The Doppler Method for the Detection of Exoplanets. Professor Artie Hatzes

power R = 67,000. (Bottom) The same wavelength segment of molecular iodine taken with an FTS with resolving power R≈1,000,000.

2.3 Charge-coupled Device Detectors

      After a spectrograph has dispersed the light into its component wavelengths, you now need a detector to record the photons. CCD detectors are universally used on spectrographs working at optical wavelengths (≈3000–10,000 Å). There are three important advantages to using CCD detectors: (1) they have high quantum efficiency; (2) they are two-dimensional devices, ideal for recording the two-dimensional format of an echelle spectrum;³ and (3) the output is in digital form, making it easier to process the data. Here we will cover just the basics of CCD detectors.

      The use of photographic plates at the turn of the last century certainly revolutionized both imaging and spectroscopic astronomical observations. Spectroscopists could not only record their observations, which is certainly easier than using your eye to detect Doppler shifts (like Huggins!), but photographic plates also enabled astronomers to integrate for long exposures, thus allowing the observations of fainter objects. In spite of this important development, photographic plates could rarely achieve S/N ∼ 20 for high-resolution spectra. If we define the quantum efficiency as the ratio of incoming to detected photons, photographic plates had a rather low quantum efficiency (QE) of 1%–3%.

      The advent of electronic detectors played the first key role in improving stellar RV measurements. The first major developments, starting in the 1940s, came with photomultipliers, which provided a QE of about 10%, or a factor of 10 increase over photographic plates. By the early 1980s, photodiodes improved the QE to ∼50%. Currently, virtually all astronomical instruments employ CCDs. These devices saw routine use in the 1990s, and a modern CCD detector can have a QE of ∼80%–100%.

      Figure 2.11 summarizes the evolution in quantum efficiency of detectors over the past decades and is the companion for detectors to Figure 1.4. Although the trend seems linear in a logarithm plot, the development came in quantum leaps (pun intended). For example, with photographic plates there was no improvement in the QE for many decades before photomultiplier tubes were developed. Since then, the developments have come much more rapidly, but still in steps.

      Figure 2.11. The increase in the QE of astronomical detectors as a function of time.

      It is interesting to reflect on how much CCDs have revolutionized observational astronomy. In the 1920s Edwin Hubble discovered the expansion of the universe using the Mt. Wilson 2.5 m telescope taking spectra of galaxies with photographic planets, often with exposure times of several days duration (and without autoguiding!). With CCDs, he could have achieved the same result with a 40 cm telescope!

2.3.1 The Structure and Operation of a CCD

      Figure 2.12 shows the basic structure of single CCD element or pixel (“pixel element”). It has two layers of silicon substrates that are in contact. One is doped such that it has an excess of electrons (n-Si type) and the other doped such that it has an excess of positive charge (p-Si type), often referred to as “holes.” On top of the Si substrate is an insulating material of silicon dioxide, and finally polysilicate gates for controlling the voltages.

      Figure 2.12. The structure of a CCD pixel and the charge readout. The pixel has three gates with an applied voltage. Below these is an insulating layer of SiO₂. The silicon substrate is doped to have excess (n-Si) or depleted (p-Si) electrons. (Left) Photons striking the substrate collect under the potential well of the central gate. (Center) A positive voltage is applied to the left gate, causing the potential well to reside under the center and right gate. (Right) After applying a negative voltage to the central gate, the charge has been completely transferred to the potential well under the right gate.

      At the p–n junction, electrons from the n-type material diffuse into the p-type and fill the extra holes. This diffusion results in the n-Si being positively charged and the p-Si negatively charged. This creates an electric field that ultimately slows and stops the diffusion process.

      The boundary where the holes and electrons have combined is called the depletion zone, and this is the photosensitive part of the CCD pixel. By applying a voltage, one can increase or decrease the electric field across the depletion zone, and this controls the flow of current between the two Si substrates. Photons striking the depletion zone dislocate electrons from Si atoms via the photoelectric effect. Because of the applied voltage (+V) of the polysilicate gate, these remain trapped in the potential well of the depletion zone.

      After exposing the CCD and accumulating charge, one needs to transfer and record the charge packet. This is done by phasing (clocking) the voltages of the adjacent gates, the process shown in Figure 2.12. Applying a +V voltage to the right gate allows the charge packet to straddle two of the gates. By changing the central voltage to −V, we have now transferred the charge packet in the pixel to the right gate. In this way, the charge packet can be transferred from pixel to pixel in the so-called “bucket brigade.”

      A schematic of the basic operation of the CCD is shown in Figure 2.13. Charge packets are first transferred along a parallel register (along columns). On the final row, charge is shifted along a serial register and an amplifier, where the charge is read, converted to analog-to-digital units (ADUs) and stored on a computer.

      Figure 2.13. Schematic of the operation of a CCD. Charge packets are first transferred along columns in the clocking direction. The last row is then read in the serial direction and into the amplifier. The overscan region is masked off so as not to record light. The numbers on each corner represent the number of recorded photons for 1000 detected photons. Losses are due to charge transfer inefficiency.

      The readout time of a CCD can range from approximately 10 s to several minutes, depending on the size of the CCD and the control electronics. Most of the time is spent along the serial register, especially the last one, where the charge is read out. The transfer along columns and then along rows is relatively fast (of order milliseconds) compared to the charge transfer along rows.

      In order to speed up the readout process, some CCDs have amplifiers at each corner of the array. This will decrease the readout time by a factor of 4. In these cases, the user has the option of using either one or four amplifiers in the readout. This speed, however, comes at a price. In this case, each amplifier and readout electronics has its own characteristics, read noise, etc. You have essentially converted your single CCD chip into four independent detectors so the data from each quadrant will have to be treated as four independent data sets. The factor of four in time savings by the reduced readout time of the CCD now translates into at least a fourfold increase in the time needed to reduce the data. For spectroscopy, this can cause problems as a spectral order now straddles two quadrants. For RV work, it is advisable not to use multiple amplifiers in the readout.

2.3.2 Quantum Efficiency

      The QE of a modern CCD depends on whether the CCD is front- or back-side illuminated. The front side of the CCD is where the polysilicon wires that control the charge transfer are attached. These absorb all the UV (or blue) photons before they can reach the photosensitive layers (left panel of Figure 2.14).

      Figure 2.14. Comparison of a front-side-illuminated CCD (left) to a back-side-illuminated CCD (right).

      To improve the efficiency, one can turn over the CCD to avoid photons striking the polysilicon wires (right panel of Figure 2.14). One can also apply an antireflection (AR) coating, which is not possible with the polysilicon wires present. However, now