HAWKER CAPT RPTS ALT EXCURSION ON TEB SID FROM TEB DUE TO EVASIVE MANEUVER TO AVOID BALLOON.

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AFTER DEPARTING TEB ON AN IFR PLAN WITH VMC CONDITIONS WE CLIMBED TO 1500 FT AND STARTING A TURN TO A HEADING OF 280 DEGS (AS PER THE SID). IN THE TURN THE COPILOT (PF) SAW A BALLOON AT OUR ALTITUDE AND IMMEDIATELY INITIATED A CLB. AT 1800 FT REALIZING WE WERE OFF OUR ALTITUDE WE BOTH INITIATED AN IMMEDIATE DSCNT BACK TO THE CORRECT ALTITUDE TO STAY IN COMPLIANCE WITH THE SID. AT 1800 FT WE DID GET A TFC ADVISORY. WHEN BACK ON OUR ALTITUDE I ADVISED ATC OF THE ADVISORY AND HE CLAIMED WE GOT IT BECAUSE OF THE ALTITUDE INCURSION. AFTER IT WAS OVER I REALIZED THAT THE TA WAS AT 1800 FT; NOT 1500 FT. THE COPILOT HAD VISUALLY SEEN A BALLOON WHEN HE INITIATED THE CLB WHICH CAUSED US TO VIOLATE OUR ALTITUDE NOT THE TA. ACTUALLY AFTER BRINGING UP THE TA TO THE CTLR I OWE HIM AN APOLOGY I WAS NOT AWARE OF EXACTLY WHEN THE TA OCCURRED. OTHER FACTORS INVOLVED WERE THE CONDITION WHEN THIS OCCURRED. THE WX WAS SEVERE CLR. THERE WAS A HEAVY MIX OF VFR AND IFR TFC. A SPECIAL EVENT WAS ABOUT TO START AT THE MEADOWLANDS (3 NM'S SOUTH OF TEB). THEY WERE IN MINUTES OF STARTING A TFR WITH BANNER PLANES AND HELICOPTERS SURROUNDING THE SOUTH END OF THE ARPT. THE SID KEEPS YOU LOW SO YOU DON'T GET INTO EWR'S AIRSPACE. IT IS APPARENT THAT IT'S A BAD SITUATION THAT DEMANDS COMPLETE DILIGENCE ON THE PART OF ALL PLTS.