S/N Characteristics of the Canon 40D DSLR

 

The Canon CMOS detector behaves very differently from a CCD in that the readout noise varies with gain (ISO).  Lower gain (higher ISO) produces less readout noise.  The following table and graph are based on my measurements and calculations using the method described in Howell’s “Handbook of CCD Astronomy”, modified for the Bayer matrix.

 

ISO

Gain

Read Noise (e-)

100

2.96

16.77

200

1.55

9.94

400

0.80

6.84

800

0.39

5.30

1600

0.20

4.55

 

 

Dynamic Range by ISO

 

Linear Dynamic Range = (number of units in range) / camera_noise (same units)

Pixel Full well = 41,400 electrons

Bias level = 1,024 DN (ADU)

Dynamic

ISO

Range

100

2263

200

2365

400

1787

800

1131

1600

658

 

ISO 100 has less Dynamic Range than ISO 200 due DN range exceeding full-well depth.  Thus ISO 100 should be avoided for all types of photography except for special effects such increased movement-blur in bright conditions.  ISO 200 produces the best dynamic range and should be favored for terrestrial photography.

 

Sky Effect on Camera Noise

 

Readout noise combines with Poisson noise from the sky background:

Total noise  = sqrt(sky + readnoise^2)

Thus camera noise has less effect on longer exposures and short exposures benefit from low camera noise (high ISO).

Effective camera noise = TotalNoise – SkyNoise. 

Effective camera noise affects dim objects and is undesirable.

This graph shows the effect of exposure time on excess camera noise (electrons), assuming sky glow = 1 electron per second per pixel, which is typical for the 40D in a semi-polluted suburban sky at f-ratio = f/5 (approx):

Effective Camera Noise by Exposure Time (sec)

 

Dynamic Range Corrected for Sky Effects

 

Background sky glow is a type of image bias – it narrows the range of sensitivity and contributes noise:

Effective Dynamic Range = (range_DN – sky_glow_DN)*gain / sqrt(camera_noise^2 + sky_glow_DN*gain)

A doubling of dynamic range represents almost 1 magnitude of brightness.

This graph shows the effect of exposure time on effective dynamic range, assuming sky glow = 1 electron per second per pixel, which is typical for the 40D in a semi-polluted suburban sky at f-ratio = f/5 (approx):

Effective Dynamic Range Exposure Time (sec)

 

Balancing ISO and Exp Time for Optimal Noise and Dynamic Range

 

Effective camera noise and dynamic range are odds with each other – longer exp times &/or higher ISO result in lower noise but also lower dynamic range whereas shorter exp times &/or lower ISO result in higher noise with greater dynamic range.  So the target should be evaluated to see which effect is most important. For example, dynamic range is important for preserving star colors in a bright cluster but low noise is more desirable for capturing dim galaxies or nebula. 

 

Another consideration concerns time limits. For example, unguided exposures must be short. Short exposures at low ISO carry a lot of noise and short exposures at high ISO have reasonable dynamic range. Thus high ISO should be favored for short exposures unless the target requires a large dynamic range.

 

A generally good approach for astronomical images that strikes a compromise between these effects is:

exposure time >= 120 sec at ISO 400-800.