Quote:
Originally Posted by SkyWatch
I suspect all this is getting quite confusing for the OP.
For the record, I have a 12" f5 scope, and the eyepieces I mentioned (even the cheap LP ones) are well corrected for coma at that f-ratio, and give sharp images across virtually the whole field, so you can watch a planet pretty much from one edge of the field to the other without having to move it into the centre of the field for the whole time.
For visual use they do a great job, and are a lot cheaper than a Paracorr or an equatorial platform... 
- Dean |
Sure, your *eyepieces* are corrected for coma, as all good eyepieces are, but your *scope* is not. The coma free zone in your scope is only 2-3mm wide in the center of the field, and adds coma linearly as you move toward the edge of the field. Even if you don't notice appreciable coma at the edge of the field (and, in 50 degree eyepieces you probably won't), the sharpness of the planetary image will slowly diminish in a linear way as it nears the edge of the field. A 10mm Plossl has about a 7.5-8mm field stop, so coma certainly won't be severe at the edge in that eyepiece. But there is coma in the image as soon as you are 1.1-1.3mm away from the center of the field. That's why tracking is so important.
Or a coma corrector if tracking isn't present.
Having said all that, though, the variation in seeing that occurs will totally swamp the best correction possible:
Perfect optics + Perfect collimation + Perfect equilibrium temperatures in the optics + perfect eyepiece + tracking + bad seeing = bad image.
Alas.