Video cameras and photometry Dave Herald Background Occultations
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 replay events • Measuring the change in brightness was not important
Background #2 • After several years experience, people became interested in measuring brightness changes v. Double star discoveries – measure the relative brightness of the components v. Occular – measure light drop and limiting magnitude, for event validation v. Mutual events of Jupiter and Saturn’s satellites – generate light curves v. Large stars – measure their diameter
Question • How reliable are our analogue video cameras for photometry?
‘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 Ø Image with linear relationship to light intensity across whole field. Precision of 0. 002 mag readily obtainable with care.
Video issues #1 • CCD with array of pixels • Array of pixels converted into an analogue signal (x-direction), in multiple scan lines (ydirection) - with number of scan lines generally different to number of rows of pixels. Ø horizontal axis is analogue representation of pixels; Ø Vertical axis – formed by combining pixels in adjacent rows of the CCD Ø 1: 1 correspondence to CCD pixels lost
Video issues #2 • Analogue signal digitised – to 8 -bits • Signal frequently compressed. Depending on compression algorithm: v. The compression is most likely not lossless; and v. May incorporate data from adjacent pixels in both the compression and decompression Ø Data integrity compromised
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 Ø Standard video images are not well suited to photometry
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
Example #2 – flat fielding • Video of star near moon, 40 cm ACF with 3 x reducer. • Plot shows field brightness in horizontal line across field & thru the star. Plot shows the background is 50% brighter in center of image
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
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}
Watec 120 N+ with Gamma ‘off’ Ratios relative to pink are: • • • 1 : 1. 7 : 3 1: 2: 4 1 : 2. 2 : 4. 7 1 : 2. 5 : 7 1 : 2. 4 : 9 => Camera response definitely non-linear with respect to intensity
Watec 120 N+ with Gamma ‘hi’ Ratios relative to pink are: • • • 1 : 2 : 5. 7 1 : 2 : 4. 2 1 : 1. 8 : 3. 7 1 : 1. 2 : 2. 3 1 : 1. 2 : 2. 2 => Camera response definitely non-linear with respect to intensity
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. Ø Normalising one object on the basis of another object of non-identical brightness will induce an apparent change in brightness as the altitude changes
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
- Slides: 16