Focusing & Hexagonal Masks

By Jim Dixon

When I first heard about these “quick focus” devices, I had no idea of their size and thought they were knobs that replaced the normal focusing knobs. After all, a larger knob would allow you finer control over focusing. When I realized they were aperture masks, I thought “yeah”. These sell for $47 or more depending on your telescope’s size although admittedly they double as a lens cap.
I wasn’t willing to spend that $50 on something that I had my doubts about. I purchased a sheet of foam core board from a hobby shop for $4 and made one that, to my amazement, actually works. The three holes form three images when the telescope is out of focus. As you close in on focus, they merge. As you can see from the picture, it doesn’t have to be very well made either.

One night I was trying to observe Sirius B, the much fainter white dwarf companion to Sirius. The difficulty in this is the closeness of the pair and the 9 or 10-magnitude difference. I heard about using a hexagonal mask to redirect the light from the bright into six spikes rather than one bright circle. I used some of the same foam core board left over from the focusing mask. It really does work. Light from the bright companion is channeled down the spikes allowing you to see objects closer to the bright companion.