Subaru FOCAS

Overview

This file summarizes several instrument specific settings that are related to the Subaru/FOCAS spectrograph.

Warning

PypeIt currently cannot reduce images produced by FOCAS in imaging mode or those taken with certain obsolete readout configurations. All image-handling assumes FOCAS images have been taken in spectroscopic mode with standard CCD readout parameters. PypeIt handles files that do not meet these criteria in two ways:

  • When running pypeit_setup, any frames not in spectroscopic mode or with non-standard readout will be ignored and should not appear in your PypeIt Reduction File.

  • If you add frames to the PypeIt Reduction File that are not in spectroscopic mode or have incompatible readout configurations, the method used to read the FOCAS files will fault.

At present,the code has only been testd on DET-ID=2 data, i.e. the second of the two detectors, which is where the primary target lands if one uses the longslit.

Deviations

The default changes to the PypeIt parameters specific to FOCAS data are listed here: SUBARU FOCAS (subaru_focas). You do not have to add these changes to your PypeIt reduction file! This is just a listing of how the parameters used for Subaru/FOCAS differ from the defaults listed in the preceding tables on that page.

These are tuned to the standard calibration set taken with FOCAS.

DETECTOR

FOCAS uses two 2048×4096 MIT/LL CCD detectors. Unlike other multi-detector instruments, FOCAS does not use mosaic construction. The detector is oriented such that the dispersion direction is along the 4096-pixel axis (y-direction) and the spatial direction is along the 2048-pixel axis (x-direction).

The detector configuration is automatically handled by PypeIt with:

[rdx]
    spectrograph = subaru_focas

This is the default configuration and typically does not need to be modified by the user.

Calibrations

Edge Tracing

FOCAS long-slit and multi-object spectroscopy requires careful slit edge tracing. The default edge_thresh parameter works well for most FOCAS configurations, but may need adjustment for:

  • Very short exposures

  • High-resolution modes (VPH gratings)

If slit edges are not being detected properly, try:

[calibrations]
    [[slitedges]]
        edge_thresh = 20

For multi-object spectroscopy (MOS), you may need to adjust additional parameters:

[calibrations]
    [[slitedges]]
        edge_thresh = 20
        trace_thresh = 10
        minimum_slit_length = 6

Multi-Object Spectroscopy (MOS)

FOCAS supports multi-object spectroscopy through slit masks. Unlike DEIMOS, FOCAS does not currently support automatic matching to slit-mask design files. Users must rely on the automatic slit detection and may need to manually verify slit assignments.

For MOS observations, consider these parameters:

[calibrations]
    [[slitedges]]
        sync_predict = nearest
        bound_detector = True
        minimum_slit_length = 8

Wavelength Calibration

FOCAS wavelength calibration depends on the grating and comparison lamp used. Common lamp combinations include:

  • HgCd lamps: Standard for most gratings

  • Ne+Ar lamps: Used for some high-resolution configurations

  • ThAr lamps: Available for high-precision radial velocity work

The default lamp selection works for most cases, but can be overridden:

[calibrations]
    [[wavelengths]]
        lamps = HgI, CdI

Common Grating Configurations:

  • 150B, 300B, 600B: Blue gratings, use HgCd lamps

  • 150R, 300R, 600R: Red gratings, use HgCd or Ne+Ar lamps

  • VPH gratings: High-resolution, may require ThAr lamps

Flat Fielding

FOCAS flat fielding generally works well with default parameters. However, for certain configurations you may encounter:

  • Fringing in red wavelengths (>7000Å): Common with red gratings

  • Illumination gradients: Particularly in wide-slit modes

  • Scattered light: Can affect faint object spectroscopy

To address fringing in red spectra:

[calibrations]
    [[flatfield]]
        slit_illum_finecorr = False
        tweak_slits_thresh = 0.9

For severe illumination gradients:

[calibrations]
    [[flatfield]]
        slit_illum_finecorr = True
        slit_illum_ref_idx = 0

Flexure

FOCAS generally has good mechanical stability, but flexure corrections may be needed for:

  • Large telescope movements between calibrations and science

  • High-precision radial velocity work

Enable flexure correction with:

[flexure]
    spec_method = boxcar
    spec_maxshift = 20

For high-precision work, consider using sky lines for flexure correction:

[flexure]
    spec_method = slitcen
    spec_maxshift = 10

Flux Calibration

FOCAS flux calibration requires standard star observations. Recommended standards include:

  • Spectrophotometric standards: BD+28d4211, HD 19445, etc.

  • White dwarf standards: G191-B2B, GD 153, etc.

Standard star configuration:

[fluxcalib]
    extinct_correct = True
    extinct_file = atm_extinct_subaru_maunakea.dat

Troubleshooting

Common Issues:

  1. Wavelength solution fails

    • Check lamp selection matches your grating

    • Verify arc lamp exposure times are adequate

    • Consider manual lamp line identification

  2. Poor flat fielding

    • Check for saturated flat field exposures

    • Verify flat lamp matches science configuration

  3. Slit edges not detected

    • Lower edge_thresh parameter

    • Verify adequate flat field signal

Additional Reading

Here are additional docs related to Subaru/FOCAS: