Exercises in Photographing Mercury in front of the Sun
and in Measuring its Position
First off all and most important:
NEVER try to photograph the Sun without using a solar filter (made from glass or film)!
Detailed annotations about building solar filters, mounting the camera, ... can be found on Tim Cole's noteworthy page Solar imaging with a digital camera, in an article of "Sky and Telescope" and in Fred Espanak's Transit of Mercury 2016 page.
On this page we only give short tips in order to motivate the participants doing exercises early before May 9th.
Digitally photographing of the Sun
- The camera should be mounted on a solid tripod.
- To avoid blurred images
- a remote-control release should be used, if possible and,
- in case of a reflex camera, the mirror should be fixed (if possible).
The focus must be as exact as possible because a bad focus causes uncertainties in determining the Sun's radius and errors in measuring the position of Mercury. But this position has to be measured with greatest precision! But: The visual impression of a good focus when looking through the viewfinder is no guarantee for a well focussed picture!
The edge of the Sun, possible sunspots and, perhaps, Mercury itself will offer the best opportunity for focussing. Using a magnifier for the viewfinder (today a feature of many digital cameras) or of the life view window of the laptop from which the camera is been operated may facilitate the focussing.
The exposure time must be suitably chosen so that the picture is neither too bright nor too dark. Some hints may be useful:
If the camera can exposure automatically it should be tested if shortening the time by one or two steps yields better results (see the examples below).
|90 seconds time difference
||Automatic exposure -1 step
||Automatic exposure -2 steps
Most often the brightness of the picture is measured in the center of the picture. Therefore, by automatic exposure, the Sun will be taken with different brightness depending on its position on the picture (see the left picture below).
Perhaps it will be better to choose a fixed exposure time. Of course, it depends not only on the aperture of the optic but also on on the Sun's elevation above the horizont and on the state of the atmosphere!
- Because of turbulences in the atmosphere it will be advantageous to make several images shortly one after the other (automatically, if possible) instead of only one and to select the best of them ex post.
Example: The Sun April 18th, 8:26 UT
The 7 images, automatically repeated within a second, show significant differences. The most right image has been taken from NASA's Solar Dynamic Observatory (SDO) from above the atmosphere; it has been taken about two hours later.
Into the complete image and into the most left cutout we have artificially mounted Mercury in it's natural size. It demonstrates the importance of preferably sharp images!
- The best of these images you may upload to the database via the according data exchange page.
Taking "twice exposured pictures"
In order to be able to determine the position angle of Mercury with respect to the direction to west we propose to photograph the Sun twice with fixed camera. Afterwards, these pictures can be superimposed using a software like ImageJ. The combined picture will show the westward motion of the Sun due to the Earth's daily rotation.
Precondition of this procedure is, of course, that the size of the Sun in the picture is small enough so that it may show 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 the orientation of the picture can be determined.
The following hints may be useful:
- For this procedure a very stable tripod is even more important than for single exposuring.
- Using an autotracking it must be possible to switch off the automatic for some minutes, because even parallactic mountings never are justified with sufficient accuracy. Alternatively, the right ascension can be shifted slightly to observe its direction on the picture.
The time difference between both pictures should be adjusted to a fixed value and maintained carefully (with a stopwatch!):
- We recommend a time difference of 150s (if possible). In this case the both solar faces don't overlap on the combined image.
- If you must choose a shorter interval test the longest duration possible because a large displacement is a precondition of precise measuring its direction. Additionally, you can in this way minimize the probality of Mercury to be positioned in the overlapping region of both images.
But be careful: The Sun tends to leave the picture in the meantime!
For the 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):
- Open the images simultaneously with the program: File → Open
- Stack the pictures: Image → Stacks → Images to Stack
- 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 seems to yield the best results. (The pictures have been taken with little experience with a new camera and at difficult weather conditions.)
- 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.
- Multiple exposuring must be exercised!
As another example the following picture shows the combination of a test serie which served to find out the optimal time difference. In order to find the orientation of the own images the time difference should be as large enough to seperate both pictures of the sun. Shorter differences are possible as well, but the accuracy will be the better the larger the seperation between the discs is.
|Serie of images with Δt=30s
The images have been superimposed with the public domain program ImageJ by using the "average intensity"-option. Other image processing software may be useful as well.
Measuring Mercury's position with respect to the Sun's center
In this way, we will try to measure Mercury's position on the solar disc.
back to the project page
- Knowing the time difference between the combined pictures you are able to determine
Position measurements with the program "evaltransitpicts" provides ρS as an additional result. For elementary methods of measuring it see the additional project Measuring the angular radius of the Sun of the project in 2012.
- the orientation of the image,
- the scale of the image (provided the decination δS of the Sun is known) and
- the angular radius ρS of the Sun.
- Example: The following photos have been taken with fixed camera on March 8th, 2016 between 9 and 10 UT (δS=-4.7°) with an exposure time of 1/4000 s. Everybody will have to find out the exposure time which is optimal for his optical system.
|09:29:29 UT||09:32:00 UT||"Twice exposured" image with Δt=151s
For this combination the "max. intensity" option has been chosen.
Because of the rotation of the sky (or of the Earth, if you prefer this viewpoint) the Sun moved by 15'*cos(δS)=14.95' per minute and, therefore, by 37.62' in 151s. Taking into account the pixel positions of the Suns and its pixel radius, we found ρS=16.13' (The correct value was ρS=16.11'.)
On that day, the sun showed two little sunspots. By using our little (Windows-) program evaltransitpicts it is easily possible to measure the positions (ρ',θ') of these spots. The angular radius of the Sun is an additional result of the procedure:
rS=16.1', (ρ'1,θ'1)=(0.319,84.2°), (ρ'2,θ'2)=(0.791,-2.75°)
In order to make the east-west-direction parallel to the lower edge of the image it must be rotated by -17.78°.
Do you get similar results for these photos?
- Those position data should be uploaded to the database via the according data exchange page.
|| last update: 21.04.2016