The Hartmann Mask

When attempting astrophotography for the first time many people find it difficult to get a sharp focus on their intended targets. This is usually due to the camera having a small and dim view finder. This can be especially problematic with DSLR's where a 'live' image cannot be seen on the camera's display screen and therefore the viewfinder has to be relied on. Although the view finder is ok for getting a reasonable focus on particularly bright objects such as the moon or very bright stars, what if you want to take images of extremely dim objects such as nebulae?

One of the easiest techniques to employ is known as the Hartmann Mask. This consists of a piece of card the same size as the telescope aperture with an arrangement of holes to allow some light though. There are various designs for the configuration, number, size and shape of the holes but the principle is always the same.



The mask is allowing light from different areas of the objective through, and by adjusting focus, when all the light converges you will have a sharply focused image. Having discrete holes enables the users to see this process happening as illustrated below.



1. Image is extremely out of focus.
2. Image is brought closer to focus.
3. Image is almost focused.
4. Image is now focused.
5. The image has now been gone past perfect focus and will have to be brought back in focus by adjusting the focuser in the opposite direction.
6. The image is back in focus. At this stage the image needs to be zoomed in to to check this is really the best focus you can get. The star points should be small and have the small peaks of the triangle just appearing at the dots three corners

The example illustrated uses two square holes and one triangular one. The triangle assists in working out the direction of focus required. The mask can be modified in many ways, perhaps having more holes, or different shapes, and the larger the holes the easier the shapes will be to see.