Hydrogen Alpha Astrophotography by Pedro Re

NARROWBAND CCD IMAGING - Pedro Ré

These pages contain CCD images obtained with narrowband filters. The images were taken either at a site with moderate light pollution (Northern Hemisphere - central Portugal) or in a location with very low light pollution (Southern Hemisphere - northern Chile) using different imaging setups.

In normal color imaging, three filters (red, green, and blue) are used to separate the primary colors of the visual spectrum. Red, green, and blue (RGB) filters are designed to approximate the color sensitivity of the human eye, so that the resulting image is true color. Each of the RGB filters covers approximately one third of the visual spectrum and the filters overlap slightly so that the whole spectrum is detected by the CCD.

Narrowband filters capture only a very small part of the spectrum. The bandpass is simply how much of the spectrum the filter allows to pass. This is usually measured in nanometers (nm). The entire visual spectrum runs, approximately, from a wavelength of 400nm (blue) to 700nm (red). Therefore, a typical RGB filter might have a bandpass of 100nm. In contrast, a typical narrowband filter has a bandpass of just 3 to 5nm. A narrowband filter is designed for a specific astronomical emission line. It only passes a few wavelengths of light along with the emission line, rejecting all other wavelengths. As a result, the contrast of imaged astronomical objects improves dramatically. Narrower filters enhance contrast by reducing the broadband light which decreases the noise. The background signal decreases linearly as the bandwidth decreases. Narrower filters make it easier to image with the moon in the sky and from light-polluted locations.

Hydrogen-alpha (656.3 nm) is the most popular narrowband filter. Hydrogen is ubiquitous in the universe being present in emission nebula, planetary nebula, Wolf-Rayet objects and supernova remnants.

Oxygen III line (500.7nm) is given off by doubly-ionized oxygen atoms, meaning the electrons are dropping two energy levels. This line is in the blue-green portion of the spectrum. It corresponds to the peak sensitivity of the dark-adapted human eye, so OIII filters are common visual accessories. The OIII line is the dominant emission from planetary nebulae.

Sulfur II 672.4nm) singly ionized sulfur emits light in the deep red part of the spectrum, beyond H-alpha. It is a weaker emission than H-alpha and OIII, but it is the most common filter used after these two.

FULL SIZE
M033. 210min, 850min, 1000min & 1300min. ATMB152 F/7.9, FF, STL11000M, self-guided, Median Sum, DDP, Paramount ME

FULL SIZE
IC5067. TMB152mm F/8, ST-10XE, Astronomik Ha filter (13 nm), self-guided, SDmask, DDP, Paramount ME. 90min, 120min, 240min, 420min.

SOLAR TELESCOPES

White Light

Most Telescopes can be adapted for white light solar observing and imaging. Unlike a nighttime scope, an instrument for solar observing is not expected to gather a lot of light. When observing the Sun, most of the effort is spent in reducing the amount of light using objective filters or Solar Herschel Wedges. Solar telescopes are usually 150 mm or less in aperture. A 125 mm aperture telescope has a theoretical resolution of 1 arc second. Smaller telescopes (50 to 100 mm aperture) are suitable for full disk observation and imaging while telescopes of 125 to 150 mm aperture can be used for high-resolution work. Objective filters are intended to cut 99.999 % of the sunlight. These are made of Mylar or foil material. Glass objective filters can also be used with good results. The sun as viewed thought these filters can have a distinct coloration (blue, yellow or white depending on the filter). Solar Herschel Wedges are without any doubt the best way to observe/image the white light sun (Continuum). These devices absorb about 95% of the incoming sunlight. The remaining 5 % have to be reduced using neutral density filters. Solar Wedges should always be used with a refractor telescope. Other filters can be used to improve the low contrast of many with light solar features (e.g. Baader Solar Continuum, UV/IR, different Wratten filters).

H-alpha and Ca-K

Narrow band H-alpha (656.3 nm) solar filters are mainly of two types: front loading and end loading. The front loading filter uses a large diameter etalon (an optical filter that operates by the multiple-beam interference of light, reflected and transmitted by a pair of parallel flat reflecting plates, based on the Fabry-Perot Interferometer) over the entrance of the telescope. The end loading etalon is smaller and it's placed inside the light path of the telescope. Each of these configurations has advantages and disadvantages. The narrower a filter's bandpass or bandwidth (the extent or band of wavelengths transmitted by a filter) the greater is the contrast of the resulting image. In order to observe prominences in H-alpha a filter with a bandpass no wider than 10-angstroms (1 nm) is needed. A narrower bandpass filter (0.15 nm) will show a certain number of features but a sub-angstrom filter is needed to observe all the details on the chromosphere. Filters for Ca-K (396.9 nm and 393.3 nm) observing can also be used with excellent results. Compared to the H-alpha line, the H and K lines are broader and thicker in appearance: a filter having a bandwidth of 2-10 Angstroms is sufficient for Ca-H or K observations.

The Baader Solar Continuum filter is designed to enhance the visibility of solar granulation and sunspot details. By transmitting a specific spectral region around 540 nm, free of emission and absorption lines, the Solar Continuum filter is able to boost contrast and reduce the effects of atmospheric turbulence.

The Baader Calcium K-Line Filter has a 8 nm passband centered at 395 nm. It can be used for high-resolution imaging of super granulation, flares, and other features that are prominent in CaK.