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Note: The use of this information is on your own risk and could damage your camera.
Gain setting with selfmade cable control
The very light sensitive B/W video camera WAT-902H2 Ultimate can be used in automatic exposure mode and with manual settings too. This is a big advantage for astronomy work. But to change the signal gain one has to screw an internal pot with a small screwdriver through a small hole on the backside of the camera case.
Because mostly one can not look to the notebook screen or to a monitor during gain setting very often several setting steps are necessary to reach the right gain. And if exposure situation changes shortly during recording it is very difficult to react in a useful time. So there could happen a wrong exposure in an important moment of the event.
Because I know about the advantage of a cable control from my WAT-120N I wanted to have a cable control at least for the most important setting, the gain, on my WAT-902H2 Ultimate too.
But the necessary electrical and mechanical changes on the camera should be as small as possible and realised in a simple way. A further condition was the removal of all changes without problems to go back to the original status. After some measurements I had found an useable solution. Only two components, a resistor and a potentiometer, are necessary to realise a cable controlled gain setting. Both components are acting as an alterable parallel resistor in the range of 1k to 11k Ohms to the internal pot. In this way the internal adjusted gain control voltage can be decreased additionally by the external circuit.
Gain setting on a modificated camera:
The external gain has to be set to the maximum (= max. resistance 11k) value. The internal gain pot has to be set to the max. useable gain or a little bit over this point. In this setting the image noise can be seen clearly. And now one can decrease this high gain with the external box down until you see a dark image. In this way it is possible to react on changing exposure situations and to set an optimal gain without touching the camera or the telescope.
|WAT-902H2 Ultimate cable remote control - circuit|
|WAT-902H2 Ultimate cable remote control box inside|
For my assembling I used a two pole cable with about four meters. This length is enough for practical use. But the length of the cable could be some meters more if needed. The gain of this camera is controlled in manual mode by a changeable DC voltage in low resistance. So this DC voltage is relative insensitive against external interferences or glitches.
|WAT-902H2 Ultimate with selfmade cable remote control|
|The cable comes out of the case through the hole of the gamma switch|
|Cable mounting with a photo screw on the top of the case|
|Plastic washers are fixing the cable in a loop around the photo screw|
The rear panel of the camera case is mounted with four cross-head screws on the corners. This screws have to be unscrewed. Now the rear panel with implemented sockets for 12V DC power and signal output can be lifted careful. Thereby a little piece of isolation tape on the printed circuit board under the gain pot becomes visible. It is used for sure isolation of the bottom side of the printed circuit board against the big nut of the BNC signal output socket.
|WAT-902H2 Ultimate with removed rear panel|
|For lack of space a thin two pole cable is used inside the camera. After lifting the small piece of isolation tape with tweezers from the printed circuit board the two pole cable can be soldered on the two right pads of the internal gain pot with the help of a magnifier. In doing so one should take care to make small soldering points.|
|Drawing of the printed circuit board with soldering points under the gain pot|
|The cable has to be soldered direct under the internal gain pot|
|The original isolation tape is placed again on the two soldering points. Two new layers of isolation tape should be added for reliability. Both solderings points must not have electrical contact to the sockets or to parts of the case.|
|Isolation tape prevents contact with BNC signal out socket.|
|After an internal loop the thin two pole cable should run out through the hole of the Gamma switch. Now the Gamma switch can not be set to "High" position. But this setting should not be used for astronomical records. The thin two pole cable was soldered by me with a bigger cable of four meters length outside the camera. The two cable soldering points were isolated and the bigger cable was mechanical fixed on top of the camera case with a photo screw and between two plastic washers very simple.|
|A cable loop inside makes room for both sockets on the rear panel.|
The modification for external gain setting with a cable remote control worked very good for me in practice during some occultation recordings. Mainly for recording events on the moon limb or on the sun this modification makes work much easier.
Additional images about the gamma switch inside the camera WAT-902H2 Ultimate
August 19, 2009
Questions and suggestions to => Gerhard Dangl