NGC 6979 – Central Veil Nebula

 

The Veil Nebula is the remnant of a supernova that occurred about 30,000 years ago. It is about 1,600 light years away in the constellation Cygnus. Here we see NGC 6979, the central portion of the nebula; north is toward the right. The eastern portion, known as NGC6992, or the Network Nebula, can be seen here.


NGC 6979

Equipment
TMB-130SS APO refractor at f/7 on an Astro-Physics 1200 equatorial mount
SBIG ST-8XM camera
SBIG CFW-10 filter wheel with Astrodon filters
Guiding: 60mm f/5 refractor and ST-402 camera
Imaging and autoguiding with MaxIm DL 4.62
Exposure
Luminance   (H-a) 3¾ hours (75 x 3 min.), unbinned, -15°C
Red   2 hours (40 x 3 min.), binned 2x2, -15°C
Green   1½ hours (29 x 3 min.), binned 2x2, -15°C
Blue   2¼ hours (45 x 3 min.), binned 2x2, -15°C
Processing Dark and flat-frame processing in CCDStack
Statistical-combined in CCDStack
Levels and curves and color-combined in Photoshop CS3
Date and Location 12 September, 2009
Montpelier, VA    N 37° 49' 12", W 77° 42' 06"


Raw and Processed

Newcomers to astronomy imaging often are confused and disappointed by what they see on the computer screen after downloading an image from the camera. Where's the target? Why is everything so grainy? I can see something, but why is it so faint? This set of images illustrates how raw images might appear, and what happens after some key processing steps (this is an older image, not the one at the top of this page).

Sample images
You might see this after downloading an image from the camera. Do not panic! This merely represents your software's current settings to display the image. You must adjust the histogram to show more or less of the image's data.

The histogram is a graph that shows all the image's data values. Pixels less than the "black point" are displayed as black, and pixels greater than the "white point" become white. Here, the black point is set higher than the nebulosity values, so the nebulosity displays as black, and can't be seen.
This is the same raw image, but the black and white points are both set low. The nebulosity is visible, but so is the background noise, and the bright stars are bloated. Your raw image may look like one of these examples, or it may be completely black or white.

Adjust the histogram so you can see the target, but don't worry about image quality at this point. Histogram settings affect only the display. The image retains its full range of data (except in Photoshop, where adjustments change the actual image).
Now the histogram is set to balance background, nebulosity, and stars. The nebulosity is very faint, and the image has "speckles" caused by electronic noise in the camera (the noise is removed later by subtracting a dark frame).

This image is fine to verify the target is framed and the equipment is working properly. Set the software to take a series of images, then sit back and relax.
The same image after subtracting a dark frame. The "speckles" are gone, but the image is still grainy and faint. This remaining graininess is caused by a low signal-to-noise ratio. There are two ways to improve this: Take longer individual exposures or combine many images.

(Although imaging software can expose and subtract dark frames while imaging, this consumes valuable time, and experienced imagers usually do it later.)
Here is the final image. It consists of 58 one-minute exposures, each with a dark frame subtracted, and combined to average-out the noise.

But these were only the first of the post-exposure processing steps. Several iterations of Curves and Levels in Photoshop applied a non-linear histogram stretch to bring up the faint areas without blowing-out the bright areas. Additional processing sharpened the image and reduced the residual noise even more.