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ISIS Blue Arm
The default CCD on the blue arm of ISIS is a thinned, blue-sensitive EEV12 array of 4096×2048 (13.5 micron) pixels.
The EEV12 detector bias frames are characterised by B(x,y,t) = f(x,y) + g(t), where f(x,y) represents the quasi-fixed pattern of pixel-to-pixel variation, and g(t) represents the variation of the mean (over x,y) bias level with time. The time variability typically amounts to up to ~10-20 counts on the timescale of hours and days, and depends on environmental factors such as CCD and controller-electronics temperatures. The quasi-fixed pattern function f(x,y) is measured from the median of the usual afternoon bias frames, and the time-variation function g(t) is measured from the overscan region of the detector from each science observation. The overscan strip of the EEV12 detector extends over rows 4105 to 4190, and is defined in the FITS headers by the usual BIASSEC keyword.
Biases and the overscan region
The table below gives the dispersion provided by each grating when mounted blaze to collimator (see the ISIS manual for more detailed information on gratings and their properties), and the spectral range covered by the EEV12 CCD. The EEV12 is mounted on the blue arm with its 4096 pixel axis along the dispersion direction, giving maximum utility of the beam width leaving the camera. However, the camera optics vignette the outer regions of the dispersed light beam such that approximately 600 pixels at either end of the CCD are vignetted. A plot of the CCD vignetting function across the chip is shown below to illustrate this effect. This function was measured in May 2011 from a flat-field exposure corrected by CCD quantum efficiency, grating efficiency and tungsten lamp spectral emissivity functions. The unvignetted region is from (spectral) pixel 665 to 3485, which is essentially the central 2820 pixels, and both the unvignetted and 50% vignetted spectral ranges in Å are given in the table below.
Wavelength coverage and spectral resolution
The dispersions of the blue-arm gratings in Å/mm are 120 (R158B), 64 (R300B), 33 (R600B), 17 (R1200B) and 8 (H2400B). The pixel size of the EEV12 detector is 13.5 microns, and the corresponding grating dispersions in Å/pixel are listed in the table, as are the slit widths that project to four pixels (54 microns) with the gratings set at blaze. The spectral resolution elements, Δλ, in Å for a 1-arcsec slit are also listed. The corresponding nominal spectral resolutions, λ/Δλ, at 4000Å with a 1-arcec slit are approximately 512 (R158B), 976 (R300B), 1980 (R600B), 4706 (R1200B) and 11429 (H2400B). These values exclude the impact of diffusion of charge between pixels, which will degrade resolution by at least 10% for wavelengths ≲6000Å. Resolutions for other slit widths and wavelengths can be computed with the SLITTOOLS calculator.
Note in the table that the slit width projecting to four pixels increases with grating resolution. This is due to grating anamorphic de-magnification, which is parameterised by cos(θ+φ/2)/cos(θ-φ/2), where θ is the grating tilt with respect to zero order and φ is the camera-collimator angle. This relation demonstrates why the slit size projecting to four pixels is larger for the higher resolution gratings; they are set to larger tilt angles for a given central wavelength, and so their de-magnification is greater. See here for a discussion of this effect.
According to sampling theory a line as recorded on the detector is fully sampled if it has at least two dispersion elements across its FWHM, and so the detector oversamples the resolution element when the slit width is set to 1-arcsec (e.g. to approximately match seeing). In this configuration the detector can be binned ×2 spectrally to increase the signal-to-noise in each wavelength bin, without loss of spectral resolution. This can also be done at the reduction stage, but binning on-chip reduces the readout noise contribution to the resolution element.
It is also possible of course to improve spectral resolution by reducing the slit width. This increases slit losses especially in moderate seeing, and in any case the slit should project to at least two pixels so that the resolution element remains fully sampled by the detector.
The blue (and red) camera is a folded Schmidt design of focal length 500mm and gives a scale of 14.9 arcsec/mm along the slit. Hence a spatial scale of 0.2 arcsec/pixel is achieved with the EEV12. It is possible to bin in the spatial direction if one is not concerned with high spatial resolution observations, indeed the seeing conditions need to be excellent to allow full advantage to be taken of using an unbinned chip with this pixel scale. The maximum unvignetted slit-length usable with ISIS is 3.7 arcmin (corresponding to 1100 spatial detector pixels, spanning ~[400:1500, 1:4096]).
These thinned chips suffer severely from fringing in the red part of the spectrum, which limits their usefulness in this region despite their continued good QE down to 8000 Å. Click here to see an illustrative flat field spectra. The following figures are illustrative:
Fringing and cosmetic defects
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 linearity test in standard readout mode with no binning showed the chip to be linear to better than 1% up to just over 60,000 ADU.
SYS@taurus> dasreset blue
Remember to check the CCD readout-speed and binning after the dasreset. For observations of spectrophotometric standards we recommend a minimum exposure of 2 sec.
The diffusion of charge between pixels during integrations degrades both the spatial and spectral resolutions. For a back illuminated CCD this charge diffusion becomes progressively worse for shorter-wavelength incident light; as wavelength decreases, absorption of photons occurs closer to the surface of the CCD where the pixel potential wells are less-well defined. Photoelectrons generated closer to the surfave have a greater tendency to diffuse into the wells of neighbouring pixels, thereby degrading resolution in both the spectral and spatial directions.
Charge spreading and its effect on resolution
In the spectral direction, using a slit-width projecting to 2 pixels on the detector results in a measured FWHM of 2.4 pixels at wavelength λ~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 λ~4000Å. This effect becomes less severe towards redder wavelengths and is negligible at around 6000Å.
The FWHM of the spatial profile in median seeing is similarly broadened from ~4 to ~4.4 pixels by charge diffusion. Note that the combination of charge diffusion and the λ-0.2 dependence of seeing means the spatial profile in the blue arm can be broader than in the red arm by several tenths of an arcsec at widely-spaced wavelengths, at optimum telescope focus.
The figure below shows the results from throughput measurements of flux standards. The Y-axis is the apparent AB magnitude of star observed at zenith which gives one detected photon per second per Angstrom. The lowest resolution grating was used (R158B) in the blue arm (without a dichroic) with a wide slit (10 arcsec). The conditions were photometric, with negligible dust levels present. This figure shows the response of the whole ISIS blue channel (i.e. WHT primary + secondary + ISIS blue optics + detector response) from ~3200 to ~8300Å.
Flux standard data and empirical throughput
Note that the H2400 grating vignettes significantly at ''redder'' central wavelengths because of its high inclination to the collimator in longer-wavelength settings, e.g., by ~20% at 5000Å, ~45% at 6000Å and ~100% at 7000Å.
The figure below shows the results from several throughput measurements of flux standards in both the blue and the red arms and with different detectors. The Y-axis is the apparent AB magnitude of star observed at zenith which gives one detected photon per second per Angstrom. In each case the lowest resolution grating was used (R158B) in the blue arm (without a dichroic) with a wide slit (10 arcsec). In each case, the conditions were photometric, with negligible dust levels present.
Quality control history
Bad pixel masks for EEV12 with different binning were created using noao.imred.ccdred task in IRAF. All masks are created for the default CCD window [585:1550,1:4200].
Bad pixel masks
An atlas of arc lines for a range of central wavelengths of the blue arm gratings is available here.
Atlas of arc lines in the blue arm
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