Abstract
We investigate the use of ultra-narrow band interference filters to enable daytime use of sodium laser guide star (LGS) adaptive optics systems. Filter performance is explored using theoretical and vendor supplied filter transmission profiles, a modeled daylight sky background, broadband measurements of the daytime sky brightness on Maunakea, and an assumed photon return from the sodium LGS and read noise for the wavefront sensor (WFS) detector. The critical parameters are the bandpass of the filter, the out-of-band rejection, and the peak throughput at the wavelength of the LGS light. Importantly, a systematic trade between these parameters leads to potentially simple solutions enabling daytime observations. Finally, we simulated the Mid-Infrared Adaptive Optics (MIRAO) system planned for the Thirty Meter Telescope with an end-to-end simulation, folding in daytime sky counts. We find that MIRAO with five sodium LGSs, commercial off-the-shelf filters to suppress the sky background in the LGS WFSs, and a near-infrared natural guide star (K ≤ 13) tip/tilt/focus WFS can attain daytime Strehl ratio values comparable to those at night.
Highlights
At wavelengths longward of ∼2 μm, the sky background in ground-based observations begins to be dominated by the thermal emission of the telescope and the atmosphere.[1]
We make the assumption that the wavefront sensor (WFS) spot size is independent of the approach we take to filtering the sky background and as such we focus on the changes to signal-to-noise ratio (SNR)
These specifications call for a total efficiency (η) of 0.42, a subaperture diameter (Dsubap) of 0.5 m, an integration time (t ) of 1∕800 s, and a pixel scale (ΩFoV) of 0.5 arc sec. bT 589 is the transmission of a given filter at 589.0 nm, the specified return wavelength of the laser guide star (LGS). cPhoto-detection events per subaperture per frame in the WFS for the Narrow Field Infrared Adaptive Optics System (NFIRAOS). dThe term Δλ is intended to represent the operation of an integral over the product of a narrow-band filter profile with our model sky spectrum. eWe always assume the largest detector size to make our estimates conservative
Summary
At wavelengths longward of ∼2 μm, the sky background in ground-based observations begins to be dominated by the thermal emission of the telescope and the atmosphere.[1]. Ground-based observatories can perform midinfrared (mid-IR) observations during the day with little impact on the sensitivity of the observation.[1] Some facilities, such as the NASA Infrared Telescope Facility (IRTF), routinely make observations during the day as a way of extending the science time available to their users. Beckers revisited these ideas in 2008 laying out both site-specific considerations and the use of such mid-IR instruments in the era of James Webb Space Telescope.[4] Hart et al.[5] performed the first measurement of the achievable contrast between an LGS and the daytime sky For this experiment, Hart et al used a magneto-optical filter (MOF), as suggested by Beckers and Cacciani. In the rest of this paper, we discuss the results of our filter specification trade study (Sec. 2), the AO performance simulations (Sec. 3), discuss particular concerns and implications of the proposed approach (Sec. 4), and present our conclusions on the feasibility of daytime LGS AO (Sec. 5)
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