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Camera
Detector Characteristics
The key component of the SBI is the bolometric
imager, which has the unique
capability to record images (320×240 pixels in size) in total light,
i.e. with a flat photometric response from the UV to the NIR. The detector
characteristics are described in more detail in
Foukal and Libonate (2001).
The detector is composed of an array of 320×240 barium strontium
titanate (BST) ferroelectric elements, each element 50×50 μm in
size. BST exhibits a strong temperature dependence of capacitance around
its Curie point (at about +30 °C). If exposed to IR radiation the
elements produce a change in output current that is proportional to the
radiation intensity. For the SBI we used a modified IR detector array
based on this effect (Hanson,
1997), commercially available for night vision cameras. For our 30 cm
aperture F/12 telescope the image scale is 0.0573 arcsec/μm. Given the
detector size, the image field of view
is about 917×687 arcsec. Thus, a
full disc image of the Sun can be
obtained with a mosaic of 10 single
tiles, with the pattern 2-3-3-2, and with a
considerable overlap between individual tiles.
To transform such a camera into a detector with flat response over the UV
to NIR range we deposited a thin (~30 μm) layer of gold black on the
monolithic light-receiving surface of the array. Gold-black films have a
spectral absorptance that varies less than ±1% from 0.2 μm to
beyond 3 μm (Advena
et al., 1993). Therefore, such a film will uniformly redistribute the
absorbed radiation in the above mentioned spectral range in the form of
thermal emission and it will be detected by the under lying thermal IR BST
imaging array. This link shows that the
measured hemispherical reflectance of a
gold-blackened BST array is extremely flat over the spectral range
between 300 and 1600 nm. This indicates that approximately 99% of the
incident light in that range is absorbed. An image of a gold-blackened
Raytheon BST detector array with fused quartz window used as a prototype
for the SBI is shown here.
Camera Operation
The detector elements are sensitive to
temperature change but do not
provide a DC response (the temperature signal is AC coupled). A chopper
(with the shape of an Archimedes spiral) modulates the scene energy onto
an AC carrier, normally at 30 Hz. Abrupt sensor output changes occur when
the chopper blade exposes or blocks the source image energy. If the scene
temperature is higher than the chopper blade temperature, the pixel element
will heat up when exposed and cool down when blocked. The opposite will
occur if the scene temperature is colder than the chopper blade
temperature, producing an apparent 180° phase shift in the signal,
however this is a perfectly acceptable mode of operation for this imager chip.
The output from each pixel amplifier goes to a sample-and-hold circuit,
and the output from the pixel multiplexer is the difference between the
current pixel amplifier and the last held value. When the next adjacent
pixel is read out, the previous pixel output is sampled and stored in its
sample/hold circuit. This means that the output frames from the detector
have alternating polarity. For example, if we define Os the output signal
when a pixel element is looking at the scene, and Oc the output
signal when the chopper blade is in front of the same element, then the
output signal S(i) for that pixel for frame i (the ith cycle of the
chopper blade) is given by: S(i) = Os − Oc + Z + noise,
where Z is an arbitrary signal offset.
The output signal S(i+1) from the next frame would then be: S(i+1) = Oc − Os + Z + noise.
By computing the difference between consecutive frames we obtain: S(i) − S(i+1) = 2(Os −
Oc) + √2 noise,
thus eliminating the offset and reducing the noise.
S(i) = Os − Oc + Z + noise,
