NGC 2174, the Monkey Head Nebula, is located in the part of the sky where the "ecliptic" passes through it. The ecliptic is a narrow band in the sky (about 20 degrees wide) through which the Sun, the planets, their moons, and most of the asteroids (also called "minor planets") can be seen in the sky. Consequently, one is much more likely to image asteroids while photographing objects around the ecliptic.

As it happens, a bright asteroid (Magnitude 13.6 asteroid #248 Lameia) was passing through NGC 2174 while I was photographing it, and it left an obvious line on the photograph. I asked for some help from members of Yahoo's Digital Astro mailing list in finding out how I could identify the asteroid in my image. Andrea was kind enough to point me to Lowell Observatory's Asteroid Plot which allowed me to identify the asteroid. Meanwhile, Jeff recognized that there was another asteroid right next to 248, and then he saw a third one on the right side of the image. Using Asteroid Plot, I was able to identify those asteroids as well:

Magnitude 16.2 #9939 (1988 VK)
Magnitude 16.9 #8070 (1981 EM30

I saw on Asteroid Plot that there could be other asteroids visible on the image as well, although these additional asteroids were much fainter. I could not see any obvious tracks on the image, so I decided to see if some image manipulation might make them visible.

To accomplish this trickery, I created two different images. One of the images had the first 2 1/2 hours of the evening, and the 2nd image had the latter 2 1/2 hours. I then used Images Plus to subtract the 2nd image from the first image. Throughout most of the image, the two images would look the same. As such, subtracting Image 1 from Image 2 would end up yielding black (with the exception of noise differences between the images). However, where there was an asteroid traveling through the field, the results would be different. In the early part of the evening, the 1st image would have a bright spot, and the 2nd image would have nothing. Subtracting the 2nd image from the first would yield a bright spot.

In the second half of the evening, the 1st image would have no data (except noise) where the asteroid passes, and the 2nd image would have a bright spot. So, subtracting the images yields a negative number, which is translated to 0 (black). So, the result of this technique ends up showing a bright line followed by a black line where there is an asteroid track.

After creating the difference image and aggressively "stretching" the image, I was able to identify 2 more faint asteroids:

Magnitude 17.8 #118973 (2000 SS277)
Magnitude 17.9 #72506 (2001 DV69)

The final two asteroids are marginally visible in the processed image, but they are clearly visible on the difference image. I was very excited to find out that I could actually detect 18th magnitude asteroids! Additionally, I was equally thrilled to know that I had an image with 5 documented asteroids.

The images below show enlargements of the 5 asteroids tracks. Hold your mouse over each of the images to display the associated difference image.