Science Operations

Enhanced Seeing Mode (ESM) with LUCI


Shown above are contemporaneous observations of M92 taken with ESM and Seeing Limited Observations. This demonstrates the improved PSF and depth ESM observations can provide in optimal conditions.

Background:

Enhanced Seeing Mode (ESM) is a mode available as part of the original FLAO (First Light AO) system and the new SOUL (Single counjugated adaptive Optics Upgrade for LBT) upgrade.  ESM provides 11 modes of corrections, which include: tip+tilt + focus, astigmatism, coma, trefoil, spherical, and one high order (Z12).   ESM might be better thought of as a way to improve the natural seeing of every observation, rather than a specific mode that is requested.  As long as there is a suitable AO Ref Star in the patrol field, there is no reason to avoid using ESM for all LUCI observations.

ESM differs from AO, in two key ways:

  1. It does not provide diffraction limited corrections, but does improve the point-spread-function (PSF) over seeing limited observations in a variety of conditions.
  2. It uses the N3.75 camera (0.12″/pixel plate-scale) and can provide corrections over the entire 4′ x 4′ LUCI field of view (FOV).

ESM offers improvements for imaging and spectroscopy.  In tests under optimal sky conditions, ESM can deliver an imaging PSF with a full-width at half maximum (FWHM) = 0.22″-0.25″ over the entire FOV.  For spectroscopy, ESM provides spatial and spectral improvements and can be used with multi-object slit (MOS) masks (which are not available for use with full-AO).  In optimal conditions, ESM will allow users to achieve higher spectral resolution using smaller slitwidths (i.e. 0.25″).  ESM also offers improvements for sub-optimal conditions.  For imaging, ESM can improve the PSF.  For spectroscopy, ESM allows for more light to be focused into larger slits (i.e. 0.5″ or 0.75″) with less slit-loss. Users may wish to use ESM in sub-optimal conditions in order to achieve their original seeing limited science goals.

Requirements:

ESM has the same setup and scripting requirements as full AO.  The overheads for ESM are similar to those for full AO, although the time it takes to close the loop with only 11 modes may be shorter.

  1. A tip-tilt reference star is required for all observations.  The star must have an Rmag < 16.51 and lie within the AO Reference Star Patrol field shown below.  The same AO Ref Star must be used for each preset.   Any dithering (either for imaging or along the slit) must keep the AO Ref Star in the patrol field if the user wishes to have corrections applied at each position.  For large objects (i.e. galaxy or globular cluster) which require dithers > the LUCI FOV, ESM will pause corrections when the AO Ref Star moves out of the patrol field and resume when the telescope returns to the target field.
  2. For programs attempting to use their science target as the AO Reference star (this would be called ON-AXIS), please be sure the object has a bright core which is CONFIRMED to be R < 16.5.  For example, if you are looking at an R=15 QSO, one needs to verify the magnitude is not the integrated magnitude but the core itself.  Verify this with SDSS or other imaging. If you are unsure, please contact sciops@lbto.org.
  3. LUCI scripts must be generated using the ESM/Diffraction-limited/ARGOS libraries – a seeing limited script cannot be used.
  4. A guidestar is required in order to collimate – the guidestar requirements are the same as with seeing limited observations (12 < Rmag < 15.5 and fall within the AGW patrol field shown below).
  5. WARNING:  The AO Ref Star patrol field is located entirely within the LUCI FOV.  Users should consider the brightness and color of the AO Ref Star to avoid saturating the detector and/or the impact of persistence on their science observations.
    Figure 1:  The AO and AGW patrol fields for both LUCIs.1 = The SOUL upgrade can access fainter AO Reference star magnitudes than FLAO.  R~16.5 should work.  However, the camera used by the AO system to “lock onto” the AO Reference star uses an R+I filter. The magnitude it reports is often fainter than the the standard R magnitude catalog value.  To be safe, users should restrict their choices to AO Ref stars equal to or brighter than Rmag=16.5 until the conversion from Rmag to R+I mag is better characterized.

    Vignetting of LUCI N3.75 FOV:

        Currently, the bottom part of the LUCI FOV is always vignetted by a platform on the W-unit stage used for AO testing.  In seeing limited mode, this impacts the bottom right corner (~70″ x ~22″) in the same way all of the time.  However, when the FLAO system is used for ESM, the amount of vignetting depends on where the AO Ref Star is located within the patrol field (this is not a problem with diffraction limited AO as only a 30″x30″ FOV is used).  This additional vignetting comes from the optics on the wavefront sensor stage. Careful planning is required to reduce the vignetting from the wavefront sensor optics (obscuration from the W-unit stage is always present, even in seeing-limited observations).
    Shown below is a comparison of the impact of vignetting.  Top left shows a raw ESM K-band image of M51 with more vignetting than normal, bottom left shows the visual planning display from the Observing Tool (OT) corresponding to this exposure.  The AO Ref Star is located in the region of the patrol field that causes vignetting.  The OT display shows this with a cross-hatched illustration.  Top right shows a raw ESM K-band image of M51, but this time the AO Ref Star is outside of the portion of the patrol field susceptible to vignetting (bottom right shows this exposure in the OT display).

    Figure 2:  A comparison of raw K-band data and the vignetting which occurs when the AO Ref Star is in various parts of the patrol field.

    ESM Performance:
    Currently, there are two use cases for ESM:

    1. Good-Optimal natural seeing (i.e. <1.2″)
    2. Mediocre to poor natural seeing (i.e. 1.2″ < seeing < 2″) 

    Recently, LBTO undertook a campaign to characterize ESM in various seeing conditions and compare LUCI-1 vs LUCI-2 image quality as well as compare ESM with the natural seeing at (or close to) the time of these observations.   As more data are obtained and analyzed, the results will be presented here.

    Overall Comparison between Poor and Excellent Conditions for ESM:

    The plot below shows a comparison between the natural seeing and ESM in poor (left) and excellent (right) conditions.  Each panel plots the measured FWHM of stars in the NE corner of M92 as a function of distance from the AO Ref Star.  The panel on the right compares the improvements for bright and faint AO Ref Stars in excellent conditions. As a caveat, improvements are heavily dependent on where the atmospheric turbulence is located.  ESM yields better improvements when turbulence is low in the atmosphere.  Currently, LBTO does not have the capability to measure the atmospheric turbulence as a function of height.  The results for LUCI-2 are consistent with those shown below for LUCI-1.  Note, that FWHM should scale with wavelength (∼λ-1/5), therefore improvements at shorter wavelengths will be less than those measured at K-band (∼2.2μm).  In excellent conditions with a bright AO Ref Star, the corrections are fairly uniform across the field.  In poor conditions, the FWHM improvements degrade as a function of distance from the AO Ref Star.


    Figure 3: Updated from Rothberg et al. 2018 comparing poor and optimal conditions and the improvements seen with ESM as a function of distance from the AO Ref Star.

    Predicting Performance at K-band and other wavelengths:

    Shown below are ESM and seeing-limited K-band data obtained with LUCI-1 and LUCI-2 in poor and optimal seeing conditions of the NE quadrant of the Milky Way Globular Cluster M92.  The plot provides an overall guidance to the expected capabilities of ESM as a function of requested seeing and the improvement expected over natural seeing conditions.

    Figure 4: The plotted points are the mean measured FWHM values of stars in a single 60 second exposure at K-band for ESM and seeing limited observations taken contemporaneously.  The errors bars are the standard deviation of the measured FWHMs of individual stars in M92 for each exposure in a set of observations.  The X-axis shows the measured DIMM (Differential Image Motion seeing Monitor) for each exposure, corrected to the same elevation as the M92 exposures.

    Shown below is an example of the improvement in J-band for ESM observations obtained in poor seeing conditions.  The AO Ref star is in the center of the FOV and the target merger galaxy (NGC 3921) is in the SW corner.


    Figure 5:  LUCI-1 J-band observation of the merger NGC 3921.  Observations were taken at an airmass of 1.31.  The V-band DIMM (corrected to elevation of the target) FWHM = 1.68″.  The ESM observations used an AO Ref Star with R=13.7.  The FWHM across the FOV is 0.7″-1.1″.

    FLAO vs. SOUL
    The FLAO system is currently undergoing an upgrade to the Single conjugated adaptive Optics Upgrade for LBT (SOUL).  The upgrade has been completed on the SX side and will be available for use in 2019B on a shared-risk basis.  The DX side will be upgraded during 2019B.  The most significant impact to ESM is that the AO Ref Star limiting magnitude is expected to be 1-1.5 magnitudes fainter.  The size of the patrol field remains unchanged, and the delivered PSF should remain the same for ESM.  Characterization and tests to confirm the expected improvement will occur during the 2019A semester.  Updated information will be made available here.

    Additional Resources:

    Preprint for AO4ELT6 Conference – Rothberg et al. 2019
    AO4ELT6 Poster Presentation – Rothberg et al. 2019
    SPIE 2018 Paper – Rothberg et al. 2018 – preliminary results as part of the Facility Instruments Update

    SPIE 2020 AO paper and conference poster link