Horsehead Nebula, etc.

Date: 20 February 2007 U.T.
Time: 4:10 U.T.
Telescope: 30cm Newtonian
Magnification: 61x
Filter: UltraBlock
NELM: 6.5
Seeing: Poor
Temperature: +25 F (-4 C)

I started out without using the above mentioned filter. NGC 2024 (also known as the Flame Nebula and Tank Track Nebula) was visible near Zeta Orionis. This nebula has a prominent dark lane running through it.

After shifting the telescope slightly to the south, Zeta Orionis left the field of view. The small, bright nebula NGC 2023 was now easily visible surrounding a star.

The next logical step was to look for IC 434 (a faint 'bright' nebula) and the dark notch within it known as B33 -- the Horsehead Nebula. These objects were visible with difficulty. IC 434 was only slightly brighter than the background sky. The Horsehead Nebula has a reputation for being a particularly difficult object to see visually without the use of a specialized (H-Beta) filter. In order to successfully see this object without a filter one must observe from a dark location that's free from the glow of city lights.

I don't have an H-Beta filter; but I do have an UltraBlock filter. The UltraBlock is a multi-layered, interference filter that selectively blocks the wavelengths of some of the more common man-made light sources (as well as some of the natural sky glow) while allowing other wavelengths to freely pass through. I screwed the UltraBlock filter into the eyepiece and took another look.

IC 434 and the Horsehead Nebula were suddenly significantly easier to see! The filter significantly darkened the background sky, but not the light from IC 434. As a result, the nebula became easier to see.

I should add that filters such as the UltraBlock do not improve the visibility of all objects in the night sky. Many objects become more difficult to see when such filters are used. Everything depends upon the wavelengths of light and their relative intensities as emitted by the various celestial objects. Furthermore, the effects of many such filters can vary significantly depending upon one's sky conditions and light pollution situation.

For more information on the Horsehead Nebula the curious reader is encouraged to visit Stephen Waldee's excellent Horsehead Nebula Main Page and associated links at:
http://home.earthlink.net/~astro-app/horsehead/index.html