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Publications of the Astronomical Society of the Pacific 94:715-721, August 1982

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Publications of the Astronomical Society of the Pacific 94:715-721, August 1982 Publications of the Astronomical Society of the Pacific 94:715-721, August 1982 THE IMPORTANCE OF ATMOSPHERIC DIFFERENTIAL REFRACTION IN SPECTROPHOTOMETRY ALEXEI V. FILIPPENKO Department of Astronomy, California Institute of Technology, 105-24, Pasadena, California 91125 Received 1982 March 15 The effects of atmospheric differential refraction on astronomical measurements are much more important than is gen- erally assumed. In particular, it is shown that relative line and continuum intensities in spectrophotometric work may be erroneous if this phenomenon is neglected. To help observers minimize these errors, the relation between object position and optimal slit or aperture orientation is derived, and practical tables and graphs are presented for use at the telescope. Key words: instrumentation—observing techniques—spectrophotometry I. Introduction Π. Atmospheric Differential Refraction Atmospheric differential refraction has long been A. Magnitude known to affect the results of a variety of measurement In this section atmospheric differential refraction as a techniques in astronomy. For example, astrometric function of wavelength is calculated for the conditions work requires observations at small zenith angles and typical of major optical observatories. comparison of plates taken through similar fil- At sea level {P = 760 mm Hg, Τ — 15 0C) the refrac- ter/emulsion combinations—otherwise considerable er- tive index of dry air is given by (Edlén 1953; Coleman, rors in the derived relative positions of objects may be Bozman, and Meggers 1960) made (van de Kamp 1967). On the other hand, solar as- 6 tronomers must often observe through the steady atmo- ("(λ^,τβο - 1)10 = 64.328 sphere shortly after sunrise, and they consequently en- 29498.1 255.4 counter severe difficulties since different regions of the + 146 - (1/λ)2 + 41 - (l/λ)2 ' disk appear along the same line of sight if viewed with a broad-band filter (Simon 1966). Similarly, in spectrosco- where λ is the wavelength of light in vacuo (microns). py a substantial amount of light may be lost at large ze- Since observatories are usually located at high altitudes, nith angles if an object is recorded at a wavelength which the index of refraction must be corrected for the lower differs from the convolution of its energy distribution ambient temperature and pressure (Barrell 1951): n and the spectral response of the human eye (and/or (η(λ)^ρ — 1) = { Wl5,760 ~ ίολ television guider system). 6 (^/ w F[1 + (1.049 - 0.0157 T)10- P] A related problem at nonzero zenith angles is that X more light may be lost from either end of a spectrum 720.883(1 + 0.003661 T) than from the center if the object is well-centered in the In addition, the presence of water vapor in the atmo- spectrograph slit at the median wavelengths. This ob- sphere reduces (n — 1)106 by viously leads to an erroneous measurement of the spec- 2 tral energy distribution if the wavelength range is large. 0.0624 - 0.000680/λ U In view of the recent emphasis placed on relative emis- 1 + 0.003661 Τ f sion-line intensities in studies of supernovae, planetary nebulae, galactic nuclei, and other objects (e.g., Baldwin, where / is the water vapor pressure in mm of Hg and Τ Phillips, and Terlevich 1981), it is important to be aware is the air temperature in 0 C (Barrell 1951). At an alti- that even at small air masses the errors may be non- tude of ^ 2 km and a latitude of ^ ±30°, average con- negligible. Although errors can easily be minimized by ditions are (Allen 1973) Ρ œ 600 mm Hg, Τ « 7° C, and rotating the spectrograph so that the slit or rectangular /^8 mm Hg. These values are used in equations (1) aperture is parallel to the direction of atmospheric re- through (3) in order to obtain η(λ), and atmospheric dif- fraction, a survey of the literature reveals that this prac- ferential refraction (arc seconds) relative to λ = 5000  tice is generally not followed. The purpose of this paper is then calculated for an object at zenith angle ζ from is to remind spectroscopists of the large effects of atmo- (Smart 1931) spheric dispersion and to present convenient tables and Δβ(λ) ^ R(X) - fí(5000) graphs (for use at the telescope) of optimal slit orienta- (4) tion as a function of object position in the sky. « 206265 [η(λ) - n(5000)] tan . © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 716 ALEXEI V. FILIPPENKO The results are presented in Table I. It is clear that the sec ζ = 1.1 is obtained with a χ 4" aperture on a importance of atmospheric dispersion increases rapidly night when 2σ = 1^5, and that the small dimension of with increasing zenith angle and decreasing wavelength, the aperture is aligned along the atmospheric dispersion. and infrared observations are rarely affected. If the star is centered in the aperture at wavelength 5000 Â, then according to Table I it will be off center by B. Effect on Spectrophotometry 0^56 at 3500  and by 0^21 in the opposite direction at Table I demonstrates that at an air mass (~ sec z) of 6500 Â. Consequently, equations (7) and (8) show that only 1.1 the image of a point source viewed at 6500  is the percentage of light entering the aperture at 5000 Â, displaced from that at 3500  by three-quarters of an arc 6500 Â, and 3500  is 81.1, 79.5, and 69.7, respectively. second, while at sec ζ = 1.3 the displacement becomes Thus, at an air mass of only 1.1 the light loss at 3500  is nearly 1.4 arc seconds. It will now be shown that this 11.4 percent greater than at 5000 Â, and it is easy to see leads to considerable relative light losses in low- or mod- that the relative light losses grow rapidly with increasing erate-dispersion narrow-aperture spectrophotometry if air mass. These errors are particularly alarming in view the aperture or slit is not aligned along the direction of of the fact that intensity ratios such as atmospheric refraction. /([Ο ιιι]λ5007)/Ι([0 ιι]λ3727) are frequently used to dis- The convolution of guiding errors and seeing produces tinguish between different ionization mechanisms in ga- a point spread function whose surface brightness μ{ή is a lactic nuclei and other objects. Gaussian with dispersion σ (King 1971): Now suppose that the same conditions apply, except 2 that the long dimension of the aperture is oriented along μΐή= -r2/2a (5) 27702 the direction of atmospheric dispersion. Then the per- centage of light entering the aperture at 5000 Â, 6500 Â, This equation is normalized such that the total intensity and 3500  is 81.1, 80.9, and 79.5, respectively. The rel- at any given wavelength is equal to unity, and σ is as- ative light loss has therefore been considerably reduced sumed to be independent of wavelength. If a star is cen- by proper orientation of the aperture. Furthermore, the tered at wavelength λ in a rectangular aperture of width losses due to differential refraction disappear entirely if 2a and length 2b (arc seconds), then the amount of light a long slit oriented along the atmospheric refraction is at λ entering the aperture is given by used to obtain widened one-dimensional spectra, since light displaced by different amounts still enters the slit. ί(λ) = XI XI ß(r)dxdy (Note, however, that problems caused by atmospheric (6) dispersion cannot be completely avoided if unwidened 2 two-dimensional spectra of extended objects are desired.) 2πσ2 χ: χ: β-{χ2 + y V^¿x¿y 5 III. Optimal Slit or Aperture Position Angle or Since atmospheric differential refraction is per- pendicular to the horizon, the slit or rectangular aper- = ( i- Ρ m \ \/2^σ ' ture of a spectrograph should be oriented along this di- (7) rection. When the object is on the meridian, the slit posi- χ tion angle must be 0° or 180°, but as it moves across the sky the angle changes. In this section the optimal slit On the other hand, if the star is displaced from the aper- position angle as a function of object position is derived ture center by x0 arc seconds along the χ direction, then and tabulated for easy reference. (Following standard practice, the position angle is defined as the angle be- « (a x tween the slit and the hour circle through the object, 2 \ ^j2mo ^~ è measured from the north through the east from 0° to <α+ 360°.) + \ Γ ^ ^ (8) + ·\/2πσ^ / Consider the spherical triangle whose vertices are de- fined by the object being observed, the zenith, and the X appropriate celestial pole. Application of the law of sines yields sm η sin h (9) With the aid of standard Gaussian integral tables the sin (77/2 — φ) sin 2: above expressions may be easily used to calculate /(λ) where η is the parallactic angle, h is the object's hour under various observing conditions. angle {h is positive if west of the meridian), 2; is the ob- Suppose, for example, that the spectrum of a star at ject's zenith angle, and φ is the observer's latitude. Equa- © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System ATMOSPHERIC DIFFERENTIAL REFRACTION 717 Table I Atmospheric differential refraction at an altitude of 2 km (aroecondg) Wavelength (Angstroms) 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500 9000 9500 10000 Sec ζ 1.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.05 0.68 0.38 0.20 0.08 0.00 -0.06 -0.11 -0.14 -0.17 -0.19 -0.21 -0.23 -0.24 -0.25 -0.26 1.10 0.97 0.55 0.29 0.12 0.00 -0.09 -0.15 -0.20 -0.24 -0.28 -0.30 -0.32 -0.34 -0.36 -0.37 1.15 1.20 0.68 0.36 0.15 0.00 -0.11 -0.19 -0.25 -0.30 -0.34 -0.38 -0.40 -0.42 -0.44 -0.46 1.20 1.40 0.80 0.42 0.17 0.00 -0.22 -0.30 -0.35 -0.40 -0.44
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