Ground cover and canopy cover were estimated using a sighting tube
with crosshairs at one end (Winkworth and Goodall, 1962).
Another possibility is to build a simple peep sight; a couple of screw hooks in a piece of scrap lumber can work surprisingly well, and even a simple cardboard sighting tube
should not be scorned.
There are no optics, just a simple sighting tube with an illuminated targeting ring near each end.
While I looked through the sighting tube with only one eye, I usually kept both open, since doing so improved my ability to see faint stars even when one eye had a blocked view.
I had to cover the front end of the sighting tube with my hand and hold my eye tight to the back end in order to see the arrowheads that led me to Venus.
Install the objective and use a sighting tube
or a Cheshire collimating eyepiece to align it as you would a standard, non-folded refractor.
The PBM consists of three components: a sighting tube
, a variable power supply, and an ordinary digital multimeter.
First, make a sighting tube
that points up at the north (or south) celestial pole.
The traditional way to adjust a reflector in the field is to do rough daylight collimation, centering the mutually reflected images of the primary and secondary mirrors seen through the empty focuser or a sighting tube
. After dark you bring a convenient star of moderate brightness into a medium-power eyepiece.