Re: [ATM] Secondary Offset

["Bob May" <rmay@nethere.com>]

Thgere are three positional directions/
1. Up and down the tube
2. left and right to the focuser
3. towards and away grom the focuser.

In addition, there is the tilt of the secondary from the desired
45 degrees as well as the rotation around the tube axis.
The reason to offset a secondary is that the cone of light coming
from the primary is larger at the distant end from the focuser
than the near end of the secondary from the focuser.  Thus the
cone of light is smaller and you don't need as much of the
reflection there. You do indeed get an elliptical shaped field as
the secondary isn't exactly in the center but that error is very
insignificant compared to anything else by a large degree.I'd
find it hard to measure that little bit of an error on any scope
and the only other problem that pops up is that the out of focus
image of a starrhas a slightly offset hole in the middle of it.
This isn't a problem of any degree as you aren't looking at out
of focus images but rather the in focus image where all of the
light the scope gathers is put into a single spot.   Even the
comatic image at the edge of the FOV isn't really impacted as the
light from the central area of the mirror is not thrown off of
the proper spot by any real valuce, usually still being within
the Airy Disk.
For visualization, use Newt and do a F1 scope for how the cone
lof light looks.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net


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