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Replacing the Desiccant
In AP series cameras, the CCD and TEC cooler are housed in a sealed chamber beneath the shutter. This chamber protects the CCD from dust and moisture. When the CCD is cold, the chamber's optical window will become colder than the surrounding air, causing it to fog over if there is a sufficient level of humidity within the camera body. This is most common in astronomical imaging, since the CCD camera is used outdoors in the damp night air, and sometimes left for long periods in a building that may not adequately keep out moisture.
To prevent fogging, and to protect the delicate camera electronics, a desiccant packet has been tucked inside the camera prior to shipping. The desiccant will maintain a dry environment inside the camera body for many months. Eventually, however, the desiccant will become saturated and no longer adsorb moisture. When this occurs, the CCD chamber window will more than likely begin to fog over each time the CCD is cooled. A telltale sign of fogging is an apparent sudden drop in the sensitivity of the camera, as if clouds have obscured the imaging target. Sometimes large, bright and dark rings will appear in images, caused by a ring of moisture on the CCD chamber window inside the camera.
Fogging of the window can be verified by opening the shutter and looking through the camera window while the camera is running with the CCD at cold temperature. In PMIS, typing "oshut" ("open shutter") on the command line will open the shutter and hold it open until the "cshut" ("close shutter") command is executed; in other software, the shutter can be held open by taking a long exposure while you inspect the inner window for fog.
When fogging occurs, it is time to replace the desiccant packet. Extra packets have been provided for this purpose. Do not remove the extra packets from their moisture-resistant sealed bags until you are ready to insert them into the camera. Replacement desiccant is available through Apogee Instruments if your supply runs out. To replace the desiccant, follow the instructions below.
Preparation
First, choose a dry day (low relative humidity) to perform this operation. This should not be a job left for a rainy day! An air-conditioned room is the preferred setting in which to change the desiccant. Work quickly so that the desiccant is not exposed to the air for a prolonged period. You will need a clean, static-free, level workspace on which to place the camera while changing the desiccant. Properly ground yourself to dissipate any electrostatic charge you may be carrying. A lint-free cloth should be kept handy for cleanup. It is preferable to have canned air (or compressed nitrogen gas, if available) to purge dust particles from the camera body and CCD chamber window before resealing. The only tool necessary is a 3/32" hex wrench, provided.
WARNING: To avoid the possibility of UV-charge damage to the CCD, never open the camera out of doors or place the opened camera in direct sunlight.
Step 1
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Using the 3/32" hex wrench, remove the twelve screws from around the perimeter of the camera lid. |
Step 2
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Grasp the round "nose" section of the lid and pull up from the camera body. This procedure may be difficult due to the vacuum grease seal. For extra leverage, you may find it helpful to screw in the 2-inch prime-focus adapter. CAUTION: There are two shutter leads that connect the shutter to the inner circuit board. Be careful not to separate the lid too far from the body or else the leads may become taught and damage the connectors. If the leads become unplugged, re-seat the connectors on the pins marked "shutter" on the circuit board (polarity does not matter). |
Step 3
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Remove the old desiccant packet. Note the way it has been folded and prepare the replacement packet so that it can be installed in the same way as the old one. Tuck the new packet down into the crevasse next to the 37-pin connector and fold the top portion over the row of capacitors next to it on the circuit board.
CAUTION: As much as possible, avoid contact with the components on the circuit board. These are extremely sensitive to electrostatic shock damage.
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Step 4
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A bead of silicone was originally applied around the edge of the lid assembly. Inspect the lid and apply a small amount of silicone to any bare spots using the supplied syringe. Do not add any silicone if there are no bare spots. It takes only a little smearing to make the seal. |
Step 5
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Replace the lid by setting one end down on the camera body. Carefully clear all wires away from the edges so that they do not get in the way of closing the lid. Also, make sure the shutter and fan leads do not drape over the CCD window. Use dry canned air (or compressed nitrogen gas, if available) to purge any dust particles from the circuit board and CCD chamber window as you close the lid the rest of the way. Reinsert and tighten down the twelve lid screws. Use a towel to wipe off excess silicone from around the lid. The desiccant change is complete. |
Other Considerations
Besides making a good seal and keeping fresh desiccant inside the camera, there are other actions you can take to safeguard against condensation:
- Allow the camera to operate for at least 1 hour prior to cooling the CCD. This will drive moisture from the enclosure to a level that should eliminate the possibility of fogging in all but the most extreme conditions.
- Establish a sealed case with desiccant in which to store the camera when not in use. Periodically refresh the desiccant in the case. This will keep the camera dry between uses and reduce the frequency at which the camera desiccant will need to be replaced.
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