Photographing the Sun and measuring exact position of Venus

The following advices of 2004 corresponded to analogously photographing (the usual method in 2004!). We have actualized them to digitally photographing by highlightening those hints valid for analoguous imaging only and by a newly added section.

Additional annotations about building solar filters, mounting the camera, ... can be found on Tim Cole's noteworthy page Solar imaging with a digital camera and in an article of "Sky and Telescope".

Tips for photographing
Additional hints for digital imaging
Position Measurements

In order to be able to determine the position angle of Venus with respect to the direction to east we propose to photograph the Sun twice on each picture with fixed camera. Precondition of this procedure is, of course, that the size of the picture is large enough for more than one image of the Sun. The smaller the Sun's face is compared with the size of the picture the longer can the time intervall be chosen and the more exactly can the orientation of the picture be determined.

In 2004 we used daylight slides (36mm*24mm) together with an optic of 800mm focal length. In this case, both exposures can be seperated by up to 6 minutes (see below).

Pictures of the Sun, taken with equivalent focal lengths of 250mm, 400mm, 1000mm and 1500mm

As our 2004 transit experiences show there are several crucial points to be regarded:

Tips for photographing

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Additional hints for digital imaging

Digital photographing is much easier!

Of course, a stable tripod is again important and the use of a timer or remote activator is very advisable in order to avoid shocks.

For processing digital images we recommend the public domain program ImageJ. But there are many other programs offering appropriate manipulating possibilities.
The following advices refer to "ImageJ".

The combination of successive photos can be done by the following steps (sequence of menu items to be followed in red):

  1. Open the images simultaneously with the program: File → Open

  2. Stack the pictures: Image → Stacks → Images to Stack
  3. Combine the pictures laying one "upon" the other (in the z-direction): Image → Stacks → Z Project.
    Following kinds of ZProjection should be tested:
    • Average Intensity
    • Max Intensity
    • Sum Images

    In most cases averaging (middle) seems to yield the best results. (The pictures have been taken with little experience with a new camera and at difficult weather conditions.)
  4. Improve brightness and contrast of the combined picture: Image → Adjust → Brightness/Contrast
    Probably, Auto will lead to a satisfying result. But there ar some other possibilities for manipulation.


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Position Measurements

In a picture with two solar discs which have been taken with fixed camera, the line connecting both discs represents the direction from (celestial) east to west. The position angle θ' of Venus is measured counterclockwise with respect to the direction to west. The position of Venus can be specified in the following two manners:

  1. by the position angle θ' and the relative distance r'=r/rS to the Suns's center or
  2. by the rectangular coordinates x'=r'cosθ' and y'=r'sinθ'.

Mercury, photographed from Namibia, May 7th, 2003 at 9.45 UT
Position measurement with "evaltransitpicts"

We offer the little program evaltransitpicts which allows to fit circles to the solar discs and to the discs of Venus. Resulting, it calculates both positions of Venus. Following the above outlined procedure it additionally calculates the angular radius of the Sun if its declination angle and the time difference between both exposures are known.

In the case of a single exposured picture the program determines the position angle with respect to the direction to the right edge of the picture unless the orientation of the picture is known.

If several pictures have been taken from the same site time and position are saved in a text file in a format which can easily be imported to an Excel worksheet. Excel than determines the line fit to the rectangular coordinates:

x' = x'0+a*t
y' = y'0+b*t

This fit makes it possible to interpolate or to extrapolate the positions to a common moment for different sites. The distance between two of those positions gives the parallactic shift β of Venus from which the solar parallax πS can be deduced following the procedure described in our basic paper.

The procedure is described in more details in our Mercury project and in the corresponding evaluation page of the measurements of 2004.
Typical results of our Transit of Venus 2004 Project can be found here.

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Editors: Udo Backhaus
 last update: 01.06.2012
Stephan Breil