Camera of astronomy

A camera of astronomy is a Caméra which was especially studied for this use.

Why specific?

Except for Sun, the Moon and of some brilliant stars, it is impossible to use a traditional numeric camera because of weak luminous flows. In order to collect these luminous flows, it is necessary to have a long Exposure time (time allowed minima: numerical fifteen minutes in , one hour in silver). However that has two important and considerable consequences:

into silver, beyond of a certain time the film loses its sensitivity;
numerically, the sensor heats much and it thus should be cooled.

Moreover one needs a sensor of big size and especially more significant, and this in others wavelengths that the visible Lumière. However no sensor with everyday usage (cinema, photography…) do not offer this possibility.

If two technologies are evoked here, one should not forget the advantages and disadvantages of each one in Astrophotographie. In the same way, if the term of CCC is more used when one speaks about the numerical sensors in this field, it is because sensors CMOS are very far from offering performances of even level.

The cooling of the sensor

There exist several ways of cooling a sensor CCC, the techniques are the following ones:

- thermoelectric cooling ;

- liquid cooling;

- mechanical cooling;

- cooling with ice dries;

- cryogenic cooling .

These techniques are classified according to the use of the camera. The camera of professional astronomy, having a sensor of big size and thus an all the more important need for dissipation of heat, uses especially the cryogenic technique. On the other hand, a camera amateur will use especially the thermoelectric technique.

In all these techniques there is an invariable common point: the sensor is connected to the cooling system by a heat-conducting element (generally coppers some).

Thermoelectric cooling

It is about the least expensive device and more used on the amateur film cameras. It is based on the Peltier effect. One uses one or more modules to transfer heat further possible from the sensor and towards a Radiateur of the solid type/air. This one can be seen very often associating a Ventilateur to evacuate heat.

Liquid cooling

This device is identical to that of the Watercooling in data processing and functions on the same principle: a water flow is used to evacuate heat. Certain top-of-the-range cameras pre-are equipped and also have with it an additional cooler, generally thermoelectric.

Mechanical cooling

This device uses Compressed air and resembles liquid cooling physically (the pump being replaced by a compressor). It alas the least effective of all and forever is very much used. Other gases (Nitrogen, freon inter alia) were used. But mechanical cooling offers neither the performances, nor the low costs of the other systems in the professional uses. This device that is used on the Réfrigérateur S and Congélateur S of family and industrial use.

Cooling with ice dries

This one is simplest of all but request more labor (recharging in ice and pressurization). The dry ice (carbon dioxide in a solid state) is contained in a tank with the back of the camera. To cool the camera, it is necessary to press the ice against the part of the reserve in contact with the hot spot. Under the action of heat the dry ice finds its gas state. To compensate for the loss in the tank, this one is provided with a system of screw press in order to maintain and compress the dry ice against the hot spot.

Cryogenic cooling

Near to the mechanical system, it is most profitable of all. It runs on nitrogen and that made that it is used the most on the professional cameras of astronomy.

The sensor

We will not speak here about the types of sensors but their characteristics.

In astronomy, the infra-red radiations and ultraviolet contain very useful information. However no normal sensor allows their measurement. Another essential point is the size of the sensor. A normal sensor has a size lower than 15 Mpixels whereas in astronomy the size depends on the use of the camera.

One comes to cameras of 100 Mpixels to see more (360 on the MegaPrime camera of the CFHT). For the following reasons:

of cost (a small number of sensors replacing large of the same a size cost less),
of maintenance (replacement of a defective sensor),
of re-use (a model developed for a camera can be begun again on another),

one prefers dividing a large sensor in a certain number of small. It is then enough to assemble them. However it is that the normal sensors do not allow this use: it is necessary that the zones receiving the light are closest from/to each other (Compare the first image of the article CCC with that of the mosaic of MegaPrime).

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