Astro-photography. Daguerreotype: on a copper plate

Transcription

1 AST 1022L

2 Astro-photography s: Photographic plates were astronomers' main imaging tool At right: first ever picture of the full moon, by John William Draper (1840) Daguerreotype: exposure using lightsensitive iodized silver on a copper plate

3 Astrometry Astrometry: study of the positions & motions of celestial bodies Used to be the only science you could reliably do with film besides detailing an object's physical appearance 3

4 Photometry Photometry mostly boils down to one question: How bright is this object...? where... could be any of the following:...relative to another object?...as a function of wavelength? (This is spectroscopy in a nutshell)...as a function of time? (This is how we find variable stars & exoplanets)...at this latitude/longitude/position?...in this polarization state? (This is how we characterize magnetic fields in space) 4

5 To do photometry, we need Charge-Coupled Devices (CCDs) This near-infrared (8900 Å) picture of Uranus was (supposedly) the first celestial object to be photographed by a CCD in Uranus s south pole looks dark due to methane absorption By the 1990s, CCDs had entirely replaced film as astronomers' standard imaging technology. 5

6 Advantages of CCDs 1: Quantum Efficiency (QE) QE = number of photons detected as a % of photons incident upon the detector Eye: 1-2% Photographic plate: 1-2% Photomultiplier tube: 20-30% IR array (HgCdTe): 30-50% CCD: 70-90% 6

7 Advantages of CCDs 2: Spatial Resolution Spatial resolution (also called angular resolution) quantifies an imaging system's ability to separate fine details. Generally, more imaging elements (e.g. cone & rod cells in your eye, or pixels in a CCD) in a given area better resolution 7 a mosaic of 4, 2024 x 2024-pixel CCDs

8 Advantages of CCDs 3: Linearity Problem with film: sensitivity to light rapidly increases & then just as quickly decreases with exposure time Therefore, trying to measure the relative brightness (magnitudes) of objects imaged with film is fruitless For CCDs, what you see is what you get: for a wide range of intensities, input signal (incident light) is directly proportional to output signal (electrons ) Can calculate magnitudes directly from flux ratios don't even need to convert from detected electrons to incident photons! 8

9 Linearity & Dynamic Range Dynamic Range quantifies detector's sensitivity range between lower detection limit & saturation Dynamic range of a CCD >> dynamic range of film 9

10 Other Advantages of CCDs 1. Very low noise (spurious signal from inside the detector itself) 2. Large spectral window (range of wavelengths that CCD is sensitive to) far-uv to near-ir 3. High photometric precision can measure relative brightness between 2 celestial bodies to ppm precision 4. Rigid (won't warp, which is both good & bad) 10

11 How CCDs Work Part 1 CCDs rely on the Photoelectric Effect If a photon strikes a [semi]conductive surface with enough energy, it can knock electrons off (i.e. ionize the atoms on the surface) 11

12 How CCDs Work Part 2 Each pixel is a capacitor: it traps & stores charged particles (in this case, the photoelectrons from the previous step). 12

13 How CCDs Work Part 3 When ready to read out, pixels become like locks in a canal voltages on adjacent pixels are raised & lowered so charges sort of pour down the rows pixel by pixel. 13

14 Imaging with a CCD 4 images of the galaxy M100 with exposure times of 1, 10, 100 & 1000 sec (taken with an 11 reflecting telescope). 14

15 Calibration & Processing Processed image below Raw image above 15

16 Image Processing: Darks, Flats, & Biases Take Raw (top left), subtract Bias (top middle) & Dark (top right), & divide difference by flat field (below left) to get final image (below right) 16

17 Why Do I Need a Flat Field? Answer: to normalize the image! 17 Raw Flat Normalized

18 Cosmic Rays: They Just Don't Care Cosmic rays are why we nearly always average/combine multiple exposures 18

19 Procedures I will demonstrate Part A at the computer Detailed instructions for Parts B (Photometry) & C (Astrometry) are on pages 5.3 & 5.4 in your lab manual I will walk you through some of the first few steps to get you started when we get to the computers For Next Week: Read Photometry of a Star Cluster Final Night Lab next Monday, 10/24: Observe the Deep Sky