SHORTLY AFTER TKOF THE CABIN DOOR ON AN ASTRA OPENS COMPLETELY. RPTR STATES DOOR WAS PROPERLY SECURED AND NO WARNING LIGHTS WERE DISPLAYED.

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I WAS SCHEDULED TO FLY AS AN FO ON A TRIP IN AN ASTRA (IAI 1125). MY PAX ARRIVED AT THE ARPT AND I TOOK THEM TO THE ACFT. THE CAPT STARTED THE #2 ENG AS THE PAX BOARDED AND I CLOSED THE MAIN CABIN DOOR AND LATCHED IT. I FELT A SMALL PRESSURIZATION BUMP AND TOLD THE CAPT; 'DOOR CLOSED AND SECURED.' I GOT INTO THE L SEAT TO FLY THE ACFT AND THE CAPT AND I FINISHED THE CHKLISTS. WE THEN CALLED FOR TAXI AND PROCEEDED TO TAXI TO RWY 24. THERE WERE NO ANOMALIES AND ALL LIGHTS ON THE ANNUNCIATOR PANEL WERE EXTINGUISHED OR NORMAL. WE WERE CLRED TO TKOF AND DID SO. EVERYTHING WAS NORMAL UNTIL 1000 FT AGL WHEN THE MAIN CABIN DOOR CAME FULLY OPEN. WE DECLARED AN EMER AND IMMEDIATELY RETURNED TO ZZZ. WE SHOT AN ILS TO RWY 24 AFTER COMPLETING OUR CHKLISTS AND LANDED THE ACFT WITH THE CABIN DOOR OPEN. I BROUGHT THE ACFT TO A STOP ON THE RWY; SET THE PARKING BRAKE; SHUT DOWN THE ENGS; AND EVACUATED THE PAX. NO ONE WAS INJURED AND WE HAD THE ACFT TOWED OFF THE RWY TO A SECURE AREA. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR EMPHASIZED THAT THE CABIN DOOR WAS PROPERLY CLOSED AND THAT SUBSEQUENT TO THE LNDG; HE WAS ABLE TO RECLOSE IT; AGAIN APPARENTLY PROPERLY SECURED. HE INDICATED THE DOOR IS NOT A PLUG TYPE BUT DEPENDS ON MECHANICAL PINS AND SLOTS FOR SECURITY. THE DOOR HAS BEEN REMOVED AND RETURNED TO THE MANUFACTURER FOR REPAIR AND REFITTING. RPTR INDICATED THAT THIS WAS AN EARLY VERSION OF THE ASTRA AND THAT HE WAS AWARE OF ANOTHER FAILURE ON AN EARLY MODEL PRIOR TO THIS EVENT. HE STATED THE DOOR WARNING LIGHT INDICATED BOTH AN IMPROPERLY SECURED DOOR AND INADEQUATE PNEUMATIC PRESSURE TO THE INFLATABLE PRESSURE SEAL. HE FELT THERE WAS SOME TENDENCY TO BE CASUAL ABOUT THIS LIGHT BECAUSE IT WOULD SOMETIMES ILLUMINATE AT LOW THROTTLE SETTINGS DUE TO THE PNEUMATIC NATURE OF THE SEAL; EXTINGUISHING WHEN MORE THRUST WAS EMPLOYED. HE EMPHASIZED; HOWEVER; THAT IN HIS INCIDENT THE LIGHT WAS NEVER ILLUMINATED. RPTR WAS CONCERNED THAT THIS VERY COMPLEX CLOSURE SYSTEM IS ONLY REQUIRED TO BE INSPECTED AT 1000 HR INTERVALS WHICH CAN BE YEARS APART IN THE STANDARD OPERATION OF THIS CLASS OF AIRPLANE. THE ACFT IN QUESTION WAS BUILT IN 1988.