Check each frame for accuracy of focus and tracking. You would be surprised at the number of different things that can go wrong in a long-exposure deep-sky astrophoto. Look for planes and satellites that may have flown across the frame during the exposure for the sub-frame. If an individual exposure is sub-standard it should not be included in the master light frame as this will degrade the final image.

I always shoot raw files and JPEG concurrently. That way, the JPEG is processed in camera, and I can examine it immediately to see if there is anything wrong with it. If there is, it will also be wrong in the raw file and it can easily be discarded. If you only shoot raw files, you can save some disk space, but you will have to apply a stretch to the raw files to examine them before processing. This is not difficult, but it's a little bit quicker and easier to examine the JPEGs for problems.

Also, I can very easily access the EXIF exposure data that is embedded in the file with a simple little freeware program called EXIF Reader. I simply put a shortcut to EXIF Reader on the Send To menu and then in Windows Explorer I can right click on a JPEG file and send it to the program and immediately see the exposure time, ISO and other data. You can also see this info in the raw file, but you need to open a more complicated program like Canon's EOS viewer utility.

It's almost hard to believe that all of the following examples could have gone wrong in a single imaging session all on the same night. Some I have explanations for, some are common, and one I have to guess at its cause.

Image 1. Trailing in right ascension in a 10-minute exposure at ISO 1600. This is a JPEG Image that has been processed and stretched in the camera, but no color correction has been applied.

Image 1 is an interesting example of what can go wrong to ruin an exposure. Trailing in right ascension, or the east-west direction is not unusual in exposures that are unguided, although it is usually less than about 30 arcseconds or 1 arc minute and it is usually due to periodic error in the right ascension gear. In this case, the 10 minute exposure was guided with an ST-4 autoguider, so periodic error should have been corrected, and would not have been on this scale.

I think what happened was that the voltage on the battery must have dropped down to just about the level where the telescope drive would not work. Perhaps it just stopped guiding for a while and then started up again. I do remember having problems with the deep-cycle battery that night because I don't think I charged it adequately enough during the day after a full night's use the night before.

As far as the strange zig-zag of the trail seen in the inserted enlargement in Image 1, who knows what happened there. Because part of the pattern is smeared so badly, I am inclined to believe that this is simply incredibly bad seeing, possibly caused by the wake of a jet aircraft that passed very nearby to M33.

Image 2. Incorrect exposure at ISO 1600.

Image 2 was supposed to be a 600 second exposure at ISO 1600. Somehow turned into a 5 second exposure. All of the exposures during this series were programmed with the Canon TC-80N3 timer. This one came in the middle of the run. All of the rest of the exposures came out correct. This one did not. What happened? I don't know. In any event, it should be discarded because it if is averaged in with the others, it will degrade the signal-to-noise ratio in the final image because it has virtually no signal in it.

Image 3. Curious North - South Trailing. This is a 300 second JPEG Image at ISO 800 that has been processed and stretched in the camera, but no color correction has been applied.

Image 3 exhibits a very curious, large trailing in the north - south direction. This is obviously not any kind of trailing due to periodic error, or even inaccurate polar alignment (the mount was accurately drift aligned).

It seems too large to have been a wind gust, not only because the mount is very solid even in high winds, but because it almost looks like the star moved and then stayed in its new position for a while. It's possible that the mount was bumped, but unlikely because it's perfectly north-south. It's also possible that the relay for the declination axis stuck in the ST-4, but I have never experienced this problem before with the autoguider, although others have. It's even possible, although extremely unlikely, that someone played with the hand controller during the unattended exposures since this was at a large public star party.

Image 4. North-South trailing of about 2 star diameters. This is a 300 second JPEG Image at ISO 800 that has been processed and stretched in the camera, but no color correction has been applied.

Image 4 exhibits a more subtle problem that might be missed if the image is not examined at 100 percent magnification. Here, the trailing is north-south, which is curious again, but only about two star diameters. This is not much, but it is enough to degrade the star images in a stack. If all of your images have trailing on this scale, then you might as well stack them all together. But in this case, it should be removed because the majority of images were tracked extremely well.

Image 5. Satellite trail. This is a 600 second JPEG Image at ISO 1600 that has been processed and stretched in the camera, but no color correction has been applied.

Image 5 shows a trail from a Earth-orbiting satellite that passed through the frame during the exposure. Because the image was taken at 4:45 am in the morning with dawn approaching, satellites in high orbit can be in the sunshine even though it is still night on the ground. These satellites are illuminated by direct sunlight and can be bright enough to be visible to the unaided eye, and also can be recorded by our DSLR cameras.

Out of 14 good frames that I had of M33, two shot early in the morning have satellite trails in them. These two frames can be included in the stack if the images are median combined, and the trails will not show up in the final image.