Dave Wallace's Adventures in Astronomy

04/04/2004

I fabricated a Hartmann Mask (see the photo below). It's made out of a plastic jar lid. For my ETX-90, I wound up using two 7/8" diameter holes on a 2-15/32" bolt circle.

Since it was raining, I used a street light a quarter mile away as a target. In my scope, this was about 1/2 the size of the full moon, but considerably dimmer. The mask works like a charm! With the mask, I can tell a difference of only a couple of degrees on the setting of the focus knob; without the mask, it's more like 10 to 20 degrees before I can see any difference. (The mask was precise enough to show me that the eyepieces I thought were parafocal weren't quite -- by about five degrees on the focus knob.)

Whenever the weather clears, I will try again with the Meade Lunar/Planetary Imager. Maybe now I'll be able to get it in focus.

Hartmann Mask