Introduction

Its been a long while since the the SC1-4 webcam modifications were made and lots has happened to bring down the cost of entry level astro CCD cameras.

The SC5 modification board is a update of the original webcam modification that allows the modified camera to feel much more like a dedicated astronomy camera.  Also modified webcams are often used to guide larger cameras the SC5 modification board has an optically isolated guide port to make connecting it to a mount quite straight forward.

As part of this update the amplifier control has been revisited (control is needed to eliminate a glow in the corner of the CCD).  As you can read below the board provides independent control of power supply to CCD and substrate biasing.   Tom How has a good page on biasing webcam CCDs http://astro.neutral.org/eq/ampoff.html

The SC5

The board has holes that coincide with mounting holes on the SPC900NC webcam.  This makes it possible to piggy back the board  to this camera and as the SPC900NC is particularly easy to modify for long exposures ( Matthias Meijer ) it makes for a straight forward conversion.

That said the SC5 board could be used with all the webcams that use Sony ICX098 CCDs (ToUCam Pros, Vestas, QC3000, most QC4000) see http://homepage.ntlworld.com/molyned/web-cameras.htm for more info on webcams and their CCDs.

The Board

1    Optically isolated guide port. 

This is a standard ST4  (LX200) compatible guide port for connection to a telescope drive system in order to issue drive corrections.  As its optically isolated the connection can be made with a simple cable to many mounts.  To use the SC5 as a guide camera a connection from the camera to a computer as well a suitable guiding software is needed.

2   USB connection.  

Rather then needing a USB and parallel port connections, as the original SC mods, this camera requires a single USB back to the computer.

3   USB Hub section. 

When connected the SC5 presents the computer with a webcam and a serial port .   To control the traffic between these sources a simple hub is used.

4   External power supply. 

So far, all the USB ports this camera has been used on can provide enough power to run the webcam and the chips on the  board.  However its somewhat inevitable that there is a computer somewhere what will not have enough power at its USB ports.   In this case the SC5 can be powered from an external 12V source.

5   Amp and substrate control section.   

The Sony CCDs used in webcams have small amplifiers within the CCD that are needed when reading an image out of the CCD.  However these also produce a small amount of light by electro luminescence that can appear as an annoying glow in the corner of a image.  Many circuits have been proposed that help to over come this and all work by lowering the voltage applied to the CCD during exposure and restoring it for readout.  When the voltage drops from the 15V normal to about 10V the glow is much reduced.  However there is a side effect.  The voltage supplied to the CCD is also used to maintain the bias on the substrate.  Put simply, this is a voltage that allows the CCD to collect the electrons produced when lights hits it.  By reducing the voltage to the CCD to get rid of amp glow we also reduce the substrate bias, which reduces the ability of the CCD to collect electrons, which can result in the camera being unable to produce high pixel levels, especially when gain is not set at max.

The SC5 board can maintain the bias on the CCD substrate independently of the voltage supplied to the CCD.  This allows the CCD supply to drop to  6.7V  (note the CCD always needs some volts) to get rid of amp glow.  The board can be used to adjust the voltages of both substrate and supply over a large range.

6  Connections to webcam circuit board.  

These are the 'standard' mod connections for control of exposure, shutter, amp etc.

7  Analogue switch.  

This is the equivalent chip that is used when doing a dead bug mod directly into the camera but is a particularly low resistance version.

8  USB to Serial port.   

Provides a port though which the PC talk talk to the SC5 board.

9  Microcontroller.   

Controls the circuit board!  Also provides digital multimeter functions to monitor amp and substrate voltages.

10  USB connection to webcam.

Software