Posts Made By: John Holt

Paul Preto said:

I have a DGM 5.5, that I have had for over a year. Generally, it gives nice crisp contrasty views, which are quite bright for the 140mm optics. My problem is I have never gotten the superb planetary views I bought it for, probably due to the trace of astigmatism that I see, even after tedious star test collimation. I spoke to Dan McShane a couple times about this, and he says it is the collimation. He also recommends using simple less well corrected eyepiece designs during collimation. I've done this, but still see the astigmatism. Can misaligned optics cause astigmatism in this type of optical system?

Since the primary is a segment of a larger f/4.3 mirror (per Dan), it has a thick end and a thin end. Dan says you point the secondary at the center of this primary section, though intuitively it seems like you would point it at the thin edge, which used to be at the center of the parent primary. I'm thinking that maybe I should be pointing my secondary somewhere other than the very center of the primary segment. Could this be my problem?

Or, what would happen if my primary segment is rotated a degree or two in its cell, so that the light cone is not hitting the secondary with the optimal geometry? I'm now considering getting a Astrosystems secondary holder to replace the non-adjustable holder that came with the scope so I can tweek the secondary during collimation (now you have to tilt the focuser to aim the laser beam). Before I waste my money, I'd like to get some feedback on my ideas and whether or not this could be a collimation issue.

Mike Alexander said:

If a barlow lense doubles the magnification of an EP, does it reduce the true field of view of the EP by 50% or is the true field of view not effected?

Thanks.........Mike