Tuesday, February 20, 2007

Borg 45ED and Borg 60ED

Previously I found there's a light baffle inside the Borg 45ED II, so I wonder if the same tube would result in vigneting when used with a Borg 60ED given it's bigger aperture.

I disassembled (just unscrew actually) the Borg 45ED II, found the light baffle to have 35mm openning. On simple calculation, it means it should be 72mm from the objective, and I found it matches the distance from the objective to the baffle.

When I plug 60mm aperture and 350mm focal length into the equation, it yields 146mm from the objective, and that probably explain why there's a tube to connect the lens cell and the same drawtube.

There won't be any vigneting when used in the standard configuration, but if one want to remove that middle tube, one is effectively stopping down the aperture.

Now, the problem reduces to focus range. My JMI-S with adapters will cost 80mm minimum, with the back focus of the 60ED of 98-148mm, that means I'll have 18mm to 68mm to spare for diagonal or blocking filter, 1.25" diagonal and blocking filter is going to work, but 2" will be hard.

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