[seooptiman]

Note that I live in a place where isha and fajr times enter.

The problem is that I have some calenders and all of them give different times for Imsak and the difference sometimes is more than 20 minutes. What to do in this case? Which calender to follow?

Also on one of the calenders it's written that I can pray Fajr 15-18 min after Imsak.

What in general is the method for determination of the time for Imsak? I mean how many degrees?

Also why do we have Imsak and Fajr?

[squicscor]

Note that I live in a place where isha and fajr times enter.

The problem is that I have some calenders and all of them give different times for Imsak and the difference sometimes is more than 20 minutes. What to do in this case? Which calender to follow?

Also on one of the calenders it's written that I can pray Fajr 15-18 min after Imsak.

What in general is the method for determination of the time for Imsak? I mean how many degrees?

Also why do we have Imsak and Fajr?

I would suggest you to follow this time-table:

http://www.islamicfinder.org/prayerD...ang=&monthly=1

With those settings (Method of calculation: Darul Uloom Karachi; 10 mins after "Zawwal"). In the link the 'Asr time is Shafi'i but you can shift it to Hanafi.