The controller/power supply unit
The Pyxis camera prototype consists of two units (three with the guiding camera); the main imaging camera and the controller box which houses both the power supply and controller board. The controller housing shown represents the most compact packaging possible for the power supply and control boards; space inside the box is very tight.
The control and power cable to the camera is a standard DB25-M to DB25-F parallel port extension cable. The video signal is routed to the controller through a 50 ohm BNC coaxial cable. The only "custom" cables are those for the filter wheel (5 lines) and the 12V power line which is terminated with a lighter plug. A separate 12V, 10A supply must be provided to operate off the AC mains. 12V DC switching supplies are commonly available for operating marine equipment at home. I have not noticed any increased noise operating off my inexpensive switching 12V supply (SEC 1212 Samlex supply, distributed by RP Electronics).
Except for the power switch, all of the switches shown in the above photo are connected to headers on the controller or power supply boards (see schematics). The filter wheel push button switches (red) step the filter wheel half a step at a time in the direction set by the filter wheel direction toggle switch (left most toggle switch). These manual controls were included so that the filter wheel could be repositioned (albeit slowly) without the controller being connected to a PC. Similarly, the shutter switch allows the shutter to be opened manually, without need for the PC. Note that the filter wheel direction switch is ORed with the filter wheel direction line from the ATTiny26 MCU on the power supply board so that the switch MUST always be returned low for proper operation of the filter wheel from the PC (I ran out of lines on the ATTiny MCU!).
The LCD is a 16 by 2 line LED backlit module using a the standard Hitachi controller. Note that it is only rated for operation down to 0°C. A lower temperature rated LCD would have required a separate negative voltage line (-5 to -7 volts). Although this might seem limiting, remember that your laptop is not rated for operation below 0°C! Most other parts in the controller use industrial temperature rated parts. Certainly, the camera itself can be operated at -20 °C.
All connections are made at the back of the controller box. An 8 cm computer fan blowing across the Peltier supplies was included as a precaution because the switching transistors in these supplies can get quite hot. A header for the fan is provided on the power supply board. I would not recommend omitting the fan because I have not tested the power supply in a tight enclosure for any significant length of time.
Although not strictly necessary it is convenient to leave an SPI programming cable in the controller so that changes can be quickly made to the firmware. The automotive noise suppression filter was included later when trying to locate a source of noise on the system when the PC and controller were operated off a common supply. The source of the noise was found to be a ground loop through the USB cable; this was solved by tying the USB ground to the controller ground through a small resistance (50 ohm). The noise filter provides no noticeable improvement in performance and can be omitted. Note that the BNC panel headers for the video connections should not be grounded to the case; insulated headers should be used.
The photo above shows the power supply inside the controller box with the controller board removed viewed from the back of the enclosure. The supply uses two custom transformers (wound on modified Amidon pot cores) and three custom wound inductors (these last three can be replaced by equivalent off the shelf inductors). The fan fits in the 1" space on the right side of the board.
The above is a view inside the controller enclosure with the main control board installed. A less obscured view can be seen here.
The imaging camera
For simplicity, the KAF-401 CCD sensor is directly mounted on the Pyxis main camera board. A hole is milled through the PC board to admit an aluminum cold finger. The LM335 temperature sensor is epoxied into the cold finger, near the CCD. The cold finger was drilled and tapped with four holes to accept two 2-56 screws to hold down either of the KAF401 or KAF1602 CCDs (see the KAF401/1602 mechanical drawings).
The 2-stage Peltier element was sealed with RTV and was bolted between the heat exchanger back plate and cold finger with two nylon screws. The space surrounding the cold finger below the board was filled with rubber foam to reduce air volume in the enclosure as well as to provide some insulation.
The fluid heat exchanger was milled into the back plate of the enclosure (0.5" aluminum plate). A thermal switch was bolted to the heat exchanger plate near the Peltier element to shut down the Peltier current above 75 °C. At room temperature, the cooler can be run to 0°C (nominal at the CCD) without using the fluid heat exchanger as long as the camera is not operated inside an unventilated enclosure. Unregulated 12V power is available at the camera board to operate a forced air heat exchanger if this is preferred.
The 12V line is also used to power two resistors epoxied to the edge of the CCD chamber window. The resistors (60 Ohms total) dissipate 5W of power, which is sufficient to prevent condensation on the CCD window. The resistors were formed by removing the ceramic encapsulation on two 5W, 30 Ohm wire-wound resistors. The resistors could then be bent to fit around the circumference of the window. A low-thermal conductivity epoxy was used to cement the resistors.
The filter wheel holds up to six filters and is driven by a 24VDC geared motor. The filter wheel position is sensed using a reflective optical sensor (an IR-diode photo-transistor pair) and a circular "encoder" with alternating dark and reflective sectors (photo to left). The encoder was fabricated by printing the encoder on an overhead transparency sheet and overlaying this on a piece of aluminized Mylar. This only allows relative positive to be sensed, so the filter wheel position may have to be reported manually to the Pyxis Capture software if the filter wheel was moved manually (i.e. using the filter wheel control switches). The software saves the phase and filter position information when it closes, so that the filter wheel should slew to the correct position when a new software session is started (as long as the manual filter buttons aren't used). The filter wheel currently has an IDAS LRGB filter set installed.
The shutter consists of a 25 mm Uniblitz shutter with 6V solenoid; it is housed directly beneath the filter wheel. A purging valve is used to fill the enclosure with dry gas; a vent (not shown) is opened during the purging operation. The purging valve is a commonly available gas-service valve.
The camera uses a Pentax thread to connect to the telescope using a Pentax T-adapter. The back focal distance from the lens mount to the CCD plane is about 7 mm shorter than the equivalent distance in a 35 mm film camera. An adapter ring (7 mm spacer) is inserted when the camera is used with lenses using the Pentax thread. A 28 mm Takumar or other short lens is handy to have when testing the camera because it results in a reasonably wide field of view of 10°.
Sealing and purging the camera
The camera was sealed using automotive RTV "gasket maker" sealant during assembly. This sealant has the advantage of being relatively easily removed if the camera must be disassembled. The DB25 connector used on the Pyxis prototype proved to be the greatest source of difficulty when sealing the camera. A ribbon cable DB25 connector was used, however direct solder connections with wires would have been much more easily sealed (simply encapsulate the wires and connector in epoxy once the connector has been assembled and tested). The camera seal was tested with a bicycle pump; do not pressurize above 40 psi! A good seal is required if the purge is to last more than a few cooling cycles of the camera. On cooling, the pressure inside the camera drops relative to the atmosphere which causes moist air to be drawn into the camera on each cooling cycle. Changes of altitude will have a similar effect.
Before purging, the camera was placed in an oven at 50°C for a few hours to help drive off water adsorbed onto the camera parts (both the purging valve and vent should be open during the heating). The camera was purged while still warm using a CO2 duster (available at most photography stores) to admit the dry CO2 gas into the camera. The gas should be admitted into the camera slowly to allow time for the dry gas to mix with the gas already in the camera before it exits through the vent. Purging requires at least one full 12 g CO2 cartridge; this corresponds to several volumes of the CCD enclosure (6 litres at STP). It is important that only dusting or food-grade CO2 cartridges be used; BB-gun CO2 cartridges contain a lubricant that will contaminate the CCD chamber. Food-grade cartridges are inexpensive but have not been tested; I expect that they contain very little water (comparable to dusting cartridges) due to the manufacturing process. Food-grade N2 cartridges are also available at slightly higher cost and should provide marginally better thermal performance (smaller heat conductivity). Note that argon, if available has an even lower thermal conductivity than nitrogen.
CAD drawings
The following drawings may be helpful when designing your CCD camera enclosure.
