Download Low cost CCD cameras for amateur astronomy

Low cost CCD cameras for amateur astronomy
G. Koralewskia, Lech Mankiewiczb, Przemyslaw Szamockic, Grzegorz Wrochnad
Independent Society of Amateur Astronomers, Szczecin; bCenter for Theoretical Physics,
Warsaw; cInstitute of Electronic Systems, Warsaw University of Technology; dSoltan Institute for
Nuclear Studies, Warsaw
a
ABSTRACT
Rapid progress in scientific research enlarges the gap between amateurs and professional scientists. Modern astronomy
is based on technologically advanced CCD cameras and large, computer driven telescopes. An investment of about
$10000 is needed for an amateur to join the club of digital observers. In this paper we describe an attempt to break this
barrier by developing entry-level systems in the range of $200-$2000.
Keywords: CCD, astronomy, robotic telescope
1. INTRODUCTION
Scientific research is dominated nowadays by large projects with 9 digits budgets, involving thousands of people for
one or two decades. This is valid for both micro- and macroscopic frontiers. In particle physics the most obvious
example is the Large Hadron Collider being build at CERN in Geneva. In astronomy the best known are the Hubble
Space Telescope and the KECK (explain abbreviation) telescope on Hawaii. There is less and less space at the science
front line for tabletop experiments.
This situation has serious educational consequences. It becomes impossible to demonstrate or even to simulate in a
classroom important scientific experiments and observations. It is often difficult even to explain the matter of Nobel
Prize winning achievements. This is one of the reasons why many people take seriously radiesthetic precautions and
why it is easier to find a book on astrology rather than on astronomy in an average bookshop.
In recent years one can observe that highly talented people more often choose non-scientific fields of activity. In the
past one of the most effective way to attract young people to science was to give them a chance to contribute to serious
research. Nowadays, rapid progress in technology used by leading scientific projects enlarges the gap between
professional and amateur scientists. In the past it was possible for a dedicated amateur to polish even half a meter
mirror. Today, even for a quite reach amateur, it is certainly not possible to build an adaptive optics device or to send
homemade apparatus to space. Are we approaching the time, when young enthusiasts of the science can only read about
scientific achievements in newspapers and jealously look at HST pictures?
Hopefully the answer is no. The two problem-causing facts, concentration of resources in large scientific project and
rapid technological progress, can be taken as an advantage for scientific education. The project described in this paper is
an attempt to realize this idea in practice.
2. CCD SYSTEMS WITH SMALL TELESCOPES
2.1 Need for small telescopes
Today’s astronomy front line is dominated by large telescopes. They are designed to study very distant (billions of light
years) objects. Therefore, they are often equipped with about 10m diameter mirrors with very long focal length. As a
consequence, they have very small field of view. Typical value is 30×30 arcsec2, which is only 10-12 of the full solid
angle.
Since most of professional astronomers rely on those devices, only a small fraction of the sky is being observed at a
given time. One can suspect that many interesting phenomena miss our attention and they are not recorded. The most
striking example is optical counterparts of gamma ray bursts (GRB) discussed in the previous article of this issue [1].
In recent years several small robotic telescopes have been developed [2] to monitor large portion of the sky. Contrary to
large telescopes they usually have short focal lengths (100mm-1000mm) and relatively wide angle of view (e.g. a
squared degree). With a typical aperture of 10 cm diameter even the name “telescopes” sounds a bit exaggerated as their
optics often consist of standard telephoto lenses.
Can those toy-like devices compete with 10 m giants? It turned out that they indeed stand for a valuable complement.
Two of the most important discoveries of recent year have been done just with 10cm telescopes. ROTSE has observed
the first (and as of today the only) optical flash associated with a gamma ray burst [3]. STARE has discovered
extrasolar planets by measuring small decrease in light intensity from an eclipsed star [4]. Yet another interesting
example is All Sky Automated Survey (ASAS) [5]. It discovered over 3000 variable stars – a record which deserves a
note in the Guinness Book.
2.2 Chance for amateurs
Although devices of this kind have much larger field of view than typical telescopes, they still cover simultaneously
only about 1% of the sky. This gives a unique opportunity for amateurs to participate in the scientific effort to cover the
full sky as deeply as possible. Even naked eye observations of variable stars are useful and for many years they have
been collected by AAVSO [6] for use in professional studies. Technological development, however, gives to amateurs
much better possibilities. Typical cost of a middle range CCD camera is several thousand dollars, which makes it
affordable for a number of interested people. Some of them participate in organized research groups like TASS [7] and
make an important contribution to scientific studies.
In this paper we address a question of widening the circle of people involved in such activities by lowering further the
cost of the system. We discuss two possible solutions. The first one, let us call it “for advanced amateurs”, with a cost
below $2000 is just briefly mentioned. The second one, “for beginners”, for about $200, is described in detail.
2.3 Systems for advanced amateurs
The cheapest commercial CCD cameras for astronomy are available for about $1000. There exist also several attempts
to design a camera, which can be constructed by amateurs themselves. The CCD Camera Cookbook [8] describes a
device based on TC211 and TC245 CCD’s by Texas Instruments. A French project Audine [9] developed a camera with
Kodak KAF-0400 or KAF-1600 CCD. It has a continuation in the US under the name Genesis CCD [10], with some
modifications for new Kodak sensors.
It turns out that quite often more expensive than a camera is a motorized mount. Usually it is a heavy-duty device
designed for 1000-2000mm focal length telescopes. The most popular commercial lightweight motorized mount is
Meade ETX series. There are many amateur constructions [11], but they often use unconventional parts and therefore
they are difficult to copy.
An interesting example is described in Ref. [12]. The Princeton Variability Survey (PVS) system is based on Meade
LX200 telescope of 12” aperture. SBIG ST-8E camera was used with Kodak KAF-1602E CCD of 1530×1020 pixels.
The device was programmed to take periodically pictures of several fields. About 3000 stars were monitored during one
month. As a result of automated data processing a new variable star was discovered.
Critical element of such kind of systems is software, the fact that is often not enough appreciated. The PVS system
mentioned above uses widely available commercial software and relatively moderate effort was needed to make all the
pieces running together. However, the total cost of the software licenses exceeded $1000 and therefore it was a
substantial part of the overall cost of the system.
Fortunately, there is some free software available and there are several open source projects, which could provide at
least partial solution. However, usually they are not “plug and play” type applications and some effort is needed to
customize them to particular needs.
Altogether, any solution for a robotic telescope based on “standard” commercial devices and software must cost several
thousand dollars, which clearly is a barrier, at least for some amateurs.
2.4 Systems for beginners
Due to progress in microelectronics technology CCD sensors nowadays can be found in many devices of everyday use:
security monitoring systems, digital cameras, webcams, etc. This fact has not escaped attention of astronomy amateurs.
Those who cannot afford cameras designed for astronomy try to use all possible CCD (and even CMOS) devices to
picture the sky and celestial objects. This hobby is especially popular in Europe where hundreds of enthusiasts
exchange ideas in a number of discussion groups and societies, the best known being QCUIAG [13], AstroCam [14]
and COAA [15].
3. WEBCAM AS A CCD CAMERA FOR AMATEUR ASTRONOMY
Webcams are the most widely used among those unconventional CCD devices because of advantages of using a
computer to acquire the data. In the following section we describe techniques developed to overcome their major
drawbacks: short exposition time, large noise and low stability. It turns out that with a moderate effort one can create a
surprisingly capable system, which allows amateurs to conduct interesting observations.
Many kind of webcams equipped with a CCD sensor are available on the market within the price range of $50-$250.
For our project we have chosen Vesta and ToUCam models1 by Philips because of their popularity among amateur
astronomers. They use ICX098AK CCD sensor by Sony of ¼” diagonal with 640×480 visible pixels. Pixel size is
5.6×5.6 µm2 and the total sensitive area is 3.87×2.82 mm2. The CCD is covered with a color filter matrix RGBG.
Information from each pixel is digitized with 10-bit ADC, but this information is not available to the user. Luminance
for each pixel is calculated as a linear combination of signals from neighboring pixels. Finally, the information is sent to
a computer via a USB port in YUV420P format. Each frame contains first the luminance Y for each pixel (8 bits/pixel),
which is then followed by 8-bit U for each 2×2 square and by V with the same resolution.
The longest exposure time available with this camera is 1/5s only 2. In such a case the image quality is determined by
electronics noise, especially because the device is not cooled. On the other hand the dark current with such short
exposures can be neglected. Nevertheless, it is still useful to subtract a dark frame (an image taken with covered lenses)
to correct for different amplifier offsets. In order to reduce the effect of noise on the dark frame itself it is highly
desirable to prepare the reference dark frame by averaging a number of images. It turns out that about one hundred is a
reasonable choice. Median is even better than the arithmetic average because it is less sensitive to large fluctuations
caused e.g. by cosmic rays.
The signal to noise ratio can be effectively improved by integrating a number of frames. Again, one hundred seems to
be a good choice for typical applications. If the camera is attached to a telescope one needs to move the camera during
exposure in order to compensate for the Earth rotation. Otherwise, the stars would leave linear traces, rather than sharp,
small circles. However, it is recommended not to compensate very accurately, so a given star is depicted in different
pixels at different frames. Off-line one can correct for that, shifting the frames before the integration. In this way one
can reduce systematic effects related to differences between pixels. With the 1/5s exposure and typical photo-lenses of
f=50mm focal length one can just use a fixed mount, because the star movement on each frame will be less than one
pixel, while it will move across several pixels for different frames.
1
Models PCVC675K "Vesta", PCVC680K "Vesta Pro", PCVC690K "Vesta Pro Scan", PCVC740K "ToUCam Pro",
PCVC750K "ToUCam Scan" are equipped with CCD, whereas PCVC665K "Vesta Fun", PCVC720K "ToUCam XS",
PCVC730K "ToUCam Fun" contain CMOS.
2
Only up to 1/25s can be set by the user. The 1/5s time can be achieved in the automatic mode.
It was found [16] that the electronic circuit of the camera could be quite easily modified to enable long exposures. Three
pins of the clock generating IC could be detached from the board and attached to wires connected to a parallel port
(LPT) of the computer. Several software applications have been developed to control the exposure time. In this case the
exposure time is limited only by the sky background and by dark current of the camera. In practice, 20s exposures can
be easily achieved.
3.1 Working modes
Philips cameras have lenses mounted with an M12×0.5mm thread. One can use this thread to attach the camera to a
telescope of photo-lenses. Appropriate adaptive rings are commercially available (e.g. from [17]).
Typical amateur telescopes have focal length of 1000-2000mm and aperture of 10-25cm. They are usually used by
amateurs to picture planetary disks, Moon surface and Sun spots (with appropriate filter). Short exposures are required
and one does not need to modify the camera.
Telephoto lenses with f=300-500mm and d=5-10cm are well suited to picture deep sky objects from the Messier
catalog, like star clusters, nearby galaxies and bright nebulas. Long exposure modification of the camera is necessary
for this purpose. Good (possibly motorized) mount is needed to track precisely the object.
The most useful for a “serious” astronomy are, however, typical photo-lenses of f=50-200mm. They offer relatively
large field of view (1º2-10º2), which contains many stars. It is useful for photometry, where reference stars are needed to
achieve high precision. One can also record and track distant planets and asteroids. The great advantage of such short
focal length is that the sky moves very slowly and there is no need for a motorized mount. Simple photographic tripod
can be used.
Original lenses with the focal length and the aperture of only a few millimeters are almost useless for astronomy, except
for recording meteors. The wide field of view of the order of 40º×30º enables monitoring of a large portion of the sky.
10-20s exposures can be taken continuously and form a “full night movie”, which can be analyzed off-line. An
automatic transient recognition programs can also be used to select on-line the interesting frames.
4. ASTRONOMICAL APPLICATIONS
Webcam based systems are used by amateur astronomers mainly to take spectacular pictures of the Moon, planets and
deed sky object. However, the digital nature of CCD images enables also more serious use of these devices. We have
already mentioned star photometry and tracking of planets and asteroids. In this section we elaborate on three kinds of
studies, which can bring results of some scientific importance.
4.1 Variability of stars
Observation of variable stars was always a domain of important amateur contributions to astronomy. Professional
astronomers had never enough time to conduct systematic, long-term observations of many stars. Amateur observations
from around the world are collected [6] and made available for professional studies. Experienced observer could reach
accuracy of 0.1m or even 0.05m. Non-modified webcam with f=50mm lenses provides comparable accuracy for stars of
5m-8m. Existing software makes it available even for a beginner. With a long exposure webcam one can achieve
accuracy better than 0.2m-0.5m for stars up to 9m-10m. An example light curve is shown in Fig.1. The VW Cephei system
consists of two stars rotating around each other at a small distance. Periodical eclipses of one star by the other cause the
visible reduction of the brightness. We have chosen this star as an example because it has the amplitude as small as 0.2m
and therefore it is difficult to detect its variability with visual observations. On the contrary, with the webcam, one can
even resolve fine details like asymmetry of minima and maxima.
Variability study can be conducted in several ways. One can select a few short-period (hours) variable stars and take a
picture of each star every few minutes. In this way, in just a few nights one can collect several full light curves.
Alternatively, one can prepare a list of long-period (months) variable stars and perform one measurement of each star
per night. After several months a large number of light curves could be completed. In both cases the measurements can
be done pointing the camera by hand (e.g. with a simple tripod) or it can be automatized with a computer driven mount.
Yet another possibility of automatic measurement with very simple equipment is to leave the camera for the whole
night and let the stars move in front of the lenses. In this way one can scan a long strip of the sky (4º wide for f=50mm
lenses). The last method is especially suitable for discovery of new variable stars.
Fig. 1. Light curve (intensity vs time) of VW Cephei obtained with webcam Vesta and f=50mm f/2 lenses.
4.2 Search for nova stars
Each year several bright nova stars exploding in our Galaxy are discovered. For example, in 2001 five discovered stars
were brighter than 9m. Hence, they were well within the reach of even non-modified webcam. Two of them were
discovered by dedicated amateur A. Pereira equipped only with a binocular on a tripod. He just has learned by heart
thousands of his own little constellations and every night he was monitoring the Milky Way for a change. Of course this
task could be performed in automatic way by a program comparing the sky to a star catalog. There is a need to develop
user friendly and efficient software of this kind dedicated for amateurs.
4.3 Gamma Ray Bursts (GRB)
Gamma Ray Bursts – the largest cosmic explosions ever seen since the Big Bang – are discussed in a grater detail in the
next article of this issue [1]. The quest for their optical counterparts is one of the most important tasks of nowadays
astronomy. So far only one flash of this kind was recorded [3]. It was as bright as 8.6m, thus again within the reach of
non-modified webcam. It last so bright for only half a minute, which makes an unguided search for such phenomena a
gambling with a rather small chance of success. However, the number of astronomy amateurs in the world is very large
and it could happen that someday one of them will win the lottery. There is nothing to loose, because such kind of
searches can be conducted as a “byproduct” of variability studies and nova searches discussed above.
STATUS OF THE PROJECT AND PLANS FOR THE FUTURE
So far we have tested the performance of selected webcams in both short and long exposure working modes. It was
proven that such a webcam could be a powerful tool for a dedicated amateur. Different operation modes were
investigated with equipment ranging from f=6mm lenses to f=4000mm telescopes with both, static and motorized
mounts. Image acquisition procedures and data analysis algorithms have been developed to overcome high noise and
low stability of the webcams. Several ideas have been prepared for amateurs to “make science” with their own data
(variability study, etc).
All the results are made available at the project WWW site [18] and have been published in several Polish journals [19].
Further articles are under preparation. Sources of all necessary hardware were identified [17, 20]. A CD with
instructions and useful software was prepared and it is commercially available [21]. A few seminars for amateurs have
been given. A course for teachers is planned as well as broadcasts on Academic Internet TV Network. We actively
cooperate with several amateurs and teachers in order to identify potential obstacles for newcomers. The project was
also presented at the “Hands on Universe” conference [22] and contacts with other countries have been established to
exchange ideas on similar projects.
Currently we concentrate our effort on the following items:
•
•
•
Testing commercially available and building a custom, low cost, computer driven mounts
Testing available software and developing some custom applications
Preparing self-guiding tutorials
5. CONCLUSIONS
We have shown, that CCD based webcams can play an important role in bridging the gap between visual amateur sky
observations and professional astronomy. The price below $200 for the simplest system qualifies them as toys, useful to
attract people to amateur astronomy. However, with a help of easily available software they can be used to perform
“serious” study, like photometry of variable stars. Their performance is undoubtedly much below cooled CCD cameras
designed especially for astronomy, but they offer a unique opportunity to learn and practice modern techniques before
investing in more expensive equipment. We propose an easy, step by step upgrade path.
1.
2.
3.
Webcam plus simple photo-lenses on a static mount for $200.
Motorized mount for additional $300-$500.
Cooled CCD camera designed for astronomy for $1500-$2000.
This path is a good illustration of the fact that possibilities of an apparatus go like a logarithm of the cost. This
logarithmic law is a serious problem for the scientific front-line. The project presented in this paper is an attempt to use
it in our advantage on the educational side of the science.
To this end it is worthwhile to mention that the special effort by qualified scientists to adapt the vast professional
knowledge about CCD technology, digital information processing and astronomy to the amateur level and spread it
through a system which is as open as possible has been one of the key elements of the success of the whole project. As
far as the popularization of science is concerned, at present many interesting ideas are being developed in different
countries [22] and one should seriously address the issue how to integrate them into a common platform. Perhaps the
organization of front-line scientific research, which emphasizes the vast and efficient distribution of any new results,
may serve as a model solution here.
ACKNOWLEDGMENT
The authors are very grateful to prof. B. Paczynski for his encouragement and support.
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