Hydrogen Alpha Filters

Coronado H-Alpha Filter
By: Jeff Thrush
Safety Rules for Viewing the SUN
• Do not view the sun without proper solar filters in place on your telescope.
• Sunlight can also damage the optics of the telescope unless a proper filter is
• Use only solar filters which cover the entire aperture of the telescope.
• Never use an eyepiece solar filter.
• Cover or remove any finder scope or Telrad-style device.
• Exercise caution when other people (both children and adults) are at or
around your telescope.
Neutral Density Filters
• Neutral density solar filters are used to view and photograph
the photosphere of the sun.
• Neutral density filters come in two varieties: glass and
• Glass solar filters produce excellent images of the
sun and are less likely to develop pinholes than film filters.
• Glass filters are more expensive than film filters.
The photosphere is the thing we see when you look at the sun using a neutral
density solar filter.
The photosphere, is at a temperature of about 5800 K. Sunspots are "cool"
regions, only 3800 K (they look dark only by comparison with the
surrounding regions). Sunspots can be very large, as much as 50,000 km in
diameter. Sunspots are caused by complicated and not very well understood
interactions with the Sun's magnetic field.
Hydrogen-Alpha Filter
• Viewing solar prominences and flares, requires a specialized filter
called a Hydrogen-Alpha filter.
• Hydrogen-alpha (or H-alpha) filters are so-called because they transmit
a specific wavelength of light in the far red end of the spectrum called
the - you guessed it - hydrogen-alpha line.
• H-alpha filters are available in a variety of bandpasses. The "bandpass“
of a filter tells how wide a region of the light spectrum is transmitted
around the primary wavelength.
• The narrower the bandpass, the more surface detail on the sun
becomes visible through the filter. A wider bandpass will show only
prominences off the edge of the sun.
The chromosphere lies just outside the photosphere, and is almost
completely transparent.
The temperature in the chromosphere rises slowly as you go further
out; from about 4300 degrees to 8300 degrees at the edge.
How Does It Work
The Coronado filters consist of three elements:
1. The energy rejection filter (ERF) which removes the UV and the
bulk of the visible spectrum including all the infra red. The ERF is
an integral part of the second element, the narrowband element.
2. The narrowband element which gives the narrow peak at Ha plus the
adjacent peaks in the spectrum.
3. The blocking filter which removes all the peaks except Ha.
How it Works
The Sun in Hydrogen-Alpha Light
The Sun in Hydrogen-Alpha Light

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