Quote:
Originally Posted by rmuhlack
Back to the GSO secondary size, I have found with my 200mm f4 that the position of focal plane is actually further away from the secondary than it really needs to be, and at that distance my secondary (according to the OWL Diagonal calculator) doesn't actually give 100% illumination even in the centre of the field. The secondary is simply too small. The solution is either a different slightly longer optical tube to bring the focal plane closer to the secondary, or a larger secondary (or just accept that the scope is performing less efficiently than it needs to). If that was the case with James' 300mm f4 too, then I guess that would necessitate the secondary mirror upgrade.  |
That's just typical of commercial scopes. They're a compromise between visual and imaging. I'm not sure why they have such long focusing units. Possibly because old style photography needed the racking space for adapters.