Astronomical imagers. ASTR320 Monday February 18, 2019

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Ferdinand Hancock
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1 Astronomical imagers ASTR320 Monday February 18, 2019

2 Astronomical imaging Telescopes gather light and focus onto a focal plane, but don t make perfect images Use a camera to improve quality of images Optics correct aberrations from telescope Record the light with a detector, usually a CCD at optical wavelengths

3 Astronomical imaging CTIO Blanco 4m telescope with DECam

Optical filters Often we want to study the color of an object in addition to the amount of flux it produces, measured in magnitudes Use optical filters to

4 Optical filters Often we want to study the color of an object in addition to the amount of flux it produces, measured in magnitudes Use optical filters to divide white light into broadband colors Think of multiband imaging as a very low resolution spectrum DECam filter transmission curves Measuring redshifts with filters

5 Optical filters Often we want to study the color of an object in addition to the amount of flux it produces, measured in magnitudes Use optical filters to divide white light into broadband colors Think of multiband imaging as a very low resolution spectrum

6 Color indices In astronomy we define the colors of stars quantitatively, using color indices Suppose we measure fluxes in two different filters: We can make a color index by subtracting: We generally write the color index as letters that denote filters, e.g.: Note that the distance cancels, so the color is the same for absolute and apparent magnitudes.

7 Color indices By convention, pick c A -c B based on Vega (an A0V type star) so that for Vega: By convention, generally write colors with the shorter wavelength passband first: B-V, U-B, J-K So that smaller numbers are always bluer and larger numbers are redder A-B<0 means bluer than Vega A-B>0 means redder than Vega

8 What color indices measure Most usefully, temperature Stars behave largely like blackbodies A hot, opaque object produces a continuous blackbody spectrum of light characterized by its temperature

9 Blackbody radiation A blackbody is an object that absorbs all light. Absorbs at all wavelengths Characterized by its temperature It is also the perfect radiator: Emits at all wavelengths (continuous spectrum) Total energy emitted depends on temperature Peak wavelength also depends on temperature

10 Blackbody curves

11 The color of a star is related to its temperature Betelgeuse: a reddish star (cooler). Rigel: a bluish star (hotter).

12 Rigel: T eff = 11,000K B-V = Betelgeuse: T eff = 3,500K B-V = 1.85

13 Stellar spectra in order from the hottest (top) to coolest (bottom).

14 Astronomical filter systems Measurements of astronomical objects are made by different telescopes and instruments all over the world In the beginning, each scientist designed and built their own unique filter system to use at their telescope Once enough astronomers began to make modern measurements of astrophysical sources, it became clear that a standardized system was needed so the measurements could be compared and confirmed

15 Astronomical filter systems We design photometric systems to maximize information that can be gleaned from extremely low resolution spectroscopy, i.e., photometry.

16 The Johnson-Morgan UBV System The UBV system was originally designed by Johnson and Morgan (1953) to understand stars (particularly hot stars). The Johnson-Morgan V band is meant to simulate and perpetuate measurements historically made by the human eye, to which it approximately matches. The Johnson-Morgan B band approximates the blue sensitivity of the original photographic emulsions to typical stars. The B-V color provided a measure of the temperature of (hotter) stars. Johnson and Morgan realized that much more information was possible by adding a third filter in the ultraviolet.

The UBVRI system In the 1960s, Johnson (and later others) extended the UBV system to the red and infrared, with R,I,J,K,L,M,N... bands. In the optical, then, we have the UBVRI broadband system.

17 The UBVRI system In the 1960s, Johnson (and later others) extended the UBV system to the red and infrared, with R,I,J,K,L,M,N... bands. In the optical, then, we have the UBVRI broadband system. It was found that the UBV system did not work well for very cool stars, like K and M spectral types since these very red stars were easier to study at redder wavelengths. So the V, R and I bands are often used to study these kinds of stars.

18 SDSS ugriz system The Sloan Digital Sky Survey used a filter set based on the Thuan-Gunn system Used in the SDSS survey, and is now commonly used by most astronomers in order to compare to the huge dataset of SDSS

19 Other filter systems Stromgren: stellar classification, absolute magnitudes, surface gravities Washington/DDO: metal abundances Tuned narrowband filters: measure specific emission line features in galaxies or nebulae, redshifts of galaxies/quasars

20 Other filter systems Filters+detector efficiency+optical response comprise total spectral response of the system SDSS DES

21 Gaia filters Gaia detects light in Gaia G (astrometric instrument), BP and RP (spectrophotometer) and RVS (radial velocity spectrograph) passbands. From

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