A tale of two Crescents

Thursday night, the 17^th, was another fairly clear night. So, out to the observatory I went. It was a night that I didn't have high expectations because of the 1^st crescent: the moon. For someone trying to image deep sky objects, the light from the moon causes the contrast to decrease significantly. Also, for us earthbound inhabitants, the sky is never really black. While it may seem inky black, it is only a very dark gray at best. That would be because of the lights here on earth that disperse in the atmosphere and light it up. It's called skyglow. OK, there are a few other contributors to skyglow, but man made light is the primary one. When I take an image, it's done by taking a series of images, usually with one each of 4 filters, for a certain length of time and then “stacking” them together. There is a stack for the red filter, blue filter, green filter, and a luminosity filter, which is basically clear, but it blocks infrared light. The 4 stacks are then combined to achieve the color image I present here on this blog. However, if I were looking through the eyepiece, all the images here would appear gray. See the entry for M20. So, if the moon lightens the background gray to the same shade as the nebula I want to see, the nebula effectively disappears. The longer light gathering time of a camera helps with pulling the nebula out of the background, but the fuller the moon, the brighter the background and the harder time I have pulling out the dim objects like a nebula.

Well, that brings us to the 2^nd crescent; NGC6888 or the Crescent Nebula. 

NGC6888

 

 

8” LX200GPS F10, QSI 683

LRGB 33,3,3,3

Imaged in Nebulosity 3

Post processed in Nebulosity 3 and Paint Shop Pro 7

(lots of 3's, huh?)

This image is a stack of 9 minutes each of red, green, and blue, and a stack of 33 minutes luminosity. (Most of the detail is to be found in the luminosity channel.) NGC6888 is formed by the fast stellar wind from the Wolf-Rayet star WR 136 (HD 192163) colliding with and energizing the slower moving wind ejected by the star when it became a red giant around 250,000 to 400,000 years ago. Thank you Wikipedia. At least, that's what I understand. The remaining star is call a Wolf-Rayet after Charles Wolf and Georges Rayet who discovered them in 1867. NGC6888 is about 5000 ly away in the constellation of Cygnus, the Swan (aka The Northern Cross). The reason for so many other stars in the image is because the Milky Way galaxy runs right through Cygnus. So all of those little dots of light are stars in our own galaxy.