Schematic, Errata and Linearity testing
The modification is rather simple with respect to the actual circuit. There are 2 versions, with version 1 being suitable for retro fitting into existing cameras that currently use either logic switching or bi-fet switching (analog). Diagram 1 shows the circuit for version 1. The parts outlined in red dash are reused from an existing amp control mod that is connected to Pin 9 of the ICX098 CCD array. It is not necessary to remove these parts but there is no need to place this modification right next to the CCD array and the parts are rather inexpensive so a rebuild is recommended. If Pin 9 of the CCD has been cut to accommodate the amp-off mod as described on Steve Chambers site then you will have to bridge it back to ground on the PCB with wire/solder. On most cameras (with exception to those who remove their CCDs from the PCB) you will have to cut Pin8 on the CCD itself as the through hole pad is often used as just a pass through to supply the vertical driver chip (1267AN) with its supply voltage.
Diagram 1
The premise behind the circuit is to switch the reference zener diode in series with the supply line during integration. This in turn provides a precision voltage drop and controls amp glow by reducing the bias on the internal output amp in the CCD array. The diode is a 5.1 volt zener and must be place with the band on the supply side (+15 volt side). The zener provides a current independent voltage drop which is also very stable over temperature vs. a resistor. The bypass capacitor may not be needed but is good engineering practice when steering supply lines via regulators or zeners. The current consumption of the circuit is less than 5mA and should operate with out putting undue stress on the +15Volt lines. The total current consumption should never exceed 10mA with the CCD supply at +10 volts and never exceed 7mA with the supply at +15 volts.
The follow chart shows apparent magnitudes for a given exposures using an IR blocked point source. The background apparent magnitude is the 50% level of the entire image and gives some indication of contrast and background brightness. lower values are an indication of better contrast when compared to the point source magnitude. The values were derived from the exposure images taken on the exposure page at different integration times. This clearly shows the CCD integrates for up to 4 minutes. The compression seen with no amp glow control and +10Volts is due to the limitation in the dynamic range of the ADC in the camera and the magnitude of the point source. Reciprocity failure in film astrophotography is seen in CCD imaging as compression or a lack of headroom during integration and manifests itself as a loss of contrast and the inability of the image array to integrate over a given exposure time. Considering most web cam derived images and the time to integrate them the above method to reduce amp glow provides adequate control with out compromising the head room of the CCD. At lower values of VDD the CCD still integrates but there is a loss of sensitivity all be it small.
