On either side of the arm 57 are {HEAD ~ switches} 58 and 59 that are engaged by {TAIL ~ the arm} 57 when the telescope 55 is swung through as small an arc as a fraction of a degree 
On either side of the arm 57 are { HEAD ~ switches } 58 and 59 that are engaged by { TAIL ~ the arm } 57 when the telescope 55 is swung through as small an arc as a fraction of a degree
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH