Detector Characteristics

4.1. General Principles

CTIO operates several types of CCDs. There is, as yet, no general purpose CCD and a CCD suited to your program will be the one scheduled. Loral CCDs require periodic UV- flooding in order to provide enhanced sensitivity below 5000Å, and must be kept cold in order to retain the improved response. The output signals from CCDs are extremely small (remember 1 electron = 1.6 x 10^(-19) Coulomb!) and the electronics must not allow significant extra noise to degrade the performance. In case of problems, electronic components can be replaced (at the board level) and if necessary, the CCD can be changed. If you are dissatisfied with the performance of the CCD system, please consult with Observer Support personnel.

At PFCCD the CCD is mounted in a downward looking liquid nitrogen filled dewar, and is kept at a constant temperature by a small heater. This temperature is displayed in the console room and should remain constant. If the temperature appears to be rising immediately alert either your Observing Assistant or Observer Support personnel. After initial power on and the dewar has been filled with liquid nitrogen for the first time, it requires at least 4 hours for all temperatures and operating voltages to stabilize. The dewar hold time is over 12 hours -filling at the start of the night will generally do for the whole night, except in winter time when, near midnight, the night assistant will ask for permission to return the telescope to the access platform in order to fill the dewar.

At CFCCD the dewar is upward-looking, otherwise procedures are as in the above paragraph. All the dewars have the fill-tube extending only half-way into the Liquid Nitrogen can. Although this has the disadvantage that the can cannot be filled to more than half-capacity, the advantage is that the dewar can be used in any orientation.

The analog signal (which can be considered to be some number of electrons) from each pixel of the CCD is first amplified, then passes through an integrator which discriminates against various noise sources, and is then converted into a digital signal by an analog-digital- converter (ADC). Hence the signal at this stage is measured in "counts" or " adu's" (analog- to-digital-units). The ratio adu/electrons is called the gain, although by convention we usually talk about the inverse gain, electrons/adu. The gain is an adjustable parameter on the CTIO CCD systems. The range of data numbers possible extends from 0 to 65535 (0 to 216-1, 16 bits).

4.2 Detector Options

A summary of the characteristics of the detectors normally used for direct imaging is as follows:

    Tek 1024 Site 2048
            
  Pixels 1024x1024 2048x2048
  Pixel size (microns) 24 24
  Readout noise (electrons) 4-6 3-5
  Electrons/ADU (typical) 1-4 1-4
  Cosmic ray rate (per min) 20 100
  U: Sensitivity at the 4m 18 54
  B:    ( U=B=V=R=I   ) 220 288
  V:    ( = 20 mag    ) 350 380
  R:    ( star, in    ) 380 356
  I:    (electrons/sec) 180 167
  Read time (quad, secs) 10 25

Notes:

1. The full well capacity is the limit above which charge "spills" out of a CCD pixel into adjacent pixels. At some level below full well the response of the CCD becomes non-linear to incident light. Some CCDs retain excellent linearity right up to full well, but for others the departure from linearity becomes severe well below full well. This will define the practical maximum charge capacity per pixel. It may depend on details (slope, amplitude, levels) of the clocking voltages and for some of our devices there is some hope of improving the values given. Typically a factor 20 more charge must be accumulated before noticeable bleeding occurs. This bleeding is stronger in the column direction for our CCDs. With normal gain settings the data system limit of 65535 counts is reached prior to entering the CCD non-linear region. Care should be taken always to operate the CCDs in the linear part of their response.

2. The readout noise is somewhat dependent on gain, larger values of readout noise correspond to larger values of e-/ADU. Similarly, the read time is given for gains of 3-4 e-/adu.

4.3 Detector Notes

4.3.1 The Tek 1024 CCDs

We have one Tek 1024 CCD, #2. It is now (2000) only scheduled in case of emergency. This is a quad-amplifier device with very low read noise. Cosmetics are superb. QE is poor in the UV and U band photometry is not recommended. Note that these CCDs do not have the serial registers and amplifier areas shielded from light, consequently on high light level flat field exposures the overscans show an exponential decay and during reduction should be fitted with high-order splines. See the software manual for more details.

4.3.2 The Tek (SITe) 2048 CCDs

We have three Tek 2048 CCDs (#3,#5, #6).These are thinned, AR coated devices like our Tek 1024s. Read noise is very low, and QE is like the Tek 1024's, except the U band response is much improved. Tek #3 has several column defects. There are bright columns on each side and three major column traps near the center. These should all be avoided. Tek 2048 #6 is the CCD normally scheduled at the 4-m PF, while #3 is dedicated to the 0.9m, and #5 is used at the Schmidt. Type "ccdinfo" ro see the characteristics of the CCD you are using.

4.3.3 The Thomson 1024 CCDs RETIRED!!!!

4.3.4 The STIS 2048 CCD RETIRED!!!!

4.4 Detector Control Options

The observer can control and change several CCD parameters. These are: the CCD readout format, the binning, the preflash time, and the gain. Observers should think carefully whether they need all the field (if not read a ROI) or the resolution (if not, bin 2x2). Even though the CCDs with quad readout have short read times by big-CCD standards, substantial gains in efficiency are possible by reducing the format.

4.4.1 CCD Readout Format

A single region-of-interest can be read out, positioned at an arbitrary place on the CCD. These operate very efficiently, for instance a 1024x1024 ROI centered on a Tek 2048 takes under 10 seconds to read out, while a 256x256 takes just 2 seconds.

4.4.2 Binning

The ARCON controllers are able to bin pixels in various ways. The readout noise per pixel remains the same as in the unbinned case. The gain calibration is unchanged from the unbinned mode. For most direct imaging applications at CTIO the CCDs tend to undersample the images, so there is little advantage in binning. The main advantages of binning are that the readout time is shorter, and less data is created.

4.4.3 Preflash Time

Default is zero. All our CCDs operate without preflash.
4.4.4 Gain Generally you will find for direct imaging a gain of about 3-5 e-/adu is a good compromise between sampling the read noise and achieving good dynamic range. Excessively high gain (ie small e-/adu) does little good since digitization noise is soon masked by CCD readout noise and photon statistical fluctuations. Different CCDs are somewhat different in their photon/adu conversion, noise and points at which non-linearities begin to occur. The method used to calculate the gain is described in Appendix II.

4.5 CCD scales at various foci

(1) Pixel sizes (arc sec)

        Tek Tek
    1024 2048
       
  1.5-m f/7.5 0.44 0.44
  1.5-m f/13.5 0.24 0.24
  0.9-m f/13.5 0.40 0.40
  Schmidt f/3.5   2.3

(2) Field Sizes (arc min per side)

 

        Tek Tek
    1024 2048
       
  4-m f/7.5 2.70 5.40
  1.5-m f/7.5 7.41 14.8
  1.5-m f/13.5 4.09 8.18
  0.9-m f/13.5 6.76 13.5
  Schmidt f/3.5   78