Once is a great while, you really will get something that goes wrong that no one really has a good explanation for. Sometimes you can only use your best guess as to what caused it.

After doing astrophotography for more than 30 years, I have seen just about everything that can go wrong. Most of the time it is simply operator error, or mechanical problems, or equipment failure. Sometimes you get something you haven't seen before and it is like a puzzle to figure it out.

Below are just a few examples of the dozens and dozens of things that can go wrong in a long-exposure image. But this is part of what makes astrophotography such a fun hobby - overcoming challenges and problems!

Dust Spots

These really are no mystery at all, and are actually quite common. They are the shadows from pieces of dust, lint and other kinds of boogers that fall on the filter in front of the sensor. They are out of focus because they are on the filter which is a little ways in front of the sensor. They may appear more or less in focus depending on the focal ratio of your optical system.

Some newer cameras have systems built into the camera that tries to prevent this dust from falling on the filter, or tries to shake it off when you turn the camera on or off. Sometimes it works, sometimes it doesn't. Sometimes it just moves the dust around

Four different examples of dust on the filter in front of the sensor cause shadows in these images.

• Solution:
  • Keep the sensor as clean as possible. Don't leave the camera body open without a lens or lens cap on it. If you do get dust shadows, try blowing the dust off with a blower bulb. Do not use canned air, or a high-pressure air hose as you may damage the sensor.

  • Send the camera back to the manufacturer to have it cleaned.

  • Clean it yourself. If the dust is really stubborn, and the camera is out of warranty, you can try using a Sensor Swab and some Eclipse cleaning solution. Note that there are different types of Eclipse solution based on the kind of coating on the filter in your camera. See this web site on cleaning digital cameras for more information.

    Warning! Note also that, done incorrectly, attempting to clean your sensor can damage it!

Depending on the type of scope and f/ratio, sometimes the mirror in a DSLR can cause a shadow that runs across the bottom of the frame that sort of looks like the band of darkness at the bottom of this image. But a mirror shadow would not cause these bands on both the top and bottom of the image. There is also amp glow present in the upper and lower right corners, but we know what that is.

In this case, these dark areas were caused by a bad sensor in the camera. DSLR cameras are highly complicated electronic devices that are mass produced, and sometimes you just get a bad one.

The bands on the top and bottom of the image are caused by a bad sensor.

• Solution: Get the sensor in the camera replaced by the manufacturer.

What is that ethereal strange light in the image below? Is it a UFO? Is it an angel?

No, sorry. It is just lens flare from the brilliant full moon. Hold your mouse cursor over the image to see the full frame.

Mouse-Over

Ghosts and flare are caused by the full moon in this image. Hold your mouse cursor over the image to see the full frame.

• Solution: Remove any filters you have on the lens. Keep extremely bright objects, or lights, outside of the frame. Even then, if they are just outside of the frame, you may still get ghosts, flare or reflections. If the bright light source is just outside of the frame and you are still getting these kinds of problems, try using a lens hood, or using your hand to shade the front of the lens.

The example image below shows what happens when you try to use an interference filter on a wide-angle lens. In this case the interference filter was an IDAS LPS light pollution filter used on a 24mm lens.

Interference filters need light that is mostly parallel. When a wide-angle lens is used, the light comes in from an extreme angle, and the bandpass of the filter shifts differentially across the frame. In this case, the result was the artifact of a large green donut of differential filtration.

This image shows the results from using a light pollution interference filter on a wide-angle lens.

• Solution: Don't use an interference filter on a wide angle lens. You can try to clean up this mess with software like GradientXterminator, but it will be extremely difficult.

Ok, these really are spooky! They scare the heck out of me.

What caused them? I don't really know. But I can make some guesses...

Weird star trails in this long time-exposure guided image were probably caused by a cable getting caught on something during the exposure.

In the example above, my best guess is that a cable somewhere on the scope, possibly a power cable for an anti-dewer, got caught on something as the scope tracked during the exposure. I don't really know for sure. Perhaps the camera or scope started to bump into the tripod leg during the exposure.

One initially perplexing thing however is that the bright star on the left side of the frame trails out of the frame and then seems to re-enter on the right side. Now, that is bizarre!

If you look at the other trails in the frame, they are all continuous. What is really going on is that the star on the left is Eta Geminorum. It goes out of the frame at left and does not return. The star that comes in at right is Mu Geminorum. Both are similar magnitudes, and lie at very close to the same declination.

• Solution: Check your cables to make sure they are not going to get caught on something during a long exposure run.

In this next image, I really have no idea what happened. The stars trailed north-south. Perhaps someone came along during the exposure, which was taken at a star party, and pressed the declination button for fun. I don't know, I was not there watching my equipment every second. I usually start an automated series of exposures and then go off for a while to observe through other scopes.

The real mystery though, is what happened near one end of the trailing. Why did the star go crazy and go all over the place? Perhaps someone was pushing on the scope with his hand while holding down the dec button. It looks like more than just the scope being bumped. Perhaps a large bird landed on the scope during the exposure and sat there flapping his wings. But probably not.

Sometimes, if you are intentionally shooting star trails, and a plane flies through the frame, you can get distorted star trails from the planes exhaust. This can make the seeing go crazy. But usually this causes the star trails to get blurry or a little bit wavy for a short while. I don't think this is what happened here however.

The weird star trails in this image are unexplained.

• Solution:
  • Sit there and monitor your exposures at a star party so that no one walks up and innocently touches your scope.

  • Throw this one away, or keep it for fun in the "weird, wild, and wacky" file.

I think I have an idea of what happened here. I can tell you that I did not hold down the right ascension button for a while, then the declination button, and then the right ascension again, and then the declination again, etc. I can also tell you that I got this exact same staircase-like pattern of star trailing on 6 consecutive 5-minute exposures.

The fuzzy out-of-focus trail is from a bright galaxy in the frame.

I also know that I can't blame this on anyone else because I was observing alone when it happened.

This was the first time that I tried to use a 120-volt camera battery adapter running through an inverter hooked up to my 12-volt deep-cycle battery that was also running the anti-dewers, telescope drive, and autoguider.

Sometimes, these types of electronic devices don't play well together.

The weird trails in right ascension and declination in this image were probably caused by using a 120-volt power adapter with an inverter on the same 12-volt battery powering the scope's drive and autoguider.

• Solution: Use a separate 12-volt deep-cycle battery with the inverter for the 120-volt battery adapter, or use a direct 12-volt adapter.

No mystery here. I know exactly what happened. I even did it intentionally.

I tried to hand-hold a 30 second exposure with no tripod.

Just call me crazy!

Mouse-Over

Stars will trail, all over the place, if you try to hand hold a long exposure. Hold your mouse cursor over the image to see a comparison between a 30-second hand-held exposure and a 30-second exposure on a tracking mount.

• Solution: place your camera on a fixed tripod, or mount it solidly on a tracking mount for long exposures.

Problem: Unexplained Mysteries - The Bottom Line Most of the time you can figure out what went wrong in an image. With experience, you will have seen many of these problems and become familiar with them. Sometimes, however, things go wrong and you can only guess at the cause. Anything involving computers, such as a laptop running an autoguider or camera, or even the computer built into your mount, can frequently act weird. Shutting them down and turning them back on again can often fix some of these problems.