Observing the sun:
- Remember, never look at the Sun without proper eye protection. There are a
variety of ways to safely observe the Sun, many of which can be found in the
section of SkyandTelescope.com.
- The Sun is one of the few objects that display a
rewarding amount of detail without a telescope or even binoculars. The only
equipment you need is an appropriate filter — a piece of No. 14
arcwelder's glass is the traditional choice. This safe filter material is
available at any welding-supply store (check your yellow pages for a local
dealer) in convenient 4-inch-wide pieces that allow viewing the Sun with
both eyes. Although welder's glass imparts a green hue to the Sun, one of
these economical filters might be all you ever need for casual observing.
But while welder's glass provides satisfactory naked-eye views of the Sun,
its poor optical quality makes it unsuitable for use with binoculars and
- The part of the Sun that we actually see is a layer called the
photosphere. It is the nearest thing the Sun has to a "surface."
The first features you are likely to notice on the photosphere are large
sunspots. Spots of this size are fairly common
when the Sun is active, and occasionally several groups are visible at the
same time. Tracking the visibility of large spots provides an interesting
project for naked-eye observers. In addition to sunspots, look for
decreasing brightness toward the edge of the Sun's disk. This limb darkening
is the result of looking through a progressively thicker cross section of
the darker, cooler upper photosphere near the Sun's limb.
- direct telescopic viewing using neutral density type filters:
- When it comes to outfitting optical
instruments for solar viewing, a number of excellent options are
available. However, there is one type of filter that is very dangerous:
the eyepiece Sun filter. These were once commonly supplied with imported
telescopes and consist of a piece of dark glass mounted in a cell that
screws into the bottom of an eyepiece. The heat from the Sun
concentrated by a telescope can shatter these filters without warning.
Today's advice is to destroy these filters to ensure they can cause no
- For safe viewing, most observers choose either a glass or Mylar solar
filter mounted in a cell that fits securely over the front aperture of a
telescope which cost ~$A150. Such filters are made with light-rejection coatings that
allow only a fraction of a percent of the Sun's light to pass. This
style of filter protects not only your eyes but your equipment too,
since the potentially harmful heat of the Sun never enters the
- Glass solar filters generally produce a yellow or orange Sun, while
Mylar filters usually yield a blue image. Aesthetics aside, there are
other differences to consider. Mylar filters tend to offer better
contrast between the solar disk and bright faculae surrounding active
regions. However, Mylar's blue-tinted image also suffers more from
scattered light and atmospheric dispersion than the orange image
produced by a glass filter. Somewhat better sunspot detail is seen with
a glass filter, but faculae are usually rendered all but invisible.
Although these filters are more alike than different, it is probably
best to give some thought to what you most want to see before purchasing
a filter for your telescope.
- Telescopic solar observing is pretty
straightforward since venders make filters sized to fit most popular
instruments. Simply attach your filter to the front of the tube so that
it cannot fall off, and you're in business. Don't forget to make sure
that your telescope's finderscope is capped at the objective end or,
better yet, removed completely. Aiming the telescope without a finder
might seem problematic but it is quite simple. Just move the telescope
around until its shadow is minimized, at which point the Sun should be
within the field of a low-power eyepiece.
- For most visual astronomy, bigger is better — the larger the scope,
the more light collected and the greater the theoretical resolution.
However, when it comes to solar observing, the playing field is tipped
in favor of smaller scopes. Light-gathering is not an issue since we are
trying to dim the Sun's intense glare, but what about resolution? Here
again, the advantages of a large instrument are essentially neutralized
by atmospheric turbulence. Daytime seeing is rarely steady enough to
permit the maximum resolution of even a 4-inch telescope.
- Glass and Mylar filters can also be used with binoculars. In addition
to making it possible to view small sunspots, binoculars will show the
limb darkening with greater ease than the naked eye alone. You can
purchase filters for many binocular sizes, and you can even make your
own from Mylar solar-filter material available from several venders.
Make sure filters are firmly affixed so that they will not fall off or
blow away in a gust of wind.
- solar projection viewing:
- One method of solar observing that dispenses
with filtration altogether is solar projection. (See "Observing
The Sun By Projection.") An eyepiece is placed in the
telescope's focuser and used to project an image of the Sun onto a
convenient flat surface. Telescopes with folded light paths, such as
Newtonians or Schmidt-Cassegrains, are not recommended since the
converging beam of light can produce enough heat to damage internal
- When it comes to eyepieces for projecting the Sun's image, the
much-maligned Huygenian design is a good choice because it does not
contain cemented elements that can be damaged by the Sun's intense heat.
Most solar projection is done onto white paper or card stock. But no
matter how white the screen, it must be adequately shaded from direct
sunlight and other extraneous light in order for the viewer to see the
finest details in the solar image. This powerful technique enables a
4-inch telescope to produce a usable image of the Sun 30 inches across.
The size and brightness of the Sun's image depend mainly on the distance
between the eyepiece and the viewing surface — the farther away it is,
the larger and dimmer the image.
- direct viewing using the amazing H-alpha filters to allow wavelengths
of only 6562.8A (H alpha):
- these provide fantastic image details of sunspots, mass coronal
ejections, etc but are very expensive:
- for example, the Coronado
filters require an objective filter and a special eyepiece filter in
$US in 2003:
- filters are 0.7A bandwidth but can be double stacked if buy two to
increase contrast and reduce bandwidth to < 0.5A.
- MaxScope40 40mm telescope OTA $US890, with filters $US2155
- MaxScope60 60mm telescope OTA $US1790, with filters $US3685
- MaxScope90 90mm telescope OTA $3990, with SolarMax90 + TMax90
filters $US9798 (<0.7A) and $US12185 (<0.5A)
- filter packages only:
- SolarMax40 + TMax40 + B5 $US1265 (nb. special blocking filters
also available for Meade ETX60 or 70)
- SolarMax60 + TMax60 + B10 $US2590
- SolarMax90 + TMax90 + B10 $US5045
Solar prominences sprang into view, with excellent contrast. A
solar flare ( an astonishing bright white feature) was clearly
visible on the Sun's disk with granularity and mottling visible
around sunspot groups. The Solar disk was surprisingly bright and contrast was easily on a par with
filters by Day-Star (read costs much more $$$) and clearly
superior cheaper to 1.5 angstrom designs.
Only having a
40mm aperture raised a question about the resolution of such a
small aperture. This was not an issue. Daytime
"seeing" was the limiting factor. The image was sharp,
with more magnification showing the limits of the seeing more
than the aperture. Our only criticism would be the small
exit pupil of the blocking filter tends to restrict the field of
view at longer focal lengths. Also as successively higher power
eyepieces were used the apparent image contrast fell away quite
quickly (not surprisingly, there is only so much light you can
squeeze through a 40mm aperture!). This made visual observation
of prominence features at high magnifications not as satisfying
as the low power views through the same instrument.
magnification limitations notwithstanding, H-Alpha viewing with
SolarMax proved to be very addictive. Low and Medium power views
clearly showed prominences: looped, hooked, fanned and
continually changing shape. The view is dynamic, and a
refreshing alternative to deep sky views enjoyed by astronomers
of the night."
- imaging with the SolarMax:
Users of smaller
CCD cameras (SBIG STV, ST5c, ST237a) will find no restrictions
when trying to image the sun through the exit pupil of the
SolarMax blocking filter.
The brightness of
the image is a little too high! with *CCD images are close to
saturation with even the briefest exposures. A Neutral Density
filter is required. SBIG ST7e owners would probably need
to obtain a support bracket to hold the camera without placing
too much stress on the 1.25" blocking diagonal. SBIG
ST8/9/10/1001 owners would find the blocking filter field
restrictive with only limited field able to be projected onto
the larger detectors of these cameras.
Users: would need an eyepiece projection tube that can be
inserted into the 1.25" blocking diagonal.
- Sunspots are relatively cool regions in the Sun's photosphere (visible
surface). They form where solar magnetic fields lock onto the Sun's churning
ionized gas and hold it in place long enough to cool. The photosphere's
average temperature is 5,700° Kelvin, but in a sunspot's dark center
(umbra), where the field is strongest, it's about 2,000°K cooler. If you
could view a sunspot by itself, it would appear blazingly bright. It appears
dark only in contrast with its even brighter surroundings. The umbra of a
spot is typically surrounded by a lighter penumbra, where the magnetic field
allows the gas more motion.
- Solar activity varies with an 11-year cycle. As the cycle progresses,
activity rises and falls, and with it the amount of detail visible on the
Sun. At solar minimum, the Sun often appears nearly featureless, completely
free of sunspots. At maximum, however, there can be hundreds of sunspots
arranged in a half dozen or more groups and plenty of faculae. Obviously,
the most exciting time to observe the Sun is in the years surrounding solar
maximum. The last solar maximum was in 2000, and we are currently seeing the
fall to the next minimum due in 2005 or 2006. Although the years ahead will
see less solar activity, the occasional appearance of a large sunspot group
can still take astronomers by surprise.
Mass Ejections (CME):
- solar flares produced by sunspot activity cause an outburst of
high-energy particles which if it is headed directly toward Earth may
produce a severe or extreme geomagnetic storm. Transformer damage and
widespread blackouts are possible at higher latitudes, though power outages
are the most severe of the possible effects. Spacecraft are also at risk,
and radio-communication disruptions are expected.
- such events may also cause aurorae.
- videos of the massive CME x17.2 directed at earth on 28th Oct 2003 taken
by SOHO showing the snow storm effect as the high energy particles travel
towards us, the bottom two videos have had the sun itself blocked so that
the CME is visible:
- Coronal holes are "dark" coronal regions of the sun with open
magnetic field structure. During the minimum years of the solar cycle they
are confined in the Sun's polar regions, while at solar maximum they can be
found at all latitudes.
- The fast-speed solar
wind originates form the coronal holes, and accordingly they are
considered the main reason for the "recurrent" type of geomagnetic