The “ Gamma Ray Astronomy at ALmeria ” ( GRAAL ) experiment uses 63 heliostat-mirrors with a total mirror area of \approx 2500 m ^ { 2 } from the CESA-1 field at the “ Plataforma Solar de Almeria ” ( PSA ) to collect Cherenkov light from air showers .
The detector is located in a central solar tower and detects photon-induced showers with an energy threshold of 250 \pm 110 GeV and an asymptotic effective detection area of about 15000 m ^ { 2 } .
A comparison between the results of detailed Monte-Carlo simulations and data is presented . Data sets taken in the period September 1999 - September 2000 in the direction of the Crab pulsar , the active galaxy 3C 454.3 , the unidentified \gamma -ray source 3EG 1835+35 and a “ pseudo source ” were analyzed for high energy \gamma -ray emission .
Evidence for a \gamma -ray flux from the Crab pulsar with an integral flux of 2.2 \pm 0.4 ( stat ) ^ { +1.7 } _ { -1.3 } ( syst ) \times 10 ^ { -9 } cm ^ { -2 } sec ^ { -1 } above threshold and a significance of 4.5 \sigma in a total measuring time of 7 hours and 10 minutes on source was found .
No evidence for emission from the other sources was found . Some difficulties with the use of heliostat fields for \gamma -ray astronomy are pointed out .
In particular the effect of field-of-view restricted to the central part of a detected air shower on the lateral distribution and timing properties of Cherenkov light are discussed .
Upon restriction the spread of the timing front of proton induced showers sharply decreases and the reconstructed direction becomes biased towards the pointing direction .
This is shown to make efficient \gamma -hadron separation difficult .