Transit of Venus: Technical Info

The disc of Venus on the sun is much larger than that of Mercury (almost ¹/32 of the apparent diameter of the sun) and so can be seen even without a telescope. Naturally, this must be done through a filter (or eclipse specs) to avoid damage to eyesight and cut down glare but was something I felt very worthwhile to do as it is all too easy to only view astronomical phenomena through instruments and forget to experience them directly! For photography, however, something more sophisticated is required for good results.

Although a dark filter can be used, the best and safest method is to project the image and photograph it directly. To avoid the distortion problems experienced when photographing the image projected onto card I used a translucent plastic screen instead. The image was projected onto one side and photographed from the other, allowing the camera to be placed directly in line with the image and thus see a round disc. As with the Mercury transit I allowed the camera to decide its own exposure, which was again satisafactory, but used a 28-70mm lens set at a point which filled the frame with the sun's disc with the camera a convenient distance from the screen. By ensuring that the disc was the same size for each shot I ensured that the final images would need least manipulation. Due to the space constraints of the location I was using it was not practical to achieve a constant orientation, however, but at least this would only require a rotation of the frame to achieve. I also took a few close-up images by use of a different lens but these were less satisfactory due to the difficulty of accurately focussing both the telescope and the camera.

The large size of the disc of Venus means that attaching a camera directly to a telescope eyepiece will be very effective, but the same precautions must be taken as with the naked eye and so a sufficiently bright image is hard to achieve. Digital web cameras solve this dilemma, being light and essentially disposable should a disaster occur! As with the astrophotographs, I used a Phillips ToUCam Pro coupled via a commercially available adaptor onto a Meade EQ114 reflector with an equatorial mount for easy of tracking. The filter problem was solved by attaching a pair of eclipse specs to the top of the telescope tube so that one eyepiece was located above a subsidiary opening in the tube cap, through which the image was taken. Remarkably, the specs fitted the telescope absolutely precisely and gave a truly excellent result! Have a look at the pictures of the transit for a better idea of how this worked.

The above precautions meant that processing the images was reasonably straightforward, if tedious. The projected images were simply re-scaled to a convenient size then rotated into the correct position: this could be quite accurately estimated by reference to a plot of Venus' motion over the sun's disc made using the StarryNight astronomical program. Slight "up & down" adjustments then finalised the placement before the images were reversed and rotated by a constant amount to bring them into the correct NSEW orientation. The digital images required just re-scaling and contrast enhancement but because of the much higher magnification achieved had only a small part of the sun's disc visible. This was solved by laying each of them onto a blank "sun disc" of appropriate size, the correct position being determined by aligning the section of disc-edge visible in each with the edge of the blank disc.

After processing, the images were mainly used to make up into animations as this seemed the best way to demonstrate the transit. Unfortunately, this produces rather large file sizes so the animations on this web-site are on the small side to reduce download times. I decided not to rotate the digital images as a whole into the correct (telescopic) orientation, as this would involve much effort for little visual reward, but I have rotated the composite image so it can be directly compared with the "naked eye" track derived from the projected views. It will be seen that whereas the digital track is a straight line the naked-eye track is curved and not on the same alignment. This is not an error on my part but a result of the different telescope mounts used to capture them.

The digital images were taken with an equatorial mount which tracks the sun, ensuring its disc is always in the same alignment relative to the camera each time an image is captured. However the naked-eye images were taken with an alt-azimuth mount which only moves "up & down" and "side-to-side", failing to compensate for the fact that the sun is actually rotating as it climbs to the zenith i.e. its North pole points to the left near sunrise, "up" only at noon, then to the right towards sunset. To see what I mean, try holding the palm of one hand flat towards yourself with the elbow of that arm on a table. Place the side of your hand next to the little finger on the table also, then rotate your lower arm upwards (with the elbow still on the table!) until the arm is straight up. You will see that your palm rotates by 90degrees, in the same way as the sun does. This rotation is not visible (as the sun is featureless) but can be seen by observing the moon in the same way. The image of Venus also moves with the sun, of course, causing its apparent position as seen by the naked eye to change, turning its track into a curve. This curve has its greatest "bend" late in the transit because at this time (around noon) the sun is rotating most quickly as it heads towards the zenith and then down again in the afternoon. Clear? Mmm, thought not - ah well, I did try! The image above shows the effect well - the telescopic track has been rotated so the sun is in the correct orientation for the early part of the transit (about 8:45am) and the naked eye track laid on top of it. At first the two tracks align but as time passes the naked eye track diverges and curves as the sun rotates in the sky.