All About Space Astronomer Book - 2014 UK

Astronomer Book

a filtered 4-inch under good seeing conditions. This texture, often referred

to as a "rice grain" pattern, is known as solar granulation and is due to the

photosphere being made up of the tops of vast convective cells that start

many thousands of miles below. Each "cell top" is approximately 600 miles

across and typically lasts for 10–20 minutes before dissipating.

Narrowband Filters

The use of narrowband filters, such as those tuned to the hydrogen-alpha

(H-alpha) wavelength of 6562.8Å (1Å = 0.0000000001m) or Calcium-K

(CaK) wavelength 3933.7Å, reveal a completely different view of the Sun.

These filters ignore the vast majority of the incoming light, concentrating

on a very narrow window of wavelengths around the central value.

Although these aren’t the only narrowband filters available for solar work,

H-alpha and CaK are by far the most popular. H-alpha can be used for both

visual and imaging applications while CaK filters tend to produce an image

which is difficult to see visually. As such CaK filters tend to be used for

imaging only.

WARNING

It’s important not to confuse deep-sky and solar H-alpha filters.

Although they have the same name, deep-sky H-alpha filters are

unsuitable and indeed dangerous for solar work.

Hydrogen-Alpha Filters

In order to work, a solar H-alpha filter needs to be manufactured to

incredibly tight tolerances. Centred on the H-alpha spectral line, the window

of extra wavelengths that the filter passes – known as the filter’s bandpass

• determines just what the filter will reveal. Typically, a solar H-alpha filter’s
  bandpass will be in the order of 1Å or less. The central optical component of
  most solar H-alpha filters is an optical resonance cavity known as a Fabry-
  Pérot etalon. The incredible tolerances needed to make this component
  work are responsible for the often rather high price tag of these instruments.
  H-alpha filters are available as sets for converting astronomical telescopes
  for H-alpha viewing or as dedicated solar telescopes.
  A hydrogen-alpha filter reveals clouds of glowing hydrogen which exist
  immediately above and beyond the photosphere. The photosphere itself
  is hidden in this view, beneath a blanket layer of hydrogen known as the

are extracted, aligned and averaged. There are various computer programs

which can do this automatically such as the freeware AVIStack and Registax.

The obvious targets for white-light imaging are sunspots and sunspot

groups. These high-contrast features are also useful as focusing targets.

When imaging the Sun, start out using a low power first using prime focus.

If the conditions are stable, optical amplifiers such as Barlow lenses can be

used to boost the magnification but it is important to match the power to the

conditions. Use too high a power under poor seeing and what you’ll end up

with will be fuzzy and low on detail.

If there are few or no sunspots visible, the edge of the Sun can be used for

focusing. The outer regions of the Sun's disc in white light look darker than

the middle due to an effect known as limb darkening. Look carefully in the

shaded edge regions because here you may see lighter patches known as

faculae. These are regions where strong magnetic fields reduce the density

of gas in the photosphere, allowing us to see deeper into the Sun where

hotter gas radiates with greater intensity. The photosphere itself is textured.

This can be seen visually with a 6-inch properly filtered scope or imaged with

5

6 7

[5] Sunspot groups visible on the white-light Sun. [6] & [7] Active regions of the Sun seen in H-alpha.