How to choose the Deva and Roopa based on the chart?
There are various principles to arrive at the proper answer. We have division based on graha, rasi, arudha, bhava and yoga in various charts of Hora.
If planet is in own sign then Tattva deva should be worshipped, for planets in neecha-avastha (debilitation) Dasa Mahavidya sadhana with the crystal-mala forms the upaya, whilst for uccha (exaltation) planets the Avatara of Vishnu with the Tulsi-Mala is perscribed. The roopas of these are based on the Naisargika Karakatva, whilst for choosing the form for the swakshetra (own avastha) we use the division of the Tattvas for the sign.
There are other yogas in the chart which call for different form of Deva, like Dhanada (Hora Yogada), second house from AL, Dharma Deva, ninth bhava and few more like purity-issues are always sustained/cured by Sri Vishnu sadhana in the form of mantra (fifth), stotra (third) or puja/japa (ninth).
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