Some darkframes with a vesta pro and a 1/4" B/W ccd chip
Testing the efficiency of the preamp control
The preamp glow is a known phenomenon: CCD chips integer a preampli that radiates and alter long exposure images by a veil, appearing in the upper left corner for the Vest Pro and ToUcam, ang gaining the whole image as exposure lengthens.
As it has been described by Steve Chambers, a solution to this problem is to cut power on the preamp when it is not needed, that is, during the entire exposure, and to power it up just before downloading the image. This page illustrates the effects of the glow on a B/W 1/4" CCD chip and the benefits of suppressing it. Let's images speak:
| Preamp off during exposure | Preamp on during exposure | |
| 3.2 seconds |
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| 6.4 seconds |
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| 12.8 seconds |
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| 25.6 seconds |
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| 51.2 seconds |
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No comment needed. Comparison of 5 images with preamp off on the left column, with preamp on on the left. 5 exposures: 3.2, 6.4, 12.8, 25.6 and 51.2s, on a vesta pro, B/W chip, gain at full range
Preamp glow can already be seen on the first image at 3.2S. The effect is dramatic over 12 s and suppressing it is a must-do if we want to exploit at its best the sensitivity of this chip, without losing fainter objects.
On the picture below by JP Bourgeay, that illustrates SC2 modif, the part that concerns preamp control appears brighter. This control is completely independent of the SC2 mod and can be made very easily. Just be careful while soldering the transistor on pin 9 of the chip: CCD are fragile in nature. Take the necessary precautions to avoid harming your cam.
And then you are go for dark darkframes...
fm, 2003/09/06