CHAPTER 4:
OBSERVING TECHNIQUE - TELESCOPIC

Observing meteors with telescopes or binoculars is one of the most valuable fields of study that the amateur astronomer can work in. It involves observation of meteor events well below the limit of either photography or naked eye observation and can cover a size range of meteor particles recorded by professional scientists using radar techniques. The restricted field of view of even wide angle telescopes and binoculars means that these observations are much more accurate than results from naked eye work. Meteor rates are rather modest but do steadily improve with experience and everyone who has taken up this field actively, agree that the long hours of patient watching are amply rewarded when a bright naked eye meteor is seen.

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, giving a more complete picture of a shower. The restricted and magnified field of view allows the paths of meteors to be determined 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 since we use analysis software to assign shower membership and to search for radiants.

There is no single best telescope or binocular for telescopic observing. The choice will depend on the quality of your observing site, your eyesight, observing goals, and how much you wish to spend or what is already available. However, there are two main factors that should influence a choice: the instrument should have a low power and a wide apparent field of view.

You must have a low magnification for a given size of objective lens or mirror. To put that into numbers, the magnification should be in the range of 1.4-2.0 times the aperture in centimeters. So for example, a 7x50 pair of binoculars has a magnification of 1.4 times the aperture in centimeters and a 10x50 has a magnification twice the aperture.

The apparent field of view is governed by the eyepiece design. You can derive it from the product of the magnification and the true field of view. For example, a 10x50 binocular, with a 6 degree true field, has an apparent field of 60 degrees. A wide field of view will encompass more of the sky, and hence you will see more meteors. The recommended range is 45 to 70 degrees, with 50 to 60 degrees being preferred. One of the principal reasons for observing telescopic meteors is to investigate radiant properties by plotting meteor paths accurately. As the apparent field of view enlarges, the average plotting accuracy goes down. So ultra-wide fields (>65 degrees) are best for determining rates, and hence deriving the time of maximum for a shower, whereas for field sizes of around 50 degrees rates are still reasonable and accurate positional data can be obtained. Given the choice between the two, you should err on the side of the smaller apparent field as it offers more flexibility and science. Also, ultra-wide eyepieces or binoculars are either very expensive if they give pinpoint images across the entire field, or give increasingly distorted images towards the periphery of the field. Below 50 degrees the loss of sky coverage starts to become important. If rates become too low, boredom and loss of concentration can soon set in.

Binocular vision is the natural way to look, and since comfort is a critical consideration for the telescopic observer, a binocular is preferred to a (monocular) telescope. Aperture is less critical, and IMO observers' apertures range from 40mm to 300mm, though most are in the range of 50-80mm. The intermediate apertures (50-80mm) seem to work best. The quality of the optics can make a big difference to the performance. Remember that you will be observing for long periods and considerations like accurate collimation and pinpoint images will reduce strain. This consideration can outweigh some of those mentioned already. For example, a quality 7 x 42 is going to let you see more meteors than a cheap 8 x 50.

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 toward the radiant, they will intersect at near right angles. This gives the best definition of the radiant. The distance of 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 the elevation of the radiant is low or other showers and geometrical considerations are involved, we can go towards the upper range. You can still see telescopic meteors well away from their radiant, however they are generally traveling 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. Other configurations are adopted when searching for minor showers.

Observe each field for about half an hour 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, a few field centers around the radiant are used 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.

Use a new report sheet each night (standard IMO forms are available). Items that are recorded include: the double date like "1996 September 10/11"; observer name and location; and the specification of our binocular or telescope, namely the aperture, true field diameter, and magnification. Also some remarks about the sky conditions are normally added. For each watch record the field or naked-eye limiting magnitude, the start and end times (in 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.

When a meteor is seen, 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 below) and annotate with an identification number. This starts at one (1) each night. Measure the duration of any persistent trains.

Path: use two well-separated pairs of stars. Each pair of stars should straddle closely to the meteor's path. 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, you will 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 brightness 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 estimate its duration and occasionally make sketches of its decay.

Depending on the weather conditions and alertness of the observer, it is best to take longer and more-frequent breaks than visual observers, as telescopic observing does require more concentration, especially when the rates are low. Many new observers don't overcome this initial hurdle and give up. With a little perseverance, many fascinating avenues of research open up.

The IMO Telescopic Commission has several sets of charts suitable for plotting telescopic meteors. Each set has its own limiting magnitude, field size, and orientation, and each is geared towards popular binocular and telescope specifications. Within each set there are 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 monitoring of new or obscure minor showers, as well as to allow the investigation of the distribution of sporadic meteors.

By measuring x-y start and end 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. This data along with the other parameters is used by Rainer Arlt's RADIANT software to analyze the distribution of meteor radiants present in the data. Observers should make these measurements on their own. I enter the values into the computer, run a couple of programs and then they are ready for analysis.

The diameters of the stars on the charts indicate their catalog brightness in the V (visual) band, and a key is provided. Variable stars are indicated as such. Each chart has an inset showing an enlarged portion of the field to a fainter limiting magnitude. This is to allow an estimate of the field limiting magnitude during a watch.

It is best if an observer obtains the above mentioned IMO charts, but other sources will do if one is waiting for them to arrive and you want to observe with a small binocular. In these cases, 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 then 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.

Telescopic meteor work takes quite a bit of getting used to and observers should persevere. Experience is rapidly gained and your hourly rates of meteors will climb steadily. There are enormous rewards, the most spectacular being a close view of a bright meteor. The basic principles of telescopic meteor observing are essentially the same as for visual observing since an area of sky is watched and a detailed record is made of meteors that are seen. The field of view should be chosen carefully and is usually recommended beforehand by the IMO Telescopic Commission Director.

To learn what fields should be chosen, or for more information on this method, contact the IMO Telescopic Commission Director Malcolm Currie.