Posted 24 January 2013 - 01:15 PM
Posted 24 January 2013 - 05:22 PM
Posted 24 January 2013 - 05:49 PM
Usually, when you rack the focuser out, increasing the distance from the secondary mirror, the secondary mirror looks smaller, so it's more likely to be fully visible. The primary mirror reflection (with mirror clips) inside the secondary mirror will often fill, and then spill over, the secondary mirror.
...When fully collimated, while looking through the collimating eyepiece if you rack the focuser all of the way out the very bare right edge of the mirror is cut off in the view. It just takes off enough of the view to remove sight of the edge of the mirror clips on that side. When racked in the whole mirror is visible.
From your description, I'm unsure which is happening. When you note that the scope is "fully collimated", do you mean the axial alignment is correct?
It sounds to me like your secondary mirror placement is still not fully corrected. If the secondary mirror looks "round" and the clipping (whichever type) is occurring on the side closer to or farther from the primary mirror, the error is due to incorrect offset, and the correction is (usually) done at the secondary mirror. But if the clipping is occurring perpendicular to the OTA axis, then the correction will probably require shimming the focuser. All of this assumes that the secondary mirror edge looks round (relative to the bottom of the focuser draw tube and the primary mirror reflection).
The focuser doesn't need to be square as much as it needs to be aligned to the secondary mirror. It doesn't hurt to have both--but getting precise squaring is unnecessary for most applications. How tall is the focuser fully racked in and fully racked out?
I just don't like losing that small sliver of mirror when racked out. I have heard reference to the focuser needing to be square. Is this the issue I am experiencing? Do I need to shim the focuser on one side?
Posted 24 January 2013 - 05:50 PM
The center of which mirror?
I dont think that is an issue. When using a sight tube the center of the mirror is directly under the crosshairs.
Posted 24 January 2013 - 06:45 PM
Posted 24 January 2013 - 07:17 PM
...When racked in all appears in the sight tube aligned and visible.
1.) Intersection of sight tube cross hairs aligned with primary mirror center spot (focuser axis)
2.) Reflection of sight tube crosshairs aligned with primary mirror center spot (primary mirror/optical axis)
Secondary mirror placement - all three circles concentric
1.) Bottom edge of sight tube
2.) Real edge of secondary mirror
3.) Reflected edge of primary mirror
The fact that the cross hairs of the sight tube are still aligned to the primary mirror center spot, we know that the focuser axis is aligned and the focuser draw tube travel is linear.
...When racked all of the way out the edge of the secondary...the upper right edge to right middle is slightly out of view by a hair, but the sight tube stays centered over the secondary and the primary center mark.
If one edge of the secondary mirror slips out of view when the focuser is racked out, then the secondary mirror is slightly offset incorrectly. If it's only slight, as you've described, I wouldn't worry about it too much, as it only affects balanced fov illumination, which will almost certainly be undetectable visually.
Keep the axial alignments in tolerance and your image performance will be unimpaired (by collimation).
Posted 24 January 2013 - 11:15 PM