OAS Executive Committee President- Dave Dunn

Ph. (801) 544-7705

Vice Pres- Lee Priest

Ph. (801) 479-5803

Secretary- Bob Tillotson

Ph. (801) 773-8106

Treasurer- Doug Say

(801) 731-7324

The March Meeting

The monthly meeting of the Ogden Astronomical Society will be held this Thursday March 8, 2001 in the Ott Planetarium on the Weber State University campus at 7:30 p.m. This month's program will be a showing of the latest planetarium program at the Ott. The new show is titled "Women Hold Up Half the Sky."

March Messier Marathon

A Messier Marathon and Star Party is planned for this month. Club president Dave Dunn has details which will be discussed at the March 8 meeting. If you are interested in participating in the marathon and are unable to attend on the 8^th, you may contact Dave at his telephone number above.

By Jeremy Mathews

The night hours of winter maintain superiority until March 20th, when the vernal equinox signals the end of twilight's domination and equality between night and day. It is in the Coma-Virgo Supercluster that the shear number of galaxies presents even the most seasoned observer with a verdant supply of deep-sky objects.

Our trek begins with Messier 98 in Coma Berenices. Located at 12h 13min R.A., 14d 53min Dec, this 10.1 magnitude lenticular galaxy displays a blue shift rather than a red shift which is somewhat uncommon in the Coma-Virgo galaxies. This extremely elongated edge-on galaxy runs NNW-SSE with a bright, extremely mottled core. In contrast with M98, Messier 99 (also know as the pin-wheel) exhibits one of the highest redshifts in the cluster. At 12h 18min R.A., 14d 24min Dec, it's current

estimated value gives it a velocity of nearly 2,400 km/s. Small telescopes show a distinct 9.8 magnitude core about 1/5th the size of it's outer halo. A 10" telescope with averted vision can reveal a vague spiral pattern. Crossing into Virgo just 3d 32min South at 12h 30min R.A., 12d 23min Dec, lies Messier 87 (also known as Virgo A). Although bright at magnitude 8.6, this galaxy has a uniform appearance and may be difficult to locate in urban skies. NGC 4536 is a 10.4 magnitude galaxy at 12hr 32min R.A., 2d 10min Dec. It is a small, dim, edge-on galaxy which may be challenging to locate. It forms one vertex of an equilateral triangle with a 7th magnitude star 10' to the ENE at one vertex, and an 8th magnitude star 10' to the North at the last vertex. It makes a good challenge for even veteran observers.

Interesting stars include Gamma Virginis (Porrima) and Phi Virginis. Gamma Virginis (12h 41min R.A., 1d 27min Dec) is a striking pair of rich yellow stars both similar in brightness. It is a very close double, and by the year 2010 will be less than 1' apart. This is a beautiful double star which vividly shows the component colors. Phi Virginis is actually a triple star, all of whose components are a different color. The primary star is a deep yellow color; the secondary shows a paler orange hue, the bluish tertiary may be more difficult to find as it shines at 12.4 magnitude and is 93" away. It is a wonder that this triplet is not more widely known, since it can show so many different colors in a single field of view.

Minutes

Ogden Astronomical Society

February 8, 2001

The February meeting of the Ogden Astronomical Society was called to order in the Layton P. Ott Planetarium by OAS president David Dunn.

The Bates Elementary School star party has been postponed to later in the year. Information to members will be available after the date is finalized.

The public star party which has been arranged for the Parks and Recreation Service at East Canyon State Park is set for June 30, 2001. There is no observing events et for this month (February).

Bob Tillotson explained to members his communication with the hoped-for speaker James Schefter who recently passed away. It was a profound disappointment that the OAS could not host his visit. A note on available web sites for Mr. Schefter was mentioned.

Steve Reicher mentioned that a static testof a Space Shuttle booster engine will be made on March 24^th. If interested in seeing the test, he has details.

Dave Dunn is searching out sites for Southern

Utah star parties and Messier Marathons. He will bring details when his search is completed.

The meeting moved on to the evening's central program "The New Cosmos" narrated by Author C. Clark.

Following the show, Steve Reicher demonstrated a CD ROM containing numerous astronomical images.

The NEAR Shoemaker spacecraft will soon attempt to land on asteroid Eros. A video on the NEAR mission was shown. Discussion followed.

The meeting adjourned at 9:00 p.m.

Bob Tillotson, Secretary

My Experiences Putting Together A Radio Telescope, Part 1

By Dale Hooper

Many of the major astronomical discoveries over the past fifty years such as the cosmic background radiation, quasars, and pulsars have been through the study of radio astronomy. Lack of funds, my wife, and my neighbors will always prevent me from building an eighty-foot radio telescope in my back yard, but there are many things that can be done in amateur radio astronomy with a converted satellite dish.

I've been interested in radio astronomy for several years now and I finally started in earnest to build a radio telescope about three years ago. We live in a tremendous time for amateur radio astronomy because even ten years ago it would have required extensive knowledge in electronics to build the required components. Every part of the radio telescope I assembled is commercially available. A lot of this is a result of the amateur SETI movement. Of course, it's still possible for those with electronics experience to build their own components.

The design of my radio telescope was based extensively on a configuration developed by the SETI League. Figure 1 shows a block diagram of my system.

Figure 1 - Radio Telescope Block Diagram (courtesy of The SETI League)

I began by acquiring a used ten-foot C Band satellite dish. Right now these can usually be had for free or at very little cost, because most people that have them are moving to the small satellite dishes. Good places to check are homes that have both a small dish and a large dish! I got mine from a neighbor of a co-worker.

There are a few modifications that have to be made to a C-Band dish before it can be used for radio astronomy. Television satellite dishes are normally set up to move along the celestial equator (declination zero degrees) so that they can be moved to various geostationary satellites. For my purposes, I raised the declination axis of the dish mount as though I was at zero degrees latitude (i.e. so that it could point straight up and be moved side-to-side). I then rotated it ninety degrees so that the dish can be pointed north-to-south. So, to look at objects, I adjust the elevation (which coincides with a specific declination) and then perform what is called a "drift scan". In other words, I let the earth rotate underneath the heavens to bring an object into view. None of the other electronics that came with the satellite dish was usable because it is set up for a different frequency range than what I wanted to monitor and the electronics just aren't sensitive enough to listen for weak signals from interstellar space.

Radio telescopes are similar to optical telescopes in that they are photon collectors. However, radio waves are much longer than the waves of visible light. I am currently observing what is called the neutral hydrogen line, which has a wavelength of about 21 cm (2.1 x 108 nm) as opposed to 400 nm to 700 nm for visible light. As a result, the beam width of my radio telescope is about four degrees! So it takes an object about 16 minutes to drift across the field of view.

There are several different electrical components that are located right at the satellite dish. We can call these the front-end electronics. My system is a classic prime focus radio telescope. At the focal point, instead of a CCD camera, secondary mirror, or eyepiece I use what is known as a feed horn. This is basically an aluminum cylinder sized to guide the photons toward a ¼ wave monopole antenna that is near the end of the cylinder. The way the feed horns are constructed makes them usable, or tuned to, a small frequency range. My feed horn has a range of about 1300 MHz to 1700 MHz. The feed horn I use also has what is called a "choke". This is basically a larger aluminum cylinder surrounding the opening of the feed horn. It is used as a baffle to reduce interference from stray signals.

Even though the collected signal is from a ten-foot diameter dish, it is still extremely weak. The signal strength is increased by passing it into a low noise amplifier (LNA). The original signal is so weak that it requires the LNA to be hooked up directly to the ¼ wave monopole antenna without even running it through so much as a foot of cable. The feed horn, choke and LNA can be seen in Figure 2. The picture on the left shows the inner cylinder, which is the feedhorn-waveguide. The outer cylinder is the choke. The small rectangular object connected to the feedhorn is the LNA. The plastic butter bowl is a weather resistant case for the LNA 12 volt power supply.


Figure 2 - Left: the entire feed horn and LNA. Right: Underneath view of feed horn, showing the 1/4 wave (5 cm) monopole.

Radio waves at a wavelength of 21 cm correspond to a frequency of about 1420 MHz. At this frequency, regular coaxial cable is extremely lossy. My computer is about 85 feet away from my radio telescope so I had to use extremely low loss cable. The cable I purchased is LMR-400 low loss plenum coaxial cable from Times Microwave Systems. At 1420 Mhz there is only about a 5 db signal attenuation over 100-feet. The signal is passed along the cable to what may be called the back-end electronics, which I'll discuss next month.

Items Discussed this time, Sources of information and products, and Notes

1. http://www.jpl.nasa.gov/radioastronomy/, Basics of Radio Astronomy, prepared by Diane Fisher Miller ­ This is an excellent reference that covers all of the basics in a very readable fashion.

2. http://www.setileague.org, This is the website for the SETI League which is dedicated to amateur SETI. Whether or not you are interested in pursuing SETI, this is an excellent source of amateur radio telescope information.

3. Used C-Band (or TV satellite dishes): These are fairly plentiful right now ­ check for people with big and little dishes ­ they usually don't mind getting rid of the "big one". I recommend a mesh dish versus a "solid" dish ­ they are lighter and have fewer problems with high winds.

4. Feed horns and chokes: Radio Astronomy Supplies, http://www.nitehawk.com/rasmit/ras20.html

5. LNA's: Radio Astronomy Supplies and Down East Microwave, http://www.downeastmicrowave.com/

6. Low Loss Coaxial Cable: I went with Times Microwave Systems LMR-400 low loss plenum coaxial cable,

http://www.timesmicrowave.com but there are also other sources.