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(meteorobs) September Telescopic Observing and Aurigids (LONG)
Since Lew raised the topic of telescopic observing of the Aurigid
showers, I thought it might be worthwhile to extract and slightly
modify some of my Telescopic Notes from WGN in 1995. The Moon is
roughly at the same phase.
For the new readers, you can observe meteors with a low-power,
wide-field telescope or binocular. They are called telescopic
meteors. The observed rate is lower than for visual meteors, and you
see fewer apparently bright meteors, so what's the point? This
technique lets us observe meteors fainter than can be detected with
the unaided eye. Such information can help understand how meteor
streams evolve because the various forces affect the meteoroids (the
dust particles in space) of difference sizes differently. The meteors
are plotted on charts. This can be done much more accurately than
visually. It lets us locate new or weak showers more clearly, and
investigate the structure of radiants, such as the Orionids.
See http://medicine.wustldot edu/~kronkg/namn/chap4.html for more
details.
The numbered charts are from the IMO set available from me. There are
different sets at roughly the same field centres for different
instruments. The field of view and limiting magnitude vary from set
to set. Lew and I experimented with making some available on the Web.
The PostScript files are quite large (about 200kB compressed). GIFs
are much smaller (about 20kB) but the quality is poor. At present I
only have the C set (suitable for Astroscans or 80 to 100-mm
binoculars) online. Uranometria charts are also satsifactory for 40
to 50-mm binoculars.
Before I put the IMO sets online I wanted to use the unified
catalogues being created at the time. These overcome some of the
problems using the SAO Catalog (another one for the list Lew?) and the
HST Guide Stars, both individually and together. I also want better
photometry of the stars for the LM estimates.
Gary Kronk will probably have more to say about the alpha Triangulids.
Malcolm
tele@imodot net
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September is my favorite time for telescopic viewing. The nights are
darker and longer than in high summer, but are often clear and not
cold; the sporadic meteor rate has such verve that it readily grips my
attention, but that is not all; there are showers too including a
complicated region in Auriga, Perseus, and Cassiopeia, and there have
been some surprises like the alpha-Triangulids in 1994. Unfortunately,
moonlight hampers prospects of follow-up observations of the
alpha-Triangulids in 1998, when rates might increase again, if the
posulated periodicity is correct. The visibility of this weak shower
of faint meteors being highly susceptible to the sky conditions.
Instead we kick off with the alpha-Aurigids. For a few days
either side of September 1, these swift meteors are best known for
their high average brightness and long paths. Even though the visual
population index is very low (probably less than the Perseids) the
shower does give weak telescopic activity implying that the index
increases as the mean magnitude dims. A first look at the 1994 data
shows that the radiant lies a few degrees north west of the visual
radiant position in the IMO shower list. alpha-Aurigids exhibit the
expected high fraction of trains; seeing the occasional brighter
alpha-Aurigid and decay of its train is an exhilarating experience.
In some years the shower does give visual outbursts tens of times the
normal peak rates lasting around a couple of hours as happened in
1935, 1986, and probably in 1994 (implying an ~8-year period).
Unfortunately because we did not have a team widely dispersed in
longitude, we have no telescopic data during the 1994 outburst. Like
the Perseids the average brightness increases during these transient
phenomena, and so may not be observable by telescopic means, but we
need out look out every favorable year to find out. To see members of
this shower it is essential to watch after midnight local time to
allow the radiant to attain a reasonable elevation. Also a
first-quarter moon will have set by then.
Lasting through the whole period are the delta-Aurigids. Like the
other showers in the vicinity during these months they comprise fast
meteors (v_infty ~ 65 km/s) and their streams have high inclinations,
possibly associated with a sun-grazing comet. There is good evidence
for four sub-components to this shower. During the first half of
September last year three sub-centers are visible during my
preliminary radiant analysis. This year best observable rates are
likely during the second half of the September, though a few early
ones can be seen at the start of the month. Again post-midnight
sessions are needed to see this shower at its best.
More prominent during early September is the lesser-known September
Perseids radiating from between alpha and beta Persei, though
moonlight is going to spoil its best offerings, which were between
September 8--13 in 1994 and stronger than the delta-Aurigids. A
number of other radiants have been suggested in this region based on
visual and telescopic from data in 1971, 1988 to 1991. The 1994
observations has some evidence for a couple of these though the study
is in its infancy. Around the start of the month the strongest source
of meteors was western Cassiopeia and Lacerta (RA = 328 deg, Dec = +44
deg), the latter also being recorded in 1989. The former might be the
beta-Cassiopeids seen best in 1988, but now faded into the strong
noise evident in Cassiopeia towards mid-September.
To summarize, there is a complex of radiants during September and
October whose activity dates and radiant parameters are at best poorly
determined and at worst unknown. Observations by all methods are
badly needed over a number of years to describe the properties of
these showers, and to investigate if any are interrelated. To cover
and indeed to delineate all these real or possible showers, telescopic
observers should watch several telescopic fields each clear dark
night, and pay special attention to careful plotting. For early
September I recommend charts 36, 48, and 49 for the
beta-Cassiopeids and Lacerta radiant, which should be the prime
targets until about 23h. For the remainder of the night use
charts 36, 37, 51, 75, 76 and 39 for the radiants in Perseus and
Auriga. The last two are best for the alpha-Aurigids. During late
September's delta-Aurigids I should select charts 19, 42, 53, 54,
and 56.
References: