29/1/2005 last updated
So you want to start taking Astro photos well read
this first
CCD =
charge-coupled device
A charge-coupled device (CCD) is a light-sensitive integrated circuit that
stores and displays the data for an image in such a way that each pixel (picture
element) in the image is converted into an electrical charge the intensity of
which is related to a colour in the colour spectrum. For a system supporting
65,535 colours, there will be a separate value for each colour that can be
stored and recovered. CCDs are now commonly included in digital still and video
cameras. They are also used in astronomical telescopes, scanners, and bar code
readers. The devices have also found use in machine vision for robots, in
optical character recognition (OCR), in the processing of satellite photographs,
and in the enhancement of radar images, especially in meteorology.
A CCD in a
digital camera improves resolution compared with older technologies. Some
digital cameras produce images having more than one million pixels, yet sell for
under £1,000. The term mega pixel
has been coined in reference to such cameras. Sometimes a camera with an image
of 1,024 by 768 pixels is given the label "mega pixel," even though it
technically falls short of the mark. Another asset of the CCD is its high degree
of sensitivity.
A good CCD can produce an image in
extremely dim light
and that’s why they
are good for Astronomy
and its resolution does not deteriorate when the
illumination intensity is low, as is the case with conventional cameras.
The CCD was
invented in 1969 at Bell Labs, now part of Lucent Technologies, by George Smith
and Willard Boyle.
CMOS vs CCD—it’s the top fight on
the digital imaging card.
In this corner,
defending its crown is proven imaging technology, a chip designed for imaging
and a track record of quality with every major digital camera manufacturer. And
in this corner, the challenger promises lower cost manufacturing, on-chip
processing, and a host of new imaging applications. The fight of the decade?
Well, maybe.
Of all the sensor
manufacturers, Kodak supplies both CCD (charge-coupled device) and CMOS
(complementary metal oxide semiconductor) image sensors. Each contestant brings
strengths and weaknesses into the fray. Regardless of whether any consumer will
ever ask a sales clerk if that cool-looking digital camera is CMOS or CCD,
understanding how these two technologies stack up is important to everyone in
the imaging business. Not so much because one will eliminate the other but
because together they define the future of imaging.
The Technologies
Both CMOS and CCD
imagers are manufactured in a silicon foundry; the equipment used is similar.
But alternative manufacturing processes and device architectures make the
imagers quite different in both capability and performance.
Developed in the
1970s and 1980s specifically for imaging applications, CCD technology and
fabrication processes were optimized for the best possible optical properties
and image quality. The technology continues to improve and is still the choice
in applications where image quality is the primary requirement or market share
factor.
A CCD comprises
photosites, typically arranged in an X-Y matrix of rows and columns. Each
photosite, in turn, comprises a photodiode and an adjacent charge holding
region, which is shielded from light. The photodiode converts light (photons)
into charge (electrons). The number of electrons collected is proportional to
the light intensity. Typically, light is collected over the entire imager
simultaneously and then transferred to the adjacent charge transfer cells within
the columns.
Next, the charge
is read out: each row of data is moved to a separate horizontal charge
transfer register. Charge packets for each row are read out serially and sensed
by a charge-to-voltage conversion and amplifier section (see image below). This
architecture produces a low-noise, high-performance imager. That optimization,
however, makes integrating other electronics onto the silicon impractical. In
addition, operating the CCD requires application of several clock signals, clock
levels, and bias voltages, complicating system integration and increasing power
consumption, overall system size, and cost.
CMOS and CCD Sensor Architectures
In short, the
champ delivers consistent clout but requires significant upkeep.
A CMOS imager, on
the other hand, is made with standard silicon processes in high-volume
foundries. Peripheral electronics, such as digital logic, clock drivers, or
analog-to-digital converters, can be readily integrated with the same
fabrication process. CMOS imagers can also benefit from process and material
improvements made in mainstream semiconductor technology.
To achieve these
benefits, the CMOS sensor’s architecture is arranged more like a memory cell or
flat-panel display. Each photosite contains a photodiode that converts light to
electrons, a charge-to-voltage conversion section, a reset and select transistor
and an amplifier section. Overlaying the entire sensor is a grid of metal
interconnects to apply timing and readout signals, and an array of column output
signal interconnects. The column lines connect to a set of decode and readout
(multiplexing) electronics that are arranged by column outside of the pixel
array. This architecture allows the signals from the entire array, from
subsections, or even from a single pixel to be readout by a simple X-Y
addressing technique—something a CCD can’t do.
Still, the
industry is only in the early stages of optimization for imaging. The challenger
has some great moves, but can it deliver the knockout blow?
Enabling New
Applications
So how fast can
CMOS sensors displace CCDs? A better question might be whether they will
replace CCDs.
The biggest
opportunities for CMOS sensors lie in new product categories for which they are
uniquely suited. Keys to their success are
Lower power usage
Integration of
additional circuitry on-chip
Lower system cost
Such features
make CMOS sensors ideal for mobile, multifunction products like Kodak’s mc3 or
imaging attachments like the PalmPix.
Still, if CMOS
sensors offer all of these benefits, why haven’t they completely displaced CCDs?
There are a number of reasons; some are technical or performance related, and
others are related more with the growing maturity of the technology. CCDs have
been mass-produced for over 25 years whereas CMOS technology has only just begun
the mass production phase. Rapid adoption was also hindered because some early
implementations of these devices were disappointing:
they delivered poor
imaging performance and poor image quality.
A pioneer in CCD
imaging technology, Kodak developed its CMOS imaging technology to the point
where it could deliver quality images before introducing commercial products.
Quite simply, Kodak scientists and engineers applied the optical science and
image processing experience derived from more than 25 years of work with CCD
sensors and digital cameras to develop and characterize CMOS sensors—and to
define modifications in standard CMOS manufacturing lines and equipment to make
low-noise, good-quality sensors.
For example, for
a standard CMOS logic process, a shallow epi layer mitigates CMOS
latch-up. For an imager, this layer can lower response in the red portion of the
spectrum. Similarly, shallow, heavily doped junctions enable dense,
short-gate-length devices—a desirable feature in CMOS logic but the cause of low
green response and high dark currents in CMOS imagers. Understanding and
accounting for these and numerous other process trade-offs has enables us to
create CMOS devices that deliver the leading imaging performance.
As the figure at
left shows the current sensor market divides itself into two areas: the
high-performance, low-volume branch, and the low-cost, high-volume branch. In
the high-performance branch are applications that will continue to be dominated
by CCD technology, but CMOS technology will find market share too, especially
for lower cost or more portable versions of these products. The second area is
where most of the CMOS activity will be. Here, in many applications CCD sensors
will be replaced with CMOS sensors. These could include some security
applications,
biometrics and
most consumer digital cameras.
Most of the
growth, though, will likely come from products that can employ imaging
technology—automotive, computer video, optical mice, imaging phones, toys, bar
code readers and a host of hybrid products that can now include imaging. These
kinds of products will require millions of CMOS sensors.
So at this moment
in time ( 29-01-2005 ) I would go for a CCD chip camera
Now this is what I used first but. . . . . . .
Fuji camera the photos are
taken down the eye piece
Jupiter & Venus taken on
December 10th at 7am 2002
Thursday night (5th- 12- 2002) cant believe it its come cloudy again after
the clear sky all day so went and amused my self on the computer and N.A.S.A
HAVE FOUND LIFE on mars but we where not to be told. But through a friend he
sneaked out a photo to show me now you must not breath a word or cause panic and
you must not tell a soul or else. But do click here and if you are easily scared
then do not look over 18's only. Life on mars
But now in 2005 some 2 years on I find that most
people are using the:
Philips
ToUcam Pro II which has the CCD chip
unscrew the lens and fit the
adaptor
Now this is on sale in most Astro
shops for around £80. . . rip off or what so I bought mine from PC World for £48
a saving of some £30 come on you guys stop ripping us off. Also on the right is
the adaptor that I bought from Green Witch for £24
this will then fit into the 2x Barlow lens and all I
want now is some clear sky's as its cloudy here in Leicester at the moment
(29/1/2005) so I'm on the P.C doing this article. But as soon as its clear I
will be out there
Free Image
Processing Software
Astro software for a stacker
program try this
http://registax.astronomy.net/ I will let you know how good / bad as
soon as I play with it