Re: Astro CCDs still dragging their pixel feet

On the cameras that I built we used LN2 as a coolant.
One of the dewars held only a fraction of a liter of ln2
and the goal was to get the hold to to last through the night.
I was very interested in reducing the heat load on the LN2 and did a bunch of tests. The CCD does not dissipate power during exposure as the clocks are held in the expose state and we dropped the input fet bias down to some level during this time to prevent corner glow.

In that state most of the thermal load was coming form the front window
thermal radiation. pull the chip out and the thermal load dropped by 5 watts if i remember for a 2K*2K 15 um pixel chip. for higher temps this will be less of an effect. The same setup with a dry gas instead of a partial vacuum would dramatically increase the heat load starting at about 0.1 miilitorr pressure. I was very important to use a lapped surface for the cold finger as thermal conductivity is a function of
contact area and pressure am i tried to reduce the pressure as much as possible. Such a method could also cause a "slow leak" as it took a while for all the air to get out of a confined space and all fasteners used were drilled down the center, so called "pump out" hard ware that produced a harder vacuum in a shorter time.

we have encountered dewars that do have a real "slow leak " and apart from reducing the hold time cause long term damage due to condensation on the ccd. I used low out gassing parts when ever possible. Some detectors were contaminated by the dry gas line that had a plastic tube going to the gass supply. during the summer when the dome was warm during the day, some of the volatile components would collect on the
detector inside window and detector.

I know people that have used TEC stacks to get a delta T better than 60 degrees and they always had a hard time maintaining it over time.

Dan



David Nakamoto wrote:
Based on my experience at JPL, it is not that easy to attach a heat sink to a CCD or CMOS chip. The requirements seem contradictory. The adhesive must be thermally conductive so the heat from the chip can be drawn away, but electrically insulated so it won't affect the electrical properties of the chip. Usually the two properties go hand in hand, thermal conductivity usually associated with some electrical conductivity. Also, the housing for most CCDs and CMOS imagers is not a good thermal conductor, so simply attaching the heat sink, thermal finger, and/or cooler to the back of the IC housing is not efficient. For normal cameras this type of housing is not a problem, since cooling is not done because the signal levels (brightness of the scene) is so high that the noise is negligible. So the solution is to take the chip out of its housing, or at least take the backing off the housing, and adhere the cooling device to the CCD or CMOS imager.

The adhesive has to have no thermal expansion in either direction, as this would place mechanical stress on the chip. This usually means the adhesive being used is has pretty much permanently attaches the CCD or CMOS imager to whatever it is that's cooling it, making replacing a damaged chip an impossible affair without replacing the heat sink or thermal finger it's attached to, a costly affair.

And this is just trying to cool the chip, a necessary requirement since this is the best way to decrease the amount of noise generated by the chip, a result mainly of the heat it generates, which it sees as part of the signal coming from the image.

An added requirement due to the low signal levels is the need for low noise electronics to read out the signals. If you look at a histogram of the number of pixels verses the signal from those pixels, on normal images (day or nighttime) most of the pixels will have a high signal value. This is just the opposite of deep sky images, or even planet images unless the planet covers more than half the pixels, where most of the pixels are at the low signal end, often in the lowest 10% of signal values if you isolate the pixels just containing the object you wanted to image. Any noise from the readout electronics will add to the difficulty in trying to get clean clear images of deep sky objects, as you can see from this discussion, since noise is also in that low 10%.

Compared to what everyday cameras need in order to generate images, astronomical cameras need higher grade electronics and other things not needed by those consumer cameras that are still a requirement for getting good images of very faint objects.

And for those that say "the professionals use this or that," I suppose if we all individually had tens and hundreds of dollars of spare change, we could also afford such large cameras, assuming we had enough image size at the focal plane of our telescopes to be able to take advantage of such large arrays. Such comments and comparisons with professional equipment are, in this case, merely tools of a troll, and the poster should be considered as such, or at least horribly misinformed or educated on the subject.

--- Dave 
.

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