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62 NIRvana
™
-LN System Manual Issue 4
Another way to prevent light from falling on the array during readout is to use a
Princeton Instruments supplied
25 mm external shutter which is typically used at a
spectrograph entrance slit. This shutter can then be controlled by the camera and is
synchronized with the exposure-readout cycle as shown in
Figure 7-1 . The shutter
compensation time (t
c
) is approximately 8 ms for the 25 mm shutter.
NOTE:
When acquisition time is on the order of ~130 μ s at 2 MHz or
~510 μ s at 500 kHz, the addition of the 8 ms shutter
compensation time slows the readout time, thus reducing the
number of frames per second. To minimize the impact of
shutter compensation time, disable shutter open in software.
Figure 7-1: Timing Diagram: Mechanical Shutter and SHUTTER MON
In Figure 7-1 :
•
t
exp
= Exposure Time;
•
t
o
= Shutter Open Compensation Time;
•
t
c
= Shutter Close Compensation Time;
•
t
R
= Readout Time
• SHUTTER MON
is the output of the SHUTTER MON connector on the Aux I/O
cable, and the TTL OUT connector when
Shutter
is selected.
7.3 Noise Sources
Primary noise sources include:
•
Dark signal - IR signal from the environment and leakage current
•
Input signal - photon shot noise
•
Readout - noise generated when the multiplexers are read out
The ideal detector contains no noise sources and is limited only by the photon shot noise.
Unfortunately, there are always noise sources that will contribute noise of one kind or
another to the total signal. The methods used to reduce the noise depend on the noise
component being addressed. Infrared noise from the detector and the environment can be
reduced by lowering the temperature of the detector and environment, respectively.
4411-
0145
_002
1
M
ECHANICAL
SHUTTER
O
PEN
E
XPOSURE
t
o
t
c
t
R
C
LOSED
R
EADOUT
t
exp
S
HUTTER
MON