What is Hydrogen Alpha?

Hydrogen is the most common element in the Universe. This is of particular interest to astronomers due to the hydrogen emission and absorption spectra. The emission lines correspond to photons of discrete energies that are emitted when excited atomic states  make transitions back to lower-lying energy levels. The Balmer series resides in the visible spectrum with H-alpha falling at 656.3 nanometers or 6563 angstroms which is at the red end of the spectrum.  It is emitted by glowing hot plasma approximately 10000 degrees Kelvin!  Other discreet emission lines are also present as indicated above however the H-alpha line is most common, therefore emission nebula are good targets to be imaged with an hydrogen- alpha filter.  Typically amateur astronomers think of solar H-alpha filters however filters suitable for study of the solar chromosphere have a passband much too narrow for imaging deep sky objects. The passband of  deepsky H-alpha filters is typically 3-10nm centered around the emission line of 6563 Å. This is also the area of the spectrum in which most charge coupled devices (CCD's) are most efficient at recording light. In recent years amateur astronomers have discovered the ability to record stunning high contrast imagery from even bright urban locations as the narrow passband of an H-alpha filter effectively blocks the sodium and mercury ( night time ) lighting emission lines and overall attenuates sky glow considerably. With this narrow bandpass even nights with a full moon are now spent imaging if its clear!

Equipment

While any decent camera lens can be employed to take stunning widefield H-alpha CCD images the long exposures required to gain a good signal to noise ratio demand a high quality mount and the ability to follow the stars to a high degree of precision. The H-alpha filter while passing H-alpha light severely attenuates broadband energy sources such as guidestars. Most  are very dim and many imagers have reported they are not able to use the autoguide function of the  dual chip line of imagers from SBIG. Many  find that a seperate guide scope and (autoguiding) CCD camera work best for locating suitable guide stars. This is really determined by your mount and the focal length you are imaging at. Working at a focal length of 530mm I have no problems using  my ST-10XME's onboard guide chip to successfully autoguide the Astro-Physics 1200 GTO mount although I have had to use guide exposures of up to 20 seconds long! Again not every mount is capable of this feat and you may have to find an alternate solution if your mount is not up to the task. Above is my gear shown setup in my front yard. Note the black cloth that is wrapped around the focuser to block unwanted stray light as well as the Robofocus unit attached to the far side focus knob on the Takahashi FSQ 106 refractor. The ability to find accurate focus is critical to capturing good images. Also through the night with falling temperatures the refractor tube will contract necessitaing the need to refocus periodically. I use Focusmax along with the Robofocuser and Maxim DL/CCD which autofocuses the system very well. On some nights I am able to leave the setup unattended and  I gather photons while I sleep <G> My present setup is very mobile with the AP portable pier and mount resting on a set of JMI wheeley bars  can be deployed and polar alighned within 15 minutes!

While nearly all modern CCD's exhibit very good sensitivity at the H-alpha emission line the microlensed Kodak KAF 3200ME used in the SBIG ST-10XME has the highest available H-alpha quantum efficiency in a front illuminated sensor. Above is a plot showing the quantum efficiency of the microlensed chip (blue) and the standard blue enhanced sensor (yellow).

This along with the excellent noise characteristics of the SBIG ST-10XME camera make it a superb choice for H-alpha imaging.
 
 

Visit the Gallery (  The images have been captured with a high resolution CCD camera and file sizes can be large- please be patient )