29
How do I get a low power beam into the camera?
The power required to saturate the camera is generally very low so we provide
ND-1,2, and 4 filters to attenuate any input and in the camera adjustments the
exposure time can be varied by a factor of 1000 to further attenuate or optimize
the measured intensity level. Low power beams can be measured, so it is not
necessary to use the 4% reflections of uncoated surfaces. We recommend using
the transmission through HR reflecting mirrors with polished back surfaces as the
least intrusive method in a typical optical set-up.
Figure 23. Example physical layout of mirrors and cameras for GuideStar II
system.
What is the largest beam the system can measure?
The camera aperture is 12 mm so a larger beam will over fill the camera.
What is the limiting resolution of the camera?
The pixel size resolution of the CMOS sensor is 5.2μm but the beam position
resolution is much smaller than this because the beam is measured over a large
number of pixels. The camera position resolution noise had a standard deviation
of 0.04μm and temperature drift coefficients of 3μm/°C vertical and 0.5μm/°C
horizontal. With a 2.7:1 telescope all these numbers get multiplied by 2.7.
What is the temperature sensitivity of the cameras?
Measurements of beam position in a quiet environment have shown temperature
drift coefficients of 3μm/°C vertical and 0.5μm/°C horizontal. With a 2.1:1
telescope these numbers get multiplied by 2.1.
What is the minimum angular change we can detect with the system?
The accuracy of the beam position specification of 25 µm is conservative. The
noise we see with a closed optical path measurement with no air turbulence over a
minute is < 0.1 μm. The angular change that this corresponds to depends on the
Power in 2mm diameter beam
needed to saturate GSII Camera
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
300 400 500 600 700 800 900 1000 1100 1200 1300
Wavelength (nm)
1 W
1 mW
1 uW
1 nW
