Video cameras and photometry Dave Herald. Background Occultations are usually step events When video introduced, it overcame issues of Personal Equation,

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

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

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

imageImage 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 (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 CCD1:1 correspondence to CCD pixels lost

Video issues #2

• Analogue signal digitised – to 8-bits• Signal frequently compressed. Depending on

compression algorithm:The compression is most likely not lossless;

andMay incorporate data from adjacent pixels in

both the compression and decompressionData 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

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, 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

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 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

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

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

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