Multiple Unguided Exposures - The Next Step

Once you know how long an unguided exposure you can take, how do you get more detail without getting trailed images?  Simply by stacking multiple short exposures.  This technique has another advantage:  it tends to reduce noise in an image.  Take the example below:

Above:  The left image shows a single 60-second exposure of M83.  The image appears a bit grainy due to noise in the image.  The image on the right it a composite of five 60-second shots.  Not only is there more detail visible, the noise is significantly reduced.

Note:  We will see how to combine these images in the Image Processing Basics section.  For now we just need to get the exposures.

The simplest way to collect multiple images is simply to set your camera control software to take a sequence of however many total images you would like.  Software packages such as MaxIm DL and CCDSoft allow you to automatically take and save a sequence of images.  CCDOPS has a feature known as Track & Accumulate which works with SBIG's cameras to automatically combine the images.  There are limitations to the Track & Accumulate feature which make the longer process of capturing and then later combing individual images worth the effort.

Tip:  For specific details on how to capture multiple images using MaxIm, or CCDSoft, as well as details on using the Track & Accumulate feature of CCDOPS, visit the Software Instructions section.

When you take multiple exposures you have the option of selecting only the best images to use when combining the files.  If you have a gust of wind ruin one of your shots, or a satellite or airplane flies through your picture, you can easily discard this shot and composite only the best images.

Above:  A sequence of four exposures, each 60-seconds long.  In the event that a super-massive black hole should suddenly create a vast, gaping whirlpool in the fabric of the space-time and devour your target object, using individual multiple exposures ensures that at least some of your images come out unscathed.  Or if, say, a satellite flies through the picture or a gust of wind bumps your scope, but those are such rare events....

Note:  For short exposures like these you don't have to feel too bad about throwing out a 60-second exposure.  However, for much longer, guided exposures, one may not wish to discard a shot which took as much as an hour to take!  For this there are other techniques to remove satellite trails, etc., from the image.  This is covered under the Advanced Image Processing section.

We usually recommend that you take some time to master black and white imaging before moving on to the next step, color images.

Next, Color Imaging....

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