New design of the Gx camera desiccant container

Main page▹Product Overview▹Astronomical cameras▹Articles	13.2.2017 \|

CCD dark current halves with every ~6°C, so keeping the CCD sensor at low temperatures is a key feature of scientific CCD cameras, allowing long exposures not ruined by excessive dark current. The CCD cooling can achieve up to 50°C below the ambient temperature, which leads to below-freezing temperatures of the sensor in all cases. This is why it is crucially important to place the cooled detector in the hermetically sealed chamber else it would be covered by the water ice within minutes. And the chamber must be dried by a desiccant, because even the residual humidity in the air is enough to create frozen ice patterns on the sensor, harming the images. New design of the desiccant containers of the Gx cameras allows more comfortable desiccant exchange and easier desiccant regeneration. Also a new variant of the container was introduced, designed to allow tool-less container replacement.

The desiccant container used in the Gx cameras allowed for easy opening, pouring-out the desiccant (typically silica-gel) and replacing it with a freshly baked one (the container is designed to easily accept even the very hot silica-gel without any possibility of temperature-induced damage of the container). However, this operation typically required removing of the camera from the telescope or at last positioning the camera (and thus the whole telescope) so it is possible to pour-out the used desiccant and then repositioning it again to be able to pour-in the fresh one. But removing the camera and connecting it again to the telescope have some unpleasant consequences, like a necessity to acquire new flat fields.

Newly designed desiccant containers keep the desiccant in the removable part. This allows to just unscrew the container with the desiccant without the necessity to remove the whole camera from the telescope. The desiccant is kept inside the container by a screwed perforated cap. This brings two important advantages:

The whole container can be “baked” in the owen to regenerate the desiccant without worrying about some damage. There are no seals (o-rings) nor soldered joints, which could be damaged.
It is easy to replace the desiccant (silica-gel) in the container in the case of wearing-out (or damaging of the silica-gel with too high temperature etc.). It is enough to unscrew the perforated cap, pour-out the desiccant, pour-in the fresh one and screw the perforated cap back. Then the container can be either baked or screwed into camera head, providing the new fresh desiccant is properly dried.

Remark:

The new construction still have one disadvantage in somewhat longer time necessary for the desiccant do dry-up the CCD chamber. It if very difficult to say how long time is necessary, because it depends on may factors, e.g. the environment temperature (in below-freezing temperatures the time can be many days).

Also different cameras have different resistance to humidity within the cold chamber. Generally cameras with sensors covered with glass ave very resistant to humidity, they can operate without frost creating on the CCD glass even if the humidity inside the chamber is very high. Still, it is highly recommended to keep the CCD chamber dry as the water inside it can cause other damage.

Warning:

The temperature at which the silica-gel has to be dried depends on the manufacturer. The newly supplied cameras use silica-gel in the form of large (3-5mm) spheres, which is bright yellow-orange when it is dry. As opposite to older variants, requiring temperature up to 160°C to dry up, the maximum recommended temperature used to dry the new silica-gel is only 120°C.

The new desiccant container construction also allowed introduction of a new modification of the container. The standard container, supplied with the camera, is equipped with a slot for a tool to allow it to be released and unscrewed and later screwed and properly tightened. This container does not overhang the camera head outline. This could be important when the camera is used in e.g. in the secondary focus of the fork-mounted Cassegrain telescope etc. If the space behind the camera is not important, the alternate variant of the container can be used. It is somewhat taller, but allows tool-less manipulation.

Camera with standard desiccant container (left) and with the alternate variant for tool-less manipulation (right)

The tool-less variant brings one limitation — it is necessary to remove it (and thus un-seal the CCD cold chamber) when the camera back shell has to be removed. Also camera cannot be laid flat on the back when e.g. exchanging filters in the internal filter wheel etc.

Also introduced is a cap, which allows sealing of the container after baking if it is not immediately mounted into the camera. The cap is equipped with the same o-ring sealing like the piece on the camera CCD cold chamber. So if it is not possible to immediately attach a container with regenerated desiccant to the camera, it can be sealed with the cap.

Desiccant container with the optional cap (left), container with slot and slightly longer tool-less container (right)

Remark:

Please note the container cap as well as the tool-less container variant is sold only as optional accessory, all Gx cameras are always manufactured and shipped with standard container with a slot.

The latest revisions of all G2, G3 and G4 cameras also use unified desiccant containers, so the very same container can be used on all models. However, with the introduction of the Enhanced cooling cameras with bigger heat sink and thicker back shell, also the desiccant container have to be longer. So there are special (longer) variants of both standard and tool-less containers for Enhanced cooling cameras available.

Standard and prolonged tool-less desiccant containers (left), all available variants with a cap (right)

Hint:

While it is obviously not possible to use standard (short) container in the enhanced cooling camera, nothing but space behind camera (and aesthetics) prevents using of longer container with standard camera.