The vacuum jacket for each camera is an electron-beam welded structure with 6 flat faces. For convenience, the dewars were designed to permit bottom-feed nitrogen injections. All of the optics are held in tubes that are secured to a vertical cold work surface via detachable pillow blocks. The lenses in each tube are held in spring loaded cells and are surrounded by numerous baffles to minimize stray light. Each lens tube can be removed and split to provide easy access to the pupil stop, which must be changed to accommodate f/8 and f/35 modes. Two filter wheels, each containing 8 filter cells, are driven by a pair of external stepper motors. The array is located near the base of the dewar, supported by rigid fiberglass stand-offs, and cooled with a cold finger attached to the LN2 can. The chip carrier assembly is housed in a separate radiation shield to block stray light.
The same DSP-based controller planned for use with all future CCDs is used to run the Redeye infrared cameras. The "Gen III" controller is coupled to a separate computer (Sparc 1E engine) that serves as an interface between the controller and the summit HP9000 computer. The Sparc engine and its accompanying 16 Mb of RAM and 700 Mb hard disk are linked to a fiber optic transmitter in a single VME Crate. The fiber link is used for high speed data transfer from the controller to the array computer, where images can be buffered, coadded, or manipulated in a variety of ways. Observers are provided with the "Pegasus" window environment to use for camera control and image manipulation.
Figure 3.1 shows the mechanical layout of the cold structure and outer vacuum jacket. The jacket uses a variety of subplates to allow coupling of exterior components to the interior mechanism as well as easy access to key components. For example, the motors are mounted on a separate plate so that, if it is ever decide to relocate the motors or totally reconfigure the interior, this can be done by replacing a single subplate instead of creating a large number of new holes in the jacket. The window is mounted in a subplate as well in order to facilitate different window sizes by simply replacing the cell without making any fundamental changes to the jacket. Access to the array is possible by removing another plate on the bottom of the dewar, then removing a radiation shield on one side of the detector cavity. Finally, a large plate covering an entire side of the dewar permits access to the vertical cold work surface, optics, filters, etc.
Modular components were used throughout Redeye's interior. The array is mounted via fiber glass tabs to the cold structure in a well shielded cold cavity. These tabs provide thermal and electrical isolation of the array from the rest of the camera while holding the array fixed to within «1 pixel of its nominal position, regardless of the camera's orientation. The detector is cooled by a cold finger, with temperature control provided by the Gen III electronics. The array is mounted on a board that can be removed from the entire camera by removing four screws and unplugging a single socket used to transmit all electronic signals to and from the array. The lens barrels are mounted in pillow blocks so that, by simply removing the top of each block and sliding the collar on the end of each tube, it is possible to remove each tube within a few minutes from the cameras. This procedure also preserves critical dimensions of the tubes so they can be replaced or interchanged between cameras while keeping everything in focus. The filters are mounted in cartridge assemblies that can be removed after loosening 4 screws in the vertical work surface. Under normal circumstances, it is not necessary to remove the filter wheels (e.g., to replace filters), but having the ability to remove and interchange wheel cartridges between cameras was seen as a desirable feature during the normal lifetime of the cameras. Access to filters is provided by small plates on the outside of each cartridge. Finally, the entire inner cold structure is mounted within the vacuum jacket on four fiberglass tabs near the bottom of the camera. After unplugging the inner electronics and detaching the cold structure from these tabs it is possible to remove the entire cold structure from the vacuum jacket rapidly and easily so that access to any part of the camera is possible. Achieving this level of modularity, precision, and interchangability between cameras, while keeping a total camera weight under ~20 kg, reflects the central design goals of the Redeye cryostats.
Figure 3.2 shows the design of the Redeye optics, including axial and marginal rays coming from the CFHT Cassegrain f/8 configuration. In each design, the field lens is located ~25 mm behind a CaF2 window in the dewar. The telescope's focal plane is coincident with the front apex of the field lens. This of course makes the system somewhat susceptible to dust being reimaged onto the array, but this problem is expected to be minimal since the field lens is in a sealed environment. Redeye was designed this way in order to allow custom field masks to be installed in the field lens cell, as may be needed for imaging FTS, cold grism slits, or coronagraphic applications. Next in each system is a filter, which is mounted in one of two wheels in the camera. Following the filter is a cold stop, which can be easily removed from the camera and exchanged with stops of various sizes and shapes. Observers who wish to use Redeye with a visitor instrument should specify the f-ratio of the beam they intend to send into the camera so a properly sized cold stop can be placed in the camera prior to their run. In the standard f/8 Cassegrain mode the cold stop has a central occulting disk that blocks thermal flux from the f/8 baffles and hole in the primary mirror from reaching the detector. This stop theoretically reduces the K-band background by ~0.5 mag, while reducing the system throughput ~1%, hence it boosts the system's sensitivity greatly. Behind the cold stop lies 4 reimaging lenses which, in the narrow field optics, reimage the telescope's focal plane onto the array with 0.7:1.0 magnification. In the wide field optics these lenses yield 1.7:1.0 minification of the telescope's focal plane. Not seen in Figure 3.2 are the ~15 baffle stops that surround the beam as it passes through the optics. These baffles, along with most of the interiors of the optical tubes, are painted black to minimize stray light.
Exact optical specifications are provided in Tables 3.2 and 3.3 for observers who would like to insert these optics into models of their own, e.g., to evaluate the performance of visitor instruments in combination with Redeye.
The 77 K refractive indices over the 1.0 - 2.5 m wavelength range for LiF and BaF2 are given by the Schott equation:
The constants ax for the lens materials used are listed in Table 3.4.
Additional details about the Redeye optics are available from the CFHT technical staff, upon request.
Referring to Figure 3.3 again, note that dimensions have been listed next to each barrel. These dimensions are the warm barrel lengths that should be used to achieve proper internal focus when the camera is cooled and the entire optical assembly contracts by ~0.4%. Normally these barrels will not change length once the locking collars are secured within the barrel subassemblies, but repeated thermal cycling may over time lead to the collars coming loose and the barrels may inadvertently change length when removed from cameras. If this happens use a precision height gauge (or a set of calipers if that is all that is available) to readjust the barrels back into their correct lengths.
There are two different sets of spacers used for mounting Redeye to the CFHT Cassegrain environment, depending on whether or not the camera is at the f/8 or f/35 focus. Figure 3.5 shows the series of spacers needed to mount Redeye at the f/8 focus. In this mode, the standard 100 mm, 280 mm, and Redeye adapter plate are used between the camera and Cassegrain environment. Figure 3.6 shows in detail the Redeye adapter plate, which has a special hole cut in its center to allow the external shutter to lie within the adapter plate. The Redeye adapter plate has 4 holes in it that are on an identical layout with the 4 holes in the top of the Redeye vacuum jacket. Using the M10 bolts that are normally left threaded into each camera yields a fairly accurate radial placement of the camera's optical axis with respect to the telescope's axis. Note that this mode does not require a tip/tilt adjustment and therefore uses only simple spacers to place the camera near the optimal f/8 telescope focus. Also note that two of the holes are 20 mm in diameter, while the other two are 10.5 mm in diameter (and when used with M10 bolts provide accurate registration with respect to the Redeye optics). This difference is due to slight machining errors in the locations of the top mounting holes. This design was adopted In order to use a single plate for all Redeye modes, and preserve optical alignment with the different hole placements in the cryostats.
Figure 3.5 also shows the configuration that should be used at the infrared f/35 CFHT focus. Once again, the same Redeye adapter plate is used to attach the camera to the spacers, except in this mode a special tip/tilt spacer is used that allows the camera's optical axis to become parallel with the telescope's optical axis. This spacer has a thickness of 150 mm so that, when used in combination with the standard CFHT 200 mm spacer and Redeye adapter plate, the Redeye focal plane is nearly coincident with the optimal f/35 focus.
The dewar should never be cooled with a warm pressure exceeding 10 mtorr so that it can effectively cryo-pump during the cool down phase. In order to reach nominal hold times, the cold pressure in the dewar should be below 1 mtorr, and ideally should drop below 0.1 mtorr so that radiative heating is the dominant thermal load on the cold structure (i.e., not conduction by residual gas molecules). A pressure test port is attached to the outside of each dewar to check the dewar pressure whenever necessary. An even better diagnostic of the dewar's vacuum quality and heat load is the boil-off rate when the camera has reached an equilibrium temperature. This boil-off rate should be around 1-2 liters sec-1 for both dewars, which corresponds to roughly a 20 hour hold time. Boil-off rates exceeding 5 liters sec-1 indicate a serious problem in the camera and a member of the CFHT technical staff should be contacted to investigate the problem.
Referring to Figure 3.7, which shows the Redeye cold structure from the side, note that the dewar uses a LN2 injection feed that is similar to that used by all of the CFHT CCD dewars. In order to maximize the LN2 throughput, dedicated injection tubes should be left in the dewars after they are mounted on the telescope. These tubes are marked and are cut to the exact length needed to splash LN2 into the can once it clears the tube. Redeye can be mounted in either uplooking or side looking configurations, depending on the insert-tube used in the dewar. The insert-tubes thread into the base of the dewar's LN2 can. When an uplooking mode is used, the longest insert tube should be threaded into the can to make the can's capacity to ~2 liters. Do not use this tube if Redeye is going to be used in a side looking mode, since the end of the tube will be immersed in liquid and a geyser of LN2 will be ejected from the can, which can be very dangerous. Only use the short insert-tube for side looking modes, which places the end of the tube in the center of the can so that LN2 is not spilt at any camera attitude. Of course the capacity is reduced to about 1 liter when this shorter tube is in place, leaving a hold time of ~10 hr.
Experience has shown that the minimum injection pressure that will work is ~15 psi, hence be sure the transfer dewar is pressurized to this level before attempting to fill a dewar, otherwise the LN2 will flash-evaporate as soon as it enters the dewar and essentially no liquid will reach the can. Since all of the summit transfer dewars have been outfitted with pressure safety-valves that open at 15 psi, getting sufficient pressure in a transfer dewar should not require any special effort. If for some reason the pressure in a transfer dewar is well below 15 psi yet the dewar is known to contain liquid, carefully place the dewar on its side for a few seconds. This will vaporize some of the LN2 (when it comes in contact with the warm top of the dewar) and restore the pressure level to 15 psi. When cold and mounted on the back of the telescope (i.e. in the normal uplooking mode), it should take about 15-20 minutes to completely fill Redeye. The dewar is full when a steady stream of LN2 exits the spill port on the injection assembly. It is not unusual for a weak steam of liquid (few drops per second) to fall from the spill port only a few minutes into the injection process, hence do not take this to indicate that the dewar is full. This weak stream comes from the slight amount of LN2 that splashes back down the injection tube and the liquid oxygen that naturally condenses (at 90 K) on the extremely cold surface of the brass injection assembly.
Filling Redeye with LN2 when it is warm requires a different technique than when it is already cold. LN2 should be injected into a warm dewar at the highest rate possible to overcome the dewar's initial thermal load and allow the nitrogen can to accept liquid. Since the pressure from transfer dewars cannot exceed 15 psi, the only way to increase the liquid throughput is to turn Redeye upside down, remove the insert-tube, then use that insert-tube (squeeze the rubber hose on the transfer dewar around one end) to inject LN2 into the dewar. This tube has about twice the diameter of the normal injection tube hence the liquid throughput is greatly increased. After roughly 30 minutes, the dewar will top off with liquid using this fill technique. If time permits, allow the dewar to cool for an hour then top it off again before threading the insert-tube into the can and inverting the dewar into its normal uplooking state. Do not let the dewar sit for more than a couple of hours after the initial injection without topping it off since, when the dewar is cooling from room temperature, the boil-off rate is extremely high and all of the LN2 will be lost in only a couple of hours. Unlike filling a cold dewar, filling a warm dewar should only be done by qualified CFHT personnel since the final step will lead to a geyser of LN2 streaming from the dewar before it is inverted.
If for some reason the dewar has boiled off its entire reserve of LN2 while it is mounted on the telescope, two options for cooling the dewar are available. First, the camera can be removed from the Cassegrain environment and cooled as illustrated in Figure 3.8. This can be cumbersome though since removing the camera from the telescope is at least a two-person job. The other option is to leave the camera on the telescope but remove the insert tube and inject LN2 with that into the camera reservoir for 5-10 minutes. This will use several liters of nitrogen while the can is cooled to the point that it will accept liquid. After cooling the can, immediately replace the insert tube and begin injecting LN2 with the injection tube. Under these conditions it could take ~30 minutes to top-off the dewar. Before attempting either of these solutions, first check the dewar's internal pressure. If it exceeds 5 mtorr, the camera must be pumped back down to < 1 mtorr before attempting to cool/fill it. Otherwise, the activated charcoal in the camera will saturate with condensed gas while it cryo-pumps during the cool down period. In any event, the Waimea technical staff should be contacted before a warm camera attached to the Cassegrain environment is dealt with.
1) By having a single type of controller that operates all of the CFHT array detectors, the long term complexity of the summit data acquisition environment is greatly reduced and hardware redundancy is increased.
2) The cost of the electronics can in large part be absorbed by a system already under development for CFHT's CCDs.
3) The only significant difference between running a NICMOS array and a CCD with the DSP system is the software that is up loaded into the controller and the dewar cabling. As a result, switching between CCD and Redeye control requires minimal effort.
4) Redeye can take immediate advantage of the software already developed for CFHT's CCD program.
Figure 3.10 shows the layout of the entire system used for Redeye. The controller, which is based on that developed by Leach (1991) at San Diego State University, is coupled to a separate computer (Sparc 1E engine) that serves as an interface between the controller and the summit HP9000 computer. The Sparc engine and its accompanying 44 Mb of RAM and 886 Mb hard disk are linked to a fiber optic transmitter in a single VME Crate. The fiber link is used for high speed data transfer from the controller to the array computer. Observers use the CFHT "Pegasus" window environment for camera control and image manipulation.
The controller electronics next to the dewar has five separate components including (1) a digital timing board, (2) one or more analog read-out boards, each one handling one read-out, (3) a utility function board that manages detector temperature regulation, shutter operations, and an assortment of other tasks, (4) a backplane communications board, and (5) a power supply. A three board set would suffice for reading out single arrays and a 22 board set would be used for reading out 20 arrays. The digital timing board contains the fiber optic communications circuitry, the DSP, the DSP program, the master clock, and interface circuitry for maintaining the backplane. The backplane is a commercially available VMEbus board whose pins and timing functions have been redefined for this application. The power supply accepts 110 volts AC input and produces DC voltages for each board, with the analog board performing extensive local regulation of noise-critical voltages.
A more detailed breakdown of the array control system is shown in the top half of Figure 3.11. For control of the NICMOS3 array we use a total of 4 analog read-out boards providing one video processor chain per array quadrant. Only one analog read-out board, quadrant one, is used for clocking the array and clocks all four quadrants in parallel. In this way the read-out time is optimized. Thus, for each pixel cycle, each of the four quadrants are clocked in parallel resulting in a burst of four digitized pixel values being shipped down the data fiber to the VME interface board. Upon arrival in the VME INF the four pixels are buffered in RAM until a block of 512 pixels has been collected. At this point a VME bus transfer (DMA write) is executed by the VME INF which places the buffered data into predefined UNIX memory. From there a software driver running on the Sparc1E adds the buffer to a growing image, and when all of the pixel values in the array have been transferred, the image is written to disk.
Clocking of the array is controlled by the DSP timing board through an onboard program. This program operates as a waveform generator clocking a bank of DACs. Each DAC output contains diode protection circuitry to prevent accidental excursion out of the NICMOS3 TTL-like operating voltage window. We have found, as noted by the array manufacturer, that direct control of several operating biases can improve device performance by optimizing dynamic range, lowering the noise floor, and reducing noise. These bias voltages are provided by DACs and are actively controlled during various parts of the array read-out sequence.
To achieve optimal system performance we employ a multiple sampling technique that provides low read noise with minimal read-out overhead. The bottom half of Figure 6.2 shows a timing diagram for a typical exposure and read-out cycle. After a rapid reset of the entire array a brief settling period is allowed, then a non-sampled read-out is made, another brief settling delay occurs, then a sampled read-out is executed. This sampled read-out is shipped to and stored in the Sparc1E memory for later use. Next the shutter is opened and an exposure is made. At the end of the exposure time the array is again read-out, but not sampled, the array is allowed to settle, then a final read-out is sampled and transferred to the Sparc1E memory. Simple image processing is then performed whereby the first read out is subtracted from the second yielding a low noise frame (correlated double sample). Finally, this image is descrambled (the pixels are reordered into quadrants) and written to disk with an appropriate FITS header.
On power-up or reset the DSP program is read in from a boot ROM, which is a single byte-wide socketed device for easy re-programming external to the controller. Programming is done on a PC- or SUN-based cross compiler supplied by Motorola in native DSP56001 assembly language. The supplied program consists of initialization code to configure the DSP in the desired mode, a command processor, testing and diagnostic routines, routines to read from and write to internal DSP memory over the fiber optic link, tables containing read-out parameters and timing waveforms, and array read-out code. A description of the commands is supplied in a separate document. Modifications to the code can be done either by re-programming the ROM or by modifying the DSP contents over the fiber optic link after the ROM program is booted. The ROM is an erasable electrically programmable part (EEPROM) that can be reprogrammed from the DSP via the serial link as well, though a special security jumper and password system is implemented to protect against unauthorized intrusion. Furthermore, a backup copy of the DSP code is resident in ROM to ensure that changes to the ROM code are not made that would render it permanently unable to be booted. Rapid, reproducible and non-intrusive changes to the array clocking voltages for optimization of the array device operation can be made while the array is operating by exercising the write DSP memory command.
Support for the backplane is provided by high current drivers and a careful timing design to ensure reliable operation when many read-out boards are installed. Since the DSP also reads the pixel data from the A/D converters on each analog board over the backplane, receivers are also included. The DSP then writes the data to the fast serial transmitter to be received by the instrument control computer. Both the transmitted and received data words consist of a high start bit followed by 24 data bits, high true, with the most significant bits transmitted first.
Clocking signals for the array transfer and reset transistor gate are generated by a bank of twelve 8-bit DACs that were chosen for their speed and low glitch energy. They provide a voltage resolution of about 100 millivolts, and an output over the range of +10 to -10 volts. The DAC output is buffered by a fast op amp that can drive large capacitive loads. A set of 12-bit DACs generates the DC bias voltages - seven of them for the array and one for the offset adjustment of the video processor. Three of these voltages are unipolar high voltages of 0 to +30 volts, while the remaining four are bipolar over the range of +10 to -10 volts. Twelve bit DACs were chosen to provide greater long-term stability than 8-bit DACs, but are only settable to 8 bits, as their four least significant bits are grounded. Long term voltage stability to better than 5 millivolts is achieved. Additional circuitry on each board provides an interface to the backplane and decodes the five board select signals (D12-D15, A0), and the four DAC select signals (D8- D11). One of these 16 codes selects a programming sequence for the 12-bit DC bias supply DACs, which is a two step programming process. Regulators are placed liberally throughout the board to minimize coupled noise and minimize switching glitches, and are located on the power supply input of every DAC, on the supply lines to the A/D converter, and on the ±15 volt supply lines to the video processor.
Note that no potentiometers are used anywhere in the controller, as all adjustable voltages are set digitally by the DSP. The analog board is implemented on a six-layer printed circuit card with careful isolation between digital grounds, the noisy analog and digital ground surrounding the clock drivers and logic circuitry, and the quiet analog grounds in the video processor. Ground planes are placed liberally throughout the circuit, and a careful physical placement of components isolates these circuits as well.
The backplane supports full 24-bit data words on both reading and writing. While the analog read-out board only read and writes 16 of these bits, other boards can be built to utilize the full 24-bit capability of the DSP56001. Four address lines, A00-A03, are carried on the backplane in order to address the A/D converter when transferring pixel image data to the DSP. This makes the backplane a D24:A04 system. It is not a bus in the normal sense since only the timing board can be a master, and no bus arbitration circuitry is needed. A complement of 22 timing signals generated by the timing board are also carried over the backplane, while only seven of these are currently used by the analog board. These additional signals could be used to operate such devices as a programmable array temperature controller, a shutter, diagnostic hardware, filter wheels and so on. Two interrupt input lines to the DSP are also available on the backplane, which can be used to implement a hardware timing circuit for overall exposure timing that would be independent of the host computer.
The power supply delivers +5, ±15 and +36 volts to the controller, the first three through the dedicated power distribution pins on the VME backplane. Normally it is expected that the power supply will be mounted remotely from the controller some distance away. Noise pickup along the power supply cables can be minimized by proper bypassing at the controller, while the extensive on-board regulation provided by the noise-critical analog read-out boards should minimize its effect.
Toomey D. W., Shure M., Irwin E. M., Ressler M. E., "ProtoCAM - An Innovative IR Camera for Astronomy," SPIE Proceedings - Instrumentation in Astronomy VII, 1235, ed. D. L. Crawford, pp. 69-81, SPIE, Washington, 1990.