Our star the Sun
Astrosolar Filters
One of the most effective ways to protect a scope for white-light viewing
or imaging is to construct a full-aperture filter for it out of a material called
Astrosolar film made by Baader Planetarium. The film is available in A4-
sized sheets or, for larger instruments, a 1.0 x 0.5m roll. Two versions of
the film are available, in different filter densities, one for visual and one for
imaging applications; the higher density (ND5) film is suitable for both uses.
Astrosolar film is safe to use on any type of telescope. Simple instructions
for creating your own custom filter cell are given on the Baader Planetarium
website. Other types of certified white-light filter are also available. The thin
film nature of Astrosolar film means that it doesn’t, and indeed shouldn’t, be
stretched across the filter cell tightly. Ripples in the film have no significant
effect on the incoming light path.
Before fitting, it’s good practice to hold the filter up to the Sun for visual
inspection. Any holes or tears indicate that the filter should be discarded and
a new one made. If the filter passes visual inspection, it should be fitted to the
front of your scope in a way that won’t allow it to lift off easily. For safety, a
piece of low-tack tape can be used to secure the filter to the scope.
Another important safety point concerns the telescope’s finder, which is a
small telescope in its own right. This needs to be capped, filtered or indeed
removed before pointing the main scope at the Sun.
With the main filter fitted, and the finder dealt with, the next job is to point
the telescope at the Sun. The safest way to do this is to look at the scope’s
tube shadow on the ground and position the telescope so that the tube’s
shadow profile is as small as possible. Once this has been achieved, it means
the telescope will be pointing at, or at the very worst, close to the Sun. Final
positional tweaks can then be done through the eyepiece.
The white-light Sun can be photographed in much the same way as the
Moon and bright planets, using the set-ups described in chapter 3 and
elsewhere throughout this book. The best solar imaging results come from
using a high-frame-rate camera because this provides a means to reduce
the blurring and distorting effects of the Earth’s atmosphere.
As the Sun heats the ground, it’s fairly common to find the view becomes
less stable throughout the day. This is evidenced by the edge or limb of
the Sun appearing to "boil". The resolution of fine detail on the Sun’s visible
surface (photosphere) is heavily dependent on the overall stability of the
view. This stability is described under the general term “seeing”. The Mount
Wilson Solar Seeing Scale, shown reproduced below, lists the different
types of solar seeing that may be encountered. An arcsecond, by the way, is
1/3600th of a degree or roughly 1/1800th the apparent diameter of the Sun
in the sky.
1 = Solar image looks like a “circular saw blade”. Completely out of focus.
Limb motion and resolution greater than 10 arcsec. Smaller sunspots will
not be seen.
2 = Solar image is always fuzzy and out of focus. No sharp periods. Limb
motion and resolution in the 5 to 10 arcsec range.
3 = Solar image about half the time sharp and half the time fuzzy. Some
short periods where granulation is visible. Limb motion and resolution in the
3 arcsec range.
4 = Solar image more often sharp than not. Granulation almost always
visible. Limb motion and resolution in the 1 to 2 arcsec range.
5 = Solar image looks like an “engraving”. Extremely sharp and steady.
Limb motion and resolution 1 arcsec or better.
A high-frame-rate capture typically produces several hundred or even
thousand still frames. In order to construct the final image, the best shots
track their change. A large spot consists of a dark central region or umbra
surrounded by a lighter area or penumbra.
When a spot reaches the limb (edge) it is carried round to the far side of
the Sun to reappear after a fortnight, if it still exists. A really large group may
persist for several rotations, though small ones will disappear after only a
few hours.
You can learn a great deal about the Sun and its spots even with a small
telescope, but for advanced studies we need more than a telescope. We
need instruments based on the spectroscope, a device astronomers use to
reveal the chemical make-up of stars.
Observing the Sun is a potentially dangerous activity and must be carried
out with great caution and care. Projection methods should only be carried
out with refracting telescopes. The use of specialist certified solar film
allows you to use any type of telescope as long as the full aperture of the
instrument is securely covered. Note that cheap eyepiece Sun filters should
not be used.
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