Thank you!
I imagine the purpose of any reticule light for any total station or theodolite is so you can see the reticule while you are doing a star sight.
Thanks guys.?ÿ So in order to see the crosshairs against a dark background?
No, it is not for dark backgrounds. When I survey underground I use illuminated targets for the backsight and foresight. If I don't have the reticle illumination turned on I won't see the cross hairs. It is because there is not sufficient ambient light at the instrument to see the cross hairs. It doesn't matter that I'm sighting an illuminated target.
In the old days when using a mining transit underground, one would hold a lamp near and to the side of the eyepiece to light up the cross hairs. My Wild T-2 has a lever on the telescope that you can slide back and forth. It acts like a rheostat for adjusting the brightness.
A correction regarding the Wild T-2.
For the old style Wild T-2, the reticle illumination is adjusted by a knurled knob on top of the telescope. For the new style Wild T-2, it is adjusted by a lever on top of the telescope (telescope in Face I).
My error in the previous post was to describe it as being "like a rheostat." It doesn't have a thing to do with electricity. The reticle illumination is adjusted by an internal rotating mirror. The battery box for these have one cable port that is controlled by a rheostat. That allows the instrument person to adjust the amount of light illuminating the horizontal and vertical circles.
Sorry for the momentary brain fog.