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NGC7000 “North America” and NGC6888 “Crescent Nebula” in narrow band
 Thomas Lelu sent us another wonderful examples of great images captured with small camera. Thomas uses his G2-4000 camera on 10" Newtonian telescope (and also G1-0301 guiding camera) to capture very long exposures through filters, passing only very small interval of wavelengths.

Because Thomas chose to capture emission nebulae, using of narrow-band filters brings great effect. It is true, that the amount of light passed by these filters is very low, so a sensitive CCD camera and many hours long exposures are necessary to achieve good signal-to-noise ratio. But the light from these nebulae is not a result of high temperature, like in the case of stars (or candle flame or classic bulb tungsten filament), but it is emitted by certain atoms only and thus it has very specific wavelength. So narrow-band filter significantly lowers the amount of light captured from thermal sources (stars), but almost all light from the nebula reaches the sensor. The result is the nebulosity is enhanced relative to stars (and other sources, like light pollution brightening our night sky).

The first image is a part of a larger nebula with catalog designation NGC7000, called “North America” in Cygnus. According to our geographical analogy, the part shown spans the majority of Mexico (the Gulf of Mexico is on top). Detailed image shows the gaseous nebula pressed by intense stellar radiation of young stars. Interstellar gas then creates filaments and trunks. This part is also called the “Cygnus Wall”. The dark part on the top of the image with much less stars is in fact very similar cloud, but we see it from the back side. It is very probable the opposite side of the cloud, facing the bright stars, looks very similar to Cygnus Wall, which we can see from the side.

The image is composed from exposures captured thought narrow-band Hα, OIII and SII filters as well as through standard Red, Green and Blue filters. Total exposure time is 23 hours.

Another narrow-band image from Thomas shows the NGC6888 nebula called “Crescent Nebula”, also in Cygnus. The nickname originated when the first images, taken on classical films, revealed the nebula shape similar to crescent. But modern cameras are much more sensitive than films and show much dimmer parts of the nebula, so the crescent shape somewhat disappears.

The nebula is formed by intense radiation of a very hot star in the nebula center designated WR136 (such hot stars are named Wolf-Rayet stars after their discoverers).

As there is almost no SII emission in this nebula, Thomas used only Hα and OIII filters for narrow-band data. Total exposure time is 22 hours.

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