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MARCONI chip on ISIS Red Arm
- General information on the device
- Spectral resolutions and wavelength coverage
- Spatial scale on ISIS
- Fringing and Cosmetic defects
- Linearity measurements
- Flux standard data and empirical through-puts
- Quality control history of ISIS Red arm
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Spectral resolutions and wavelength
coverage
The table below gives the dispersion provided by
each grating when mounted blaze to collimator (see the
ISIS
manual for more in depth details on gratings and their properties),
and the spectral range covered by the MARCONI2 CCD. The MARCONI2 is mounted on
the red arm with it's 4610 pixels axis along the dispersion direction,
giving maximum use of the beam width leaving the cameras. However the camera
optics vignette the outer regions of the dispersed light beam such that
approximately 1000 pixels are seriously vignetted (from 50% to 80% attenuation)
at the detector. Hence the central 2750 pixels are essentially unobstructed,
with ~475 pixels at either end suffering a sharp drop off in response.
A plot of a flat-field
exposure across the chip is shown here to illustrate this effect.
ISIS wavelength coverage and resolutions with MARCONI2 |
Grating |
Blaze |
Dispersion (Å/mm) |
Dispersion (Å/pix) |
Total Spectral range (Å) |
Unvignetted range (2750 pixels) |
Slit-width for 54 mu at detector (in arcsecs) |
Slit-width for 27 mu at detector (in arcsecs) |
R158R |
6500 |
121 |
1.63 |
7530 |
4492 |
0.84 |
0.42 |
R300R |
6500 |
62 |
0.84 |
3858 |
2302 |
0.88 |
0.44 |
R600R |
7000 |
33 |
0.44 |
2054 |
1225 |
0.97 |
0.48 |
R1200R |
7200 |
17 |
0.23 |
1055 |
631 |
1.24 |
0.62 |
The pixel size of the MARCONI is 13.5 microns. According to sampling theory a line can be considered resolved if it has 2 dispersion elements at it's FWHM,
we are oversampling the best resolution that we can achieve with this spectrograph
setup when the slit width is set to match the seeing (normally around 1 arcsec). As a way of increasing the signal in each wavelength bin one can
then combine the signals of each pair of adjacent pixels without losing
important resolution information. This can be done at the reduction stage,
but to decrease the readout noise contribution one can bin the CCD at readout
so that two pixel elements are combined to produce one 'dispersion element'
which has the readout noise of approximately one pixel. The above
table includes a column for the slit-width which projects a FWHM of 4 pixels
at the detector (54 microns), the observer can choose to bin the chip at
readout in the spectral direction by 2 to maximise signal detection, and
reduce read-out noise contributions.
It is theoretically possible to make use of the
smaller pixels of the MARCONI to improve the actual spectral resolution.
This of course requires the Observer to use quite a narrow slit-width,
and results in significant light loss (assuming moderate 1" seeing conditions).
Spatial scale on ISIS
The red (and blue) camera is a folded Schmidt design of 500mm and gives
a scale of 14.9 arcsec/mm along the slit. Hence a spatial scale of 0.20
arcsec/pixel is achieved with the MARCONI2. 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
a 1110 detector pixels, spanning ~[600:1710, 1:4610]).
Fringing and Cosmetic defects
MARCONI2 is a low-fringing high-QE CCD. Fringing begins at about 8000 Å on the MARCONI chips, and the ripples are visible at about 2-3 % level. Fringing increase to ~10% at 9000, rising to to ~15-20% at
10000 Å .Click here to see an illustrative normalised
flat field spectra
Flux standard data and empirical through-puts
The figure below shows the results from recent through-put 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 (R158R) in the red arm (without
a dichroic or any filter) with a wide slit (10
arcsec). The conditions were photometric, with negligible dust levels present.
This figure shows the response of the whole ISIS red channel
(i.e. WHT primary + secondary + ISIS red optics + detector response) from
~3800 to ~9200Å.
Quality control history
The figure below shows the results from several through-put 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. In each
case the lowest resolution grating was used (R158R) in the red arm (without
a dichroic or filter) with a wide slit (10
arcsec). In each case, the conditions were photometric, with negligible dust levels present.
This shows the response of the whole ISIS blue channel
(i.e. WHT primary + secondary + ISIS red optics + detector response) from
~3200 to ~9200Å.
This page is undergoing continual change and enhancements, if there
is anything you wish to see changed or added then please mail me.
