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Astronomical Imaging : Tools of the Trade The following
is a (non-exhaustive) list of digital cameras suitable for Astronomical
Imaging. Quoted prices are believed to be correct at the time of writing, but
will not remain correct for long. The information
presented is given in good faith and is believed to be correct, but I cannot
accept any responsibility for its accuracy or otherwise. Competitively
priced, self assembly cooled CCD cameras. All models are based on the same
basic design, with the option to install different CCD chips according to
your requirements and pocket. See http://www.artemisccd.co.uk/Artemis_Cameras.htm for up to date
information.
Note that these
cameras also require a separate power supply at £35 when purchased with kit. I have a beta
version of the ART-429. It works well with very low noise. See ATIK below
for pre-assembled alternatives. The webcam of
choice for planetary and lunar imaging. Philips make
a number of different webcams. Only the top-end devices feature the more
sensitive CCD chips.
You will
usually require an IR/UV blocking filter when using a webcam. You will also
need some means for mounting the webcam on the telescope. Suitable adapters
can be sourced in a number of places including http://www.telescopehouse.co.uk. The Toucam and several other webcams can be modified to do
long exposures. This requires some dexterity and skill with a soldering iron.
See for example http://homepage.ntlworld.com/molyned/web-cameras.htm for an excellent
summary of web cameras and links to further information. Starlight
Xpress Starlight Xpress offer a huge range of professionally built cooled
CCD-based cameras. They are available from most astronomical suppliers. See http://www.starlight-xpress.co.uk.
Starlight Xpress also produce a number of other products to assist
with astronomical imaging including an Adaptive Optics Unit, Star2000
self-guider and a separate guide camera. I have used the
MX5C and MX716 cameras and have found them both to be reliable and good
cameras. Terry Platt who set up Starlight Xpress is
very responsive to user queries. (*) as quoted
on Starlight Xpress site. Size quoted in Sony
datasheets and Artemis site disagree.
DSI PRO model
can be bought with a colour filter set to perform
tri-colour imaging.
ATIK
instruments offer commercialised pre-assembled
versions of some popular webcam modifications and ready-built versions of the
Artemis cameras.
Note the ATK-16
and ATK-16HR are pre-assembled versions of the ART-429 and ART-285 models
from Artemis. The other
models are essentially webcams with forced air-cooling and long-exposure
modifications. Suppliers: www.modernastronomy.com, http://www.iankingimaging.com This section is
not complete. SAC103.3MP
ICX262AQ SAC10 NGC11-429
ICX429ALL SAC10 NGC12-422
ICX422AL SAC10 NGC13-285
ICX285AL Suppliers www.opticstar.co.uk Conventional
Digital cameras. Have the advantage you can use them for your holiday snaps
as well. 300D £500 CMOS
6Mpixel 15.1 x 22.7mm 350D £660 CMOS
8Mpixel Prices with
lens. Canon produce an astronomical variant on these cameras the EOS 20Da,
which has better H-alpha sensitivity and has been tweaked for
astrophotography. Available from
telescope house for £1700 (body only). TODO http://www.wodaski.com/wodaski/pick_a_camera.htm http://www.covingtoninnovations.com/dslr/EOS300Dastro.html Astronomical
imaging has its own jargon words. The following are some that you will
commonly encounter. CMOS –
complimentary metal oxide silicon. A technology used in the manufacture of
silicon chips of all kinds. Light sensors made using the CMOS process tend to
be noisier and less sensitive that CCD devices, but they are also cheaper. CCD – charge
coupled device. A silicon technology that converts photons of light into
electrons which are stored in the chip until read-out. QE – quantum
efficiency. The proportion of photons that hit the sensor that are converted
to electrons. QE often varies with frequency (colour)
of the light. Well depth –
image sensors can only capture a finite number of photons before they
saturate. Once saturated, no more information can be gathered by the pixel.
The end result is that detail in bright areas is lost. The well-depth also
affects how many brightness levels can be represented by each pixel. ADC – analogue
to digital conversion. This is the process whereby the charge stored in the
sensor chip is converted to a digital count. The number of bits of resolution
in the ADC determines how many brightness levels can be captured. An 8-bit
ADC can produce 256 brightness levels. A 16-bit device produces 65536 levels.
For planetary and lunar work, 8-bits is often
sufficient. For deep sky work where very faint objects are being imaged, it
is important to have more bits. Noise – all
sensors suffer from noise of various kinds. In particular, image sensors
suffer from thermal noise and read-out noise. Noise introduces a random
factor into the captured image, which is different at each pixel. When the
signal is small (e.g. a very faint object), then the noise can dominate. Even
with a strong signal, noise can be evidenced by images that are grainy
looking. Thermal noise –
this kind of noise is introduced due to heat in the sensor chip. It can be
reduced by cooling the chip. Read-out noise
– this kind of noise is generated during the transmission of data from the
sensor chip to the computer. A well-designed camera will minimise
read-out noise by careful placement and choice of components and by making
sure that noise is eliminated from the power supplies. Peltier – a semiconductor
device that when a current passes through it, produces a temperature difference
between its sides. If the hot side is cooled (e.g. by attaching it to a heat
sink or the camera case, and perhaps fan assisted air cooling), then the
temperature at the cold side can be as low as 20-30 degrees below ambient. |