Quote:
Originally Posted by jase
Also Richard....
What is your opinion of cooling and longevity of the CCD chamber seal? I'm aware many (telescope rental companies and education institutes) don't warm up the cameras during the day light hours as by doing so it apparently prolongs the longevity of the CCD chamber seal.
Fact or Fiction?
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I don't think there's a single answer that is true for all designs.
It really depends on how the camera and seal are made
I can say that leaving the sensor biased up 24/7 will increase the liklihood of random radiation damage.
The sensors are bombarded constantly by radiation. ]Under the right circumstances some of it can cause hole-electron pair generation in the gate oxide. This is very undesirable, but the damage can be self-repairing (annealing) under the right conditions.
when an energetic particle passes through the gate oxide and collides with atoms in it, it loses energy and that can create hole-electron pairs within the gate oxide. If these oppositely charged "particles" remain physically close to each other, there's a statistical probability that they will recombine, and if they do there's no lasting damage.
However the hole has very low mobility compared to the electron and what often happens is that the electron is swept away by an electric field (like if the sensor is under bias) and that leaves a hole trapped in the gate oxide. That causes a shift in the flatband voltage if enough of them are so trapped and that can cause the sensor's dark current to increase significantly in pixels so damaged.
Leaving the sensor under bias simply increases the probability that the recombination does not happen. So if you have two identical sensors sitting next to each other, one powered up and one not, the one that is powered up will age faster than the other one....
It is certainly possible to design a camera that can be cooled separately from powering up the electronics, so in theory it should be possible to leave one cooled down but not under bias, so as to minimize the risk, but none of the amateur Astrocams I have seen work that way.
the FLI ML and PL cameras have very good seals. Thermally cycling them seems to have no effect on the integrity of the seals.
I've heard that the reason Apogee takes a half hour to cool down is out of concern of the integrity of their seals. I haven't personally compared the design of the seals of the two brands to see how they differ. I suspect that the Apogee seal design is reasonably robust and using a half hour to cool down is being unnecessarily conservative from the perspective of chamber seal integrity.
Perhaps there's another reason for the long cool down like thermal runaway?
That's poked its ugly head up occasionally in certain camera designs in the past. One thing is certain; the fans aren't arranged in such a way to efficiently shed heat from the heatsink in the Uxxx designs: the fans' airflow stagnates at the bottom of the heatsink in the one I examined.
From a marketing perspective, it seems easier to swallow telling someone to accept slow thermal slews to keep the chamber seal good than to prevent thermal runaway....
claiming the former will tend to cast doubt on the operational integrity of your competitor versus the latter which reveals a design deficiency in your own product: two different ways to spin the same issue....