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lect24 - Spectrograph Optics 12:45 — 1:10 25 min •...

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UCF Physics: AST 5765/4762: (Advanced) Astronomical Data Analysis Fall 2009 Lecture Notes: 24. Spectrosopic Gratings and Extraction 1 Check In: 12:30 — 12:35, 5 min Questions? 2 Median Combination (Review): 12:35 — 12:45, 10 min Can use median combination to make flats, darks, biases, skies Sigma-rejection technique from HW5 is even better, given enough images If sky varies and you are making skies, add indented normalized median combination steps Take many dithered images Subtract dark frame (cold shutter, “closed” filter) Divide each by its median (median is now 1, may use median region rather than full frame) Loop over pixel positions Find median of all images at each pixel position Astronomical sources, cosmic rays, bad pixels rejected! Noise reduced! Save as normalized median Multiply by median (in region) of a dark-subtracted image (is now sky for that object frame) Subtract from dark-subtracted image 3
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Unformatted text preview: Spectrograph Optics: 12:45 — 1:10, 25 min • Collimated beam at grating, or beam and dispersion angles mixed • Review of making a spectrum from an image • Input image now smeared in λ • R determined by PSF size: “true” spectrum convolved by PSF • BUT, we can pass light through a slit to reduce PSF 1 • Spectrograph optics – Discard light, but improve R – Avoid nearby objects contaminating spectrum – Can use mirrored slit, send rest of image to slit-viewing camera – Camera can guide exposure – Camera can monitor slit placement 4 Long-Slit Spectroscopy: 1:10 — 1:25, 15 min • 14specextract • Slit usually covers entire detector (why else?) • Star is only part of the slit • Can get sky background spectrum next to star • Extended objects bigger, may or may not fill slit • Scan slit: spatial-spectral hypercube • Can also allow planet to rotate under slit 2...
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lect24 - Spectrograph Optics 12:45 — 1:10 25 min •...

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