Quote:
Originally Posted by strongmanmike
Thanks Roger
Well you can...just costs more money 
It is a 2.5 deg diagonal field at 1.57"/pix 
I think I could still tweak the corrector spacing and maybe collimation a tad, the rest is image plane tilt and flexure, not sure I can ever eliminate the last two 100% with this OTA design, it is already acceptable to me but once I get her permanently under a dome (soon) and have more time, I will play with that corrector spacing and maybe I can shim something...?
Mike |
Yes, the corrector / chip spacing is quite sensitive, and the technical specs by some of the manufacturers leave a lot to be desired. I've found that collimation can be a bit tricky as well as you can't just rely on laser collimators to get it right; that's why the classic star test is recommended, but even then the standard visual star test at the centre of the field won't guarantee pin-point stars to the corners of a large field. I am still tweaking this for my own scope; I have used the laser collimation tests to get close, and then I have fine collimated based on the appearance of stars in the corners of test images that I then take. I then use the laser collimator again to see where the centre of the paraboloid is to simplify future collimation. When I spoke with Mark Suchting he commented that the centre of the paraboloid moves around the geometric centre of the mirror by about 3mm or so during final figuring (correct me if I've misquoted you, Mark) which makes sense and accounts for the fact that a simple laser collimation using a centre mark doesn't guarantee perfect collimation. The other thing that I've realised out of this is that having defined the centre of the paraboloid and collimated the primary accordingly, that the secondary also needs to be recollimated on the same spot or some image tilt may be noticeable (I'm still doing some testing on this but the clouds aren't co-operating at the moment). Finally, with these mid/large newtonians I wouldn't necessarily assume that any "flex" is all in the way the camera is supported; the optics themselves can't be superglued in place (well, unless you want severe astigmatism!) and may have some slight play (for me probably a limiting factor in using an external guidescope for longer than 15-20 min exposures). Eg. if the silicone supporting the secondary mirror is too thick it may tilt slightly under the weight of these larger secondary mirrors. ... anyway, food for thought ...