|K3's Astronomy - Exposure|
|"When I consider your heavens, the work of your fingers, the moon and the stars which You have set in place, what is Man that You are mindful of him?" -- Psalm 8:3,4|
|As an amateur astronomer I am
using webcam for astrophotography. It is Philips Vesta
Pro (PCVC680K) model. It uses SONY ICX098AK CCD chip.
Image acquired from Vesta Pro webcam is 24-bit RGB bitmap (AVI file uses 12-bit codec). Although 24-bit BMP format has 16777216 possible colors, it has only 256 levels of brightness - it means, that the ratio between the brightest pixel (level 255) and the detectable darkest pixel (level 1) is only 255:1 (48dB). In magnitude scale it means the difference of 6.01 mag.
The range of brightness levels on Moon' disk during the first quarter is much more than 1:255. More expensive specialised astronomy CCD cameras use 16-bit A/D converters, which guarantee the brightness ratio 65535:1 (96dB). In magnitude scale it means the difference of 12.04 mag. It allows to obtain more precise astronomy photographs, but the problem of displaying such photographs on computer monitor is the same - graphic cards work in 24-bit (eventualy 32-bit) mode, which allows 256 levels of brightness.
The possibility of displaying images with such range of brightness is to convert the original 16-bit levels image to 8-bit levels. Using a simple cutting lower 8 bits will cause similar effect as a picture was captured at 8-bits - the dark parts of image will disappear. Solution of the problem can be converting 16-bit picture to 8-bit according to logaritmic scale - the displayed brightness ratio of result picture will be still 255:1, but darker parts will be visible.
There is a possibility to record more brigthness levels with 8-bit webcam, too. It can be achieved by superposition of two or more pictures of the same object with different exposure settings.
The above pictures show the procedure of obtaining
picture, which enable to display parts of the moon with
higher brightness ratio than 255:1 with webcam.
Superposition was done in Corel PhotoPaint
Computer generated images, real images, drawings and texts are property of the author and may not be reproduced or used without permission of author.
Last Update: 11.4.2001