Make a Quick and Simple Hartmann Mask
by Bruce Johnston
First, find a cardboard box, the size of one side of which, is larger than the diameter of your OTA. With luck, you may be able to find a box wih a removable top, where the top is larger than your OTA. If so, grab it! It saves you a little work!
If you've found only the full cardboard box, no problem. Simply close the top and tape it closed near the edge. Then, cut the 'face' of the box off, so that it looks the way the top would look. Make the remaining edge about two or three inches wide. Either way, by now you have your 'top' piece.
Take your newly created "top" piece and cut off two of the four sides.
The new 'top' to your soon-to-be-mask, is where two sides meet. Take the cardboard and just flop it over the end of your OTA, with the 'top' pointing up. Make a decent guesstimation on where to put the two or three holes. I used three holes, as shown. I arranged them so that they were roughly 120 degrees apart, and that they were out to nearly the edge of my corrector plate. I made the holes about 2" in diameter, but I didn't actually bother to measure the angles, the spacing, nor the hole size. It just isn't that critical!
Some people prefer a two hole mask; others prefer three holes. Some use MANY holes. It really is subjective and mostly a matter of taste, since they all seem to work equally effectively. I think that three or more holes make it easier to spot any misfocus when you get down to that last slight 'tweak' of the focussing adjustment.
The holes needn't be round, either. There are a variety of combinations that people use for various reasons.
Once you've completed cutting the holes, you're ready to go. From now on, you can just grab your mask by it's 'top' and flop it over the end of your OTA or dew shield. Since it is used for focus only, there is no proper 'up' nor 'down'. You simply set it on the scope so that the 'top' is aimed toward your overhead. (It stays on the scope better that way, is all.)