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Measuring Sunspot Rotation - by Ricky Leon Murphy:

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This was my first project years ago for my degree. This project used photographic data of the Sun to determine rotation of the Sun.

Enjoy...

            Using commercially available products, viewing the Sun safely with the unaided eye or with photographic equipment can show the surface and its sunspots. Viewing the changes in sunspots over time can reveal the rotation speed of the Sun as well as the life span of sunspots. While the equipment used to view the Sun can be expensive, there are alternate inexpensive methods available to study the Sun safely. The methods I used to capture images of the Sun are using a telescope with a solar filter and a digital camera. With the higher magnification available from a telescope, it is possible to view very subtle changes and track the progression of sunspots more accurately. Safety is extremely important. Viewing the Sun with the unaided eye without protection or through a telescope without filters can permanently damage the eye. If you suspect you may have looked at the Sun inadvertently, stop your solar viewing immediately and consult an eye care specialist.

            Viewing the Sun through a telescope with a solar filter is a very exciting experience. Of course, you do not need a telescope and a solar filter to view the Sun and its sunspots. One of the simplest methods of viewing the Sun is using developed un-exposed color slide film. A roll of slide film can cost around $4.50 and development of the film can cost an additional $4.00. Of course, prices vary. Buy your roll of slide film (any brand will do) and take it to the photo lab to have the un-exposed film developed. Remember to ask for develop only; asking for slides will cost extra, and slides are not helpful. What you will get is a very dark roll of film. Cut the film long enough to cover both eyes when being held by the hands. Overlay at least three pieces of cut film over each other. With this film over your eyes, you may look at the Sun safely. When viewing the Sun without magnification, it is possible to view sunspots, but only the larger sunspot groups (if there are large groups to be seen!). To study the rotation of the Sun and the life span of sunspot groups, this will suffice. A safer method of viewing the Sun is using a shoebox camera. With a shoebox held length wise, place a white piece of paper at one end, and punch a pin hole at the other end. Aim the pinhole towards the Sun, and you will see the Sun displayed on the white sheet of paper. The problem with this method is the difficulty in viewing sunspots. Usually this method is best for viewing annular or total solar eclipses. Viewing the Sun at sunset (or sunrise) may also be an option since the atmosphere acts as a natural filter, but the disk of the Sun is so distorted it may not be possible to view sunspot changes. The safest method of viewing the Sun is viewing the Stanford Solar Center website (http://solar-center.stanford.edu). This is an excellent resource for everything solar, and the cost is the fee for your internet service provider! Of course, it lacks the personal touch and the excitement of discovery. There are even more advanced methods of viewing and imaging the Sun. This method uses a very costly Hydrogen Alpha filter available from specialty companies like Coronado. With the Hydrogen Alpha filter, it is possible to view the granularities on the Sun’s photosphere, and more detailed views of sunspots. I have never looked through a telescope with such a filter in place, but I have seen breathtaking photographs.

            My method of choice for studying the groupings of sunspots is with a telescope and a solar filter. There is a benefit to this method: sunspot groups of any size are visible with a telescope and measurements of sunspot change will be more accurate since more subtle changes are viewed. Many of us interested in astronomy may already have a telescope. To make the telescope ‘solar ready,’ a solar filter must be purchased. Technically these are called white light solar filters, but catalogs and telescope retail stores simply call them solar filters. This filter fits over the objective lens of the telescope. Consult your telescope manual or telescope retailer to make sure you get the correct size; keeping in mind the dew shield has a larger diameter than then objective lens. This is a solar filter:

This particular model was purchased from Orion Telescopes (www.telescope.com). Viewing the Sun with the filter in place does not degrade the image at all, so any magnification may be used to view the sun. While I can certainly view and photograph the sunspots with high magnification, it is best to view the Sun as a whole. It is easier to capture the progression of sunspots across the surface of the Sun using a lower magnification.

            For documentation purposes, the changes of the Sun’s surface must be drawn or photographed. I am a very poor artist, so I chose photography as my method of documentation. Documenting changes using a pencil and paper will suffice for documenting changes of the sunspot groups – this is how the Astronomer’s of old documented their observations. Photography is much faster, and in my opinion, more accurate, especially when capturing subtle changes in sunspots. Film based camera or digital cameras can be used when photographing the Sun. For an SLR (single lens reflex) camera back, a T-adapter is required. The T-adapter allows the camera to connect directly with the telescope in place of the eyepiece, making the telescope a very long telephoto lens. This is called “prime focus” photography, so the image scale will be limited to the focal length of the telescope. Eyepiece projection uses the eyepieces magnification scale to view the image – what you see is what you get. Since I do not own a SLR camera body, I chose to use my existing digital camera. I would recommend a digital camera over an SLR camera[i] for several reasons: CCD technology has caught up with film with regard to image quality and CCD detectors are more sensitive than film. The main reason for the recommendation is the instant results that can be evaluated to ensure the best quality image has been captured. A film based SLR camera will require development of film which can take hours. A CCD is a “charged coupled device” and is the heart of a digital camera. An additional item that is needed is an eyepiece adapter, shown below with an eyepiece:

An eyepiece adapter allows a digital camera to mount on an eyepiece. The eyepiece of choice is a Meade Super Plossl 32mm (although any eyepiece will do, as long as the adapter will fit) and the adapter fits in place of the eye cup. This 32mm eyepiece, when used on my telescope with a 700mm focal length gives a power of 21 (21x)[ii]. The adapter is made by Scopetronix, and is available to fit a variety of cameras. The adapter next to the hex key screws into the lens of the digital camera, and that assembly screws into the adapter in the center of the image.

            Unlike viewing or photographing objects in the nighttime sky, viewing the Sun (in the day light of course) does not require equatorial mounts, drive systems, polar alignment or any of those painful to set-up checklists. The Sun is easily seen, and photography of the Sun, even with the filter, takes a fraction of a second. My telescope does come with an equatorial mount, but for studying the Sun, exact alignment is not required. This photograph shows the telescope, pointed roughly north, with filters and eyepiece in place:

I have a lovely view of the ocean (as seen by the extreme upper edge of the photo), but try to ignore that! California weather has given me the opportunity to view the Sun on a daily basis (with only a few days of partially cloudy weather). To photograph the Sun, my camera of choice is a Nikon Coolpix 995 digital camera. This is an excellent consumer grade camera that is very capable as an astrophotography camera. Since I had to use the camera to photograph the telescope set-up, a mirror was used to photograph the camera:

Although it is difficult to see, the silver circle is threaded to allow the addition of the eyepiece adapter. Minor changes to the camera should be made prior to using the camera on the telescope. It is important to turn off the automatic focusing, and to put the camera into shutter priority mode. By turning off the automatic focusing, the camera will not search for something to lock on so focus can be adjusted. When using a digital camera for astrophotography, the majority of subjects are dim so the automatic focus will not work. By setting the camera to shutter priority, the user has the ability to adjust the speed at which the image is captured. The sun is very bright, even with a filter, so a shutter speed of 1/500 should be selected (sometimes more or less). Dimmer objects, like stars, can be photographed with a shutter setting of 1” (one second) or more. Consulting the owner’s manual of your camera will give detailed instructions on how to do this.

            Prior to photographing the Sun, it is important to aim the telescope toward the Sun. Remember, safety is important, so I designed a method to allow for safely aligning the telescope toward the Sun. This is a two step process. The first step is to use the shadow of the optical tube:

The second step is to use the viewfinder[iii] so the brightest part of the Sun is displayed on the hand:

Once aligned using this safe manner the Sun should be within or very near the field of view when looking in the eyepiece. With my Nikon camera, and the low power of the telescope eyepiece, I am able to view the Sun on the LCD screen on the back of my camera. Once centered and focused, begin capturing images. You may need to take many photographs at different exposures and focus to find the right image. A technique I learned as a medical photographer is called image stacking – the process of reducing the transparency of a single image, and layering images on top of each other until the total transparency level is 100%. This technique is also used in Astronomy by using software used to “register” images for layering automatically (Registar is one software title that comes to mind, though it is not very good at stacking the image of the Sun). One good image per day is good enough to evaluate the changes of sunspots. One other important aspect of daily photography of the Sun that must be considered is the time of day in which the Sun is to be photographed. For accuracy, the Sun should be photographed during the same time everyday. In the length of time to photograph the Sun (about 2 weeks), the same position in the sky is about the same time of day – give or take a few minutes. There will naturally be an error of a few minutes since the best photograph may not have occurred at the same exact time as the day before. This method is using sidereal time. Once calculated, the solar rotation, based on sunspot changes, will be the sidereal rotation. If this had been the synodic rotation, images would have to be captured when the sunspots are at the same location on the surface of the Sun, and that would be useless in our experiment. A single photograph of the Sun is very dramatic, as seen below:

This photograph was taken on May 4, 2003.

To study changes in sunspot groups, and determine the rotation of the Sun, we must view the changes of the Sun’s surface on a daily basis. I have photographed the Sun over a period of 19 days, and concentrated my attention on two particular groups of sunspots which, luckily, begin on the extreme left side of the image and end on the extreme right of the image. The first group of sunspots is circled in blue, and goes through the most dramatic changes on a daily basis. The second group of sunspots is circled in black, and remains somewhat stable throughout the semi-rotation of the Sun. Each photograph was taken around noon-time on each day:

24 April 2003 25 April 2003
26 April 2003 27 April 2003
28 April 2003 29 April 2003
30 April 2003 01 May 2003
02 May 2003  03 May 2003
04 May 2003  05 May 2003
06 May 2003 07 May 2003
08 May 2003 09 May 2003
10 May 2003 11 May 2003
 
12 May 2003  

These 19 images show the gradual movement of sunspot groups over the Sun’s surface. The first group (circled in blue) appears on the far left of the 24th of April image and disappears on the far right of the 7th of May image. This group remained visible for 14 days, the duration of the semi-rotation of the Sun. The second group of sunspots (circled in black) appears on the 30th of April on the far left of the image, and disappears on the far right on the 12th of May. This sunspot group remained visible for 13 days – the semi-rotation of the Sun. Since the Sun is a sphere, multiplying the number of days by two will give the full rotation of the Sun. Based on the study of the above images, the solar rotation seems to be about 26 to 28 days, however the life of the sunspots can only be inferred around 14 days for the longest lasting spots – although the exact life span of both groups of sunspots is not certain. Very small sunspot groups appeared and disappeared in a matter of days, however the very large group remained for the duration of the semi-rotation, and changed dramatically on a daily basis. This information is very close to the actual rotation period and the life of the sunspots based on two sources of information: the HET602 text book, Universe by Roger Freedman, and the Stanford Solar Center website (http://solar-center.stanford.edu).  According to the Universe text, the solar rotation at the equator is 25 days, and the rotation near the poles is as long as 35 days. The groupings of sunspots observed in the images above appear to be above and below the equator of the sun, and this corresponds to the official data; however, it is difficult to pinpoint a measurement of sunspot life since they can vary from hours to months. Since the Sun is spinning, the shape is more oblate resulting in a more rapid equatorial rotation, called differential rotation. In order to mentally visualize the equator of the Sun, I used the Stanford Solar Center website. I was able to confirm to correct orientation of the photographs, so the rotation is accurate from left to right. If these photographs were used straight from the camera without orientation, we would still be able to assess the same periods for rotation and sunspot life but the rotation would be from bottom to top (because of the refractors natural ability to invert the image, and the right angle of the diagonal on the telescope to flip the image, and the angle of the telescope on my equatorial mount during the time of day).

            Safety in viewing the Sun is very important. Solar filters must be in place prior to viewing through a telescope, and the Sun should never be viewed with the unaided eye. At the Stanford Department of Ophthalmology – for which I am the Senior Ophthalmic Photographer, we had a case of a young 21 year old man who presented with a history of decreased vision after viewing the Sun with the un-aided eye. While he looked at the Sun for just a fraction of a second, the damage was done, and is not correctable. His vision, with glasses, was measured at 20/80 in both eyes (that means a normal person is seeing a group of letters at 80 feet as this young man sees them at 20 feet) – for comparison, normal vision is 20/20. A color photograph and fluorescein angiogram was performed on both eyes to document the change. First, we will show a normal color photograph of the retina, and a frame of a fluorescein angiogram (different patient):

Now for a color photograph and a frame of an angiogram of this young man; pay special attention to the small yellowish circular area in the very center of the image (circled in black). This is the solar “burn” on the macula. Also, note the lack of dye in this same area on the angiogram. This is the result of “capillary dropout” in the macula area that is permanent:

There is currently no treatment for Solar Retinopathy.

This project has demonstrated that with basic and consumer available equipment, data can be collected to determine some basic vital statistics of the Sun. With daily observation of the Sun, it is possible to measure the rotation period, as well as documenting changes and determining the life cycle of a group of sunspots. We have learned the Sun rotates at approximately 26 to 28 days near the equator, and groups of sunspots can last from a few days to 13 days, sometimes longer (according to the Universe text, page 406, sunspots can last a few hours to a few months). We have also confirmed our findings with official data, and discovered that the variability of rotation speed from our experiment is a result of differential rotation. We have also learned that safety is vital when viewing the Sun as damage to the eye is not correctable. With the appropriate safety equipment, anyone can enjoy viewing the Sun. A telescope is not necessary to enjoy the views of our nearest neighbor, but can provide some spectacular views. One can view the Sun using un-exposed film as a filter, a shoebox camera, or even a website. For the more serious solar observer, a telescope and a solar filter should be considered.

References:

Freedman, Roger A. Universe 6th Edition. W.H. Freeman and Company, 2002

The Stanford Solar Center. Internet. http://solar-center.stanford.edu. 2003
 

Websites of Interest:

Scopetronix: Quality Astronomy Products - http://www.scopetronix.com/

Coronado Solar Filters - http://www.coronadofilters.com/

Orion Telescopes - http://www.telescope.com

ESA SOHO website - http://sohowww.nascom.nasa.gov/

Registar Image Stacking software - http://www.aurigaimaging.com/

Adobe PhotoShop - http://www.adobe.com/products/photoshop/main.html
 

Special note: The transit of Mercury occurred on May 7, 2003. This transit was not visible in California so no transit is visible on any photographs captured during this time.


[i] Nikon makes an SLR digital camera. Only SLR cameras need a T-adapter. Digital cameras with integrated lens will require an adapter to mount to an eyepiece, provided they have a threaded lens. Such digital camera cannot mount directly to a telescope.

[ii] Telescope power is determined by the formula: F / f = x; where F is the telescope focal length, f is the eyepiece diameter, x is the power rating.

[iii] The telescope viewfinder should already be adjusted to the telescope. Consult the telescope manual on the process of aligning the viewfinder.

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