Photometry using CCDs

Transcription

1 Photometry using CCDs Signal-to-Noise Ratio (SNR) Instrumental & Standard Magnitudes Point Spread Function (PSF) Aperture Photometry & PSF Fitting Examples

2 Some Old-Fashioned Photometers

3 ! Arrangement for a Star/Sky Background Photometer Two-Star Photometer

4 Example: a simple field with 2 bright stars

5 Apertures: star+sky and sky

6 The star s light actually extends quite a ways..

7 Image Saturation See for more about artifacts

8 Need to know what precision is required Ex: Want 3-sigma measurement on a 1% effect: need 0.3% precision or SNR~300 Ignoring dark current & background fluctuation uncertainties & scintillation noise: net N star = net star counts N sky = sky counts N read = read noise g = gain SNR = net gn star net gn star 2 + ngn sky + nn read n = # pixels in aperture If the star s shot noise dominates: SNR = net gn star net gn star + ngn!"# + nn 2 sky $ read SMALL SMALL = gn net star net gn star net = gn star

9 Aperture Photometry with ATV.pro

10 An example of a PSF-fitting photometry routine ( StarFinder ) Strictly-speaking: PSF = Point Source Function, spread before the detector response PRF = Point Response Function - spread after folding in the detector response

11 Warnings about PSF/PRF fitting 1. Sometimes the PSF is only a few pixels wide. Example - The Infrared Array Camera (IRAC) on the Spitzer Space Telescope: Photometry using IRAC data is no different from that with any other high-quality astronomical data. Both aperture photometry and PRF-fitting work successfully. Aperture photometry is most commonly used, so we will discuss it briefly Point source fitting to IRAC data has proven problematic as the PSF is undersampled, and, in channels 1and 2, there is a significant variation in sensitivity within pixels. 2. Sometimes the PSF has a bad shape. Example: if the stars are really bright, it may be necessary to de-focus the telescope to avoid saturation. Here is an example from the Rapid Eye Mount (REM).

12 An extreme case of de-focus!

13 Getting to Standard Magnitudes What you measure is the response of your instrument - this can be converted into an instrumental magnitude m = C 2.5log 10 N star+sky N sky + 2.5log 10 dt

14 The number you get depends on the aperture size

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16 How to place this on a standard system, which is (in effect) defined arbitrarily? Calculating the Transformation Coefficients V v = ε BV ( B V ) + ς v ( B V ) ( b v) = η ( 0 BV B V ) + ξ bv + etc. standard color index 1.0 ( B V ) V v ( b v) 0 instrumental extinction-corrected color index ( B V )

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18 The Reality of Photometry

19 Filters in the Infrared... J H K L M

20 Example of a Modern Instrument The Infrared Array Camera (IRAC) on the Spitzer Space Telescope InSb Detector Pickoff Mirror Doublet Lens Lyot Stop Ge Filters Si:As Detector Ge Beamsplitter Lyot Stop MM Figure 2.2. IRAC optical layout, top view. The layout is similar for both pairs of channels; the light enters the doublet and the long wavelength passes through the beamsplitter to the Si:As detector (Channels 3 and 4) and the short wavelength light is reflected to the InSb detector (Channels 1 and 2). Telescope Beam Fiducial Channels 2 and 4 Pickoff Mirror MM Channels 1 and 3 Figure 2.4. IRAC optics, side view. The Si:As detectors are shown at the far right of the figure, the InSb arrays are behind the beamsplitters.

21 IRAC Instrument Handbook Oversampled IRAC pixel scale Actual IRAC pixel scale Figure 4.5. The in-flight IRAC point response functions (PRFs) at 3.6, 4.5, 5.8 and 8 microns. The PRFs were reconstructed onto a grid of 0.3 pixels, ¼ the size of the IRAC pixel, using the drizzle algorithm. We display the PRF with both a square root and logarithmic scaling, to emphasize the structure in the core and wings of the PRF, respectively. We also show the PRF as it appears at the IRAC pixel scale of 1.2. The reconstructed images clearly show the first and second Airy rings, with the first Airy ring blending with the core in the 3.6 and 4.5 µm data.

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23 Image of the Helix Nebula, taken with IRAC (blue) and MIPS (red) How does one convert images taken at invisible (or other) wavelengths into RGB? Visit Hubble s Color Toolbox!: index.php (Note: This page keeps moving and getting modified!) Want to calculate signal/noise ratios for IRAC? Go to SENS-PET tools/senspet/ Observing with the Rapid Eye Mount (REM)?: and click on the Exposure Time Calculator (ETC).

24 Aspects of Planning Matching Airmass with Calibrators (an airmass program written in IDL)

25 Is the Moon going to be a Problem? (StarAlt) Target Moon Distance of Moon from Target

26 Spacecraft Field of Regard (solar and anti-solar avoidance zones) Example: James Webb Space Telescope

27 The SAA - Bane of Spacecraft Astrophysics

28 Kepler Photometer and Spacecraft The Kepler photometer is a simple single purpose instrument. It is basically a Schmidt telescope design with a 0.95-meter aperture and a 105 deg2 (about 12 degree diameter) field-of-view (FOV). It originally pointed at and recorded data from just a single group of stars it original mission.

29 The photometer is composed of just one "instrument," which is, an array of 42 CCDs (charge coupled devices). Each 50x25 mm CCD has 2200x1024 pixels. The CCDs are read out every three seconds to prevent saturation. Only the information from the CCD pixels where there are stars brighter than mv=14 is recorded. (The CCDs are not used to take pictures. The images are intentionally defocused to 10 arc seconds to improve the photometric precision.) The data are integrated for 30 minutes.

30 Kepler K2 Mission In May 2013 Kepler lost of one of its reaction wheels used for precise pointing. Kepler's pointing now is being slowly moved across the sky in a new "mission" referred to as "K2". It continues to discover new planet candidates.