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Re: (meteorobs) Hello from Nashville...
Ooops!
I forgot to attached my photography article.
George
PHOTOGRAPHING METEORS
by: George J. Zay
Revised: August 1996
Photographing meteors involves a great deal of luck for the most part. In
turn, the results have the potential for producing the most accurate data.
There are a few things you can actively do to improve your chances at
photographically capturing a meteor. I'm going to try to summarize things to
do, rather than try to explain the reasons.
# PURPOSE
1) Enjoyment - This usually involves nothing more than mounting a camera
onto a tripod, opening it's shutter with a cable release for an extended
period of time and hoping for the best. Data about the meteor seldom goes
beyond noting it's brightness. The intention here is simply to catch a
meteor and hopefully add some composition as well.
2) Scientific - Photographing meteors for scientific purposes also garnishes
a measure of enjoyment...at least the satisfaction of accomplishment. What
can be gained are accurate radiant determinations, meteor durations,
velocities and approximate magnitudes. Also orbits can be figured by the
mathematically gifted for meteors that were photographed by two different
cameras separated by at least 25 to 50 miles.
# EQUIPMENT
What you basically need is a good sturdy tripod, camera with a normal to 28
mm lens, appropriately fast film and a lot of patience.
1) CAMERA - I recommend the standard 35 mm camera. But make sure that the
camera's shutter is not electronically controlled. Time exposures can give
you a dead battery in a couple hours or less. I have two preferances for
lenses. They are a 28mm lens with an f/stop of 2.8 and a 50mm lens with an
f/stop of 1.8 .
a) PROS AND CONS - A 50 mm f/1.8 lens catches meteors of 0 mag or brighter
quite well...when they cross the camera's field of view. That is the
problem. A 50 mm lens field of view is rather small. Quite often you may
only get part of the meteor. On the other hand, a 28 mm lens has a wider
field of view. You increase the odds of a meteor crossing your
camera's field. But, you also made it so that in order for your camera to
capture a meteor, the meteor must be brighter. I find meteors to be
captured with a 28mm lens generally have to be in the -2 magnitude range.
This may vary slightly, dependent upon the meteor's velocity and angle of sky
your camera is aimed. Some folks use a fish eye lens to cover all the sky.
I personally don't use these because of the distortion to the image and the
fact that meteors now have to be even brighter just to register on film.
Instead of capturing -2 magnitude meteors with a 28mm, your fisheye magnitude
threshold is probably along the order of about -4 or -5.
b) WHICH LENS TO USE - I choose the lens I'm going to use based upon the
expected meteor showers Population Index "r". The Population Index are
included in most shower calendars. The r - value indicates how many more
times, meteors of magnitude m+1 appeared, than of magnitude m. If a shower
has an r - value near 3.0, there will be a higher percentage of dim meteors .
If the r - value is near 2.5, the percentage of bright meteors are greater.
The rule of thumb I use is this: If the r - value is 2.5 or less, choose the
28mm wider angle lens. If r is 3.0 or numerically larger, choose the 50mm
normal lens. If the values are somewhere between 2.5 and 3.0, take a wild
guess based upon the r - value/lens relationship I mentioned previously.
2) FILM.
a) Color - Often color is used because that's what the film makers try to
sell and that's what we remember the most. Also, there is potential for
colorful effects from the surroundings of a captured meteor. Most of the
light from a meteor is from the blue to Ultraviolet part of the spectrum. I
would take off any UV filters. If any UV light gets thru the lens, I want it
to expose the film. However, your lens absorbs most UV light anyway. The
visible white light is...white. If you have color film, the meteor image is
still going to be white on the print. If the meteor appeared green, blue or
red, the image will still be white. So...the only color will be from the
stars...and I agree this is appealing in itself. But if you are trying to
capture the color of a meteor, remember all meteors will be white on print.
b) BLACK AND WHITE - I use Black and White film exclusively. And I prefer
HP-5. But T-max 400 and T-max 3200 are good also. On moonless
nights, I use HP-5 developed in T-max developer for the equivalent of 3200 ASA. It's fast, and the grain is not as bad. I wouldn't make very large prints for esthetics, but
5X7's come out quite pleasing. I haven't tried making 8"X10"'s with this film
simply because I haven't a need. I still develop for ASA equivalent of 3200
when there is a quarter moon. I just have to make sure I don't aim the camera in a
direction where moonlight will harm the exposure.
Sometimes I'm forced to observe a major meteor shower during a full moon. I
consider this period to be 5 days before and after the actual full moon.
I still use HP-5, but develop for ASA 400. This emulsion rate is also fast and the moonlight can easily effect it. You still have to manage your camera's direction and
exposure times. I will discuss both of these later.
For scientific purposes, one of the things that can be determined is the
meteors magnitude. But this can only be done with Black and White films.
3) TRIPOD AND CABLE RELEASE.
It can't be over emphasized the importance of a sturdy tripod and a good
locking cable release. Most people have relatively flimsy tripods. Part of
their tripod problem is that they are so light that they don't really rest
on the ground firmly. There is a little "spring" to it as it rests. One
possible remedy is to suspend a heavy weight under the tripod's head. You
might want to prepare for this eventuality before attempting meteor
photography? I have a sturdy tripod, but sometimes I'm affected by strong
mountain winds. I drilled a few holes that allowed me to attach a short
piece of nylon cord. With a loop on the end, I'm able to suspend a 1 gallon
bucket full of dirt. It's quite stable. All you might need to suspend, is a
well chosen rock.
As to cable releases...just like the motto for some plastic cards..."Don't
Leave Home Without Them!" Just make sure that they can be locked after the
shutter is tripped.
# METHOD
1) UNGUIDED - Most people simply slap a camera onto a tripod, set camera to
the "B" setting and lock the cable release "until the cows come home".
The camera equipment stays motionless, but the sky moves. This is unguided.
For Science purposes, there is a way to determine Right Ascension and
Declination points for both the Beginning and End of the meteor's trajectory.
But times are very important here. You must know to within 3 seconds the
beginning and end of the exposure and again within 3 seconds of the meteors
appearance. Also you need to know where the camera is pointed and be able to
identify at least 6 stars. Here it's best to frame a couple easy to find
stars, such as the one's in Orion, or the Pleiades. From there,
the other stars won't be too difficult to determine. With this method, make
exposures 15 minutes or less.
2) GUIDED - Guided is a hassle while making the photo, but when you get
something, it's a lot easier to make "heads or tails" from it. You still
have to write shutter start/stop and meteor appearance times, but you can be
within 30 seconds of accuracy. It is still prudent to be as accurate as
possible. You can "Piggy back" a camera onto a properly aligned telescope or
use a device like the "Vista Star Stepper". You can attach this to a sturdy
tripod and make your polar alignments from there. I find the "Vista Star
Stepper" an ideal guided platform for a 35mm. It's battery powered and
portable. I use a 28mm lens that permits me to make at least 25 minute
exposures and on good alignment nights, I can get 1 hour before stars begin
to show any trailing. Here if you get a meteor, you can look at the print
later to find it's Begin and End points. Combining data from multiple
images, accurate radiant positions and diameters can be determined.
3) PRINTING - If you capture a meteor, be sure to make your print at least
5"X7" in size. Also, include all edges of the negative in your print. You
should make the print lighter than you normally would. It won't look pretty.
Remember, this is for scientific measurements. Esthetics is not considered.
On the back of your print, write the following information:
# Your Name
# Day, Month, and Year (Use Double Date)
# Time of Meteor Appearance. Use Universal Time or Local with the Time Zone
shown.
# Apparent Magnitude
# Apparent Velocity (Very Slow, Slow, Medium, Fast, Very Fast)
# Location: Site Latitude, Longitude and elevation and with verbal location
(ex. Sequoia National Park; Yuma, Arizona etc.)
# Camera Lens: (ex. 28mm lens, f/2.8)
# Film: (ex. HP-5 pushed developed as 3200 in T-max developer for 11 1/2minutes at 68 deg F).
# Estimated Print Center: (ex. RA: 8h 30m, Dec: -8 deg)
# Identify any particular brite stars by tapeing a thin sheet of paper over
print so labeling can be done. This is not necessary for guided images.
You can send this print with all the above info to:
International Meteor Organization
c/o Jurgen Rendtel
President
Gontardstrabe 11
D-14471
Potsdam,
Germany
He has the proper equipment to make precise measurements for the unguided
images. This photo and information will then be included into IMO's
Photographic Archives for future studies.
4) EXPOSURES - How long should you expose the film? There are several "It
all depends" to consider. First, what is your purpose? If you want to be
creative, you might just expose one frame for the whole night with an
unguided camera? If the camera was aimed at Polaris, you can end up with the
traditional celestial "Bulls Eye" created by the Earth's rotational star
streaking abilities. Besides all the concentric circles, there's a bit of
luck as to what will crisscross the skies. Aircraft mostly, with a few
meteors and an occasional brite satellite to create interesting patterns.
They are like snowflakes...no two will be the same. Vary the exposures and
aiming points slightly and things can change dramatically. To be creative,
there is no concrete rule for exposure durations. I personally find longer
exposures to be more pleasing. For unguided scientific purposes, try to keep
the exposures between 5 to 15 minutes. If there is an exceptional rate of
meteoric activity, keep exposures at 5 minute increments.
For Guided Cameras, long exposures can produce beautiful milky way
compositions. If you get a meteor, it just enhances the image. Of course,
you have to make a judgement first as to how accurate your polar alignment
is. The rule of thumb I use is this: The wider angle the lens, the longer
the exposure can be before the stars start to trail off. With my 28mm, I
can get 25 minute exposures with pinpoint star images. At 30 minutes the
stars just start to trail off. So, in the early evening prior to midnight,
meteor activity is rather low. During this slack time, I let the camera run
for 30 minutes. I gamble the last 5 minutes to stretch out a roll of film.
If I get something during the last 5 minutes, it's still useable. If I
should see a possible bright meteor cross the camera's view prior to 15
minutes exposure, I let it run for a whole 15 minutes to give the negative
some star detail. If a meteor crosses between the 15 and 30 minute period, I
stop the camera as soon as possible and prepare for the next exposure. This
involves re-doing my polar alignment before each exposure. It takes me about
a minute to do this. After midnight, when meteor activity begins to
increase, the only thing I do different is that I shorten my longest
exposures to 25 minutes. If you use a 50 mm lens, you'll
have to make the exposures shorter. I make 20 minutes my maximum exposure
here. I have a 200 mm lens that I make max exposures of 5 minutes. That
should give you a good range of lenses to give you an idea about the
lens/exposure relationship with the "Vista Star Stepper".
The previous discussions pertained to moonless nights. Moonlight is a
condition I have to deal with accordingly. When the moon is smaller than the
quarter moon phase, I still use HP-5 developed as ASA 3200 and aim the camera towards most
parts of the sky...but I give the moon some free space. I don't want the
moon or any nearby moonlight to fog the film. I basically have no problem
here. Brighter than Quarter moon all the way to Full moon, I develop for 400 ASA.... 3200 is just too fast for all that light. I normally don't even
observe 5 days before or after a full moon. But on occasions, a major shower
will peak near a full moon. Miserable as it is, meteor photography is still
possible. Irregardless what major shower is active, a full moon will
obliterate the entire sky except for one little area. Under these
conditions, for both observing and photography, I do this: First of all, the
darkest part of the sky will be in a Northerly direction and up at about a 50
degree angle. At 9pm, direct your attention to the Northwest. At midnight,
due North. And at 3 am, face Northeast. With a 28mm f/2.8 lens and HP-5
400 film, make exposures no longer than 15 minutes, but try to keep near 10
minutes. Longer exposures will produce "Bullet Proof" negatives. With 10
minute exposures, you won't get too many stars for reference points, but you
will get enough to get fairly accurate meteor positions.
When I operate a camera, I have a cassette recorder nearby. I make verbal
entries to the exact second when I start or stop the exposure. I also
mention where the camera is aimed. When I state the Start/Stop times, I make
sure it coincides with the exact minute and zero seconds. It just makes my
record keeping easier. When a meteor appears, I have a stop watch that I
start the moment I see a bright meteor. I can gather all the pertinent data
and then when I'm ready for the time, I stop the stop watch at an exact time
on my wrist watch. I'll enter how much time has passed and enter the
minutes or seconds down with the understanding that I'll subtract this time
later. This usually gives me times within 3 seconds or better.
Before the sun sets, I usually write with a magic marker onto an 8X10 sheet
of paper, pertinent information about that nights photographic efforts.
Such as: date, my name, lens and f/stop used, film type and location. I make
the first exposure by photographing this info. Often with a 36 roll of film,
I can get 2 and sometimes 3 nights worth of photography. This also gives me
a begin and end point for any successive nights.
5) WHERE TO AIM THE CAMERA?
As I said before, photographing meteors involves a lot of luck, but there are
three things you can do to actively increase your odds. One dealt with the
r-value and the other two involves where you center your camera's field of
view. If you aim the camera straight up (the Zenith), you have only a narrow
wedge of sky where a meteor begins to produce light. But the light will be
more intense. Whereas, if you aim the camera between 50 - 70 degrees above
the horizon, your camera will cover a larger portion of sky. If you aim
lower, you will cover even more sky, but the meteor will most likely be
further away and have less light to expose the film. So...the happy medium
here is at a 50 -70 degree angle.
The other thing you can do is to aim your camera in an area that is about 30
- 40 degrees from the radiant itself. This area represents a very compressed
area from which a meteor will be coming from. If you aim at the radiant
itself, it's even more compressed, but most meteors don't become visible
until it's a little ways away from the radiant. And that will be the 30 to
40 degree region. If the camera is aimed further from the radiant, you will
have less shower meteors passing thru because they radiate further away from
a central point.
One other thing I try to do and that is how you align the long axis of your
film plane to the radiant. I figure you can cover more potential meteor
crossing area, if you align the long axis so that a meteor coming from the
center of a
radiant will be perpendicular to the frame. If the long axis is parallel,
you will have less potential meteors.
6) METEOR VELOCITY vs EXPOSING FILM - The faster a meteor travels, the
brighter it must be in order for your film to be exposed. I personally
accept -2 meteors of very fast velocity to be my threshold for a 28mm f/2.8,
3200 ASA equivalency set up. If a meteor was traveling fairly slow, I can pick up
-1's.
6) ROTATING SHUTTERS - These are relatively simple devices that resemble a
fan. As a matter of fact, mine is made from a large fan. I tore it apart
just keeping the motor, wiring and control buttons intact. I removed the fan
blade and cut out an 18 inch diameter circle from a 1/8 inch thick sheet of
door covering. Dividing this circle into fourths, I then cut out 2 quarters
diagonal to each other. Using your imagination, mount the blade onto the fan
motor. In use, the blade will pass about 1 inch in front of the camera lens.
With an open shutter, the light entering the camera lens is opened and closed
many times per second. For my fan motor at it's slowest speed, I have an RPM
of around 1150. I calibrate this with a strobe light at least twice during
the night. The RPM may fluctuate during the night. An RPM of 1150 will
give me 38.33333 open and closings per second with a two bladed shutter.
Determining RPM's with a stroboscope appears simple enough. The flashing lite of the strobe can be adjusted until the blades of your rotating shutter appears motionless. The equivalent RPM's can then be read either digitally or off an analogue dial. However, there are some difficulties in it's use that must be understood. If you adjust the flashing strobe, you will discover that more than one position will "freeze" the same number of blades that the shutter is made up of. To determine which represents the proper RPM, first mark one of the blades clearly. Start the shutter and strobe. As you increase the strobe frequency, you will pass thru a number of harmonics. The highest speed at which you see a single stationary marked blade is the actual rotational speed of the shutter blades.
In use...when a bright meteor passes thru
your camera's field, the end result will be a meteor image chopped up. From
this, it is possible to compute the meteor's duration within ten thousanths of
a second. To do this, divide shutter revolutions/second times two shutter
blades, into the number of meteor breaks. My shutter works well
for very fast meteors, but the chops merge somewhat for the slower meteors.
An ideal RPM for a motor would probably be in the range of 1000. If you
happen to be working as a team with camera's separated by 15 - 50 miles and
accurate times are being recorded, meteor altitudes, velocities, entry
angles, and orbits can be determined. With single stations, only certain
bits of information can be deduced and more if you assume certain shower
characteristics. The hardest part about doing something like this is the
finding of a 2nd team.
# YOUR PHOTO ARCHIVE
It is very important to keep an accurate log for each photographic night. I
include all the times for each frame. I also include where the camera was
aimed and meteor number for any captured meteors that was simultaneously
observed so that I can associate any pertinent info with the photo. I also
indicate what lens and film was used and whether I had a Rotating Shutter in
operation. If you use a guided camera, no telling what other things you may
have unwittingly photographed. You may have photographed a nova or even a
comet that went unnoticed until somebody makes a discovery and old photo's
are re-examined. In short, It is very important to keep an accurate
archive.
# SUCCESS RATE
Most material discussed are items that I've personally learned from my own
experiences. I don't know how many photographic hours I have in trying to
capture meteors, but it should be close to the number of hours I observe
visually. I have over 1700 observing hours, so my photographic hours is at
least 1000. To date I have over 70 meteor images. So, my photographic rate
is near 20 hours per successful meteor capture per camera. I've heard rates in the
neighborhood of 100 hours per meteor capture as average. I feel quiet lucky,
but I also realize that a lot of effort went into this success. If anyone
desires to do photographic meteor work, I wish you luck and must emphasize
that the methodology is simple but laborious. If you would like to discuss
this area with me, I will be happy to exchange thoughts.
e-mail: GeoZay@aol.com