December 2020 | Science Reporter | 45

Hydrogen-Alpha System

Debasis Sarkar

Sun through H-Alpha Filament and Prominence

A Window to Sun

THE Sun is an energy-belching fireball – the only star that we can see as a dish, rest all other stars appear as dots. Although the Sun has always fascinated humans,

it keeps itself hidden behind its unimaginable brightness.Using materials like shoot glass, welder glass or

projection of solar image to cut down the extremely high intensity of Sun for visual observation have all been tried out. But all these show the Sun as a flat dish – without much of its surface details. The ‘Hydrogen-Alpha Telescope’ or in short the ‘H-Alpha telescope’ is an effective tool to study the Sun.

The H-Alpha telescope system brings us hidden details, it shows us Solar prominences or filaments which are large, bright features extending outward from the Sun’s surface into the Sun’s hot outer atmosphere, Corona. It gives stunning views of Solar Eclipses. Shooting this giant blazing ball during transits of inner planets or International Space Station over its face is a fascinating challenge for serious sky enthusiasts.

Colour of SunlightThe Sun spews out a huge bundle of electromagnetic waves of wide range of wavelengths. Out of this complete range or spectrum, only a tiny portion from Red to Violet comes within our visual perception. While red has a wavelength of 700 Nanometer or 7000 Angstroms (Å = 1/,00,00,000 millimetre), that of Violet is 400 nm or 4000 Å. Thus, when broken down using a prism, almost white incidental Sunlight shows us a band of different colours from Red to Violet in output. This is a smaller spectrum of colours in visible light. Each colour in that spectrum has its own distinctive wavelength.

Brightness of SunlightThe first objective before studying the Sun is to reduce the intensity of sunlight that reaches us. This is done using Sun filters that can bring down the intensity to 1/100,000 times. At this diminished intensity, the Sun appears no brighter than the Full Moon. As all the colours of sunlight get almost equally attenuated with these filters, we find the Sun as a near-white dish.

But it appears almost static and featureless as details of Sun’s dynamic top surface ‘Chromosphere’ get washed out due to the much brighter underlying ‘Photosphere’ layer. Here comes the role of the H-Alpha system.

What is H-Alpha?If we break Sunlight into a spectrum through a spectroscope, we find many definite dark lines in the multicolour band due to the absence of a few colours or lights of particular wavelengths. These dark lines are caused by elements that absorb the lights of those particular wavelengths. Most prominent one among them is caused by absorption or influence of Hydrogen. That portion of the spectrum at the wavelength of 6562.8 Angstroms is commonly known as H-Alpha. The Sun’s top surface or Chromosphere greatly influences this particular portion of the spectrum as it is actually a layer of glowing hydrogen gas.

So, if we see the Sun only through the H-Alpha portion of the spectrum by eliminating all other lights, we will be able to ‘see’ disturbances, influences or details of Chromosphere. But to do that, instead of simple filtering we need a system that at first can attenuate sunlight to a safe level. Then it needs

featuRe ARticle

46 | Science Reporter | December 2020

Sunspot Solar transit of International Space Station in H-Alpha (India’s first H-Alpha ISS Solar Transit image)

Annular Solar Eclipse (26 Dec 2019) composite image in H-Alpha. (India’s first Solar Eclipse image in H-Alpha)

Solar surface features and size of Solar System objects against them

to block all colours of the spectrum while allowing a very narrow portion or band only to pass. To work properly, this bandpass needs to be only 0.7 Å wide centred at the Hydrogen Alpha spectral line (6562.8 Å).

A time tested method of doing this is by using a Spectrohelioscope. But that is too complicated. The other and more common one is H-Alpha telescope that allows a bandpass of 0.5 to 1.0 Å centring at 6562.8 Å.

The H-Alpha System consists of:• Energy Reduction Filter (ERF)• Fabry-Perot Etalon• Blocking Filter (BF)

Energy Reduction Filter (ERF): ERF is the first part of an H-alpha system. It cuts down the intensity of light almost equally for the entire spectrum. This filter is usually placed in front of a refractor telescope of aperture ratio f/10 or higher.

Fabry-Perot Etalon: Named after its inventors Charles Fabry and Alfred Perot, this special kind of arrangement of two optical discs is the heart of the system. It allows only specific sections of the spectrum to pass through.

Etalon is a stack of two plane-parallel optical surfaces with a tiny space in between. With proper and careful placing,

this stack can cause interference of light that comes on it. And then it gives out a series of narrow bandwidth (Near 1 Å) spikes of light components in its output. One of these spikes comes with 6562.8 Å wavelength H-Alpha line at its centre. But this output is not yet ready for observation as it is still a mix of many light components of different frequencies from the full spectrum.

Blocking Filter (BF): This is a semi-narrow bandpass filter that allows a small portion of spectrum around the H-Alpha line while blocking all others. When the series of spikes coming out of the Etalon are passed through this BF, it blocks all but allows one spike only with 6562.8 Å wavelength at its centre. This final narrow band light coming out the BF carries the sharp signature of the hydrogen-rich Chromosphere or Solar surface.

2nd Stack of Etalon: To make the image further sharper, an even narrower bandwidth is needed. To ensure that the second stack of Etalon is added to the system. When properly tuned, the second etalon marginally cuts down the bandpass. With a carefully maintained and tuned double Etalon system, we can have a narrower bandpass of even 0.5 Å.

Understandably, images formed through H-Alpha system are essentially Monochromatic. Only one colour is there

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Spectrum of Visible light H-Alpha Telescope with Double Stack Etalon

which is predominantly Red because of its frequency 6562.8 Å, very close to that of Red (7000 Å). Thus, to make them visually pleasing the images are usually false coloured instead of keeping them monochromatic or black-and-white.

Using H-AlphaThe H-Alpha is used like common refractor telescopes used for either live viewing or photographing. But tuning an H-alpha system is a tedious job that demands deep patience. The attachment of the second Etalon sharpens the image, but at the cost of brightness and by making it more complicated to handle. Eventually, viewing becomes tougher with an H-Alpha system with the second stack of Etalon.

As it is with all other optics, the quick movement of the Sun within the field of vision is a major hurdle. A properly polar aligned RA axis motorised mount can keep the system in sync with the Earth’s movement and thus freeze the Sun’s movement in the viewing field.

Photography with H-AlphaThere are different techniques to attach a camera to an H-Alpha system. It can be ‘Prime Focus’ style where the eyepiece of the H-Alpha system and camera lens are not used. The main objective of the system works as the camera lens while the camera body takes the photo. Or ‘Afocal’ style where the image formed by the objective is thrown by the eyepiece

Solar features through H-Alpha

Prominence: Large, bright, gaseous tongues extending outward from the Sun’s surface. Anchored to the Sun’s surface in the Photosphere, Prominences extend outwards into the Sun’s corona.

Solar filaments: These  are dark lines or curves on the solar surface.  Filaments  are actually prominences seen from a different perspective. They appear dark over the background of the much brighter solar surface.

Sunspots: Dark spots on the solar surface. These are cooler areas compared to the surroundings. 

Solar flares: These are a sudden explosion of energy due to tangling, crossing or reorganising of magnetic field lines near sunspots.

Solar granulation:  Grainy features visible on the surface of the Sun. Thermal currents in the thermal columns cause the formation of granulation.

Mr Debasis Sarkar is working as Special Correspondent with a national financial daily newspaper holding the responsibility of the media organisation’s news bureau operation for entire Northern West Bengal, Sikkim, Bhutan and East Nepal. Address: Subarna Kutir, 57, Aban Tagore Sarani, Deshbandhu Para, Siliguri, West Bengal-734004. Email: debasisslg@gmail.com

Images credit: Janmejoy Sarkar (Sky Watchers Association of North Bengal)

on the camera lens. Eyepiece projection is also common to ‘project’ the image formed by the objective through the eyepiece directly on to the film or electronic image sensing panel inside the camera without its own lens.

But whatever the attachment pattern, filament and other surface details demand grossly different exposure settings of an H-Alpha camera set-up. Thus, multiple shots with different properly calculated exposures are taken. Then specialised image post-processing techniques are adopted to blend those images to develop an image with proper replication of surface details consisting of different features.

Now many companies offer H-Alpha systems at an affordable price ranging from USD 600 to USD 4000. But high import duty or shipping costs escalate its price in India. As it is with almost everything else, ease of using them or other features of different models of different brands varies with the price. And so for the output quality too.