[Prev][Next][Index][Thread]
(meteorobs) Ari-Tri: Beginner's Guide to Telescopic Meteors
Greetings meteor observers,
Lew Gramer suggested that I circulate some brief notes on telescopic
observing in the context of the Ari-Tri campaign. These are probably
much longer than he intended... and rather hurriedly written.
This contribution to meteorobs is long overdue as I regard meteorobs
as a welcome development for the exchange of information and ideas
about meteors and their observation, despite the recent setback in
NAMN.
My future messages in meteorobs will be much shorter, 'onest 'guv'.
Malcolm Currie
mjc@ast.star.rl.acdot uk
Beginner's Guide to Telescopic Meteors (from the Ari-Tri radiant)
=================================================================
What are Telescopic Meteors?
----------------------------
Put simply a telescopic meteor is a meteor too faint to see with the
unaided eye. Another definition is a meteor seen through a telescope
or binocular.
Why Observe Telescopic Meteors?
-------------------------------
It is fun. The thrill of seeing a bright meteor pass through the
field and seeing its train drift and distort is unforgettable. The
opportunity to learn more about meteors is another reason compelling
reason for me. But why telescopic instead of visual? There are
scientific and technical reasons summarized below.
By observing with a telescope, we extend the size range of meteor
particles recorded. This, for instance, can give insights into
evolutionary effects which segregate the particles by mass. It should
also be possible to determine meteor fluxes for the low-mass
particles, and giving a more complete picture of a shower.
The restricted and magnified field of view allows the paths of meteors
to be determined significantly more accurately than visually. This
lets us investigate the properties of meteor radiants, detect minor
showers more easily, and find new showers.
As the meteors are plotted we can always reanalyze the data, possibly
comparing results from different epochs. There is less bias involved;
we use analysis software to assign shower membership, and to search for
radiants.
Wouldn't the observed telescopic rate be low?
---------------------------------------------
The reduced field of view of the telescope compared with the naked eye
will indeed reduce the observed rate of bright meteors. However,
telescopic meteors are exceedingly numerous, and the diminished area
of sky under inspection is offset by bringing these fainter meteors
into view.
Rates will depend on the specification of the telescope aperture
compared with its field of view and magnification, the time of year
and night, observer comfort, and the sky quality. The last factor is
even more important than for visual observing. The typical range is 6
to 12 per hour though some of us have seen over 20 per hour around
Geminid maximum under +7 skies. Initially new observers see few
meteors, but if they have a little patience, the rate increases
steadily with experience and as they become attuned to the higher
angular speeds. Yes those fleeting flashes really are meteors.
Rates are more uniform because at telescopic magnitudes most meteors
are sporadic, even during the peak of the Perseids. Many of the
familiar major showers are similarly deficient in faint meteors. In
compensation there are minor showers than are more prominent; some
even appear to be telescopic-only showers. A `major' telescopic
shower would yield a rate comparable to the contemporary sporadics.
Showers giving twice or more times the sporadic rate are rare.
Using a 5-inch comet seeker at 19.5x the Ari-Tri radiant in 1994 gave
better rates than the Perseids---around 3 per hour at best for +6.5
skies.
Instrumentation
---------------
I noticed that my FAQ from WGN has already appeared in meteorobs, so I
won't repeat that information except to say that a wide-field,
low-power telescope or binocular is needed. Something like a
comet-seeker telescope or 7x50/10x50/11x80 binocular is fine.
The critical factor is the mounting and the location of the eyepiece(s).
You must be able to concentrate for 20 to 30 minutes at a time looking
for meteors without strain. If you have a pain in the neck or back,
cold extremities etc. the observed meteor rate will plummet. I cannot
stress too highly the importance of observer comfort.
In addition to the normal visual equipment you'll need a clip board
for the charts and report sheet.
Where to look
-------------
In the simple case where we want to follow a known shower, we select a
pair of charts, preferably above the radiant, in a configuration like
in this schematic. The elevation of the fields should be at least 35
degrees.
--- ---
/\ \ / / \
Field 1 \ \ / \/ / Field 2
--- ---
\
*
*** Radiant
*
The idea is that if we extend back the paths of shower meteors seen in
the two fields towards the radiant, they will intersect at near right
angles. This gives best definition of the radiant. The distance from
the field to the radiant is about 10 to 30 degrees. For faster
meteors we go closer. Normally I choose 15 to 20 degrees, but
sometimes because of the elevation of the radiant is low or other
showers and geometrical considerations we can go towards the upper
range. You can still see telescopic meteors well away from their
radiant; however, they are generally travelling faster through your
field and hence appear dimmer and are harder to see. Also any error
in your estimate of the orientation of the meteor gets magnified when
extrapolated back to the radiant. The radiant distance is a
compromise to yield high accuracy for the positions, but also to see
sufficient shower meteors. If you looked towards the radiant, the
rate would be very low.
In the case of the Ari-Tri radiant, I've selected four charts:
IMO numbers 50 (01h38m, +48), 73 (00h36m, +31.5), 75 (02h56m,+38.5),
and 92 (01h00m, +21). The best pair are 50 and 73.
Prior to observation you should mentally note where each field is
located, and think how you will direct your telescope or binocular
to them, say by `star-hopping'. I use Norton's for this. It's much
easier to memorize this before going out, than fumbling about in the
dark.
We adopt other configurations when searching for minor showers.
Observing Method
----------------
We observe each field for about half an hour (called a `watch'), and
alternate between the two. This enables us to pinpoint the location
of nearby radiants, and gives the observer a change of scenery to help
reduce boredom when rates are low, and a chance to stretch, or take a
short break. In practice we normally use a few field centers around
the radiant to try to reduce artifacts from the reductions or
occlusions when looking at areas where there are many radiants in
close proximity, such as in Aquarius and Capricornus during the
summer.
We use a new report sheet each night. (Standard IMO forms are
available from me). On it we record the double date like "1996
September 10/11"; our name and location; and the specification of our
binocular or telescope namely the aperture, true field diameter, and
magnification. Also we normally include some remarks about the sky
conditions.
For each watch we record the field or naked-eye limiting magnitude,
the start and end times (UT please), the sum of any breaks, and the
effective observing time in hours. To compute the last of these
observers need to estimate or measure their dead-time, that is the time
while they are not actually looking at the sky. For me, it's about 40
seconds per meteor.
During a watch we concentrate on looking for meteors. This takes
practice (think "I am looking for meteors"). This switch from `casual'
to `formal' meteor watching has a dramatic effect on the observed
rates. The casual rate, say as seen by someone hunting for comets or
novae is around 2 per hour. An experienced telescopic observer can
expect to see more like 10 per hour. It also appears that if you
concentrate very hard you can achieve even higher rates than normal,
but it is very tiring. It is like doing a sprint as opposed to a
long-distance race, you go faster but you can't sprint for long.
When a meteor is seen, you freeze and try to replay in your mind what
you've just witnessed. Record the brightness, speed, the type, time
of appearance, and plot the position and direction of the trail on a
chart (see the heading IMO charts) and annotated with an
identification number. This starts at 1 each night. Measure the
duration of any persistent trains.
Path: we use two well-separated pairs of stars. Each pair of stars
should straddle closely to the meteor's path. We estimate the
fractional distance of the meteor's path between the two stars in a
pair, for example midway, or 30% from the lower to the upper star.
Repeat for the other pair. After some practice we find that this
comes naturally, and it gives accurate results.
Brightness: The magnitude comes from comparison with field stars,
though after a while it is possible to judge the brightnesses of most
meteors directly.
Speed: The angular speed is on a scale from A to F; A being the
slowest equivalent to about 2 degrees per second, and F is the fastest
corresponding to 25 or more degrees per second. Numerical estimates
are too difficult given the magnification. The velocities are needed
in the radiant analysis.
Type: The type is a code as to whether the meteor started and/or left
the field of view. OO means it traversed the whole field. 10 means
that the meteor started within the field, but moved outside.
Train: If there is a persistent train we estimate its duration and
occasionally make sketches of its decay.
Depending on the weather conditions and alertness of the observer, we
take longer and more-frequent breaks than visual observers, as
telescopic observing does require more concentration, especially when
the rate is low. Many don't overcome the initial hurdle and give up.
Even I had a number of false starts. A little perseverance and many
fascinating avenues of research open up.
IMO Charts
----------
The IMO Telescopic Commission has several sets of bespoke charts for
plotting telescopic meteors. Each set has its own limiting magnitude,
field size, and orientation; each is geared towards popular binocular
and telescope specifications. Within each set there 164 fields
scattered mostly over the northern sky. The chart number defines the
region of sky irrespective of the chart set. The chart centers
were selected not only with specific showers in mind but also
to allow searching and then monitoring of new or obscure minor
showers, and to investigate the distribution of sporadic meteors.
By measuring x-y start and positions of the meteors from the charts,
and the distance between four fiducial crosses, it is easy to
calculate the R.A. and Dec. of the meteors. These data along with the
other parameters are used by Rainer Arlt's RADIANT software to analyze
the distribution of meteor radiants present in the data. Observers
should make these measurements to spread my work load. I enter the
values into the computer, run a couple of programs and then they are
ready for analysis. This is a big improvement over the days when
observers drew their own charts. Data reduction was very tedious if
not impossible.
The diameters of the stars on the charts indicate their catalog
brightnesses in the V (visual) band, and a key is provided. Variable
stars are indicated.
Each chart has an inset showing an enlarged portion of the field to a
fainter limiting magnitude. This is allow an estimate of the field
limiting magnitude during a watch.
I hope that selected charts will soon be available via the IMO Web
pages as well as the post.
I don't have the IMO charts. Can I still observe the Ari-Tri shower?
---------------------------------------------------------------------
Yes provided you want to observe with a small binocular. There are
other star atlases they will suffice. The Uranometria 2000 star atlas
is a good substitute. It's best if you photocopy the relevant page,
then use a correcting fluid to remove the R.A. and Dec. lines in a
region slightly larger than your field diameter about the chosen
center. This becomes the master chart. Subsequent to the observing
you will need to measure the start and end points of each meteor in
equatorial co-ordinates, and enter these on the report form instead of
x-y positions.
Concluding Remarks
------------------
There is a lot to digest in these notes, and in my haste I may have
been unclear or omitted a vital point. Please don't hesitate to mail
me, and I'll endeavor to make things clearer.