DESCRIPTION |
Instrument Overview
===================
The Descent camera (DESCAM) is mounted on the
lander radar bracket and is pointed downward during lander descent.
It has the exact same optical design as the Navcams and Hazcams. The
DESCAM will be used to take images of Mars as the lander is
descending.
The DESCAM is made of a detector head and an electronics box. The
detector head houses a lens assembly and a Charge Coupled Device
detector (CCD). The electronics box contains the CCD driver
electronics, the 12-bit Analog to Digital Converter (ADC),
camera/rover interface electronics, and a heater resistor that will
keep the box above the minimum operating temperature of 218 K. The
DESCAM uses a broadband visible filter and produces 1024 x 1024
pixel images.
Information in this instrument description is taken from the Mars
Exploration Rover Engineering Cameras paper[MAKIETAL2003]. See this
paper for more details.
Instrument Objectives
=====================
The chief objective of the DESCAM is:
1) to acquire images of Mars as the lander descends towards the
surface.
Calibration
===========
The DESCAM has been calibrated over the flight range of
temperatures. It was calibrated with respect to: geometric flat
field response, detector dark current, camera absolute responsivity,
detector noise performance, and detector gain.
Detectors
=========
The DESCAM uses a 1024 x 2048-pixel CCD with 12 micron-square pixels
and a 100% optical fill factor. The CCDs operate in frame-transfer
mode, dividing the detector into two regions. One of the regions
is a 1204 x 1204 pixel photosensitive imaging region where the image
is recorded. The other region is a 1204 x 1204 shielded storage
region where the recorded image is shifted and stored during
detector readout. It takes 5.1 msec to transfer data from the
imaging region to the storage region, and 5.4 seconds for readout of
data from the storage region. Each CCD includes 32 non-imaging
pixels in the serial readout registers, which allow the monitoring
of the CCD electronics offset and detector noise performance. The
RMS read noise at cold temperatures (218 K) is approximately 20
electrons, and the detector systems have gain values of
approximately 50 e-/DN, which results in a system with approximately
.5 DN of RMS read noise.
The absolute CCD quantum efficiency (QE) of the CCDs has been
measured between 400 and 1000 nm at four operating temperatures
ranging from 218 K and 278 K. The QE of the MER CCDs is typical of
a silicon CCD detector, with sensitivity peaking at 700 nm with a QE
value of approximately 43%. The SNR of the detector system is
essentially Poisson-limited because of its low readout noise and
small dark current rates in the Martian operating envirionments.
Electronics
===========
The DESCAM CCD uses a 'clocked antiblooming' readout technique,
instead of having anti-blooming circuitry. The proper choice
of autoexposure parameters prevent the blooming effect.
Filters
=======
The DESCAM uses a broadband filter with the center at
approximately 750 mn and a Full Width at Half Maximum (FWHM)
of approximately 200 nm.
Optics
======
The DESCAM has a f/12 optical system with a 14.67 mm focal length.
It has a 45 x 45 degree field of view, and a 60.7 degree diagonal
field of view. The angular resolution at the center of the field of
view is .82 mrad/pixel.
Location
========
The DESCAM is mounted on the lander radar bracket.
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REFERENCES |
Maki, J.N., et al., Mars Exploration Rover Engineering Cameras, J. Geophys.
Res., 108(E12), 8071, doi:10.1029/2003JE002077, 2003.
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