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Photographing the Leonids

Hello John, Brad, Gaetan, Walter, Matteo, Stu and List!

Here is a primer of meteor photography culled from Sky & Telescope.
Although it is about the 1993 Perseids, Dennis di Cicco's words are of
general and fundamental importance to meteor photography.

Dennis di Cicco (1993) Observer's Page: Photographing the Perseids (Sky
& Telescope, August 1993, pp. 97-98):

The prospect of a Perseid meteor storm on the night of August 11-12 is
sure to excite anyone who has ever pointed a camera at the night sky.
Just imagine catching dozens of meteors raining down from the heavens on
a single photograph. I suspect that even casual skywatchers will want to
have a camera on hand during this year's Perseid display (see page 43).
Meteor storm or not, the circumstances surrounding the 1993 Perseid
shower have prompted novice and experienced photographers alike to
ponder the best way to capture a meteor on film. Information on the
subject is difficult to find. With few exceptions all popular
astrophotography books, old and new, treat meteor photography with a few
cursory paragraphs. They almost always offer the same advice - use fast
film and a fast, wide-angle lens; open the shutter for as long as sky
conditions permit; and hope that a bright meteor crosses the field of
view. Some authors recommend aiming the camera about 45° from a meteor
shower's radiant to better the chances, but I've seen spectacular
photographs made with cameras aimed in every conceivable direction,
including directly at the radiant point itself. While the advice in the
books is reasonably good, let's consider for ourselves what it takes to
capture a meteor on film.
By and large, meteor photography consists of recording a moving point of
light. It is the same situation as recording stars with a brief
exposure. This type of astrophotography involves concepts quite contrary
to those that apply to scenic low-light work, where a faster focal ratio
(lower f/number) always means a brighter image on the film. While it is
true that for a given lens a faster f/ratio means a brighter star image,
it is also true that an f/5.6 lens of 200-millimeter focal length gives
the same star brightness as does a 50-mm f/1.4 lens. The reason is that
the brightness of a point source is governed primarily by the effective
light-collecting aerea of the lens and not its f/ratio. The table on
page 98 gives the diameters and areas of popular lenses for 35-mm
cameras.
Star brightness is measured in magnitudes, where each magnitude step is
a change in intensity of 2.51 times. Thus, a lens with 2 1/2, times the
collecting are of another (regardless of their f/ratios) will record
stars one magnitude fainter as long as the exposure time and film remain
the same. Keep in mind that this statement holds only where sky fog is
not a Iimiting factor. (Sky fog is a function of f/ratio.) A good rule
of thumb that takes into account the reciprocity failure of modern
emulsions is that each decrease of one f/stop allows exposures about
three times longer. (There is an excellent discussion of limiting
photographic magnitude under various conditions in Patrick Martinez's
Astrophotography II, published by Willmann-Bell and available from Sky
Publishing Corp.)
>From the table we see that lenses.of longer focal length generally have
larger apertures (when used wide open) and therefore produce brighter
meteor images on the film. But there's a catch. In addition to covering
smaller areas of the sky (reducing the chance of capturing a meteor), a
longer focal length means a greater image scale. The greater the image
scale, the faster a meteor's image crosses the film. The faster a meteor
moves, the less time it has to expose a given spot on the film and the
effective total exposure is reduced.
It is a delicate balancing act but one with interesting implications.
Let's compare two lenses in the table. A 50-mm f/1.4 lens has 17 times
the aperture area of a 24-mm f/2.8 lens, which means meteors will appear
17 times brighter on the film. But meteors will also streak across the
film about twice as fast with a 50-mm lens (an image scale of 1°.15 per
millimeter versus 2°.39). The 50-mm lens thus has a net gain in total
exposure of about eight times, which means it can record meteors that
are some 2 1/4 magnitudes fainter than the 24-mm lens can.
This is significant, since even conservative estimates indicate that
there are about 2 1/2 times more rneteors within each fainter magnitude
step (some estimates place the value as high as 4 times more). Thus, a
gain of 2 magnitudes means at least a 10-fold increase in the cumulative
number of meteors within reach of a camera. The increase is somewhat
offset by the fact that the 50-mm lens covers only one-quarter the sky
area of the 24-mm, but this means we still have a net increase of about
three times more meteors with the 50-mm lens.
During the last two decades I've seen more meteor photographs submitted
to Sky & Telescope taken with 50-mm lenses than with any other focal
length. Whether this supports the arguments above or reflects the fact
that more camera owners use a "standard" 50-mm lens is debatable,
however.
Furthermore, it is difficult to reach a general conclusion about what
lens is best to use. As a case in point, consider two lenses from my own
collection that I frequently use for astrophotography. One is a 16-mm
f/2.8 fisheye fitted to a special filmholder providing a full 180° field
of view. (Pointed at the zenith, it records the entire vault of sky with
a single exposure.) The other lens is a 1970-vintage Minolta 58-mm
f/1.2. It has a whopping 71 times the effective collecting area of the
fisheye. Although meteors move more than 3 1/2 times faster across the
film with the 58-mm lens, the net gain is still about 20 times more
exposure. This means the 58-mm lens will record meteors 3 1/4 magnitudes
fainter and thus puts at least 20 times more meteors within range of the
camera.
As impressive is it seems, this gain is almost completely offset by the
fact that the fisheye records the entire sky, whereas the 58-mm lens
records less than 1/20 of it. Also, a picture with several faint meteors
will likely pale in comparison to one with a bright fireball, and every
fireball is sure to be within the field of the fisheye!
While the advice in books about using wide-angle lenses for meteor
photography is good, it is no reason to shun the venerable 50-mm lenses
that virtually every photographer already owns.

Lens               Effect.Apert.     Image Scale   Relative
Focal   f/ratio Diam.  Area        °/mm*         Exposure**
Length             (mm)  (mm^2)
(mm)
16       2.8         5.7    25.6          3.58                 8
24       2.8         8.6    57.7          2.39                12
35       2.8        12.5   123           1.64                18
50       1.4        35.7   1,002        1.15                100
135     2.8        48.2   1,826        0.42                 67

* Image scale is calculated for the center of the field. Many wide-angle
and fisheye lenses have  somewhat different image scales at the edge of
the frame compared to the center.
** Relative exposure for meteor photography is based on the image's
apparent brightness and rate  of motion across the film, as explained in
the text. Larger values can record fainter meteors.

The photo on page 97 gives awonderful impression of what to expect.
Here's the accompanying text describing the myriads of meteors streaking
across the film!!

Leonid meteors literally rained from the sky on the morning of November
17, 1966, when the shower briefly turned into a major storm. Many hope
that this year's Perseids will produce a similar display. A. Scott
Murrell of New Mexico State University Observatory recorded this scene
with a 50-mm f/1.9 Miranda lens and Kodak Tri-X (ISO 400) film. He made
the 10-to-12-minute exposure with the camera attached to a telescope
that tracked the sky's motion.

Photo on page 98:

On the morning of August 13, 1988, Colorado amateur Michael Shin
captured this stunning Perseid of magnitude -8. He used a 50-mm f/2.8
lens and Kodak VR 1000 film for the 7-minute exposure that ceased
immediately after the meteor appeared.


Best wishes and enjoy (photographing) the Leonids,

Bernd

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