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Sunday, 26 August 2007

Webcam Modifications 3


One of the side effects of the Amp-off Mod is increased image noise. Discussions on the QCUIAG group led me to experiment with noise reduction techniques. This ultimately led to a complete reduction in CCD noise. Unfortunately this was coupled with a permanent loss of signal from the CCD. That is what happens when one shorts pin 8 to pin 9 on an ICX098 CCD......

Desperate for replacement, and with no ready supply of 680s, I modified a ToUcam Pro. I wont go into the details here, but overall this is an easier camera to modify and is more sensitive than the Vestas. Ashley Roecklein's website gives a good explanation of how to carry out this modification. Although I constructed an SC2 daughterboard I did do the mainboard mod for the ToUcam; once bitten twice shy.

I used the Peltier arrangement from my prototype 680, to cool the airspace within the ToUcam's case, a plastic Maplin project box. The results were very disappointing, with hardly any reduction in thermal noise. I noticed, however that the main chip on the rear of the main PCB got really hot after a few minutes of operation. I figured that this was either due to heat transmission from the CCD, or was internally generated. Either way it must directly or indirectly contribute to thermal noise in the result images. I tested the hypothesis by extending the Peltier's cold finger so that it was in contact with the chip. Bingo, immediate and dramatic reduction in thermal noise levels. This arrangement was not practical, but served as a prototype for a new Cooled ToUcam Pro, described below.

ToUcam Pro Cooled SC2

Outline Specification

Rugged, yet light weight construction to suit operation either at prime focus of an ETX90, or with 35mm camera lenses on a barndoor tracker. Also scalable as a first CCD camera on an 8" scope, which I hope to be my next step up the ladder.

Capable of low noise operation at 75% gain for exposure times of up to 120 seconds. (I plan to standardise on 1 minute exposures, when I have the scope to match)

Buildable, at home without any specialist tools.

Design Concept

I started from the point of my Vesta 680 prototype. In this design I sandwiched the CCD between the front and rear of a alloy case. In this way there was no need to fix the CCD to the coldfinger. I also wanted the minimise the back focus distance, so that the camera could be used with a Newtonian scope (keeping my options open). The case was to have the minimum n

umber of openings in order to reduce the likelihood of dewing/frosting within the camera.

Here is an external view of the front of the completed camera. The alloy case is available from Maplin. It has just sufficient room to hold all the electronics. A larger case would have allowed silica gel drying. The PK bayonet mount is the front of an old tele-converter. The IR Cut filter is salvaged from one of my Vestas. (All lenses were removed). The mount for the IR filter is cut from the front of the original ToUcam egg, and glued to the alloy box lid in front of the CCD. The mini-Din socket is the connection to the parallel port.

And here is the rear of the camera. The cooler is a Golden Orb unit, available from Maplin. The small white project box, also from Maplin houses the connections to the Peltier and fan. There is no fancy control circuit yet, just a full-on 5V supply.

Here is the inside of the box showing the peltier and coldfinger elements. The Peltier is a 5V Greenweld unit. It is 30mm square, which fits nicely between the two piece of aluminium cut from a 30mm wide strip, available from most DIY superstores. The cold finger is a 10mm square off-cut from the same strip material. This is stuck together with heatsink compound and mounted on the round base of the golden orb. I used the original fixings of the Orb to fix it to the

box. It protrudes through a 32mm hole in the case. The spring in the Orb's fixing pushes the cooler assembly onto the PCB, when the box lid is screwed in place. Note the Vesta tripod bush to the left of the Peltier. This is used for Mounting on the Barndoor tracker, or piggybacking the camera onto a .scope.

The electronics are mounted on the back of the lid. This gives good access during construction and testing. Note the white patch where the cold finger rests on the large chip on the PCB. The PCB is connected to the case using the original ToUcam screws. The PCB is held square to the lid using an insulated aperture made from a nylon (?) off-cut. This is approximately 20mm square and 3 mm thick. A rectangular hole is cut in its centre, that is exactly the size of the CCD's glass window. This isolates the front of the CCD

from the cold interior, thus minimising frosting and dewing, and provides a stable mount for the PCB.

Note that I have isolated the PCB from the lid. I used shrink wrap tubing. The earth loop between the PCB screen and the lid was a source of electrical noise, manifesting itself as the dreaded 'diagonal patterning' or 'herringbones'. Isolating the screws removed all traces of noise.

Above the PCB is the SC2 circuit. To the right if the stripboard is the Amp-off relay. I have used the Burri-Behrens pin 8 mod, which uses a Zener to reduce voltage to the on chip amplifier during exposure. Instead of the PNP trainsistor I have used a relay in an attempt to improve noise immunity. I still need to work on this part of the camera. I think that the Zener, a 6.2V type could be raised in value, as there is still some glow in 100% gain, 120 second exposures.

Possible Enhancements

A machined coldfinger may improve thermal efficiency, though 120 second darks are acceptable.

A larger case would allow Silica Gel drying.

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