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Video cameras and photometry Dave Herald

Video cameras and photometry Dave Herald. Background. Occultations are usually step events When video introduced, it overcame issues of Personal Equation, increased the timing precision, and provided a mechanism to re-play events Measuring the change in brightness was not important.

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Video cameras and photometry Dave Herald

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  1. Video cameras and photometryDave Herald

  2. Background • Occultations are usually step events • When video introduced, it overcame issues of Personal Equation, increased the timing precision, and provided a mechanism to re-play events • Measuring the change in brightness was not important

  3. Background #2 • After several years experience, people became interested in measuring brightness changes • Double star discoveries – measure the relative brightness of the components • Occular – measure light drop and limiting magnitude, for event validation • Mutual events of Jupiter and Saturn’s satellites – generate light curves • Large stars – measure their diameter

  4. Question • How reliable are our analogue video cameras for photometry?

  5. ‘Traditional’ CCD camera • CCD with array of pixels • No anti-bloom gating on CCD (linearity issue) • 1:1 correspondence between pixel and image file. Imaged represented at 16 bits • Bias frame, dark frame and flat field applied to image • Image with linear relationship to light intensity across whole field. Precision of 0.002 mag readily obtainable with care.

  6. Video issues #1 • CCD with array of pixels • Array of pixels converted into an analogue signal (x-direction), in multiple scan lines (y-direction) - with number of scan lines generally different to number of rows of pixels. • horizontal axis is analogue representation of pixels; • Vertical axis – formed by combining pixels in adjacent rows of the CCD • 1:1 correspondence to CCD pixels lost

  7. Video issues #2 • Analogue signal digitised – to 8-bits • Signal frequently compressed. Depending on compression algorithm: • The compression is most likely not lossless; and • May incorporate data from adjacent pixels in both the compression and decompression • Data integrity compromised

  8. Video – issues #3 • CCD presumably has anti-bloom gating => non-linear response • Video traditionally has a gamma correction applied => non-linear response • Darks and flat fields are not usually applied => non-uniform fields • Standard video images are not well suited to photometry

  9. Example #1 - Derek Breit video • Yellow star at 3,600 – Tycho V = 6.7 • Pink star at 1,200 = 0.33 of yellow = 1.2 mag fainter => mag 7.9. HOWEVER Tycho V= 7.3 • Pink star should be at height 2070 => Measured brightness  star brightness

  10. Example #2 – flat fielding • Video of star near moon, 40cm ACF with 3x reducer. • Plot shows field brightness in horizontal line across field & thru the star. Plot shows the background is 50% brighter in center of image

  11. Example #2 – flat fielding • Limovie & Tangra both measure a changing star brightness as it crosses the field – in this case more than a 50% change Variation in background illumination and star brightness consistent with a need for a flat field

  12. Example 3 – camera response • Integrating camera • Change integration period – keeping stars in the same position. Measure star brightness • If camera response is linear, ratios of star brightness should remain the same {Using brightness ratio avoids any need to precisely determine the exposure duration. Keeping stars in the same position avoids any flat-fielding issues}

  13. Watec 120N+ with Gamma ‘off’ • 1 : 1.7 : 3 • 1 : 2 : 4 • 1 : 2.2 : 4.7 • 1 : 2.5 : 7 • 1 : 2.4 : 9 Ratios relative to pink are: => Camera response definitely non-linear with respect to intensity

  14. Watec 120N+ with Gamma ‘hi’ • 1 : 2 : 5.7 • 1 : 2 : 4.2 • 1 : 1.8 : 3.7 • 1 : 1.2 : 2.3 • 1 : 1.2 : 2.2 Ratios relative to pink are: => Camera response definitely non-linear with respect to intensity

  15. Effect of gamma on long recordings • Gamma changes the recorded brightness depending on the brightness of the object • Star brightness changes as air mass changes (i.e. a star gets fainter as its altitude decreases) • Without gamma, two stars (or moons) of similar brightness retain same brightness ratio • With gamma, ratio of recorded brightness changes as star altitude changes. • Normalising one object on the basis of another object of non-identical brightness will induce an apparent change in brightness as the altitude changes

  16. Summary • Compared to usual CCD photometry, analogue video photometry presents serious challenges • Video systems are usually non-linear – need to understand the non-linearity in the recording system as a whole (camera + avi creator + recording software/hardware) • Flats and darks are essential for decent photometry

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