K3's AstroPhotography
"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

Philips Vesta (PCVC675) SC2 cooled camera tests

Here are some test results of my cooled Vesta SC2 camera.

Test A: Cooling down
Test B: Switching amplifier ON/OFF
Test C: Comparing sensitivity of camera with amplifier ON/OFF
Test D: Saturation of image with amplifier OFF
Test E: Influence of resistor between collector and emitor of CCD Amp switch transistor

Test A: Cooling Down

Ambient temperature: 26C
Exposure: 120 seconds
Gain: 100%
CCD Amplifier ON
The camera was attached to computer for 30 minutes to achieve its working temperature. The ambient temperature was 26C. The first frame was exposed without cooling. Then the cooling system was switched on immediatelly. The temperature of CCD chip was stabilized after 8-10 minutes of cooling.


0 minutes (start)

After 2 minutes of cooling

After 4 minutes of cooling

After 6 minutes of cooling

After 8 minutes of cooling

After 10 minutes of cooling (temperature stabilized)

Test B: Switching Amplifier ON/OFF

Ambient temperature: 26C
Exposure: 120, 300, 600 seconds
Gain: 100%
CCD Amplifier ON/OFF
The camera was attached to computer and cooled down for 20 minutes to achieve its working temperature. The ambient temperature was 26C.

Amplifier ON Amplifier OFF

120s, gain 100%

120s, gain 100%

300s, gain 100%

300s, gain 100%

600s, gain 100%

600s, gain 100% (some glow is visible at top-left)

Test C: Comparing Sensitivity of Camera with Amplifier ON/OFF

Ambient temperature: 26C
Exposure: variable
Gain: 100%
CCD Amplifier ON/OFF
The camera was attached to computer and cooled down for 20 minutes to achieve its working temperature. The ambient temperature was 26C.
Image is reverted, because of camera position (ventilator on top). I didn't want to revert it to proper position because I wanted to keep the amplifier glow position at top-left. The room was totally dark, only my laptop screen was a little shining (in oposite direction than photographed scene). That's why image has some blue tone. Some very low light is penetrating through our blind from our city (visible on the wall near vase).

The image with amplifier OFF is darker than the image with the same exposure, but with amplifier ON. As my constructed image shows, it is caused by bias caused by glowing amplifier. By addition of two frames - one with scene with amplifier OFF + one blurred dark frame (because of removing hot pixels) with amplifier ON we obtain the very similar image like with scene with amplifier ON. So it seems, that the sensitivity of camera is the same with amplifier ON and OFF.


90s, gain 100%, amp ON

120s, gain 100%, amp OFF

120s, gain 100%, amp ON

150s, gain 100%, amp OFF

Construction of 120s, gain 100%, amp ON (inPhotoPaint). Constructed from
Photo 120s, gain 100%, amp OFF picture +
blurred DarkFrame 120s, gain 100%, amp ON (blurred because of removing hot pixels from DF)

240s, gain 100%, amp OFF
Some granulation in wall is visible - it is some problem with amplifier OFF mode and bright areas.

Test D: Saturation of image with amplifier OFF

Ambient temperature: 26C
Exposure: variable
Gain: 50%, 100%
CCD Amplifier ON/OFF

After doing SC2 modification I tested the camera in daylight. I set amplifier gain to 0%. At first I thought, that it was not working, and all my work was useless. The picture was still of the same intesity regardless of exposure length. When I tested the exposure with amp ON, the image was totaly white (overexposed), when I used longer exposure time (it's correct behaviour).

I experimented with switching amplifier ON/OFF during exposure and found out, that switching amplifier OFF causes limitation of video signal to some saturation level. I used 10s exposure time. When I switched amplifier OFF for 5 seconds at start of exposure, the result image was similar as if I used 5s exposure with amp ON. When I switched amplifier OFF for 5 seconds at the end of exposure, the result image was very dark and grainy.

After the first disappointment I tested the camera in the evening. When I set amplifier gain to higher values, I noticed that amp OFF mode is working with some limitations.

The camera was attached to computer and cooled down for 20 minutes to achieve its working temperature. The ambient temperature was 26C.
Image is reverted, because of camera position (ventilator on top). I didn't want to revert it to proper position because I wanted to keep the amplifier glow position at top-left. The room was lighted by constant light, exposure time was altered from 2s to 120s.

The experiment shows some strange behaviour of webcam with amplifier OFF - if low gain is selected, than the image become saturated in some level of exposure and further lengthening of exposure time have no influence to light parts (only darker parts become lighter). Furthermore in saturated parts some granularity is visible. Fortunatelly, the phenomenon doesn't not occur if I set aplifier gain to 100%. The lower gain is set, the lower is saturation level in amp off mode. When gain is set to 0%, then picture is saturated also when short exposures are used (0.5s) and is grainy and dark.

Images were captured in 320x240 resolution and were resampled to 50% size.

Here is full size of saturated image (120s, gain 50%, amp OFF). Granularity is apparently visible.

 

Test E: Influence of resistor between collector and emitor of CCD Amp switch transistor

Ambient temperature: 26C
Exposure: variable
Gain: 100%
CCD Amplifier ON/OFF

If some problems occur, there is an advice on Steve Chamber's pages to add auxiliary resistor between collector and emitor of CCD amplifier switch transistor.
Although the resistor helps to solve the problem of saturation of my CCD chip in lower gain, it also causes that amplifier is not switched off totally, thence some amplifier glow is visible. Here is comparison of 180s and 600s exposures with various resitors.


180s, Amp OFF, no resistor

180s, Amp OFF, 680 ohm resistor

180s, Amp OFF, 390 ohm resistor

180s, Amp ON

600s, Amp OFF, no resistor

600s, Amp OFF, 680 ohm resistor

 

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Last Update: 14.09.2005