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Technical Tid-Bit #2 – Hartman Masks
By Aaron Clevenson, AL-Cor
You probably have seen some of the CCD photographers in the club using a mask on the front of their telescope that has two or more holes either circular or triangular. You possibly have wondered what it is. Well, it is an optical focusing mask.
First a little history. It was actually invented in 1691 by Christopher Scheiner, although most people know of it as a Hartmann Mask not a Scheiner Mask. It is also known as a Shack-Hartmann Mask and I have even heard it referred to as a Kiwi Mask (but only by one person). These masks are often used by very large professional telescopes as well. The concept is that the light coming through the two or more holes will form one image for each hole if the telescope is out of focus. The Scheiner Mask is the simplest form. It has two circles. When the images converge you are in focus. A Hartmann Mask actually has many holes, but the concept is the same.
You can buy one (Kendrick sells one called a Kwik Focus for $50 to $100) but they are very easy and inexpensive to make. A piece of cardboard cut to the proper size with 2 or more cutouts is all you need. But how many, how big, where, and what shape should be the cutouts? First a simple one; you can put two pieces of masking tape at 90 degrees to each other across the front of your telescope. When you are in focus, the image will change from a square to a circular star with 8 diffraction spikes coming out of it. They should be clean and well defined.
But back to the masks… What about the holes? First question is how big. The larger they are the brighter the stars will appear when out of focus and the more pronounced the final results will be when you are in focus. So bigger is generally better. The literature has them anywhere from 1/10th to 1/3rd the diameter of the objective. Next question is where. You want them to be as close to the outer edge of the telescope field as possible. The farther apart the holes are, the farther apart the out of focus star images will be. This should give you a better focus. Third question is how many? You need at least 2, and more than 4 is probably unnecessary unless you are planning on doing optical wavefront analysis. I think two is plenty, so we’ll recommend two. No matter how many you choose, they should be equally spaced around the edge of the mask. (Three would form an equilateral triangle.)
The last criterion is shape (and we’ll also elaborate on layout a little). Circles will give you no diffraction spikes (with 2, 3, or 4 circles). You focus until you get the smallest image possible. Triangles though make for a more informative image. A triangle will cause 6 diffraction spikes. Two triangles that are not oriented the same way (60 degrees out of phase) will give you 12, and three (40 degrees out of phase) will give you 18. This makes for a very fancy image, and you can use the spikes to tell you where the centers of the stars are by interpolation. However, if you put two or three triangles exactly in phase with each other you will get the same 12 or 18 spikes, but they will converge to 6 bright spikes when the telescope is in focus. You can actually see them converge and know exactly when you are at precise focus.
I use a focusing mask that has two triangles, 1/4th the diameter of my telescope, oriented in phase, at the outer edge of the front end of the telescope (on the corrector plate). The longer the exposure the more pronounced the spikes will be. Be sure to include a mechanism to remove it when you are done focusing. I use a small dot of Velcro that I can attach to with a piece in my hand.
One other layout I have heard recommended highly is two circles and a triangle (that fits within one of the circles). Aim one of the points of the triangle towards an imaginary line between the two circles. When you pass optimum focus the pointer triangle will jump from one side of the circles to the other. This tells you immediately that you have gone too far.