Ryukyu Astronomy Club Newsletter
Volume 1, Issue 3 May 2002

Next Club Meeting - May 11th
Conference Room B at the Camp Lester Naval Hospital

April Meeting
The April general meeting of the Ryukyu Astronomy Club was held the 13th of April, 2002 in the Navy Hospital's Conference Center. Attendance was eight, with several new faces.

The group discussed holding an event at a local shopping center (Camp Foster Exchange) on Astronomy Day, April 20th. Discussion was also held regarding the upcoming community bazaar at Lester Middle School. It was decided that the Club would definitely sponsor the Astronomy Day event, but the Lester bazaar would be dependent upon as yet unknown availability of members to attend.

Mike Swanson gave a presentation on use of filters for astronomy. The presentation is available in PowerPoint format at the club's web site.

After the presentation there was a question and answer session which proved to be quite useful and popular. It is recommended time be set aside at most meetings for this purpose.

After the meeting. the group traveled to Awase Meadows Golf Course for a couple of hours of observing. Mike Swanson (11" Schmidt-Cassegrain and 3.1" achromatic refractor), David Weissgerber (8" Schmidt-Cassegrain and 4" (?) achromatic refractor) and John O'Briant (3.1" achromatic refractor) set up their scopes on a fairway that was free from any nearby local lights. The seeing was OK, but the transparency was not too good as witnessed by the obvious skyglow surrounding the golf course. Everyone had a good time, with many folks getting their first views of several objects. Special thanks goes out to Eric Burns for making the trip to the golf course possible!

Astronomy Day 2002!
April 20th was celebrated worldwide as Astronomy Day 2002. The Ryukyu Astronomy Club, eager to do its part, secured permission to setup a table and telescope at one of the local shopping centers in the community (the Camp Foster Exchange). Last month Mike Swanson requested and received 100 "How to Start Right in Astronomy" brochures from Sky and Telescope magazine. Mike also brought his 80mm refractor and solar filter to allow direct viewing of the sun. Dave Weissgerber provided a model showing the interactions of the Sun, Earth and Moon as they travel through space. Tommy McGee and Mike Swanson manned the table from 2 PM to 6 PM.

Approximately 80 people stopped by for a look. About 60 of the astronomy brochures as well as a flyer introducing the Club were distributed. Some of the passersby were very interested and asked question after question about astronomy, cosmology and even astrobiology! It was a very successful event, introducing many people to just a few of the wonders of our universe and undoubtedly kindling (or rekindling) a strong interest in several individuals.

Things to See This Month
This month will be quite a show for the five naked-eye planets (those we can see without optical aid). Throughout the month, they will be playing tag with each other and the Moon. Look to the west any night after sunset and start by picking out the brightest object in the sky - Venus. During the beginning of the month you will be able to find Mercury, Mars and Saturn within 10 degrees (the width of your fist at arms length) of Venus. Jupiter is about 30 degrees up and slightly to the left.

The evenings of the 6th and 7th of May will result in a truly special view - Venus, Mars and Saturn will form a tight triangle of about 3 degrees. All three planets will provide a spectacular view in binoculars (mount them on a tripod for stability) or in a short-tube, wide-field telescope. On the evenings of May 10th and 11th, Venus will pass within 1/3 degree of Mars, resulting in a rare chance to see both in a high-magnification telescopic view.

On May the 14th, the Moon joins the show as a sliver-thin crescent Moon passes within about one degree of Saturn just after sunset. At 9 PM local time on the 16th, the crescent Moon will be less than two degrees from Jupiter, resulting in another wide-field telescope treat. June 2nd and 3rd will result in Venus passing within two degrees of Jupiter as well.

Speaking of Jupiter, May will yield the final good telescopic views of the "amateur's planet" until it returns to us from the glare of the Sun in September. By the end of the month, Jupiter will be too low in the sky to give much detail in the eyepiece. Jupiter is aptly named the amateur's planet due to the satisfying detail that can be found in even the smallest telescopes.

In addition to all the planetary action, be sure to spend some time looking at the many fine globular clusters available this time of year. Next month we will explore globulars in more detail, but for now seek out M3, 5, 13, 92, 10 and 12 and see if your views agree with the descriptions we will have for you next issue!

For more objects of interest and the locations of those listed above, download the latest chart from or visit and customize the online star chart for Okinawa's general location of 128 degrees East longitude and 26 degrees North latitude.

Field of View in a Telescope
After spending just a little time exchanging the eyepieces in a telescope, you will soon find that more than just the magnification varies with each eyepiece. One of the primary differences is the total amount of sky you can see with each eyepiece, also known as the field of view or FOV.

The field of view is the circle of sky visible through the eyepiece. Generally speaking, as you exchange eyepieces to get a higher magnification, the field of view is a smaller piece of the sky. We measure FOV in degrees or fractions of a degree. Usually astronomers refer to the actual field visible in the eyepiece as the true field of view or TFOV. Knowing the TFOV of each of our eyepieces is very useful since we can then compare what we see in the eyepiece to printed or computerized star charts to help us identify what we are seeing.

Also, some objects require a wide field of view to show the entire object, so we need to choose the eyepiece that will let us 'take it all in'. Herein lies the popularity of wide field eyepieces. At any given magnification, they allow us to see more of the sky. Extended objects like open star clusters, many nebula and some nearby galaxies are only visible in their entirety with a wide-field view.

Calculating the TFOV is not too hard, but there are a few things we need to proceed. First, we need to know the focal length of our telescope and the subject eyepiece. These two are easy as they are generally marked on the side of the scope and eyepiece. But we also need to know the apparent field of view (AFOV) of the eyepiece. This is generally obtainable from the manufacturer of the eyepiece, but it is useful to know that most Plossl eyepieces (the most common type on the market) have an AFOV of 50 degrees.

Armed with this information, the calculations are quite simple. First calculate the magnification of the eyepiece:

MAG = Scope Focal Length / Eyepiece Focal Length

Then you can directly calculate the true field of view:

TFOV = Eyepiece AFOV / MAG

Let's say the focal length of our scope is 1000mm and the focal length of our Plossl eyepiece (50 degrees AFOV) is 10mm:

MAG = 1000 / 10
MAG = 100x

TFOV = 50 / 100
TFOV = .5 degrees

To help with all this math, there is a "Scope Calculator" available in the Downloads section of our Club web site. This Excel spreadsheet performs all the tedious work, you just provide information such as the focal lengths of your scope and eyepieces and the apparent field of view of your eyepieces. The result is a table showing the magnification and the approximate true field of view of each eyepiece. Print a copy and place it in your eyepiece case for quick reference.

Clear Skies!

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