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(meteorobs) purchasing image intensifiers and more
Hello folks,
please calm down a little! Rome wasn't built in one day, and an image
intensifier alone makes no working video system... ;-)
> Seems like meteor (and other) video observers the world over would have
> sufficient numbers to justify a bulk purchase from some supplier of these? This
> would definitely reduce the entry cost for this type of observing! Maybe we
> could coordinate a purchase through the IMO or informally over the net?
Lew, you got the point. It is both easier and cheaper, if a number of
interested persons purchase image intensifiers together and distribute
them afterwards. We have started already such an initiative, as I will
explain you later. However, I first want to make you aware what problems
have to be solved beside that, before you can start video observations of
meteors. The problem is described in detail in several papers (WGN, IMC
Proceedings), so I'll summarize it here only.
The most important part is an image intensifier, of course. Even high end
video cameras do not have sufficient power to achieve a good limiting
magnitude as needed for meteor observation. There are three generation of
image intensifiers.
* first generation intensifiers have the highest gain from all of
them (>50.000), if they incorporate three levels of amplification.
Intensifiers with only one level of amplification cannot be used.
First generation devices have strong noise, a strong distortion of the
field of view and a variable sensitivity within the field of view. So
they normally are second best choice only.
* second generation intensifiers (MCPs) do not reach as much gain
as first generation devices (often <10.000), but they are the best choice,
anyway. They have much less noise, image distortion and an almost
constant sensitivity within the field of view. For automatic meteor
detection as intended in the future, MCPs are absolutely essential.
* third generation intensifiers are comparable to the second generation,
but reach their maximum of sensitivity in the infrared. So, they are of
little use for us.
Image intensifiers in general should have as large a photo cathode and
phosphourus screen as possible. A diameter of 20 mm or more would be
fine, but also 17 mm (those are currently available) are fine. Some image
intensifiers have a curved output screen. That's no problem if your video
module has a projection lens with a sufficient focal length.
New image intensifiers are too expensive for our wallets. So we have to
stick to second hand offers from the army. Some suppliers distiguish
between 'professional grade', 'astronomy grade' and even lower quality.
Astronomy grade *might* be enough, but that strongly depends on the
actual intensifier you get. We once bought seven intensifiers, and 1 of them
could not be used for our purpouses.
Lower quality means, that there are big dark spots in the field of
view. You may also have bright spots (like hot CCD pixels) and maybe
also parts of a crosshair are burnt into the image intensifier. This
gives you an idea, what for the intensifier was used before you got it. :-)
Our current best offer is $320 for a good image intensifier, but I'll
come to that later.
The easiest part to obtain is the lens. You do not need as fast lenses as
we apply (f/0.75), but standard photo lenses with f/1.2 and a focal
length of 50 mm will work as fine. You should contact the next second hand
photo shop for it, it will cost you less than $100 or so.
If you really insist in ultra-fast lenses, you should try to find a guy
from Holland, who may get them. So far we know about an Dutch and a
German enterprise, which produced such lenses many years ago. However,
pay attention that the optic designed and is corrected for infinity!
If you apply lenses with longer focal lengths, the field of view will go
down (<20 deg), but the limiting magnitude becomes better. On the other
hand, you get a larger field of view with shorter focal lengths, but the
limiting magnitude decreases significantly. So you either need really
fast wide angle lenses, or a first generation intensifier with higher
gain, if you want to do wide angle observations.
The third component is the video camera. To purchase such a camera is no
problem, you may obtain it from electronics dealers. The sensitivity
of such a video camera is not important, since they are always sensitive
enough to record the background illumination of the intensifiers. Thus,
the power of a video system is *not* limited by the video camera.
A cheap camera with automatic gain control will probably cost a little
more than $100. If you want to apply a better camera with manual gain and
black level control, you need to pay about $300 with the projection lens.
It was asked, in how far American cameras differ from European
ones. Due to the different video standard in the States (NTSC with 30
frames/s in contrary to Europe's PAL with 25 frames/s) your records will
be different from ours. However, that only implies that we cannot
simply exchange video tapes. All the other things (automatic tape
inspection, computer based analysis) will be possible equally with both
standards.
At that stage, your camera works in theory. However, in practice some more
things have to be done. Everything has to be mounted together, to be
fixed on a telescope mounting or a similar device. You will need a lens
heat as well. Here we used the 'famous' Kendrick dew remover (see the ad
in S&T) and made good experiences. Another problem is recording the time.
We found the ultimate solution when we contacted observers of lunar
occultations. They have developed an electronic time inserter: You feed
the video signal and the time signal into the inserter, and out comes a
modified video signal with an extremely accurate clock superimposed to the
bottom of the image. This inserter supports only the German DCF-77 time
signal so far, so it can only be used in Central Europe. However, the
constructer currently thinks about supporting the American WWV time
signal as well, to offer it also to American observers.
Currently, the price for such an inserter together with the time signal
receiver is about $200.
Last but not least, you need a video recorder which records everything,
which is normally available at home. Also a portable TV set is needed.
At this stage, you can operate your video system, but the only thing you
obtain is a huge pile of video tapes. So we now come to the more critical
part of the story...
As described in the last WGN issue, I've succeded with a prototype
software for automatic meteor detection. You can, of course, inspect your
video tapes manually (that's the way we are currently doing it), but this
is not effective for regular recordings. I intend to make a proper
inspection software of the current prototype and support newer PCI frame
grabber cards, but that will be done not before next year.
Second, you need to analyse the meteors you have found. For that, Marc de
Lignie has expanded his AstroRecord software, so a solution is already
available. AstroRecord reads Windows AVI files, thus, you need a PC
with a frame grabber and a control software which supports that format.
I guess, that's it!
I hope that I have not completely discouraged you to try to start video
observation. However, you should think in advance, how much time and
money you want to spend for your system. It is nice to obtain beautiful
records of meteors, but that is not be the main goal for video observers.
It is easy to forget 'boring' visual observation when dealing with video
observation, but you know how important naked observations are in the moment.
Video observation will play a more important role in IMO in the future.
Observing aims will be fixed and organisational structures will be
created soon. I am happy about every new meteor observer who wants to
contribute to this certainly thrilling new development in meteor
astronomy, but I also want to warn you. We needed about three years from
the first light of our camera until we obtained the first reliable
scientific results. Do not expect, that you buy yourself an image
intensifier and that's it. Due to the large amount of data, a central
analysis of video tapes is absolutely impossible. You need to inspect
and analyse the tapes yourself, and this is the real challenge.
Is there still anybody willing to do video observations? :-)))
Right, it is also much fun, of course... Watching meteors in slow motion,
replaying events that were especially nice and impressing friends with
your video tapes is always very encouraging. ;-)
As I mentioned above, we have started to collect orders for image
intensifiers. In fact, my friend Mirko Nitschke (nitschke@argos.ipfdddot de),
who already built the German camera series, is doing that. So far, there
are about 5 interested groups here in Europe. I suggest you inform him
about your interest in the image intensifier, and we will keep you
informed privately or via the mailing list.
Currently, the best offer comes from the States. If you look in recent
issues of Sky&Telescope you will find the ad of Stano, who sells 17mm
MCPs at $320 or so. Mirko has ordered one of those devices earlier, and
it works fine. Maybe we will find a better offer (if you have any good
offer, *please*, tell us about it!) in the next weeks, but so far this
looks most promising. If we really decide for Stano, it might be one of you
(Lew?) who actually purchase the devices and distiubutes them among
the observers, but that can be decided later.
So long for today, I guess it's time to go back to work. :-)
Cheers, Sirko
**************************************************************************
* Sirko Molau * __ *
* Str.246 Nr.16 * " 2B v 2B " *
* D-13086 Berlin * *
* smo@informatik.tu-chemnitzdot de * Shakespeare *
* http://www.tu-chemnitzdot de/~smo * *
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