The Horsehead Nebula in Orion is shown here to illustrate the noise reduction and SNR increase obtained from image additions.
The image on the LEFT is a single 20 min exposure with an ST-7 in red light using a 6-inch f/7.5 A-P EDF in basically average seeing.
The image on the RIGHT is a co-added image created from 4-20 minute exposures which were shifted and rotated for registration using IDL S/W written by the author.
For non imaging visitors, the image above is of a nebula in Orion.
What you are seeing is a glowing cloud of hydrogen gas ( the brighter
areas of the image ) which has a dark obscuring gas cloud in front of
it. This dark cloud of gas just happens to have a shape that our minds
recognize as a silhouette of a horse's head ( or more like a stylized Knight
chess piece ! ). This faint nebula is located near the left most
(eastern) star of the 'belt' of Orion ( the Hunter ) in our winter sky
(or Summer sky for Souther observers!).