge and the B-spline model often includes the sky background in the fit.

Finally, the default box radius is 4 pixels, or 1.728 arc-seconds. This can be reset using the boxcar_radius parameter in the extraction section of the PypeIt file:

[reduction]
    [[extraction]]
        boxcar_radius = 1.296 # 3 pixels

Inspecting Files

Calibrations

The first set are Calibrations. The APF Levy spectrograph has no moving parts that will will change where the spectra land on the detector. This fixed format means that the trace and wavelength calibrations are stable.

Slit Edges

PypeIt will map the slit edges using the trace frames. The orders locations of the orders to be extracted are pre-defined in the spectrograph file inside PypeIt. This means that, even if the order is not detected in the trace frames, the order will still be extracted.

Wavelengths

One should inspect the PypeIt QA for the wavelength calibration. These are PNGs in the QA/PNG/ folder.

Note: there are multiple files generated for every slit. When the reduction is complete, you may prefer to scan through them by opening the HTML file under QA/.

The final wavelength solution is a two dimensional fit with pixel along the order one axis and the order itself being the second.

Remember, the default calibration is in vacuum wavelengths.

Spectra

The code will generate 2D and 1D spectra outputs. One per science frame, located in the Science/ folder.

One can inspect the one dimensional spectra with pypeit_show_1dspec.

Using that tool, you can examine the individual orders.