CTIO operates several types of CCDs. Some of these are installed in special dewars and are only used for a single purpose. The Loral 3K CCD in the Air Schmidt camera, the Loral 1K CCD used with the 1.5-m spectrograph, the SITe 2Kx4K CCD in the Hydra camera, and the 8Kx8K Mosaic Image CCDs are in this category. The CCDs used for direct imaging at the smaller telescopes and for the 4m Echelle long camera are in Universal Dewars and are interchangeable. Presently these are all SITe (ex-Tek) 2K CCDs, with a SITe 1K available as backup.
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.
Almost always you will find your CCD at the correct operating temperature and ready for use. 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 well over 12 hours, and filling at the start of the night will generally do for the whole night. You should check with Observer Support, or your Telescope Operator, whether a middle-of-the-night fill is advisable.
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). By convention the ratio electrons/adu is called the "gain", although this is really the inverse gain. No matter... The gain an adjustable parameter on the CTIO CCD systems. The range of data numbers possible extends from 0 to 65535. At some gain settings some CCDS will saturate before this data system limit is reached.
You should type "ccdinfo" to find a gain table and other useful information for the CCD you are using.
A summary of the characteristics of the detectors normally used for direct imaging is as follows:
Tek 1024 Tek 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 10 30
B: ( U=B=V=R=I ) 180 239
V: ( = 20 mag ) 350 348
R: ( star, in ) 380 380
I: (electrons/sec) 200 205
Read time (quad, secs) 12 25
Full well (e-) 150000 150000
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. 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 some 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 (usually gain setting = 1 for ARCON system).
We have two Tek 1024 CCDs, #1 and #2. At the present time only #2 (a quad amp devices) is being scheduled. Read noise is low. 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.
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 similar to the Tek 1024's except the U band response is much improved. Tek #3 has several column defects, with bright columns on each side and three blocked columns 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., #5 is usually at the Schmidt. #3 & #6 have four low-noise amplifiers and thus have short read time, typically 30 seconds. #5 has one amplifier noisier than the rest, type "ccdinfo" to see the characteristics of the CCD you are using.
#6 has a peculiar fault sometimes seen when reading the lower amplifiers (eg in QUAD mode). See HERE for more details.
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.
(9.1) Pixel sizes (arc sec)
Tek SITe SITe
1024 2048 2x4K
4.0-m f/2.85 0.27
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
(9.2) Field Sizes (arc min per side)
Tek SITe SITe
1024 2048 2x4K
4-m f/2.85 35
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