The "thermal lance" is a laser cutter (named a Sonarscope in The Mark Of Archanon script) seen in The Mark Of Archanon, The Bringers Of Wonder part 2 & The Immunity Syndrome. The prop is obviously a telescope, and was seen as such in Year One.
In Year One, Victor Bergman's telescope is seen in Ring Around The Moon (in his quarters, LQ12), and in Another Time Another Place on Earth. It seems to be a Newtonian reflecting telescope (probably 6" aperture) with an equatorial mount, tripod, two eyepieces and finder scope- all features still seen on the thermal lance. The primary mirror extension rods are closed up when seen on the thermal lance.
(Thanks to Fred:) The "thermal lance" is in fact a Japanese Eikow STH4000 or STH5000 telescope. It is a 114 (STH4000) or 115 (STH5000) mm (4.5") reflector with a focal length of 1200 mm. It was sold in Europe in the 60s under the brand names of Paralux, Dauer and Admiral and in Japan under its original Eikow name. It was not a great success because the design suffers a huge weakness in the stability of the collimation. It disappeared from the Eikow catalogue after 1973.
It is in Victor's lab in 1996 in Dragon's Domain (left in both these images)
In The Mark Of Archanon, we see the thermal lance being used to analyse rock and break apart surface layers. The beam looks like a spotlight and causes rock to steam and shatter. Although rapid changes in temperature can cause erosion, in reality it's not a very practical mining tool. The text (seen clearly in the other two episodes) calls it a "thermal laser". The operators use the finder scope, and fiddle with the eyepieces while pointing the wrong end of the telescope at the rock.
Later in the episode, the thermal lance is seen in TechLab 3
In The Bringers Of Wonder part 2, the thermal lance is used again, and we see some close ups of the other side (a "Danger:Beam" sign points to the wrong end of the telescope, and the legend "Thermal Laser" is seen). Again it works like a spotlight, this time melting electronics.
In The Immunity Syndrome, the spotlight is gone and now we see a green laser beam. The lens cap on the end of the telescope reads "Thermal Lance".
Copyright Martin Willey