Low energy γ-ray observations with the MISO telescope

The MISO telescope, which has been described elsewhere, (Baker et al. 1979), has a sensitive area of 560 cm 2 and an aperture of 3° f.w.h.m. in both the azimuthal and zenithal planes. An alt—azimuth orientation system was used to point the telescope with a precision of ± 20'. A passively shielded hard X-ray detector (20—280 keV) having an effective area of 600 cm2 was also mounted in parallel with the axis of the main telescope and had a field of view of 3° x 3° f.w.h.m. This instrument was used to study the region of the sky containing NGC 4151 on 30 September 1979 between 15h01 U.T. and 20h40 U.T. A series of five drift scans were performed to survey the region contained within the coordinate points 11h32 and 12h33 in right ascension. The declination was centred on + 39.8°. The Seyfert galaxy NGC 4151 was in the field of view of the telescope for a total period of 7.5 x 103 s. The total time spent on the background measurements associated with the data presented here, was 8.6 x 103 s. The mean float altitude during the observation was close to 4 mbar (400 Pa) and the telescope was set at a series of zenith angles between 9° and 30°.