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