Adaptive optics will almost completely remove the effects of atmospheric turbulence at 10 microns on the Extremely Large Telescope (ELT) generation of telescopes. In this paper, we observationally confirm that the next most important limitation to image quality is atmospheric dispersion, rather than telescope diffraction. By using the 6.5 meter MMT with its unique mid-IR adaptive optics system, we measure atmospheric dispersion in the N-band with the newly commissioned spectroscopic mode on MIRAC4-BLINC. Our results indicate that atmospheric dispersion is generally linear in the N-band, although there is some residual curvature. We compare our measurements to theory, and make predictions for ELT Strehls and image FHWM with and without an atmospheric dispersion corrector (ADC). We find that for many mid-IR applications, an ADC will be necessary on ELTs.
Deep Dive into A Direct Measurement of Atmospheric Dispersion in N-band Spectra: Implications for Mid-IR Systems on ELTs.
Adaptive optics will almost completely remove the effects of atmospheric turbulence at 10 microns on the Extremely Large Telescope (ELT) generation of telescopes. In this paper, we observationally confirm that the next most important limitation to image quality is atmospheric dispersion, rather than telescope diffraction. By using the 6.5 meter MMT with its unique mid-IR adaptive optics system, we measure atmospheric dispersion in the N-band with the newly commissioned spectroscopic mode on MIRAC4-BLINC. Our results indicate that atmospheric dispersion is generally linear in the N-band, although there is some residual curvature. We compare our measurements to theory, and make predictions for ELT Strehls and image FHWM with and without an atmospheric dispersion corrector (ADC). We find that for many mid-IR applications, an ADC will be necessary on ELTs.
As we approach the Extremely Large Telescope (ELT) generation of telescopes, adaptive optics is becoming increasingly important to the general astronomical community. Large telescopes have small diffraction limits, and achieving these limits is a major goal for instrument builders. The mid-infrared wavelengths, in particular, stand to gain substantially from larger telescopes-at the diffraction limit, S/N ∝ Diameter2 for background-limited observations of point-sources. Today's ∼8-meter class telescopes are close to diffraction-limited in the mid-infrared, even without adaptive optics, but maintaining the diffraction limit as telescopes continue to scale upwards will be challenging.
For ground-based telescopes in the mid-infrared, seeing is often considered a minor effect, and other (smaller) atmospheric effects are completely ignored. Kendrew et al. (2008) have predicted several atmospheric properties that may limit image quality on ELTs, including mid-infrared atmospheric dispersion 2 , visible atmospheric dispersion for wavefront sensing, and water vapor turbulence (see Devaney et al. (2008) for a similar discussion in the nearinfrared). So far, these effects have not been adequately measured. The 6.5 meter MMT, with its unique mid-IR adaptive optics system (MMTAO) provides a powerful testbed for mid-IR AO on ELTs. Effects that will severely limit image quality on ELTs are just measurable with MMTAO due to its highly stable PSF. By removing the largest atmospheric effect (seeing), we measure the second largest effect (atmospheric dispersion) with the newly commissioned spectroscopic mode of the MMT’s Mid-Infrared Array Camera (MIRAC4-BLINC) and adaptive optics.
Atmospheric refraction is a well-known phenomenon at visible wavelengths, where it is typically treated as a smooth curve that flattens quickly longward of K-band (Edlén 1966;Ciddor 1996;Bönsch & Potulski 1998). However, more detailed treatments show that molecular resonances from CO 2 and H 2 O (amongst others) dominate the infrared refractivity curve (Hill & Lawrence 1986;Mathar 2004;Colavita et al. 2004;Mathar 2007). These authors show that each infrared window (L,M,N) is bracketed by molecular absorption and has an atmospheric refraction curve characterized by an S-shape superimposed on a stronger linear trend.
In this paper, we measure the atmospheric dispersion curve on the short wavelength side of N-band (8.26µm-11.27µm) using spectroscopy and adaptive optics. In previous studies, Livengood et al. (1999) measured refractivity at one wavelength (12µm) while Tubbs et al. (2004) interferometrically measured refractivity throughout the N-band but were insensitive to the overall trend. Our spectroscopic result has the benefit of measuring all wavelengths simultaneously, so that the overall trend and curvature of the effect throughout N-band is unambiguous. By directly measuring the atmospheric dispersion curve, we can assess how the effect will limit image quality in the mid-infrared for ground-based ELTs. This is useful for instrument builders, who will have the option of using atmospheric dispersion correctors (ADCs) to suppress the effect.
Our data were obtained March 4, 2009 UT with the 6.5 meter MMT and its deformable secondary adaptive optics system (MMTAO -e.g., Lloyd-Hart 2000;Wildi et al. 2003;Brusa et al. 2004). We used the newly commissioned spectroscopic mode of MIRAC4-BLINC. The instrument is a combination of the Mid-IR Array Camera, Gen. 4 (MIRAC4) and the Bracewell Infrared Nulling Cryostat (BLINC - Hinz et al. 2000) which for these observations, is used in its “imaging” mode. MIRAC4 is functionally similar to previous incarnations of MIRAC (e.g. Hoffmann et al. 1998) with the main new feature being a DRS Technologies 256 x 256 Si:As array. Some of the relevant details of this new instrument are described below.
MIRAC4-BLINC was use to observe the mid-infrared standards α Her and γ Aql. Both targets were bright in the visible (3.06 and 2.72 V magnitudes for α Her and γ Aql, respectively) which allowed us to run the MMTAO system at full sampling speed (550 Hz). At longer wavelengths the MMTAO system can produce nearly perfect diffraction-limited images with extremely stable point spread functions (Kenworthy et al. 2004;Hinz et al. 2006). At N-band, typical Strehls of up to ∼98% can be obtained under good seeing conditions (Close et al. 2003). Conditions were non-photometric with moderately high winds (bursts up to 30 mph). However, the adaptive optics system was consistently able to stay locked on bright sources. Data from the MMT weather station showed an average temperature of 7.8 • C, an average pressure of 745 mbar and an average relative humidity of 44.3%. Detailed weather descriptions for each observation are shown in Table 1.
The MIRAC4-BLINC optics are enclosed and cooled in two attached cryostats. Reflective reimaging optics in the BLINC portion of the system create an image of the secondary on an articulated mirror.
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