The Surface Brightness distribution of an object determines its contrast and visibility as well as the exposure times needed by a camera to record it on film or an electronic sensor.
Introduction to Surface Brightness of Deep-Sky Objects
The Pleiades, M45 shows beautiful nebulosity. This page shows surface brightness data for the reflection nebulae in this star cluster.
This image was obtained within the Denver metropolitan area with a 700mm focal length 5-inch aperture lens and a Canon 10D digital camera, 27 minutes of exposure time. See the astrophoto gallery for more information on this image.
A calibrated M45 image was used to measure surface brightnesses at multiple locations, below.
You can see from the spot measurements that the nebula has peak brightnesses of magnitude 20 / square arc-second in the blue and is fainter in the red. Not by the definition of magnitudes (the star Alpha Lyra = magnitude 0 at all wavelengths), red = blue magnitude would still be higher intensity in the blue because Alpha Lyra is a blue star. Spot measurements are averages in a 9x9 pixel (20 arc-second) box.
Scotopic magnitudes (what you see with your dark-adapted eye on faint subjects) are, to first approximation the average of green plus blue magnitudes in the chart above.
| Spot | Scotopic Magnitude (approximate) |
|---|---|
| A | 20.5 |
| B | 20.0 |
| C | 20.3 |
| D | 20.9 |
| E | 21.3 |
| F | 20.6 |
| G | 20.5 |
| H | 20.8 |
The profile above, a to b, is graphed below.
Surface brightness magnitudes on this page are estimated to be +/- 0.2 magnitude when brighter than magnitude 20. The error increases to about +/- 0.5 magnitude at magnitude 22. The red channel is noisier than green and blue, mainly due to the higher sky background due to light pollution in the image which was obtained from the Denver metro area.
Last Updated April 10, 2004