TAOS II has procured three telescopes from DFM Engineering, Inc. in Longmont, Colorado. Each of the three identical telescopes has a 1.3 meter diameter primary mirror and a Cassegrain focus. The wide field telescopes are F/4, and image a 2.3 square degree field onto a circle of 154 mm diameter at the focal plane. The optical quality of the telescopes is such that 80% of the enclosed energy of a star will fall within a circle of 0.8" on axis, and 1.0" at the edge of the field.
The TAOS II cameras need to be capable of high-speed readout on a large number of stars. To read out a typical astronomical-grade CCD imager at our target cadence of 20 Hz is difficult, if not impossible, to achieve without introducing an unacceptable level of noise into the readout electronics. Furthermore, if we were to cover our entire focal plane with 16 μm pixel imagers, the total focal plane would comprise over 72 million pixels. With three cameras, and 2 bytes per pixel, we would be reading out over 250 terabytes of raw image data per night! It would be extremely difficult to handle this data rate.
Given the difficulties of high-speed readout using CCDs, we have opted instead to construct the TAOS II cameras using CMOS imagers. CMOS imagers have built-in readout electronics in each pixel, allowing random access to any individual pixel at any time (with some constraints). Thus one can read out only those pixels in sub-apertures around the individual stars targeted by the survey, while ignoring the rest of the field. This will reduce both the pixel readout rate and the data volume by 99%, while still reading out image data for over 10,000 stars at 20 Hz.
CMOS imagers have typically been of limited use in astronomy due to two drawbacks. First, the readout electronics on the pixels are on the photo-collecting surface of the imagers, and the area of a pixel containing the readout transistors is not photosensitive. Therefore these imagers collect much less light than a CCD. Second, the read noise on a CMOS imager is typically five to ten times as high as that on a CCD imager. However, back-illuminated CMOS imagers are now available, and the collecting surfaces of such devices are fully photosensitive. CMOS imagers are now nearly as sensitive as CCD imagers. Furthermore, recent developments implementing correlated double sampling have helped reduce the read noise to levels near those achieved using CCDs. Modern CMOS imagers are now ideal for the TAOS II project.
After a significant effort to research all of the available options, the TAOS II collaboration has opted to use custom CMOS devices, each with a 1920 x 4608 (31 mm x 74 mm) array of 16 μm pixels, which will be produced by e2v technologies. Each device provides the capability to sample 1000 arbitrary small windows at a 20Hz readout rate while maintaining similar noise performance and sensitivity as normal scientific CCDs. A contract to procure 40 of these CMOS imagers was signed by Academia Sinica on August 7, 2012.
Each camera will comprise a 2 x 5 array of these custom devices to cover the 154 mm viewable field, as shown in Figure 3. The first engineering device (see Figure 4) was delivered in September 2014, and all of the science devices were delivered by January 2017. The TAOS II cameras (see Figure 5) are scheduled to be completed in October 2017.
TAOS II will be installed at the Observatorio Astronomico Nacional (OAN) at San Pedro Mártir (SPM) in Baja California, México. The observatory is located at longitude 115°27´49" E. 31°02´39" N., and is at an altitude of 2,830 meters. SPM, run by TAOS II partner UNAM, has excellent observing characteristics. The weather is good, averaging around 290 clear nights per year. The nominal seeing is 0.6 arc sec, and the sky is very dark (V = 21.5 mag/sq. arc sec, R = 20.7 mag/sq. arc sec). The state of Baja California has recently enacted a light pollution ordinance, so the sky should remain dark for the foreseeable future.
TAOS II will be installed on a hilltop to the southeast of the ridge where the three existing SPM telescopes are located. The three telescopes will be place on the corners of a triangle, with separations between each of the telescopes ranging from 129 m to 323 m (see Figure 7).