Tandem white light & H-alpha Solar set-up
OBSERVING THE SUN IN H-alpha
When you observe the Sun in white light all you see is the Photosphere,
the visible surface of the disc. Detail visible in white light is limited to
sunspot umbrae and penumbrae, granulation (in excellent seeing), flocculi,
and the occasional white light plage or even less common white light flare.
There are several tried and tested methods to safely observe the Sun in
white light. These range from projection using a small refractor and a simple
Huyghenian or Ramsden eyepiece, to either metallised or polymer film or metallised
glass objective filters. My personal preference is the Herschel Wedge combined with
an ND3 filter & a variable polariser to adjust the light level so it is comfortable.
A significantly greater amount of low contrast detail can be seen in white light
using a Herschel Wedge. The image is polarised by its reflection off the first
surface, and then cross polarised by the variable polariser. This enhances low
contrast features.
However direct viewing via a Herschel Wedge is only practical with a refractor.
The secondary &/or corrector plate of either a Newtonian or Cassegrain
or a Maksutov or Schmidt-Cassegrain maybe damaged. For these types of
telescope an objective filter must be used.
During a Total Solar Eclipse the white light view of the Sun is completely transformed
because of the Moon's obscuration of the dazzling Photosphere. During those fleeting
seconds of Totality, Prominences and the Inner & Outer Corona can be seen.
The reason it is not possible to recreate a Total Solar Eclipse in a conventional
telescope is because the atmosphere and the optics scatter far too much light
which swamps the illusive Corona.
It is however possible to observe the comparatively brighter Prominences using a
special type of filter fabricated so as to transmit a very narrow slice of the Solar Spectrum
centred on Å6562.8. This wavelength corresponds to the Fraunhofer "C" line
in the deep orange, one of the Balmer transition emission lines due to ionised Hydrogen.
By restricting the portion of the spectrum to just the "C" line scattered light
from the Photosphere is almost completely blocked, enabling Prominences and the
Chromosphere to be seen clearly.
The "C" line is quite wide but the H-alpha signal is almost all contained within the Å0.1 core.
A filter with a passband no wider than Å0.1 would show very high contrast
Chromospheric detail and very sharp and contrasty views of Prominences.
Unfortunately, although filters are made with such tight passbands they are
prohibitively expensive.
The "C" line is Å1.4 wide @ 60% of the Photospheric continuum. A passband
wider than Å1.4 but narrower than Å4.0 can be used in a Prom 'Scope, an attachment
placed at the prime focus of a telescope which is fitted with a Lyot stop sized to occult the
Photosphere yet transmit light adjacent to the limb. With such an instrument Prominences
may be observed projecting beyond the silhouette of the Lyot stop. A commercial version of
the Prom 'Scope is sold as a Coronagraph, but a Coronagraph is a special refractor with
extremely transparent, scatter free lenses, used at very high altitude, intended to permit
observation of the Inner Corona in white light. No Coronograph would work at sea level,
because there is far too much atmospheric scattering by water vapour and aerosols.
Filters with passbands wider than Å1.4 are birefringent interference filters usually made from
synthetic Calcite or optical Mica (Muscovite). They are used in combination with a deep red
objective filter, typically plane polished Schott RG645, and a birefringent infrared blocking
filter made from Calcite vacuum coated with silver. These types of filter are relatively inexpensive.
Passbands narrower than Å1.2, at which the transmission in the "C" line is 50% of the
Photospheric continuum require a Fabry-Perot etalon, either single or twin stage, depending on
the width of the passband at full bandwidth half maximum (transmission) or fbwhm.
Again these filters require an infrared blocking filter and a deep red Energy Rejection Objective Filter.
Etalons between Å1.2 & Å0.5 may be tilt tuned. Tilt tuned etalons can be tuned over
±Å2 - ± Å4, corresponding to a Doppler shift of 100km/sec - 200km/sec. Following the
components of Prominences with line-of-sight velocities that give rise to such Doppler shifts is
straightforward. Chromospheric detail is difficult to see when the passband is wider than Å0.7
and even at this passband contrast is low. As the passband is narrowed towards the core Å0.1,
contrast increases dramatically. Image contrast relative to the core bandwidth is shown in this diagram:
H-alpha contrast-mod.pdf (76Kb)
Etalons with passbands narrower than Å0.5 cannot be effectively tilt tuned. Tuning becomes
extremely critical at such narrow passbands and the etalon stack must be temperature
controlled. The unit is mounted in a thermo-electrically stabalised oven, operating at a
temperature well above ambient, typically between 40°C & 50°C.
Some manufacturers also use a Peltier cooler so the filter can be held on band in a
very hot climate.
Detuning narrow passband etalons is problematic. So much so following Doppler shifted
components of Surges & Sprays can take several minutes. It is for this reason that
no single filter can enable all of the Prominence and Chromospheric activity to be seen.
Prominences are best followed with passbands between Å0.7 & Å4.0,
Chromospheric detail with passbands less than Å0.6.
Ideally the light beam entering the filter should be parallel. This condition can be realised using
a telecentric amplifier giving an effective focal ratio slower than f/28. When the passband is
wider than Å0.8 the effective focal ratio can be anything slower than f/20 and a Barlow lens
is effective. It is important when using a narrow passband filter to ensure the telecentric is
located correctly within prime focus and that the eyepiece is at the desired distance behind
it's top surface, otherwise the image may not on band uniformly across the fov,
becoming blue shifted off axis. The etalon must also be held square to the optical
axis, with no racktube sag.
A Fabry-Perot etalon mounted in front of the object glass operates at an effective focal ratio of f/110.
The etalon plates are made from fused silica and are hard dielectric coated. They have a much longer
lifespan than a tail filter which has the etalons made from cleaved optical mica which can only be soft coated.
The coatings are hygroscopic and over the years absorb atmospheric moisture, even if the unit is kept in a warm,
dry place. Eventually the etalon gap increases and you see Newton's rings when you look at it, and you cannot
tune it to the "C" line any more. However it is less costly to fabricate a narrow pass tail filter unit
than a full aperture front mounted etalon, and the tail filter units can be refurbished.
I have noted on some astronomy forums that when a tail filter fails it is because the infra-red
blocking filter has degraded. This is highly unlikely because the dielectric coatings on the
calcite plates are hard, since the calcite can be heated to a far higher temperature,
typically 300°C, and therefore they are far more durable than the soft coatings
that can only be applied to the mica plates of the etalon stack. The only way the infra-red
blocking filter would be likely to fail is if the unit was dropped onto a hard surface.
I have purchased a pair of H-alpha etalon filters. A tilt tuned Å0.9 passband from
Thousand Oaks
and an oven/Peltier tuned SO1.5 Å0.3 passband from
Mark Wagner @
SolarSpectrum Inc. I have set up both on my TEC140APO, together with
a TO ND5 metallised glass objective white light filter mounted on my 90mm
Vixen Guide 'Scope. The TEC140APO is stopped down to 2.75-inches with a Baader cool ERF
and operates via a Baader x2 telecentric amplifier @ f/28 and provides whole disc views at x60.
The white light 'scope works at full aperture, and I get a similar sized image using
a Nagler 16mm @ x 60. Both 'scopes are set up along side each other so I can quickly
compare the white light and H-alpha image.
Observing Solar activity in H-alpha is fascinating, and alarmingly addictive.
Eruptive Prominences and active regions giving rise to Surges, Sprays, Ellerman bombs
and Moreton waves can change before your eyes in a matter of less than a minute.
Prominences can be pushed quite rapidly up into the Inner Corona over an hour or so,
steadily becoming fainter. In very narrow passbands Prominences seen on the face of
the Chromosphere, and then termed Filaments, take on a 3D appearance because you
can see the brighter lower Chromosphere beneath them. And where a Filament goes over
the limb, the rest of it is seen as a bright Prominence against the black sky.
However the "C" line is not the only Fraunhofer absorption line into which one may tune.
There are the Calcium H & K lines @ Å3968.5 & Å3933.7, the H-beta "F" line @ Å4861.3,
the b4 line @ Å5167.4 and the D3 Helium line @ Å5875.6, amongst others.
If you were to have filters made for each of these lines of interest you would be left with
a mighty big hole in your bank balance. The only way to go if you want to tune into these
various spectral lines is to build yourself a Spectrohelioscope.
And that is my current project.
Stay tuned.
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