Contents
NOTE: All of the information and data on this page is preliminary. Our intent is to keep the users informed about the Red Channel upgrade so that they can make decisions and rough estimates for use in proposals beginning with the first trimester of 2007. More precise information will be provided following the camera optimization during the upgrade.
- Introduction
- Motivation for the Upgrade
- Advantages and Disadvantages
- Quantum Efficiency Comparison
- Current CCD: ITL/STA CCD Characteristics
- Deep Depletion Detector Characteristics
- Cosmic Rays
- Acknowledgements
- Questions or Concerns?
Introduction
The CCD detector in the Red Channel Spectrograph will be replaced with a deep depletion CCD in the fall of 2006. This page summarizes the motivation behind the upgrade and provides some preliminary characteristics for the possible replacement devices. The information posted here may be used by astronomers proposing to use Red Channel in the first trimester (Jan - April) of 2007. However, please see the NOTE above. Eventually this page will also include on-sky calibration data and performance characteristics.
Motivation for the Upgrade
The optics and coatings for the MMT spectrographs have been
optimized for the channel in which they are used: Blue or Red.
However, in recent years, both of the channels have utilized thinned,
backside illuminated CCDs and therefore, from a detector standpoint,
much of the Blue/Red distinction has disappeared. In order to
reestablish that distinction, the Red Channel spectrograph will
be upgraded to use a new Lawrence Berkeley Labs deep depletion CCD.
Deep depletion devices differ from typical CCDs because they use
thick, high-resistivity silicon which has a much higher opacity
to red photons. They require high bias voltages of as much as 90
volts in order to reduce diffusion of charge into adjacent pixels.
The result will be quantum efficiency values that are more than
double that of the current Red Channel CCD beyond 9000 angstroms.
In addition, fringing longward of 8000 A has been a major
problem with the current device. Because the silicon is as
much as 300 microns thick in the deep depletion devices there
will be no fringing in the upgraded Red Channel.
This upgrade will also include the use of an SDSU gen2
controller which has already been purchased through the generous
support of the Packard Foundation (X. Fan).
Because the devices are thick, they suffer from a higher rate of
cosmic ray depositions than thinned devices. This will likely limit
the maximum integration time for the upgraded system. Charge
localization is also reduced, resulting in a slightly larger PSF.
However, the Red Channel resolution should not be affected
significantly in typical seeing conditions because of the
oversampling inherent in the optical design. Finally, the read
noise of the devices is larger than the current device; 10 electrons
versus 3 electrons.
Advantages and Disadvantages
Advantages
- Much better QE in the red than the current device.
- No fringing.
- Higher rate of cosmic ray interactions limiting exposure times.
- Slightly wider PSF.
- Higher read noise: 10 electrons versus 3 electrons.
Quantum Efficiency Comparison
Click here for a pdf plot of QEs.
Current CCD: ITL/STA CCD Characteristics
Click here for detector information.Deep Depletion Detector Characteristics
Of several devices tested at ITL, two were deemed useful for scientific applications. The format of the devices is 1112 x 1012 with a pixel size is 18 microns. The table below illustrates the preliminary performance for the devices through both amplifiers.
| Device sn5864 | Device sn5867 | |||
| UL Amplifier | UR Amplifier | UL Amplifier | UR Amplifier | |
| Gain | 0.94 e/DN | 1.04 e/DN | 1.20 e/DN | 1.30 e/DN |
| Read Noise | ~10 e | ~10 e | ~10 e | ~10 e |
| Dark Current | 93.5 e/pix/hr | - | 9.36 e/pix/hr | - |
Notes:
- The values here are preliminary and will be updated after optimization.
- The high dark current value for sn5864 is probably due to excess charge; the final camera will have a more reasonable value.
Cosmic Rays
The image below shows the effect of cosmic rays in an hour long dark exposure.
Acknowledgements
The MMTO would like to acknowledge the hard work of Mike Lesser,
Roy Tucker, and the ITL staff in evaluating and preparing the
devices, controllers, and dewar for this project.
The SDSU gen2 controller for this project was purchased by X. Fan
through the generous support of the Packard Foundation.
Questions or Concerns?
Users who have additional questions or concerns should contact Grant Williams (ggwilli at mmto.org) or Tim Pickering (tim at mmto.org). Xiaohui Fan (fan at as.arizona.edu) and Arjun Dey (NOAO) can answer questions about their experience with the MARS spectrograph on NOAO 4-meter which uses a similar device.