Lawrie
Joined: 25 Sep 2008
Posts: 13
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 Posted: Mon Jan 12, 2009 4:49 pm Post subject: Meridian Sweeping |
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Sweeping along the meridian and identifying objects as they came into view was what Sir William Herschel often did since this involved using only the Altitude movement of his ungainly telescopes. As a method of exploring the heavens in a systematic way it is excellent. Noting the Sidereal Time when an object is in the centre of the field of view gives the object's Right Ascension and the angle of altitude, minus 38 degrees ( my latitude is 52 degrees North) gives the Declination.
This method of exploring the celestial sphere appeals for its precision and simplicity. I decided to try the same using a 150mm Dobsonian which, on a far smaller scale, is much the same telescope as Sir William's. Orienting the telescope involved first pointing the telescope at Polaris and then turning the telescope around 180 degrees to align the telescope due south.
My sidereal clock is an alarm clock with a sweep second hand, modified with a switch which interrupts the battery supply to stop the movement with the second hand on 12, ready to restart on the minute.
The US Naval Observatory website has a Local Sidereal Time web page. Enter your longitude and back it comes with your sidereal time.
The meridian adjustment can be refined by observing a star of known Right Ascension and keeping it in the centre of the field of view until the sidereal time on the clock is the same as the star's Right Ascension. The telescope is then set precisely on the meridian . The azimuth of the telescope is clamped so that cannot move or jogged accidentally from this position.
A sweep is commenced from the horizon upwards. Any bright star, group or other interesting-looking object is allowed to drift to the centre and a note made of the Sidereal Time, i.e the RA of the object, and the altitude. Entering the RA and Dec into a planetarium program identifies the object.
A 32mm 2 inch eyepiece gives an FOV of 1,5 degrees and means an object is observable for about 5minutes.
An electronic inclinometer gives an altitude accurate to within 0.1 degree. It is zeroed at the start of each session with a small spirit level.
Results
Although you can check beforehand what you are likely to see using the planetarium program, it is more fun to discover things for yourself. Apart from observing some interesting stars in the more obscure constellations I frequently come across lesser known NGC clusters and many attractive asterisms.
Lawrie |
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