Hi, me again
I have the Bresser AR127L achromatic telescope which is supposed to have a focal length of 1200mm, making it an F9.44.
However, the measurement from the front of the objective lens to the furthest end of the focusser racked fully out is only 1100mm.
Am l missing 100mm, is the last 100mm from the star diagonal up to the bottom element in the EP, is 1200mm just and advertising lie or am l missing / not understanding something here?
I'm in the process of changing focusser from the 2.5" Hexafoc to a Bintel/GSO Crayford and l need to get the new focuser in the correct position. Given the new focusser has twice the movement range I'm confident l can put it roughly in the same position as the old one, maybe even a bit closer, but l need to ensure I'm not cutting off any of the focussed light due to the difference in internal aparture of the new focusser- see photo of old and new focusser below. Old on top, new bellow.
In my head l am seeing the objective lens as the flat base of the triangle and the focal point being the pointy top of the triangle, therefore l can determine the required internal aparture required at any point in the light path to ensure l am not cutting off any of the light.
So with my 1100mm measurement Vs 1200mm stated and the smaller internal aparture of the new focuser, I'm a bit stuck.
I plan to have an adaptor made to fit the new focusser to OTA.
Original focusser 45mm range, 140mm overall length racked in, 65mm internal aparture.
New focusser 80mm range, 145mm overall length racked in, 50mm internal aparture.
Any help greatly appreciated.
Rod.