Comparing two potential meteor cameras – the
Mintron and the Watec 120N
European Space Agency, ESA/ESTEC, SCI-SB, Keplerlaan 1, 2201 ZH
presented at the International Meteor Conference 2003, Bollmansruh,
Germany, Sep 2003 - updated 06 Jan 2004 with respect to the version that
will be in the IMC proceedings!
Recent developments coming from the security and surveillance sector resulted
in new video cameras with very sensitive detectors, which make meteor observations
without image intensifiers possible. This paper compares the image quality
of two of these cameras, the Mintron and the Watec 120N.
Around the year 2001, the Mintron camera was announced with a very light-sensitive
detector from Sony, the Sony ICX249AL (cameras with CCIR format. The EIA
format uses the ICX248AL - datasheets see (3)). It is based on a technology
called “HAD”, Hole-Accumulation Diode (1). This detector has an enhanced
sensitivity to infrared photons. Also, micro lenses in front of the individual
pixels increase the collection area of one pixel. Both of these features
allowed amateur astronomers to use video cameras to image meteors or other
faint objects more easily. In the meantime, another camera, the Watec 120N,
was produced, also using this sensor. It has a more compact housing and
different electronics. In the following, the two cameras are compared and
their advantages and disadvantages listed.
Figure 1 shows a photograph of the two cameras. The Mintron used is a Mintron
12V1C-EX CCIR. It is about 5 cm x 5 cm x 10 cm in size and contains the
complete electronics. On the back of the camera, there are four arrow buttons
and one central button, which allow selecting different camera settings.
To discuss all the different options is out of scope; here we’ll concentrate
on the delivered image quality. Just to summarize: The Mintron allows a
maximum integration of 128 frames, i.e. about 5 seconds. The Watec allows
up to 256 frames. While the Mintron allows setting the exposure time, the
Watec has a fixed exposure time of 1/50 s. With its high sensitivity, this
exposure time is too much for e.g. imaging Mars with a 6” Refractor,
the image is hopelessly overexposed. The Mintron would give the possibility
to reduce the exposure time. The Mintron has a menu which can be switched
on to be visible on the display. The user buttons are at the back end of
the camera. If the camera is attached to a telescope, pressing the buttons
will introduce vibrations to the telescope. The Watec has a separate control
box, connected to the camera via a cable of 3 m length. This has the advantage
that the settings can be changed in the comforting surroundings of a heated
room, where the video monitor is also located.
Figure 1: The Mintron (front) and Watec WAT120N (back) cameras with
the used objective lens. Figure 2 shows a front view of the cameras without
a lens. The Sony CCD detector (1/2”) is clearly visible.
Figure 2: Mintron and WAT120N seen from the front without objective
The cameras were tested on 14 Sep 2003 in the author’s backyard, in the
fairly light-polluted Noordwijkerhout in the Netherlands, half way between
Amsterdam and Rotterdam. The visible limiting magnitude during the night
of the test was around 5 mag. Both cameras were tested with the same lens,
a 25 mm f/0.85 Fujinon C-mount lens. This lens has the disadvantage of
a large final lens, it is thus difficult to mount in a standard CS thread.
However, both the Mintron and the Watec have adapter rings that could be
unscrewed far enough to allow a correct mounting of the lens. With regular
C-mount or CS-mount lenses, no problems should occur. As a target for the
camera test, the constellation Lyra was chosen. It contains stars of all
magnitudes and conveniently fits the camera field of view with this lens.
As an additional benefit, it is aesthetically pleasing. The outside temperature
was about 10 °C. Both cameras were mounted on a Manfrotto tripod (see
Figure 3). They were focused on objects at the horizon before being pointed
to Lyra. The cameras were connected to a 300 MHz Pentium PC with a Matrox
Meteor II frame grabber card. The software ‘grab’ by Sirko Molau was used
to digitize and store images on the hard disk of the PC.
Figure 3: The Mintron camera mounted on the tripod.
Figure 4 shows an image of the constellation Lyra taken with the Mintron
camera. Figure 5 shows and image of Lyra taken with the Watec. Note that
due to the way the cameras were mounted the images are rotated by 90°.
Both images are displayed showing pixel values between 140 and 255 only,
the background thus is black. To display the images more clearly, they
are shown inverted, i.e. the white stars are displayed as black,
the black background is shown as white. The faintest stars that can be
seen in the Mintron image are about 9.5 mag, with the Watec around 9.7
Figure 4: Lyra record at 128x with the Mintron.
Figure 5: Lyra recorded at 128x with the Watec.
It can be seen that the Watec seems to have a slightly higher sensitivity.
Both images are displayed using the same scaling. The Mintron image shows
a white background. The Watec image shows a grey background and also more
stars. The reason for this is unclear. The main difference between the
two cameras is the way the stars are displayed. An enlargement of some
typical stars shows this, see Figure 6. This image shows very ‘blocky’
stars for the Mintron (left). In the Mintron image, always two lines show
about the same pixel values. This is an indication that the Mintron only
works with fields,
i.e. every second of the interlaced images. Assuming
that only the even line numbers are sampled, the odd line numbers are interpolated
or copied. In addition, the Mintron image shows dark circles around the
stars. This can be a result of a sharpening algorithm or a bad electronic
design, resulting in a filtering effect. This effect is weakly visible
also in the Watec image (mainly on the top and the bottom of the star –
note that the image was rotated by 90° to allow a better comparison
between the two images) but much less pronounced. Both of these properties
clearly make the Watec the better choice of camera if, for example, photometry
is the aim of the video capture. If, however, planetary imaging is the
goal of the user, the Watec may have the problem that the exposure time
cannot be reduced.
Figure 6: Enlargement of typical star, Mintron image (left) and Watec
The Mintron is also known to show a very uneven background, see Figure
7. The left part of the image shows much more background signal than the
rest of the image. There are two possible reasons for this. Either the
detector is warmer on that side, or we see the photoluminescence effect
of the readout amplifiers. This reduces the aesthetic value of the images,
but also makes photometric measurements much more difficult if not impossible.
Figure 7: Stretched image obtained with the Mintron. The high brightness
on the left side is a result of the inhomogeneous dark current.
The Watec clearly shows better image quality than the Mintron camera. Especially
for photometric measurements, the Mintron is more or less unusable due
to the dark rings around the stars and the uneven background. The Watec’s
separate control box makes adjusting the camera settings easier. For those
that want a Mintron camera with a separate control box, Astrovid offers
a modified Mintron camera called StellaCam-EX with a detached control box.
The available camera settings are similar, but the Mintron has more possibilities
than the Watec. In summary, for scientific applications the Watec is the
clear winner even though it is a little bit less flexible due to the fixed
exposure time setting. It is, however, about 30 % more expensive.
- Information about the Sony CCDs (note: The ICX249 is not on that list
(2) Data sheets of the ICX248AL (page 1,
2) and the ICX249AL (to come)
(3) Specification of the Watec 120N,
downloaded from their web site 06 Jan 2004