The EEV10 CCD for use on IDS

The EEV10 CCD was originally used on ISIS on the WHT during the first half of 1998. This is a thinned, blue optimised device which proved very popular for 'blue' science on the WHT (i.e. approx. < 6000 Angs). We recieved delivery of a new similar device (EEV12) and decided that because of EEV12's better noise characteristics that it should be used as the defualt option on ISIS, leaving EEV10 available as an option for INT spectroscopy with the IDS. This CCD has now been commissioned on IDS, and is available to users, but they should note the various problems below. Potential users should read these pages carefully and contact Bego~na Garc'ia-Lorenzo bgarcia@ing.iac.es with any further queries. A request to use the device should be done through the usual channels with your Support Astronomer.

  1. General information on the device
  2. Vital statistics - gains, readout noise, etc
  3. Spectral resolutions and wavelength coverage
  4. Spatial scale on IDS
  5. Fringing and Cosmetic defects
  6. Linearity measurements
  7. Charge spreading variations and effects on spectral resolutions
  8. Flux standard data and empirical through-puts
  9. Quality control history of EEV10


1. General information on the device

The device with the ING label of EEV10 is an EEV42 type, science Grade1 CCD. The silicon area is 4096 x 2048 pixels, which are each 13.5 microns square. It is a thinned chip with excellent blue sensitivity matching anything currently available on the market. A quantum efficiency curve is available on the ING detectors information page for this device , but as this is usually meaningless to the Observer without the response curve of the rest of the optical system, you are referred to the Flux standard data which is a real measure of what you expect to see while observing with this chip. It has two overscan regions covering the areas [1:4200,2049:2148] and [4097:4200,1:2148]; where these coordinates are in the form [xstart:xend,ystart:yend].

While the chip itself can support two gain settings this feature is not currently available on the INT DAS system. It is likely it will be implemented in the near future.

The information on this page is geared towards the prospective Observer using IDS with this particular device mounted. Hence it has been driven by considerations of the whole system performance. For a fuller, more technical and general description of the EEV10 chip (and the other devices available on La Palma) see the ING Detector Group's home-page.

2. Vital statistics - gains, readout noise, etc

In the following tables the values for gain are given in e-/ADU, the read-out noise in e-, and the read-out time is in seconds for a windowed chip which covers the full IDS slit length (columns 800-1300 approximately).

Note: there are some unresolved problems when using this chip in it's various modes:

.
HIGH GAIN
Unbinned  Binned 1x2 (spectrally)  Binned 2x1 (spatially)  Binned 2x2 (both) 
Speed gain (e-/adu)  Noise (e-) Time (s) gain Noise Time gain Noise Time gain Noise Time
STANDARD  0.63 7.7 80 0.6 6.3 45 N/A N/A N/A N/A N/A N/A
QUICK 0.65 8.9 75
TURBO 0.72 8.9 70

3. Spectral resolutions and wavelength coverage

The table linked here gives the dispersion provided by each grating when mounted on the 235mm and the 500mm cameras. Observers can calculate the dispersion provided, given that the pixel size is 13.5 microns. The EEV10 is mounted on both cameras with it's 4096 pixel axis along the wavelength direction, giving maximum use of the beam width leaving the cameras. However the camera optics severely vignette the outer regions of the dispersed light beam such that approximately only 2275 of the CCD pixels are clear and unvignetted. The attenuation runs rises steeply at each end of the spectra. The unvignetted portion is roughly from pixel number 870 to 3145 in the spectral direction. Here is an example plot of a Lamp Flat on the 235mm camera. The points were vignetting starts, and the full free range are marked. The Y-axis is a real measure of the attenuation ratios. 
A ps version of the plot is available here

Observers should note that the 13.5 micron pixels allow a higher spectral (and spatial) resolution to be achieved than when using the TEKS. However this is of course at the expense of slit width. Since the two cameras have different magnifications, and hence different slit-width specfications for a particaular required resolution, Observers should think carefully about what is the best option for their programs. For example, it is now possible to reach a dispersion of higher than 0.8 Angstroms per pixels with the 235mm camera and this chip. The best focus we have achieved so far on the 235mm camera is a FWHM of 2.2 pixels per arc line, with a 1 arcsec slit-width. This is unlikely to decrease to exactly 2.0 pixels during normal setups, due to the accuracy that one can adjust the capstans, and the slight charge spreading visivle with these thinned chips. Observers should contact Stephen Smartt sjst@ing.iac.es if they need any advice regarding the most effiecient use of IDS and the EEV/TEK detector choice.

4. Spatial scale on IDS

A spatial scale of 0.4 arcsec/pixel and 0.19 arcsec/pixel is achieved on the 235mm and the 500mm cameras respectively. In the future it should be possible to bin in the spatial direction if one is not concerned with high spatial resolution observations, indeed the seeing conditions at the INT need to be excellent to allow full advantage to be taken of using an unbinned chip with this pixel scale. See above for binning characteristics in the spatial direction. The maximum unvignetted slit-length usable with IDS is 3.3 arcmin and 1.6 arcmin for the 235/500mm respectively (corresponding to a 500 detector pixels, spanning ~[800:1300, 1:4096].

5. Fringing and Cosmetic defects

These thinned chips suffer severely from fringing in the red part if the spectrum, which limits their usefulness in this region despite their continued good QE down to 8000Å. Some illustrative flat field spectra will be linked from here soon, but in the meantime the following numbers should serve as a reference guide to the severity of the problem : 
 
Wavelength     Peak-to-Peak Amplitude
6500Å                 5%
7000Å                 15%
7500Å                 30%
8000Å                 50%   
8500Å                 60%
9000Å                 60-70%
There are a few cosmetic defects on the surface of the chip, but nothing particularly severe. A flat field image plus bias image will be visible from here shortly.

6. Linearity measurements

A linearity test on 17/9/98 showed that EEV10 is linear across the range 0 - 60000 adu to better than 1%. It is advisable to keep the adu count level below 60,000. Further it is better than 2% linear at very low light levels (i.e. below approximately 1000-2000 adu).

7. Charge spreading variations and effects on spectral resolutions

The diffusion of charges between pixels during integrations causes a degrading of the spatial and spectral resolution. For a long-slit spectrograph like IDS, with the INT's mean seeing (of around 1.0-1.5") spatial degradation not a significant worry with the pixel size of the EEV10, but it is a consideration in the spectral direction. For a back illuminated CCD this charge diffusion (often referred to as the Modulation Transfer Function; MTF) becomes progressively worse for shorter wavelength incident light. For example, using a slit-width projecting to 2 pixels on the detector results in a FWHM measured of 2.4 pixels (measured at ~4000Å) when the spectrograph is at best focus. Similarly a slit-width projecting to 4 detector pixels will produce a FWHM of ~4.4 pixels (again at ~4000Å). This effect becomes less severe towards redder wavelengths and is negligible at around 6000Å.

8. Flux standard data and empirical through-puts

9. Quality control history of EEV10

Coming soon .....!
This page last updated: 21 Sept. 1998

 

 

Stephen Smartt (IDS Instrument Specialist) sjst@ing.iac.es