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Kodaikanal Observatory is a veritable treasure trove of data, with the datarepository covering almost 100 years of observations. For the years 1909-2007,we use calibrated Ca II K spectroheliograms from the Kodaikanal Observatory todetect the plages, fix their heliographic coordinates and also estimate theplage areas. We adopt the following procedure. After ensuring that, for all theyears, Kodai calcium images have very negligible ellipcity, a circle is fittedand two central coordinates and radius of calcium images are determineduniquely. For each pixel of the calcium image, we then fix heliographiccoordinates and extract plages along with their weighted average coordinates.The heliographic coordinates of these extracted plages are then compared withthe heliographic coordinates of photospheric sunspots from the Greenwichsunspot database and chromospheric magnetic plages detected from the SOHO/MDImagnetograms. We find that the heliographic coordinates of calcium plages matchvery well with the heliographic coordinates of sunspots and magnetic plagesauthenticating our method of detection of plages and computation of positionalcoordinates. A code is developed in Python and all the nearly century scaleplages data, with accurately estimated heliographic coordinates and areas, isavailable to the public.

Kodaikanal calcium images: Detection of plages, Fixing the heliographic coordinates and Estimation of Area