= Solar limb-darkening drift-scan experiment
A motivating measurement used the McMath solar <optical telescope> at Kitt Peak. The <Sun>'s image drifted across a $10\,\mathrm{mm}$-high, $0.2\,\mathrm{mm}$-wide <optical slit>; a double-pass <spectrograph> selected <wavelength> intervals from $0.0015\,\mathrm{nm}$ wide at $303\,\mathrm{nm}$ to $0.007\,\mathrm{nm}$ at $1099\,\mathrm{nm}$. An EMI 9750 QB <photodetector> or <silicon> <photodiode> supplied a digitized signal: $1\,\mathrm{kHz}$ samples were summed into 4096 positions per crossing, including dark and off-limb sky readings. <Solar limb darkening 1986–1990 (λλ303 to 1099 nm) by Heinz Neckel and Dietrich Labs>[§2, p. 93, instrumental compilation: slit dimensions, double pass, detectors, sampling and drift-curve layout; p. 113, acknowledgements identifying the telescope]{ref}
Controls included daily realignment of <optical telescope> mirrors, <wavelength> settings between <spectral lines> insensitive to the rotational <Doppler shift>, selection of detectors without detectable <hysteresis>, and measured electronic nonlinearity corrections below $0.1\%$ of maximum signal. Limb-position calibration combined centres of gravity of the edge <derivative> profiles with almanac semidiameters; the preliminary edge shifts were consistent with <astronomical seeing> of approximately $5$ <arcseconds> <full width at half maximum>. <Solar limb darkening 1986–1990 (λλ303 to 1099 nm) by Heinz Neckel and Dietrich Labs>[§2, pp. 93–94, precautions (1), (6)–(8); §3.1, p. 94, two-step limb positions and seeing estimate]{ref}
The decreasing centre-to-limb readout gives a <specific intensity> profile, constraining stratification of the <photosphere> under a <radiative transfer> model. <Astronomical seeing>, instrumental <scattering of light>, and <solar granulation> limit this inference; brightness alone does not uniquely determine <temperature> versus geometric height. These observations need not follow the stipulated <linear limb-darkening law>.
Back to article page
