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The CCD imager for ALIS

Based on the discussion in the previous sections, it might be concluded that an ICCD imager is better for video-rate observations, mainly due to the long read-out time of a full-frame CCD. On the other hand, the large dynamic range, linearity and long time stability of a scientific-grade unintensified CCD make it an interesting choice for absolute spectroscopic imaging measurements of column emission rates down to about 1 s time resolution. Therefore it was decided to select a thinned back-side illuminated quad-read-out CCD for the ALIS imagers. At the time of specification no commercially available imager met the requirements of temporal resolution. One company was willing to modify their dual-read-out CCD camera head to a quad-read-out system, resulting in the imager system finally procured for ALIS. This section presents a short summary of some of the most important technical details of the six imagers. For further technical information about the imager hardware, etc., see Preston [1995]; AstroMed Ltd. [1993]; Preston [1993] and AstroCam Ltd. [1995]

The camera head

The CCD camera head (CCH) is of dimensions $ 125 \times 140
\times 144\ mm^{3}$ (Figure 3.6).
Figure 3.6: The six ALIS imagers. In the right forefront a sideview of an imager is shown. The front lens, telecentric part with filter wheel, and camera lens connected to the CCD camera head are seen. The white box on the left is a camera control unit.
"White box, black hats, high noon, main street" - unauthorized comment, BG
A slightly larger front plate ( $ 155 \times 170\ mm^{2}$) enables mounting the camera head to the structure supporting the camera head, optics and filter-wheel. Centred on the front plate is the optical interface, which consists of a removable ring with a Canon lens-mount. By replacing the ring, the lens-mount can be reconfigured to a Nikon lens-mount. Thus it is possible to attach a wide variety of standard objective lenses. The thinned back-side illuminated quad-read-out CCD is mounted onto a Peltier cooler inside a hermetically sealed compartment filled with an inert gas under low-pressure. Light enters through the front-plate hole, a mechanical shutter, and a high-quality optical window onto the CCD inside the sealed compartment. Note that the CCD itself has no optical window exposing the chip and bonding threads to the environment of the compartment. Therefore, the sealed compartment may not be opened, unless in a clean room environment with adequate electrostatic discharge (ESD) protection measures as CCDs are extremely sensitive to ESD discharges. Also, proper equipment for venting and re-sealing the compartment needs to be present.

A vendor-provided CCD test report accompanied each CCD-chip, and excerpts from these reports, as well as some measurements provided by the camera manufacturer, are provided in Appendix B as Tables B.1-B.6 as well as in Table 3.2 above.

The camera head also contains a circuit board with low-noise preamplifiers and buffers for the four analogue read-out channels, etc. A heat-sink and a cooling fan on the back of the camera head cools the hot side of the Peltier cooler. Three temperature sensors measure CCD, ambient and heat-sink temperatures. Two round 2 m 37-way screened twisted flat-cables with ``DSUB'' connectors in each end connect the camera head to the camera control unit3.4. These cables are individually matched to each imager system and may not be interchanged. A third cable is powering the Peltier cooler.

The camera control unit

The camera head is powered and controlled by a Camera Control Unit (CCU). This unit includes analogue and digital signal processing chains for the four identical signal processing channels, as well as other control electronics and power supplies (Figure 3.6).

The CCU is controlled by a T222 Transputer, which communicates with its host systems over a Transputer-link (a kind of serial interface). CCD clocking and pixel read-out is controlled by an Erasable, Programmable Logic Device (EPLD) that is capable to read-out up to $ 4\times 4096$ pixels without pausing. The analogue pixel values are digitised using four fast 16-bits Analogue to Digital Converters (ADCs). The data is then stored in four 4 kByte FIFO memories for subsequent transfer to the host computer, either on the Transputer link, or by way of a 16-bit parallel port. The maximum possible theoretical pixel rate is $ 1.5 \mu s/ \mathrm{pixel}$. If desired, any CCD quadrant, or region of interest can be read-out separately.

Configuring the imager

All configuration settings of the CCD imager are under software control. A configuration file (, see stored on the controlling computer is uploaded to the CCU during its boot procedure. This file contains configuration settings for CCD-size, clock timing, voltage and gain settings, etc. The configuration file is unique to each CCD and imager. By changing appropriate configuration settings, it is possible to reconfigure the imager for various situations. As each of the four signal processing channels have software selectable gain settings as well as DCS (Section 3.1.7) time constant and slope settings, this allows for a wide range of speed, gain and noise settings. Table 3.3 gives a brief overview of some of the

Table 3.3: CCD-read noise at various pixel clocks for the six ALIS imagers. The values are from the vendor-provided configuration files for the imagers.
ccdcam pixels $ \frac{\mu s}{\mathrm{pixel}}$ $ \overline{n}_{e^{-}_{r}}\ [{e^{-}_{RMS}}]$ $ t_{\mathit{read}}[s]$ Notes
1 $ 1124\times1024$ 6 12-14 1.7 a)
1 $ 1124\times1024$ 8 9-12 2.3 a)
1 $ 1124\times1024$ 13 8-9 3.7 a) default
1 $ 1124\times1024$ 23 7 6.6
2 $ 1124\times1024$ 1.6 28 0.5
2 $ 1124\times1024$ 2.5 11 0.7
2 $ 1124\times1024$ 4.1 10 1.2
2 $ 1124\times1024$ 10.5 8 3.0 default
3 $ 1124\times1024$ 1.6 25 0.5
3 $ 1124\times1024$ 2.5 13 0.7
3 $ 1124\times1024$ 4.1 10 1.2
3 $ 1124\times1024$ 7.0 9 2.0 default
4 $ 1124\times1024$ 1.6 25 0.5 b)
4 $ 1124\times1024$ 3.8 13 1.1 b)
4 $ 1124\times1024$ 6.7 10 1.9 b)
4 $ 1124\times1024$ 12.5 9 3.6 b) default
5 $ 1056\times1024$ 1.6 28 0.4
5 $ 1056\times1024$ 2.5 11 0.7
5 $ 1056\times1024$ 4.1 10 1.1
5 $ 1056\times1024$ 10.5 8 2.8 default
6 $ 1076\times1024$ 1.6 25 0.4
6 $ 1076\times1024$ 3.8 13 1.0
6 $ 1076\times1024$ 6.7 10 1.8
6 $ 1076\times1024$ 12.5 9 3.4 default

possible trade-offs between noise performance and read-out speed. For a more detailed understanding of these configuration possibilities, please refer to the imager documentation and configuration files. See also Table 3.2, Equations 4.7 and 4.8 in Section 4.2.1.

The user port

``Okay, Houston, we've had a problem here.''
As the T222 Transputer links are capable of no more than 20 Mbits/s, image-data transfer at maximum pixel-clocking would be too slow. To remedy this the camera controllers are equipped with a 16-bit parallel interface. Originally it was intended that data from this interface would be read directly by the NIPU, however, due to the fast technical development, an extra computer responsible for receiving and storing the image data took over the role of the NIPU (see also Section 2.2.2). A prototype fast read-out interface card for a PC computer was built and tested. During these tests it was found that the user port omitted the 50 last pixels of each line on each of the four read-out channels (corresponding to a total of 102400 pixels in the centre 100 columns of the image) thus rendering the image data useless. Contacts were taken with the camera manufacturer to remedy this. This resulted in a new version of the camera controller software, fixing some of these problems, but now severe intermittent problems arose. Great effort was made to find the cause of these data-losses but without success. A solution to these problems still remains to be found, and meanwhile, the fast read-out mode had to be abandoned.

Operational remarks

At the time of writing, the ALIS imagers have been in operation for up to ten years. During this time they have produced a large number of images resulting in scientific findings as reported in Chapter 6, and references therein. The main technical obstacle has been poor frame-rate as discussed above.

The oldest, prototype imager, (ccdcam1) still works well, as does the newest imager (ccdcam6). The second camera (ccdcam2) developed a problem with ice on the CCD in 1999, probably due to a breach in the hermetic compartment. This problem should be fairly easy to resolve. The remaining cameras have had intermittent reliability problems related to the electronics. The occurrence of these problems has increased during the last couple of years. Another issue is that much of the camera electronics is becoming obsolete, severely affecting spare-parts availability. On a short time-scale (3-5 years), a limited maintenance operation will most likely bring 4-6 imagers back into operation. However, in a longer perspective, the now rather old imager electronics need to be completely replaced. During such a renovation it must be investigated if the most expensive part of the system (i.e. CCDs), and even the camera heads can be reused. All six CCD detectors are in good condition, and still among the best imaging detectors available for the scientific objectives of ALIS.

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