Using Multiple Exposures

In the days of film, if you wanted a long exposure, you had to open the shutter for a long period of time. This has a certain simplicity, but it is not necessarily the best way to image with a digital camera.

In astronomical imaging two difficulties are often encountered. First of all, the telescope must remain aligned with the sky to a fraction of an arc-second throughout the exposure. This is extremely difficult (expensive) to achieve mechanically, so in most cases some form of guiding is required. Secondly, bright stars within the frame will rapidly saturate and bloom. This can affect not just the saturated pixels but also adjacent ones as the excess electrons become trapped and spread along the column during readout.

A third problem may arise due to the camera itself. Cooled CCD cameras can take extremely long exposures without being overwhelmed by dark current. Cameras that do not have a cooled sensor, such as DSLRs, will eventually saturate from the dark current alone.

These problems can be greatly reduced by taking a series of shorter exposures, then combining them together afterwards. MaxIm DL provides support for this in the form of the Stack command. This command allows you to combine a number of images into one equivalent long exposure. The image alignment feature allows you to remove the effect of telescope drift, and the floating-point representation used means that saturation can be avoided.

Many telescope worm gears have positions where there is some relatively rapid drift. If some of the shorter images are unsuitable due to trailing, they can be examined and removed from the sequence using the Stack command. All of the images should be calibrated before combining them.

When imaging an object with high dynamic range on a DSLR camera, it may be necessary to combine images taken with different exposure lengths, and different ISO settings to reduce the saturation of brighter portions of subjects, while keeping fine detail in dimmer regions. The ISO level can be controlled through the Sensitivity selector.