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Naked Eye Sun-Spotting by John Walker |
Image: SOHO EIT |
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Naked Eye Sun-Spotting by John Walker |
Image: SOHO EIT |
I've long been interested in a subject in amateur astronomy with few potential objects: naked- (or unaided-) eye observations in daylight or twilight. The potential “life list” of such objects is rather short. Over last several decades I've managed to check off most of the obvious ones: the Moon, Venus, Mercury, and a solar eclipse. Other candidates, such as fireball meteors and supernovæ bright enough to be seen in the daytime sky, require luck more than assiduous observation—you just have to be in the right box of spacetime. One less elusive daytime phenomenon which evaded me for more than thirty-five years was a naked-eye sunspot—a sunspot (or, more precisely, sunspot group) large enough to be glimpsed without any optical aid other than suitable eye protection for viewing the Sun.
The probability of observing a naked-eye sunspot closely follows the solar activity cycle. The average number of sunspots varies with a period of about 11 years (embedded in a 22 year cycle in which the magnetic polarity reverses). Sunspot activity is measured in Wolf numbers, defined as the number of sunspot groups multiplied by 10 plus the number of individual sunspots. Here are the Wolf numbers over the last four complete solar cycles and the approach to the current solar maximum, as compiled by Royal Observatory of Belgium's Sunspot Index Data Centre:
When I started looking for naked-eye sunspots around 1965, the chances of spotting one were next to nil; this was near the solar minimum, where few sunspots are observed, and those which do appear are usually small. I did try to keep an eye on the Sun around the solar maxima of 1981 and 1992, but other distractions and cloudy skies caused me to miss whatever opportunities presented themselves on those occasions. As the solar maximum expected around 2000 approached, I made an effort to observe the Sun whenever the sky was clear (which is to say not all that often) and I had the time (rarely).
On Saturday, September 23rd, 2000 I made my customary scan of the Sun with an 80mm apochromatic refractor and Orion full-aperture solar filter. I immediately noticed a huge sunspot group near the centre of the Sun's disc. After observing it with a variety of eyepieces, I decided to see if if, at long last, I'd found one large enough to observe without any optical assistance.
I took the full-aperture filter off the 80mm
refractor, held it up
to my eye, et voilà, a naked-eye sunspot, hurrah! With a
good quality aluminium film solar filter or
welder's glass it was not even a difficult object.
The picture to the right, image processed from
slides I took of the Sun later that day, approximates the visual
appearance of the Sun; actually, the large sunspot group near
the centre of the Sun's disc seemed somewhat darker and more point-like
than in this rendering, but it isn't far off the mark.
Next I tried a pair of “eclipse specs” I used to
observe the partial phase of the
1999 solar eclipse in Iran,
and although they had far more scattered
light and fuzz, the sunspot group could still be
clearly seen, albeit with somewhat more effort and
patience.
I also tried observing the Sun with a little 8×25mm
monocular with eyepiece projection. Totally hand-held,
I aimed the gizmo at the Sun by manipulating it until its
shadow was as small as possible, then held a piece of paper
behind the eyepiece to catch the projection of the Sun's
image. With a little fiddling to get the distance
right so it would pop into focus, I was rewarded with a
clear, albeit jiggly, view of the spot group about
a clear as through the “eclipse specs”.
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Having just succeeded at spotting a naked-eye sunspot, I
immediately proceeded to try to photograph it, attempting
to capture the visual impression (not to mention to obtain
that ever-so-important desideratum for all visual observers
of unusual phenomena, evidence). I mounted a
500 mm f/8 fixed-aperture catadioptric “mirror lens” on my 1974 vintage
mechanical Nikkormat camera, loaded with Kodachrome 200
slide film. (This was a suboptimal choice, but it's
the best I had on hand. Ideal would be a slow, very fine grain
black and white film [the Sun shows no colour details in a
white light image] or, for colour, the higher resolution
Kodachrome 64—one thing about photographing the Sun:
there's no shortage of illumination!) I placed the Orion
solar filter on the front of the 500 mm lens (it didn't fit
tightly, but I decided to take the extra care not to
dislodge it rather than spending the time to improvise some
elegant way of securing it such as jamming rolled up toilet
paper in the gap), aimed and focused, then shot a 36
exposure roll of film bracketing the exposure all over the
reasonable range of shutter speeds. Different makes of
solar filters vary substantially in transmission and while
this isn't very apparent to the eye, which responds
logarithmically to intensity, it substantially affects the
optimal exposure of photographic reversal film with linear
response and narrow latitude. Examination of the resulting
slides showed that exposures of 1/250 and 1/500 second
yielded the best images.
Click on image for larger scale view. |
When the slides came back from Kodak, I selected
the best and took them to Photo Numérique Service
in Neuchâtel
where they were scanned into 3612×2352 36 bit per pixel RGB
TIFF files of about 50 megabytes per slide. Since the solar
image on these frames was only about 512 pixels in diameter
(a 500 mm lens is a bit short for full-disc solar photography),
film grain was rather prominent in the raw images, so I
arithmetically added two of the best images,
which averages out the grain while enhancing genuine
features common to both images. The resulting image is shown
to the left at half scale; click it to view the full-size
image. The colour is that resulting from the spectral
transmission properties of the Orion filter, which is biased
toward yellow-orange. The light of the Sun is, of course,
white, but most solar filters tint the light they
transmit—the eclipse specs, for example, show the Sun as
light blue.
This image shows three principal sunspot groups and hints of
two or three other less prominent groups (only the largest
was visible to the unaided eye). Photographing the
Sun with a small-aperture lens through the roiling
atmosphere is not conducive to sharp images. For a
clearer view, let's blow this spheroid with its
murky and fickle atmosphere in favour of a vantage
point where “the Sun don't set”…a halo orbit around
the L1 Lagrangian point between the Earth and the Sun
where the gravitational attraction of the two bodies
balances. Objects at this position are in a metastable
orbit; spacecraft must use thrusters to periodically “trim”
their orbit to compensate for perturbations. Since 1996,
the
Solar
and Heliospheric Observatory
(SOHO) has been circling L1, continually observing the Sun with a
diverse array of instruments in a variety of wavelengths.
Real time
images and a complete data archive are available on the Web.
SOHO images are courtesy of the SOHO/CDS consortium. SOHO
is a project of international cooperation between
ESA and
NASA.
Below is a full-disc white light (intensitygram) image captured by the Michelson Doppler Imager developed by the Stanford-Lockheed Institute for Space Research. This image was captured by SOHO a couple of hours before I took my own Earth-bound pictures.
Comparing the two images, the subtle smudges below the main sunspot group (certainly) and to the left of the disc (maybe) on my photo are marginally resolved sunspot groups which are obvious on the SOHO image.
Features big enough to see with the naked eye against the surface of the Sun at a distance of one astronomical unit are large. The equatorial diameter of the Sun's photosphere is about 110 times that of the Earth's, so given that the large sunspot group subtends about 10% of the Sun's disc, it is hence about ten times the size of the Earth. I've enlarged the sunspot group from the SOHO image and inserted an image of the Earth to scale to show their relative sizes.
Click on detail to view full Sun image. |
You don't have to go into space to produce high resolution
images of the Sun. The
Big Bear
Solar Observatory, situated in a man-made reservoir at an
altitude of 2000 metres in southern California, enjoys more than
300 cloudless days per year and excellent seeing due to smooth airflow
across the lake. Like SOHO, the Big Bear Observatory makes
real time images
and a
comprehensive
archive available on the Web.
Big Bear observes the sun in a variety of wavelengths with
two vacuum telescopes (evacuating air from the tube
eliminates turbulence which would degrade image quality): a 65 cm
reflector and 25 cm refractor. A separate 20 cm refractor
produces images of the full Sun in the Hydrogen-alpha
spectral line. Attached to the bottom of the 20 cm telescope
is a 9 cm refractor which photographs the full disc in
both visible and near ultraviolet light. The image at the right is
a detail of the large sunspot group from a visible light
image taken by the 9 cm a few hours after I observed the
sunspot from Europe; click the image to display the full
solar disc.
If you want to add a naked-eye sunspot to your personal list of things very few people have ever seen, there's no better time than the present; the solar maximum period around the years 2000–2002 should provide more candidate large sunspot groups than the years which follow. If you miss this opportunity you'll probably have to wait 11 years until the next solar maximum.
Be sure to take adequate precautions when viewing the Sun—never observe the Sun without a filter made for the purpose or a welder's filter (number 13 or 14) that renders the Sun dim enough to view comfortably. Improvised solar filters such as smoked glass, fully exposed black and white photographic film, CD-ROMs, and floppy discs all pose substantial risks. The value of your eyesight renders the cost of a welder's filter negligible.
Never point any kind of optical instrument at the Sun unless it is fitted with a full-aperture objective filter expressly designed for viewing the Sun. The filter must be securely attached so it cannot come loose while you're observing. With the objective of your telescope protected by a solar filter, make sure the objective of the finder scope and any auxiliary optics are capped. Many solar observers enraptured by the spectacle of our neighbourhood star have found themselves rudely snapped back to Earth upon discovering their hair set afire by the Sun focused through a finder scope; don't let this happen to you. It's easy to aim your telescope at the Sun even without a finder. Just adjust the tube so that it casts the smallest shadow on the ground behind the tripod. This should place the Sun within the field of view of a wide angle eyepiece, whereupon you can centre it with the slow motions before switching to a higher magnification.
Even near the solar maximum, sunspot groups large enough to be visible with the unaided eye are rare. Observing one used to require a modicum of dedication, checking the Sun every few days, ideally first with a telescope, to see if a candidate sunspot group was visible. Near real-time solar imagery on the Web now permits routine monitoring of the Sun without ever going outside. By viewing the on-line imagery from SOHO or Big Bear Solar Observatory, you can see if there's a sunspot group on the solar disc large enough it might be glimpsed without magnification. The European Space Agency has even developed a free screen saver which shows “live from the Sun” images captured by SOHO whenever your computer is idle.
Stargazers sometimes neglect the closest star, whose ever changing face provides fascinating sights when all the other stars are chased by its coruscating brilliance. Sunspots visible to the naked eye are rare but, when they appear, are visible to anybody on the Earth with a clear sky and a safe solar filter. Further, they persist for days, so there's plenty of time after discovering one crossing the face of the Sun to wait for the clouds to part and reveal it. Naked eye daytime astronomy may be a subject with few objects, but its rewards are great for those who've seen something in plain sight that few other people have ever glimpsed.
All original images in this document are in the public domain and may be used in any manner without permission, restriction, attribution, or compensation. Images from the SOHO spacecraft and Big Bear Solar Observatory must be used according to the conditions of use of their creators. Back links to Naked Eye Sun-Spotting are welcome.