ISIS Red Arm

The default chip for ISIS red arm is RED+. It is a red-sensitive array of 4096×2048 (15.0 micron) pixels with almost no fringing. Our current policy is that this particular device will remain, almost permanently, at this focal station. This has many advantages, in particular:

• Observers will have constant access to a state-of-the-art, large-format, red sensitive detector with almost no fringing.
• The stability of having one device permanently mounted allows ING staff to monitor its performance in detail over long periods.
  

1. General information on RED+ CCD.
2. Spectral resolution and wavelength coverage
3. Spatial scale
4. Fringing and cosmetic defects
5. Linearity measurements
6. Shutter effect
7. Bad pixel masks

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 REDPLUS CCD. The REDPLUS is mounted on the red arm with its 4096 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 800 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 pixel 800 to 3200 (calculated so that the flux differences are in within 5%), which is essentially the central 2400 pixels.

ISIS wavelength coverage and resolution with REDPLUS
Grating	Blaze	Dispersion (Å/mm)	Dispersion (Å/pix)	Total Spectral range (Å)	Unvignetted range 2400 pixels (Å)	50% unvignetted range 3285 pixels (Å)	Slit-width for 60 mu at detector (in arcsecs)
R158R	6500	121	1.81	7530	4412	6038	0.94
R316R	6500	62	0.93	3858	2260	3094	0.98
R600R	7000	33	0.49	2054	1202	1646	1.08
R1200R	7200	17	0.26	1055	618	846	1.38

The pixel size of the REDPLUS is 15 microns (0.22 arcsec/pixel). According to sampling theory a line can be considered resolved if it has 2 dispersion elements at its FWHM, hence 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 (60 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 REDPLUS to improve the actual spectral resolution. This of course requires the observer to use quite a narrow slit-width, which results in significant light loss (assuming moderate 1 arcsec seeing conditions).

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 0.22 arcsec/pixel is achieved with the REDPLUS. 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 1000 detector pixels, spanning ~[600:1600, 1:4200]). REDPLUS  is a very low-fringing high-QE CCD. Fringing begins at about 8000Å, and the ripples are visible at about 1% level at 9000Å. Here you will find more information. The Prontor shutter opens the aperture radially symmetrically. The same shutter effect as for the blue arm was measured for the red arm. Bad pixel masks for RED+ 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 mask bin 1 1           bad pixel mask bin 1 2           bad pixel mask bin 2 1

bad pixel mask bin 2 2           bad pixel mask bin 3 1           bad pixel mask bin 3 2

bad pixel mask bin 4 2