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The 2012 International Transit of Venus

Re-observation

 

Undergraduate Thesis 2009 by Matthew Cooper

Supervised by Dr Craig Roberts

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Venus in transit *

 

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Observation methods

 

The need for a graphical recording system

 

Methods of observing the transits before 1874 were simple – an observer viewed the transit either directly (looking through a telescope with a solar filter) or indirectly (by projecting the image of the Sun through a lens, usually a telescope, onto a flat surface). Timing of second and third contacts was obtained if Halley’s method was used, and time of each contact was determined at the discretion of the observer. It was up to the observer’s judgement to determine the instant of contact.

 

The end result of a non-graphically recorded observation of a transit is simply duration of the transit between contacts (Halley’s method), or absolute time of one of the contacts (Delisle’s method), and the coordinates of the observing locations for both. The data can be processed for different locations to obtain values for the Astronomical Unit, and the value for this can be compared to the accepted value today. However, one cannot review the visual observations themselves, since there is no record of them. For instance, one cannot see exactly what the observer saw at the exact moment when they determined that a contact occurred. Some observers drew pictures of what they saw, describing distortions in the image and perhaps how they determined when a contact occurred. These images, subject to artist’s impression as they are, may be of use in indicating the viewing conditions on the day of the transit at that location, which corresponds with the quality of timing that could have been achieved.

 

David Gault’s Setup

 

This setup is based around a 10” Newtonian reflector telescope. The telescope sits on a sturdy three pointed base which is able to be levelled. Within the base is a motor system, controlling movement in the azimuth and zenith directions of the telescope. A CCD camera is attached to the “eyepiece” of the telescope, capable of recording video images of what is seen through the telescope. A laptop computer links the video recorder with a basic GPS unit, which together insert a digital time signal into the video footage. The end result is high quality, high detail moving images that can be post-processed to draw the exact time that an astronomical event occurs.

 

One such astronomer is David Gault, of Hawkesbury Heights NSW. Gault routinely observes lunar occultations of distant stars, submitting his data to a higher level scientific body which combines his data with observations from other astronomers around the world of any one event. These simultaneous observations of the one lunar occultation allow the outline of the edge of the moon to be plotted accurately.

 

Gault’s observatory he set up in the backyard of his home is an example of the equipment at the disposal of an amateur astronomer. His observatory allows him to record and time these occultations, in a similar manner to one recording and timing a transit of Venus. If astronomers such as Gault observe the transit in 2012, at various locations around the world, simultaneous recordings of the transit will allow post-processing of all the observations and calculations to be performed to obtain a value for the A.U. 

 

The telescope

 

Built by Gault himself, the telescope is constructed of an aluminium frame with a glass concave mirror, 10 inches in diameter, housed in a plywood cylindrical body of internal diameter 285mm. The telescope is 1285mm long from end to end.

 

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Figure 2 – Gault’s 10” Newton reflector telescope and mount (Gault)

 

Base

 

The Newtonian reflector sits on top of an equatorial mount, with motorised control of azimuth and zenith.

 

Control of the telescope

 

The telescope is controlled by electric motors, moving the telescope horizontally (azimuth) and vertically (zenith). While the telescope can be aimed manually using the up/down, left/right control buttons on its base, it is set up to track astronomical objects automatically, as determined by the user.

 

Automatic tracking movement of the telescope is controlled through two computers. The first computer is a human interface, containing planetarium software with almanacs of stellar objects. The user can specify an object to track, and tell the telescope to track it, upon which the command is sent to the second computer, whose function it is to control movement of the telescope.

The second computer receives commands from the first computer – i.e. position and movement of the celestial object. This computer then converts the positional data into instructions to the motor controller unit to produce the movement required to follow the object as specified by the first computer.

 

The movement instructions from the second computer are fed into the control unit for the electric motors as mentioned above. The motors are powered according to the desired angular velocities in azimuth and zenith directions, which would change as a celestial object passes across the sky.

 

The result of this tracking system is that an observer, looking through the eyepiece of the telescope or at the video capture image, will see the tracked object stay in exactly the same position in the field of view for the entire duration that the object is being watched.

 

Video recording device

 

The design of the 10 inch reflector telescope results in an eyepiece located on the side of the telescope near the top (where the light comes in). On Gault’s example a fitting is attached here that accepts an eyepiece (for direct observation) or two types of CCD video camera. Fittings were designed such that all components could be easily interchangeable.

 

Gault used a security video camera as the recording device. In this case, however, it has been modified with a gain control added, with which a user can manually control the level of exposure in the images. For instance, when observing the Sun gain needs to be reduced so that the images are not ‘washed out’ with the Sun’s brightness, while at night gain would need to be increased to detect fainter stars. Increasing the gain has the effect of allowing the CCD to capture more light in each frame.

 

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Figure 3 – The modified CCD camera used to record the telescope image during occultations (Gault)

 

The video image has time inserted onto it, as explained below, then is sent to a VHS video cassette recorder, which according to Gault is the only recording system he has been able to get to work correctly. A live video output image is displayed on a small security monitor in the observatory so that a user can monitor the event while it is occurring and being recorded.

 

Time insertion

 

Time comes from various sources, explained in the section following. In this case, Gault has derived time from a GPS receiver, one of the most accurate time sources available (N.Z. Aus. Time Resources 2008). A Trimble SV6 receiver was used and hooked up to a small GPS antenna (resembling a miniature Leica survey-grade receiver antenna). For Gault’s observatory, this receiver provides both location of his observatory and 1PPS (Pulse Per Second) time output.

 

The 1PPS output is then fed through a piece of hardware designed by Geoff Hitchcox which converts the input time pulses to a digital ‘time stamp’ which is inserted into each frame of the video image (and thus recorded as part of the image). The function of this unit is explained in chapter 3.2.4.

 

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Figure 4 – The Trimble SV6 receiver (out of view) with external GPS antenna, linked to Hitchcox’s digital timestamp insertion device (the transparent box) (Gault)

 

The result is that each frame has data and time (to thousandths of a second) recorded at the bottom. This information provides the user with an exact time (in UTC) of any celestial event.

 

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Figure 5 – The on-screen display of the telescope image, with time digitally inserted. Image is of an occultation of the moon and a star (Gault)

 

 

Quick Facts about the 2012 Transit of Venus

 

Transit will commence at approximately 22:00 (Universal Time)

 

Transit duration will be about 6 hours 40 minutes

 

(Times above are approximate and will vary according to observer’s location)

 

For Sydney:

First contact: 8:16 AM

Last contact: 2:44 PM

The transit and the distance from Earth to Sun

History of transit observations

Observation methods

Observing conditions

Collaborating data

References

Contacts

 

 

 

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Matthew Cooper 2009

Last modified 30 October 2009