Transcript
Equinox Sky Camp— Trips / Events Please see the website for more details. Ideas for trips and events always welcome! Contact: Chris Bowden
[email protected]
Talks in the Area: ♦ 20 Oct BNSS—Durlston
Sky Watcher Club News
Welcome back after the WAC summer break. During this 2009 International Year of Astronomy, there are still plenty of events and activities being held around the UK for amateur astronomers to attend and be involved with. Up to the 15th of November, the Hershel Museum in Bath is hosting ‘BRIGHT STARS, DAUGHTERS OF URANIA’ to celebrate women in Astronomy. An excellent exhibition and worth the trip to Bath.
14” Telescope: Alan Jef-
Thisferis includes free evening viewing. Doors ♦ 21 Oct CADAS— Travelling Planetarium Winter Skies: Bob Mizon
♦ 3 Nov WAS— The Universe Unveiled: Robin Catchpole
♦ 17 Nov BNSS—Known Knowns and Known Unknowns in Cosmology: Chris Lintott
♦ 18 Nov CADAS— Tides: James Fradgely
Upcoming Events:
20 November What is new in the Solar System—Robin Catchpole 11 December Astro Quiz 22 January Astroimages and Time: Rob Hodgkinson 19 February Finding your way around the sky: Bob Mizon (Fleet Farm) OBSERVING NIGHTS Fleet Farm — Open viewing is monthly on the last Saturday, weather permitting. Check the website for details.
Please note the next viewing night is 31 October
Just a reminder to WAC members and anyone interested in joining. The membership runs from June—May and remains just £10 per year. If you haven’t joined yet, we would love to welcome you to the club.
Volume 4, Issue 4 16 October 2009
Until the next meeting in (7.4 million miles). One of farthest moons, November—Starry Skies! SB Saturn's Phoebe, circles within the newfound ring, and is likely Astro News Bites the source of its material. Saturn's newest halo is NASA's Spitzer thick, too -- its vertical Space Telescope has height is about 20 times the discovered an enormous ring diameter of the planet. It around Saturn -- by far the would take about one largest of the giant planet's billion Earths stacked m a n y r i n g s . together to fill the ring. The new belt lies at the far reaches of the Saturnian system, with an orbit tilted 27 degrees from the main ring plane. The bulk of its material starts about six million kilometers (3.7 million miles) away from the planet and extends outward roughly another 12 million kilometers
SARSAT to the Rescue
Harvest Moon 6 Oct. 2006.
If a plane crashes in the woods and nobody hears it, does it make a sound? Never mind contemplating this scenario as a philosophical riddle. This can be a real life or death question. And the answer most of the time is that, even if no people are nearby, something is indeed listening high above. That something is a network of satellites orbiting about 450 miles overhead. The “sound” they hear isn’t the crash itself, but a distress signal from a radio beacon carried by many modern ships, aircraft, and even individual people venturing into remote wildernesses. In the last 25 years, more than 25,000 lives have been saved using the satellite response system called Search and Rescue Satellite-aided Tracking (SARSAT). So what are these life-saving superhero satellites? Why they are mild-mannered weather satellites. “These satellites do double duty,” says Mickey Fitzmaurice, a National
Oceanic and Atmospheric A dm i nis t ra tio n (N O A A ) s ys tem s engi neer f or SARSAT. “Their primary purpose is to gather continuous weather data, of course. But while they’re up there, they might as well be listening for distress signals too.” In February, NASA launched the newest of these Polarorbiting Operational Environmental Satellites (or POES) into orbit. This new satellite, called N-Prime at launch and now dubbed NOAA-19, prevents a gap in this satellite network as another, aging NOAA satellite reached the end of its operational life. “The launch of N-Prime was a big deal for us,” Fitzmaurice says. With N-Prime/NOAA-19 in place, there are now six satellites in this network. Amongst them, they pass over every place on Earth, on average, about once an hour. To pinpoint the location of an injured explorer, a sinking ship, or a downed plane, POES use the same Doppler
www.weymouthastronomy.co.uk
effect that causes a car horn to sound higher-pitched when the car is moving toward you than it sounds after it passes by. In a similar way, POES “hear” a higher frequency when they’re moving toward the source of the distress signal, and a lower frequency when they’ve already passed overhead. It takes only three distress-signal bursts — each about 50 seconds apart — to determine the source’s location. Complementing the POES are the Geostationary Operational Environmental Satellites (GOES), which, besides providing weather data, continuously monitor the Western Hemisphere for distress signals. Since their geostationary orbit leaves them motionless with respect to Earth below, there is no Doppler effect to pinpoint location. However, they do provide near instantaneous notification of distress signals. In the future, the network will be expanded by putting receivers on new (Continued on page 2)
Sky Watcher
Volume 4, Issue 4 Page 2
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Global Positioning System (GPS) satellites, Fitzmaurice says. “We want to be able to locate you after just one burst.” With GPS, GOES will also be able to provide the location of the transmitter. Philosophers beware: SARSAT is making “silent crashes” a thing of the past.
A novel astronomical telescope viewfinder
4. Shows the roof prism in the prism carrier after unscrewing the One of the problems with astronomical telescopes is eyepiece. All 4 retaining screws are just visible. that the objects they observe are frequently at an inconvenient 5. The eyepiece with focussing mechanism. angle such as near the zenith. They therefore tend to have right-angle viewing systems, either at the side of the telescope in a Newtonian, or at the eyepiece. For ease in use the viewfinder eyepiece should have the same orientation. Meade make a finder with a right-angle eyepiece but though the orientation may be correct the problem is then to recognise the small area of sky shown. One solution is to present an unmagnified sky view in the same orientation as the finder view to the other eye. It is then easy to move the telescope until the object of interest in the plain sky view coincides with the larger image in the finder view. 6. Shows the front prism rotated and fixed back in the mount with Reflection in mirrors and prisms is very confusing. A some car body filler. The silvered surface is now on the lower left mirror transposes about its plane. If the plane is vertical of the prism. transposition appears horizontal and if the plane is horizontal transposition appears to be vertical. If the view is successively 7. The prisms seen from the objective end of the telescope. reflected in an even number of mirrors whose planes intersect 8. The sliding objective carrier is removed by undoing the locking horizontally or vertically the transposition is neutralised. If the screw and the end of the finder tube is sawn off at an angle of plane of a mirror is at 45° to horizontal, the horizon appears 45°, rotated through 180°and cemented back with epoxy so that vertical. Viewed in a second mirror parallel to the first the view the optic axis of the prisms and objective coincide approximately. is untransposed but as the second mirror is rotated until its With the objective back in its sliding tube the tube length is plane is at right angles to that of the first mirror the view adjusted with the eyepiece focus in its mid position. The objective rotates twice as fast and ends up transposed both vertically slide is then locked or glued in position. The finder now has a and horizontally. This is how the Amici roof prism (1. below) is right-angle focussing eyepiece and gives a normally orientated used in expensive right angle finders to erect the inverted field. image formed by the objective. The final stage involves cutting two small pieces of Here is a d.i.y equivalent which also provides eyepiece, handbag mirror which are glued at 45° on a piece of bendable focussing mechanism, tube and objective. A pair of cheap metal in turn fixed to the finder with the lower mirror slightly straight-through binoculars will almost certainly use a Pechan tipped up. This works like a pentaprism and it can be fixed on the prism system. There are two prisms. The first turns the light tube or to one side of the eyepiece at a separation dependent on path through 45° by internal reflection (the 2nd reflecting the distance between the observer’s eyes. As the eyes are surface is silvered) and the roof prism turns the beam back parallel when focussed at infinity, if one eye looks through the through 45° and erects the image. 2. shows the Pechan prism eyepiece the other will see the same view in the same orientation combination as it would be in a telescope with the eyepiece to unmagnified. The finder is aligned with the telescope by adjusting the left and objective to the right. 3. shows both prisms rotated the mount and the mirror carrier is aligned with the finder by bending. No crosshairs are needed. When a star seen in the wide sky view coincides with its image in the centre of the finder view it will also be in low-power telescope eyepiece view. The final picture shows the finder fixed to a Newtonian reflector which is being used for solar observation. A piece of welding glass filter stuck on a tube covers both light paths. This doubles as an objective cap, and when handling the telescope by day stops sunlight accidentally reaching the eye. clockwise through 90° so the eyepiece is now upwards. Now rotate the R hand prism a further 135°clockwise. The light path through it is reversed. The objective is still to the right. Instead of the two 45° deviations of the beam cancelling they now add to 90° and produce the same final result as the Amici roof prism. The finder is made from one half of the binoculars sawn free of the linking hinge. One eyepiece rotates for matching focus. It screws in and this screw thread is locked by one tiny screw. Loosen the screw and unscrew the eyepiece. The Pechan prism mount is held by three more tiny screws which also allow slight adjustment of position. If they are loosened the holder can be lifted out.
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Patrick Barrington Weymouth September 2009
