You, too, can make useful and beautiful astronomical images at Mees: Lesson 1

Transcription

1 You, too, can make useful and beautiful astronomical images at Mees: Lesson 1 Useful references: The Mees telescope startup/shutdown guide: The AST 142 Projects manual: AST 203 lectures 19 and 24: Rob Gendler (ed.) 2013, Lessons from the Masters (New York: Springer- Verlag), especially the articles by Adam Block (p. 159) and Stan Moore (p. 1). Software: TheSky, or Stellarium ( CCDSoft v June 2016 CCD imaging lessons 1

2 Some recent Mees results to whet your enthusiasm. You all know the Big Dipper: Van Gogh June 2016 CCD imaging lessons 2

3 Top ten galaxies The Big Dipper near the Big Dipper M101 M82 M81 M51 M63 M94 M109 M106 M June 2016 CCD imaging lessons 3

4 M109 LRGB, 4.0 hours total 24 May 2016, DMW 13.1 x11.3 N E 16 June 2016 CCD imaging lessons 4

5 M51 LRGB, 4 hours total 10 June 2016, DMW 13.9 x12.0 E N 16 June 2016 CCD imaging lessons 5

6 M101 LRGB, 4.5 hours total 15 June 2016, DMW 15.4 x13.1 N E 16 June 2016 CCD imaging lessons 6

7 The short answer: a recipe to make such images Identify a date range and a favorite object which is high in the sky at night for at least 3-4 hours on those dates. Compile or plan to take the calibration data. Dark and bias frames are compiled for you on the CCD-camera computer, as long as you use the CCD camera at either T = -10 C or -20 C. Flat field frames are provided too but it s a good idea to take new ones: frame sequences in each filter, pointed at clear sky near zenith, starting at sunset. For scientifically useful data: identify some 8-10 mag A stars near your target. Plan to autoguide on all deep-sky targets; identify a nearby 6-12 mag star about 12 arcminutes east of your target. 16 June 2016 CCD imaging lessons 7

8 For scientifically useful images: Recipe (continued) Equal numbers of frames in R, G, B, interspersed occasionally with short R, G, B frames on calibration stars, all binned 2x2 pixels. As many frames as you have time for. 5 minute exposures in moonlight; 8-10 minute exposures in dark skies. All will be averaged together in the end. For pretty pictures: as above but 3-4 times as many L frames as any of R, G, B, in 1x1 binning. Again, as many frames as you have time for in > 4 hours. No need to do the standard stars. Note that most APOD images involve >> 4 hours. 16 June 2016 CCD imaging lessons 8

9 Recipe (continued) These Lessons, of which today is just the first, are intended to teach you the details of following the recipe: how to use the camera SW, how to autoguide, etc. the simple physics behind imaging observations. the tricks of data reduction useful in scientifically useful imaging (e.g. IDL, CCDStack, MEM deconvolution) and those useful in making pretty pictures: the above, plus FITS Liberator and Adobe Photoshop. All software to be discussed in all lessons is installed on the two Intro Astronomy Lab laptop computers; some is also site licensed; other useful programs can be had for Free. Now for some of the technical details 16 June 2016 CCD imaging lessons 9

10 Sensitivity and signal-to-noise ratio Sensitive means large ratio of signal and noise. Signal = photocurrent from celestial objects. For small bandwidths that is, filter width λ λ, ( 1 ) = ε τη λp S IS gqe hc At visible wavelengths and with CCDs, one can take emissivity ε to be zero and photoconductive gain g to be 1. The other terms are τ Transmission of optics; range = 0-1 η Quantum efficiency of detector; range = 0-1 λ Wavelength; range = nm for visible light h, c, q e physical constants, by their usual symbols 16 June 2016 CCD imaging lessons 10

11 Sensitivity and signal-to-noise ratio (continued) Because of the finite, quantized electron charge, and random arrival time of electrons at given points in a circuit, there is noise in photocurrent. Shorthand for this effect: shot noise. qi q I = I = = I + I + I t t qe λτηqe = ( PS + PB) + ID t hc ( ) ( ) 2 2 e e N S B D Here, we refer to a single pixel or group of pixels in the CCD, and P S P B I D t power from target, incident on pixel incident background power (e.g. moonlight, city lights) dark current: current drawn by pixel even when no light shines exposure time: time over which current is averaged 16 June 2016 CCD imaging lessons 11

12 Sensitivity and signal-to-noise ratio (continued) So the signal-to-noise ratio is S I λp q λτηq N I hc t hc ( ) ( ) S S e e = τηqe PS + PB + ID N λτη = PS t 2 2 hc hc P + P + I S B D λτηqe Usually one of the terms in the denominator dominates the others: Background-limited: P P,hcI λτηq. B S D e 12 S N BL λτη = PS t PS η t hcp B 16 June 2016 CCD imaging lessons 12

13 Sensitivity and signal-to-noise ratio (continued) Source limited: P P,hcI λτηq. S B D e S N BL λτηp S = η S hc t P t Dark-current limited: ID λτηpb hc, λτηps hc. S λτηp q N hc I DL = S e η S D t P t Note that in all cases S/N increases with t. To increase S/N by a factor of two, one needs to increase the exposure time by a factor of June 2016 CCD imaging lessons 13

14 Mees 24-inch Cassegrain telescope. Our system f/13.5, 25 arcsec per mm in Cassegrain focal plane. Unvignetted field of view 24 arcmin in diameter. Collecting area 2700 cm 2. Santa Barbara Instrument Group (SBIG) STX CCD camera. Frame-transfer CCD, 4096x4096, 9 µm pixels (0.224 arcsec/pixel, 15.4 arcmin on a side, 21.7 arcmin diagonal), plus separate interline autoguiding CCD, 657x495, 7.4 µm pixels. η = across the visible band. 16-bit output; 1.27 electrons per data number (DN). I D = electrons per sec at T = -20 C. 9-electron read noise. 16 June 2016 CCD imaging lessons 14

15 Our system (continued) Baader Planetarium filters: L, R, G, B, Hα; peak τ June 2016 CCD imaging lessons 15

16 Objects in the sky, or on TheSky Secret astronomer unit: the magnitude. All you need to know is that, for two objects A and B, their fluxes (power per unit area, in real physics units) and magnitudes are related by m m =. F F 2 5log ( ) A B B A Past that, one just needs the conversion to/from physical units for a zero-magnitude star (Vega). Here, for the Johnson filters: 16 June 2016 CCD imaging lessons 16

17 Examples 1. What is the power that the Mees telescope collects from a 10 th magnitude A0V star, within the bandwidth of the G filter? From the spectrum above we see that the G filter covers wavelengths λ = nm. Thus its center wavelength is λ 0 = 546 nm and its bandwidth is λ = 89 nm. This is very much like the Johnson V filter in the table above, so we ll assume the same F λ for G for the zero magnitude star: F0 = F λ λ = W cm µ m µ m 13 2 = W cm. and the tenth-magnitude flux F 10 is given by ( ) 10 0 = 2. 5log F F F = 10 F = W cm The telescope s collecting area is a = 2700 cm -2, so P = F a =.. S W 16 June 2016 CCD imaging lessons 17

18 Examples (continued) 2. Atmospheric turbulence (seeing) blurs the images of stars taken with uncorrected ground-based telescopes, typically to a diameter of 2 arcsec at Mees. Suppose for simplicity that this image is uniform in brightness. How many pixels of the array does it cover? The solid angle of this 2-arcsec uniform blur is 2 arcsec Ω seeing = π = π arcsec 2 and that of a pixel is ( ) Ω pixel = arcsec = arcsec so the number of pixels is N =Ωseeing Ωpixel 63. In reality, the seeing-broadened stellar image would be brighter in the center and have a diameter between half-peak-brightness points of 2 arcsec, typically June 2016 CCD imaging lessons 18

19 Examples (continued) 3. Suppose the star in Example 1 produces the image in Example 2. How many electrons are collected in each pixel of the star s image in a t = 100 sec exposure? By how many data numbers (DN) does the star s image exceed the background sky level, in the displayed image? The total charge in the star s image, collected within t, is QS = IS t. Thus the number n of electrons in each of the N = 63 pixels is n Q I t τη λ P Nq Nq N hc S S 0 S = = = e e 14 ( 096. )( 06. ) ( 546 nm)( W) ( ) t See page 14 for other values. = 100 sec = 24 electrons 63 hc DN = 24e 19 DN. = 1. 27e 16 June 2016 CCD imaging lessons 19

20 Examples (continued) 4. With the telescope pointing 60 degrees away from a first-quarter moon, the moonlight produces a background observed in blank, star-free sky to be 50 DN per pixel in a 300-second exposure in the G filter. Show that this is much larger than the dark current accumulated in 300 seconds; then calculate the corresponding noise current, in electrons per second, within the size of the stellar image as in Example DN is n = 63.5 electrons, for a current in one pixel of IB q e = sec. The dark current per pixel at a CCD temperature of T = -20 C is sec. So background dominates dark current. In N = 63 pixels, the noise current is therefore (see page 11): I I q N N e qei qe Nnqe qe = = = t t t t = sec 1 Nn 16 June 2016 CCD imaging lessons 20

21 Examples (continued) 5. So what would be the magnitude of a star barely detected (S/N = 5) in the G image of Example 4? See pages 10 and 12: S I τηq λ P 5hcI = = = = = N I hci q S e 0 S N 19 5 P S W. N N τη eλ0 From Example 1 we know that the power collected from a zeromagnitude star is P 10 S,0 = W. Thus letting the barely-detected star be A and the zero-magnitude one be B in the magnitude equation we have FS,0 PS,0 m = 2.5 log = 2.5 log = FS PS 16 June 2016 CCD imaging lessons 21

22 Next time How to use CCDSoft to control the camera. In particular, how to set the camera up for operation, including focusing the telescope; calibrate and use the autoguider; work efficiently using the Color data-acquisition mode. Subsequent lessons on the data-reduction and image-prettification software, and on the optics and detection physics behind why the terms of the recipe are chosen. 16 June 2016 CCD imaging lessons 22